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Evolutionary constraints may significantly bias phenotypic change, while “breaking” from such constraints can lead to expanded ecological opportunity. Ray-finned fishes have broken functional constraints by developing two jaws (oral-pharyngeal), decoupling prey capture (oral jaw) from processing (pharyngeal jaw). It is hypothesized that the oral and pharyngeal jaws represent independent evolutionary modules and this facilitated diversification in feeding architectures. Here we test this hypothesis in African cichlids. Contrary to our expectation, we find integration between jaws at multiple evolutionary levels. Next, we document integration at the genetic level, and identify a candidate gene, smad7, within a pleiotropic locus for oral and pharyngeal jaw shape that exhibits correlated expression between the two tissues. Collectively, our data show that African cichlid evolutionary success has occurred within the context of a coupled jaw system, an attribute that may be driving adaptive evolution in this iconic group by facilitating rapid shifts between foraging habitats, providing an advantage in a stochastic environment such as the East African Rift-Valley., Modular, rather than integrated systems are classically thought to allow functional diversity to evolve rapidly. A study of cichlid fish shows integration between divergent jaw systems at the phylogenetic, population, and genetic scales, suggesting integration can and does facilitate rapid, coordinated trait evolution.

Synopsis Ray-finned fishes have two jaw systems, the oral and pharyngeal jaws, which perform functions associated with prey capture and processing, respectively. The structural independence of the jaw systems is recognized as having broad implications for the functional and ecological diversity of the radiation. Cichlids (and a few other lineages) possess a modified pharyngeal jaw system that enhances prey processing versatility and capacity. This innovation, pharyngognathy, is hypothesized to have freed the oral jaws to diversify in terms of prey capture. We test the relative role of prey capture properties (e.g., evasiveness) and prey processing (e.g., crushing) in driving divergent selection in the oral and pharyngeal jaws using a macroevolutionary model fitting framework. Evolutionary outcomes were asymmetric. All transitions between different properties of prey capture had a corresponding transition in properties of prey processing. In contrast, fewer than half the transitions in the properties of prey processing had a corresponding prey capture transition. This discrepancy was further highlighted by multi-peak models that reflect the opposing function of each jaw system, which fit better than null models for oral jaw traits, but not pharyngeal jaw traits. These results suggest that pharyngeal jaw function can change independently from the function of the oral jaws, but not vice versa. This finding highlights the possibility of ecological limits to the evolutionary decoupling of jaw systems. The independent actions of prey capture and processing may be decoupled, but their respective functional demands (and evolution) are not. Therefore, prey likely impose some degree of coordinated evolution between acquisition and processing functional morphology, even in decoupled jaw systems.

Adaptive radiation is the likely source of much of the ecological and morphological diversity of life1–4. How adaptive radiations proceed and what determines their extent remains unclear in most cases1,4. Here we report the in-depth examination of the spectacular adaptive radiation of cichlid fishes in Lake Tanganyika. On the basis of whole-genome phylogenetic analyses, multivariate morphological measurements of three ecologically relevant trait complexes (body shape, upper oral jaw morphology and lower pharyngeal jaw shape), scoring of pigmentation patterns and approximations of the ecology of nearly all of the approximately 240 cichlid species endemic to Lake Tanganyika, we show that the radiation occurred within the confines of the lake and that morphological diversification proceeded in consecutive trait-specific pulses of rapid morphospace expansion. We provide empirical support for two theoretical predictions of how adaptive radiations proceed, the ‘early-burst’ scenario1,5 (for body shape) and the stages model1,6,7 (for all traits investigated). Through the analysis of two genomes per species and by taking advantage of the uneven distribution of species in subclades of the radiation, we further show that species richness scales positively with per-individual heterozygosity, but is not correlated with transposable element content, number of gene duplications or genome-wide levels of selection in coding sequences. Analyses of molecular, anatomical, pigmentation and ecological characteristics of nearly all of the approximately 240 species of cichlid fishes in Lake Tanganyika show that the massive adaptive radiation occurred within the confines of the lake through trait-specific pulses of accelerated evolution.

Food resource specialization within novel environments is considered a common axis of diversification in adaptive radiations. Feeding specializations are often coupled with striking morphological adaptations and exemplify the relation between morphology and diet (phenotype–environment correlations), as seen in, for example, Darwin finches, Hawaiian spiders, and the cichlid fish radiations in East African lakes. The cichlids' potential to rapidly exploit and occupy a variety of different habitats has previously been attributed to the variability and adaptability of their trophic structures including the pharyngeal jaw apparatus. Here we report a reciprocal transplant experiment designed to explore the adaptability of the trophic structures in highly specialized cichlid fish species. More specifically, we forced two common but ecologically distinct cichlid species from Lake Tanganyika, Tropheus moorii (rock‐dweller), and Xenotilapia boulengeri (sand‐dweller), to live on their preferred as well as on an unpreferred habitat (sand and rock, respectively). We measured their overall performance on the different habitat types and explored whether adaptive phenotypic plasticity is involved in adaptation. We found that, while habitat had no effect on the performance of X. boulengeri, T. moorii performed significantly better in its preferred habitat. Despite an experimental duration of several months, we did not find a shift in the morphology of the lower pharyngeal jaw bone that would be indicative of adaptive phenotypic plasticity in this trait., We performed a reciprocal transplant experiment in which we forced two highly specialized cichlid fish species from African Lake Tanganyika to live on their preferred as well as on an unpreferred habitat. We found that one of these species (the rock‐dweller Tropheus moorii) performed significantly better in its preferred habitat. We did not find evidence for adaptive phenotypic plasticity in trophic structures in these species.

Morphology plays a central role in determining diet. Knowledge of ecological and morphological relations between a species and its ecosystem is highly relevant. The aims of this study are to describe the diet and the lower pharyngeal jaw (LPJ) of Cichlasoma portalegrense and to test for the existence of ontogenetic and spatial patterns in its diet. Samplings took place in four sampling sites in Viamao, including two at the Refugio de Vida Silvestre Banhado dos Pachecos. Collected individuals were grouped in accordance with size categories and dissected. Food items were identified and quantified for each size category in each sampling site, according to the volumetric method, related to frequency of occurrence. Four linear measurements of the LPJ were carried out. The quantity and quality of their teeth were ascertained. Teeth arrangement was also analysed. Cichlasoma portalegrense is omnivorous with a tendency to carnivory and feeds on pelagic and benthic items from different origins. It shows neither ontogenetic patterns nor spatial variation in its diet. The species can feed on any possible food resource, thus being an advantage regarding the others. The LPJ has five types of teeth, is generalist without specializations, which is an adaptive advantage, since it is already adapted to selection pressures from different kinds of food. The shape of the LPJ and teeth distribution clearly show a same arrangement, regardless of fish size categories.

Innovations may provide access to new resources but often result in significant trade-offs. Pharyngognathy is a classic pharyngeal jaw innovation in which the left and right lower pharyngeal jaw (LPJ) bones are united into a single structure, producing a strong bite but reduced gape. Throughout cichlids, pharyngeal suturing occurs along the entire medial border between LPJ bones, except in peacock bass (Cichla), where these bones are connected by ligaments only in their anterior region. We show that this limited attachment permits the jaw bones to spread apart and we link this feature to an increase in pharyngeal gape that is comparable to non-pharyngognathous species. The capacity of the LPJ bones to spread apart is strongest in juveniles and is mostly lost during development. Juvenile Cichla exhibit size-specific pharyngeal gape similar to non-pharyngognathous percomorphs; however, adults exhibit pharyngeal gape on par with other predatory cichlids. Relaxation of pharyngeal suturing offsets a major deleterious consequence of pharyngognathy by reducing gape limitation and we propose this may accelerate the ontogenetic transition to piscivory. Partial reversal of the classic cichlid pharyngeal jaw innovation highlights the functional trade-offs that often accompany innovations and may be a major cause of variation in their macroevolutionary consequences.

Cichlidae have a magnificent ecological versatility that provides the species to explore many trophic niches, and behavioural biology. This versatility was achieved by modifications in the cranial and pharyngeal apparatus of cichlids. Accordingly, we aim to examine the relationships between pharyngeal morphology and diet of three species of the genus Crenicichla. We analysed the stomach contents and the morphology (through the geometric morphometric and teeth types) of their lower pharyngeal jaws (LPJ). We tested the hypothesis that LPJ morphology predicts the diet of Crenicichla species in streams. Our results indicated that the diet and the patterns of pharyngeal jaws were different among the species of Crenicichla, mainly the posterior process of the LPJ, the region of muscle insertion of LPJs with the neurocranium and the curvature of LPJ. Variation among the lower pharyngeal jaw shapes was shown to be directly associated with the use of food resources. Crenicichla species had a carnivorous diet based on fish and invertebrates (Aegla and aquatic insects). Higher fish consumption was observed in species with elongated lateral process in the LPJ and larger numbers of teeth, mainly recurved and conical unicuspid and bicuspid teeth (C. scottii and C. lepidota). On the other hand, higher consumption of Aegla was associated with a robust LPJ and molariform teeth (C. punctata). The total number of teeth was positively correlated with higher fish consumption. In addition, our results point to the need for investigations about spatial-temporal variations in Crenicichla diets to understand this group’s success in several freshwater environments.

Bone-producing osteoblasts and dentin-producing odontoblasts are closely related cell types, a result from their shared evolutionary history in the ancient dermal skeleton. In mammals, the two cell types can be distinguished based on histological characters and the cells’ position in the pulp cavity or in the tripartite periodontal complex. Different from mammals, teleost fish feature a broad diversity in tooth attachment modes, ranging from fibrous attachment to firm ankylosis to the underlying bone. The connection between dentin and jaw bone is often mediated by a collar of mineralized tissue, a part of the dental unit that has been termed “bone of attachment”. Its nature (bone, dentin, or an intermediate tissue type) is still debated. Likewise, there is a debate about the nature of the cells secreting this tissue: osteoblasts, odontoblasts, or yet another (intermediate) type of scleroblast. Here, we use expression of the P/Q rich secretory calcium-binding phosphoprotein 5 (scpp5) to characterize the cells lining the so-called bone of attachment in the zebrafish dentition.scpp5is expressed in late cytodifferentiation stage odontoblasts but not in the cells depositing the “bone of attachment”. nor inbona fideosteoblasts lining the supporting pharyngeal jaw bone. Together with the presence of the osteoblast marker Zns-5, and the absence of covering epithelium, this links the cells depositing the “bone of attachment” to osteoblasts rather than to odontoblasts. The presence of dentinal tubule-like cell extensions and the near absence of osteocytes, nevertheless distinguishes the “bone of attachment” from true bone. These results suggest that the “bone of attachment” in zebrafish has characters intermediate between bone and dentin, and, as a tissue, is better termed “dentinous bone”. In other teleosts, the tissue may adopt different properties. The data furthermore support the view that these two tissues are part of a continuum of mineralized tissues. Expression ofscpp5can be a valuable tool to investigate how differentiation pathways diverge between osteoblasts and odontoblasts in teleost models and help resolving the evolutionary history of tooth attachment structures in actinopterygians.

Virtually all fish rely on flows of water to transport food to the back of their pharynx. While external flows that draw food into the mouth are well described, how intra-oral water flows manage to deposit food at the esophagus entrance remains unknown. In theory, the posteriorly moving water must, at some point, curve laterally and/or ventrally to exit through the gill slits. Such flows would eventually carry food away from the esophagus instead of towards it. This apparent paradox calls for a filtration mechanism to deviate food from the suction-feeding streamlines. To study this gap in our fundamental understanding of how fish feed, we developed and applied a new technique to quantify three-dimensional patterns of intra-oral water flowsin vivo.We combined stereoscopic high-speed x-ray videos to quantify skeletal motion (XROMM) with 3D x-ray particle tracking (XPT) of approximately neutrally buoyant spheres of 1.4 mm in diameter. We showed, for carp (Cyprinus carpio) and tilapia (Oreochromis niloticus), that water tracers displayed higher curvatures than food tracers, indicating an inertia-driven filtration. In addition, tilapia also exhibited a ‘central jet’ flow pattern, which aids in quickly carrying food to the pharyngeal jaw region. When the food was trapped at the branchial basket, it was resuspended and carried more centrally by periodical bidirectional waterflows, synchronized with head-bone motions. By providing a complete picture of the suction-feeding process and revealing fundamental differences in food transport mechanisms among species, this new technique opens a new area of investigation to fully understand how most aquatic vertebrates feed.

Functional innovations are often invoked to explain the uneven distribution of ecological diversity. Innovations may provide access to new adaptive zones by expanding available ecological opportunities and may serve as catalysts of adaptive radiation. However, diversity is often unevenly distributed within clades that share a key innovation, highlighting the possibility that the impact of the innovation is mediated by other traits. Pharyngognathy is a widely recognized innovation of the pharyngeal jaws that enhances the ability to process hard and tough prey in several major radiations of fishes, including marine wrasses and freshwater cichlids. We explored diversification of lower pharyngeal jaw shape, a key feature of pharyngognathy, and the extent to which it is influenced by head shape in Neotropical cichlids. While pharyngeal jaw shape was unaffected by either head length or head depth, its disparity declined dramatically with increasing head width. Head width also predicted the rate of pharyngeal jaw evolution such that higher rates were associated with narrow heads. Wide heads are associated with exploiting prey that require intense processing by pharyngeal jaws that have expanded surfaces for the attachment of enlarged muscles. However, we show that a wide head constrains access to adaptive peaks associated with several trophic roles. A constraint on the independent evolution of pharyngeal jaw and head shape may explain the uneven distribution of ecological diversity within a clade that shares a major functional innovation.

The structural details of some muscles, ligaments, aponeurotic structures, and osteological features of the pharyngeal jaw apparatus of ember parrotfish Scarus rubroviolaceus were studied. During the preparation, the evidence of the presence of a palatal organ in this species, which is convergently similar to that of benthic fish species of Cyprinidae and Catostomidae, has been found. The indirect effect of contraction of m. geniohyoideus and m. sternohyoideus on the operation of the pharyngeal jaw is discussed. Based on the results of anatomy and analysis of the literature, the interpretation of previously obtained morphofunctional data characterizing the apparatus of the pharyngeal jaws of parrotfish (Scaridae) is corrected using S. rubroviolaceus as an example.

Background Understanding how variation in gene expression contributes to morphological diversity is a major goal in evolutionary biology. Cichlid fishes from the East African Great lakes exhibit striking diversity in trophic adaptations predicated on the functional modularity of their two sets of jaws (oral and pharyngeal). However, the transcriptional basis of this modularity is not so well understood, as no studies thus far have directly compared the expression of genes in the oral and pharyngeal jaws. Nor is it well understood how gene expression may have contributed to the parallel evolution of trophic morphologies across the replicate cichlid adaptive radiations in Lake Tanganyika, Malawi and Victoria. Results We set out to investigate the role of gene expression divergence in cichlid fishes from these three lakes adapted to herbivorous and carnivorous trophic niches. We focused on the development stage prior to the onset of exogenous feeding that is critical for understanding patterns of gene expression after oral and pharyngeal jaw skeletogenesis, anticipating environmental cues. This framework permitted us for the first time to test for signatures of gene expression underlying jaw modularity in convergent eco-morphologies across three independent adaptive radiations. We validated a set of reference genes, with stable expression between the two jaw types and across species, which can be important for future studies of gene expression in cichlid jaws. Next we found evidence of modular and non-modular gene expression between the two jaws, across different trophic niches and lakes. For instance, prdm1a, a skeletogenic gene with modular anterior-posterior expression, displayed higher pharyngeal jaw expression and modular expression pattern only in carnivorous species. Furthermore, we found the expression of genes in cichlids jaws from the youngest Lake Victoria to exhibit low modularity compared to the older lakes. Conclusion Overall, our results provide cross-species transcriptional comparisons of modularly-regulated skeletogenic genes in the two jaw types, implicating expression differences which might contribute to the formation of divergent trophic morphologies at the stage of larval independence prior to foraging. Electronic supplementary material The online version of this article (10.1186/s12862-019-1483-3) contains supplementary material, which is available to authorized users.

The Nicaraguan Midas cichlid species complex is a natural experiment where fish from a large source population from turbid and shallow great lakes very recently (

The two toothed jaws of cichlid fishes provide textbook examples of convergent evolution. Tooth phenotypes such as enlarged molar-like teeth used to process hard-shelled mollusks have evolved numerous times independently during cichlid diversification. Although the ecological benefit of molar-like teeth to crush prey is known, it is unclear whether the same molecular mechanisms underlie these convergent traits. To identify genes involved in the evolution and development of enlarged cichlid teeth, we performed RNA-seq on the serially homologous-toothed oral and pharyngeal jaws as well as the fourth toothless gill arch of Astatoreochromis alluaudi. We identified 27 genes that are highly upregulated on both tooth-bearing jaws compared with the toothless gill arch. Most of these genes have never been reported to play a role in tooth formation. Two of these genes (unk, rpfA) are not found in other vertebrate genomes but are present in all cichlid genomes. They also cluster genomically with two other highly expressed tooth genes (odam, scpp5) that exhibit conserved expression during vertebrate odontogenesis. Unk and rpfA were confirmed via in situ hybridization to be expressed in developing teeth of Astatotilapia burtoni. We then examined expression of the cluster’s four genes in six evolutionarily independent and phylogenetically disparate cichlid species pairs each with a large- and a small-toothed species. Odam and unk commonly and scpp5 and rpfA always showed higher expression in larger toothed cichlid jaws. Convergent trophic adaptations across cichlid diversity are associated with the repeated developmental deployment of this genomic cluster containing conserved and novel cichlid-specific genes.

Some species of durophagous moray eels (Muraenidae) have been documented emerging from the marine environment to capture intertidal crabs but how they consume prey out of water is unknown. Here, we trained snowflake morays, Echidna nebulosa, to undulate out of the aquatic environment to feed on land. On land, snowflake morays remove prey from the substrate by biting and swallow prey using pharyngeal jaw enabled transport. Although snowflake morays exhibit smaller jaw rotation angles on land when apprehending their prey, transport kinematics involving dorsoventral flexion of the head to protract the pharyngeal jaws and overall feeding times did not differ between terrestrial and aquatic treatments. We suggest that their elongate body plan, ability to rotate their heads in the dorsoventral and lateral directions, and extreme pharyngeal movements all contribute to the ability of durophagous morays to feed in the terrestrial environment., Summary: Body elongation and pharyngeal transport facilitates prey capture and swallowing on land for the snowflake moray, Echidna nebulosa.

BackgroundAdaptive radiations are characterized by extreme and/or iterative phenotypic divergence; however, such variation does not accumulate evenly across an organism. Instead, it is often partitioned into sub-units, or modules, which can differentially respond to selection. While it is recognized that changing the pattern of modularity or the strength of covariation (integration) can influence the range or rate of morphological evolution, the relationship between shape variation and covariation remains unclear. For example, it is possible that rapid phenotypic change requires concomitant changes to the underlying covariance structure. Alternatively, repeated shifts between phenotypic states may be facilitated by a conserved covariance structure. Distinguishing between these scenarios will contribute to a better understanding of the factors that shape biodiversity. Here, we explore these questions using a diverse Lake Malawi cichlid species complex,Tropheops,that appears to partition habitat by depth.ResultsWe construct a phylogeny ofTropheopspopulations and use 3D geometric morphometrics to assess the shape of four bones involved in feeding (mandible, pharyngeal jaw, maxilla, pre-maxilla) in populations that inhabit deep versus shallow habitats. We next test numerous modularity hypotheses to understand whether fish at different depths are characterized by conserved or divergent patterns of modularity. We further examine rates of morphological evolution and disparity between habitats and among modules. Finally, we raise a singleTropheopsspecies in environments mimicking deep or shallow habitats to discover whether plasticity can replicate the pattern of morphology, disparity, or modularity observed in natural populations.ConclusionsOur data support the hypothesis that conserved patterns of modularity permit the evolution of divergent morphologies and may facilitate the repeated transitions between habitats. In addition, we find the lab-reared populations replicate many trends in the natural populations, which suggests that plasticity may be an important force in initiating depth transitions, priming the feeding apparatus for evolutionary change.

Dental convergence is a hallmark of cichlid fish adaptive radiations. This type of repeated evolution characterizes both the oral jaws of these fishes as well as their pharyngeal jaws that are modified gill arches used to functionally process prey like hard-shelled mollusks. To test several hypotheses regarding the evolution of cichlid crushing pharyngeal dentitions, we used X-ray computed tomography scans to comparatively examine dental evolution in the pharyngeal jaw of a diversity of New World Heroine cichlid lineages. The substantial variation in erupted tooth sizes and numbers as well as replacement teeth found in these fishes showed several general patterns. Larger toothed species tended to have fewer teeth suggesting a potential role of spatial constraints in cichlid dental divergence. Species with larger numbers of erupted pharyngeal teeth also had larger numbers of replacement teeth. Replacement tooth size is almost exactly predicted (r = 0.99) from the size of erupted teeth across all of the species. Mollusk crushing was, therefore, highly associated with not only larger pharyngeal teeth, but also larger replacement teeth. Whether dental divergence arises as a result of environmental induced plasticity or originates via trophic polymorphism as found in the species Herichthys minckleyi, there appear to be general rules that structure interspecific divergence in cichlid pharyngeal erupted and replacement dentitions.

The present investigation was designed to describe the surface ultrastructure of the gill system of tilapia Zilli. The gill system is formed from four gill arches and each gill arch carries a row of gill filaments on its convex border and two rows of the gill rakers on its concave border. The quadrilateral interbranchial septum has elevated part at the level of the third gill arch. By SEM observations, the gill arch was divided into three regions: rostral, middle and caudal region. The caudal region contained two characteristic structures: oval leaf-like structure and rounded-shaped structure. Each oval leaf-like structure carried two lateral rows of the triangular pointed spines separated by a median groove. All surfaces of gill arches, rakers and filaments were covered with a mosaic of the polygonal pavement cells, in addition to the opening of chloride cells and mucous cells. The gill arch and gill raker had only one appearance of taste buds named type I. Meanwhile, the filaments contained two types of different appearance of the taste buds named: type I and type II. Type I was the main common and similar to that present in gill arch and raker and characterized by its blunt end, while type II had hair-like structures that projected from the volcano-shaped depression. The gill rakers were formed from central axis surrounded by two lateral lobulated regions which carry pointed spines, taste buds and the opening of chloride cells. The surface of triangular lower pharyngeal jaw carries numerous teeth-like papillae which originated from the socket-like depression.

This study compares sand shiner (Notropis stramineus) and silverjaw (Ericymba buccata) minnows, in terms of the morphological shape changes of the upper, lower, and pharyngeal jaws over ontogeny. These two species of minnows initially feed on midge larvae and undergo an ontogenic prey shift. The traditional morphometrics measured-total length, snout-to-vent length, eye diameter, premaxilla length, lower jaw length, gape-were regressed onto total length to test for allometry. Digital pictures were processed with tpsDig and further analyzed with MorphoJ utilizing a regular geometric morphometrics procedure using principle component analyses. We examined gut contents for 16 fish of each species. For the silverjaw minnows, we found all jaw variables to exhibit positive allometric growth with increasing total length, while most of the jaw variables for the sand shiner exhibited negative allometric growth with increasing total length. This correlates with an ontogenic prey shift for both species. Sand shiner minnows have been found to be more omnivorous, feeding on algae later in life, while silverjaw minnows undergo a prey shift to larger invertebrates. These species lack oral dentition causing an increased reliance on the pharyngeal apparatus. Principle component analyses revealed elongation of pharyngeal jaw elements in the silverjaw minnows and a relative shortening and bulking of pharyngeal jaws in the sand shiner minnows. The ontogenic dietary shifts observed in these two species provide possible explanation for the morphological changes over ontogeny in jaw elements, which are likely enabling these species to occupy the same habitat with little niche overlap.

Although the form and function of feeding apparatus structures and diet composition are linked in adult fishes, it is often not clear when during ontogeny ecomorphological patterns develop, enabling early life history stage (ELHS) fishes to partition foraging niches and reduce competition. Species of drums (Sciaenidae) demonstrate tremendous variability in the structure of the feeding apparatus, which allows sympatric species to exploit a broader range of foraging habitats as adults than any other family of fishes in the coastal systems of the western Atlantic. The feeding apparatus (i.e., oral and pharyngeal jaw elements) and stomach contents were examined in ELHS and juvenile sciaenids from the Chesapeake Bay representing three foraging guilds: pelagic, generalist, and benthic. Stomach contents were examined and specimens were cleared and double-stained to measure elements of the feeding apparatus. Regardless of foraging guild, a single diet shift was observed in the size range of fishes examined. Benthic sciaenids were the first to undergo a diet shift at 16 mm SL, which was attributed to changes in the feeding apparatus. Pelagic sciaenids underwent a diet shift at 17 mm SL, which was attributed to a previously studied shift in the sensory modality. The generalist sciaenids were the last to undergo a diet shift at 20 mm SL, which was attributed to changes in both the feeding apparatus and sensory abilities. These ecomorphological patterns identify the developmental events to the feeding apparatus and sensory systems necessary for ELHS sciaenids to begin partitioning their foraging niches, reducing their susceptibility to competition and bottom-up processes in estuarine ecosystems.

Drums (family Sciaenidae) are common in tropical to temperate coastal and estuarine habitats worldwide and present a broad spectrum of morphological diversity. The anatomical variation in this family is particularly evident in their feeding apparatus, which may reflect the partitioning of adult foraging habitats. Adult and early life history stage sciaenids may display ecomorphological patterns in oral and pharyngeal jaw elements but because sciaenids are hierarchically related, the morphological variation of the feeding apparatus cannot be analyzed as independent data. Morphological patterns have been identified in three sciaenid genera from the Chesapeake Bay but it is not known if these patterns are present in other genera of the family and if such patterns are constrained by phylogenetic history. In this study, phylogenetic comparative methods were applied to two sets of oral jaw data obtained from growth series of 11 species of cleared and double-stained Chesapeake Bay sciaenids and alcohol-preserved museum specimens representing 65 of the 66 recognized genera to determine the magnitude of phylogenetic dependence present in the structure of the oral jaws using a recent molecular phylogeny of the family. Pagel’s lambda, a measure of phylogenetic signal, was low for pelagic sciaenids in premaxilla, lower jaw, and ascending process lengths, indicating influence of selective forces on the condition of these traits. Conversely, for benthic sciaenids, phylogenetic signal was high for lower jaw and ascending process lengths, indicating significant phylogenetic constraint for their condition in these taxa. Pagel’s lambda was intermediate for premaxilla length in benthic sciaenids, suggesting that the length of the premaxilla is influenced by a mix of selective forces and phylogenetic constraint. Although the ecomorphological patterns identified in the oral jaws of scaienids are not entirely free of phylogenetic dependence, selective forces related to foraging are likely driving the evolution of these structures.

We examine the structure of the bone of the pharyngeal jaws of a large fish, the black drum (Pogonias cromis), that uses its tooth-jaw complex to crush hard-shelled bivalve mollusks. During mastication huge compressive forces are concentrated in a tiny zone at the tooth-bone interface. We report on the structure of this bone, with emphasis on its contact with the teeth, at different hierarchical levels and in 3D. Micro-CT shows that the molariform teeth do not have roots and are supported by a circular narrow bony rim that surrounds the periphery of the tooth base. The lower pharyngeal jaw is highly porous, as seen by reflected light microscopy and secondary electron microscopy (SE-SEM). Porosity decreases close to the bone-tooth interface and back-scattered electron (BSE-SEM) microscopy shows a slight elevation in mineral density. Focused ion beam - scanning electron microscopy (FIB-SEM) in the serial surface view (SSV) mode reveals a most surprising organization at the nanoscale level: parallel arrays of mineralized collagen fibrils surrounding channels of ~100 nm diameter, both with their long axes oriented along the load direction. The channels are filled with organic matter. These fibril-channel arrays are surrounded by a highly disordered mineralized material. This unusual structure clearly functions efficiently under compression, but the precise way by which this unique arrangement achieves this function is unknown.

Two monotypic haplochromine cichlid genera (Teleostei: Cichlidae) of the Pseudocrenilabrus group are described from northern Zambia. One new genus is Palaeoplex gen. nov., with Pa. palimpsest sp. nov. as the type species, from the Luongo and Kalungwishi Rivers (Upper Congo drainage, Luapula subdrainage). It is diagnosed by a unique combination of morphological characters: (1) a fully developed infraorbital series without a distinct gap between the lachrymal and second infraorbital bone, (2) fused hypuralia 1+2 and hypuralia 3+4, (3) molariform teeth on the sagittal series of the lower pharyngeal jaw, and (4) a large maximum size. The second new genus, Lufubuchromis gen. nov., with L. relictus sp. nov. as the type species, is restricted to the upper Lufubu River catchment (Upper Congo drainage, Lake Tanganyika subdrainage). It is diagnosed by a unique combination of morphological characters: (1) a fully developed infraorbital series without a distinct gap between the lachrymal and second infraorbital bone, (2) fused hypuralia 1+2 (rarely with a visible suture) and fused hypuralia 3+4, (3) a unique male coloration pattern, i.e. deep crimson red colored areas on the anterior ventral flank parts, chest and belly and on the lower head; remaining parts of flanks and caudal peduncle bluish), and (4) a Pseudocrenilabrus blotch present in both sexes. Both new genera are compared with all remaining taxa of the Pseudocrenilabrus group and with all representatives of all other major haplotilapiine lineages.

Hybridization drives rapid speciation by shaping novel genotypic and phenotypic profiles. Genomic incompatibility and transcriptome shock have been observed in hybrids, although this is rarer in animals than in plants. Using the newly sequenced genomes of the blunt snout bream (Megalobrama amblycephala [BSB]) and the topmouth culter (Culter alburnus [TC]), we focused on the sequence variation and gene expression changes in the reciprocal intergeneric hybrid lineages (F1–F3) of BSB × TC. A genome-wide transcriptional analysis identified 145–974 expressed recombinant genes in the successive generations of hybrid fish, suggesting the rapid emergence of allelic variation following hybridization. Some gradual changes of gene expression with additive and dominance effects and various cis and trans regulations were observed from F1 to F3 in the two hybrid lineages. These asymmetric patterns of gene expression represent the alternative strategies for counteracting deleterious effects of the subgenomes and improving adaptability of novel hybrids. Furthermore, we identified positive selection and additive expression patterns in transforming growth factor, beta 1b (tgfb1b), which may account for the morphological variations of the pharyngeal jaw in the two hybrid lineages. Our current findings provide insights into the evolution of vertebrate genomes immediately following hybridization.

Haplochromis pharyngalis and Haplochromis petronius, two endemic cichlids from the Lake Edward system (Uganda, Democratic Republic of the Congo), are very similar in general morphology but have been reported to differ in pharyngeal jaw morphology and distribution. This study analysed 51 morphometrics and various qualitative characteristics of 48 specimens from different localities. The morphological traits of both species strongly overlap, and differences in the pharyngeal jaw morphology correspond to a geographic morphocline. We conclude that all specimens belong to one valid species, H. pharyngalis, and consider H. petronius to be a synonym.

Durophagous fish consume a diet based primarily on hard-shelled animals, mainly mollusks. In order to successfully perform this task, they are equipped with an extra set of jaws located in their throat called pharyngeal jaws. Here we present the results of a study of the structure of the bony material of the exceptionally powerful lower pharyngeal jaws (LPJs) of the black drum Pogonias cromis which generate the highest biting forces documented in bony fishes. In particular, we studied the two long and slender struts that support the entire dental plate and teeth of the LPJ, in order to determine how this structure withstands the huge stresses it encounters repetitively and for long periods of time. We describe the hierarchical structure of the struts of lower pharyngeal jaw of P. cromis at a wide range of length scales, and show how it is adapted to successfully achieve its high mechanical performance. In particular, we show that the bone material of the strut is neither cortical nor cancellous, and although it is highly porous, its complex and layered three-dimensional arrangement of thick lamellae sheets, which are inter-connected by thin plates, is perfectly tailored to withstand extremely large but directionally-consistent forces. STATEMENT OF SIGNIFICANCE: The diet of some fish consists of hard food, like mollusks and shells. In order to accomplish the task of cracking this type of food, they have an extra set of bony jaws located in their throat, called pharyngeal jaws. Here we describe the hierarchical structural elements of these jaws which allow them to withstand huge forces repeatedly over long periods of time. Surprisingly, the structure is very porous, but its architectural design is superbly adapted to handle consistently-oriented forces. This structural motif defines a new bony material which is neither cortical nor cancellous.

Coral reefs are complex marine habitats that have been hypothesized to facilitate functional specialization and increased rates of functional and morphological evolution. Wrasses (Labridae: Percomorpha) in particular, have diversified extensively in these coral reef environments and have evolved adaptations to further exploit reef-specific resources. Prior studies have found that reef-dwelling wrasses exhibit higher rates of functional evolution, leading to higher functional variation than in non-reef dwelling wrasses. Here, we examine this hypothesis in the lower pharyngeal tooth plate of 134 species of reef and non-reef-associated labrid fishes using high-resolution morphological data in the form of micro-computed tomography scans and employing three-dimensional geometric morphometrics to quantify shape differences. We find that reef-dwelling wrasses do not differ from non-reef-associated wrasses in morphological disparity or rates of shape evolution. However, we find that some reef-associated species (e.g., parrotfishes and tubelips) exhibit elevated rates of pharyngeal jaw shape evolution and have colonized unique regions of morphospace. These results suggest that while coral reef association may provide the opportunity for specialization and morphological diversification, species must still be able to capitalize on the ecological opportunities to invade novel niche space, and that these novel invasions may prompt rapid rates of morphological evolution in the associated traits that allow them to capitalize on new resources.

Convergent evolution is at the forefront of many form-function studies. There are many examples of multiple independent lineages evolving a similar morphology in response to similar functional demands, providing a framework for testing hypotheses of form-function evolution. However, there are numerous clades with underappreciated convergence, in which there is a perceived homogeneity in morphology. In these groups, it can be difficult to investigate causal relationships of form and function (e.g., diet influencing the evolution of jaw morphology) without the ability to disentangle phylogenetic signal from convergence. Leuciscids (Cypriniformes: Leuciscidae; formerly nested within Cyprinidae) are a species-rich clade of fishes that have diversified to occupy nearly every freshwater trophic niche, yet are considered to have relatively low morphological diversity relative to other large freshwater clades. Within the North American leuciscids, many genera contain at least one herbivore, insectivore, and larvaphage. We created 3D models from micro-computed tomography scans of 165 leuciscid species to measure functionally relevant traits within the pharyngeal jaws of these fishes. Using a published phylogeny, we tested these metrics for evolutionary integration, phylogenetic signal, and correlation with diet. Measurements of the pharyngeal jaws, muscle attachment areas, and teeth showed strong positive evolutionary correlation with each other and negative evolutionary correlation with measurements of the inter-ceratobranchial ligament (ICB ligament). Using diet data from published literature, we found extensive dietary convergence within Leuciscidae. The most common transitions we found were between herbivorous and invertivorous taxa and between insectivore types (aquatic vs. terrestrial). We document a trade-off in which herbivorous leuciscids have large teeth, short ICB ligaments, and large muscle attachment areas, whereas insectivorous leuciscids showed the opposite pattern. Inverse patterns of morphological integration between the ICB ligament the rest of the pharyngeal jaw correspond this dietary trade-off, which indicates that coordinated evolution of morphological traits contributes to functional diversity in this clade. However, these patterns only emerge in the context of phylogeny, meaning that the pharyngeal jaws of North American leuciscids converge by similar means (structural changes in response to dietary demands), but not necessarily to similar ends (absolute phenotype).

Investigations in the form–function relationships of vertebrate feeding systems have a long and illustrious history of inferring function from anatomical structure and specimen manipulation, and a shorter but highly successful history of measuring function directly in living animals with sophisticated methods such as electromyography, bone strain gauges, high-speed cinematography, and cineradiography. Two new methods, X-ray Reconstruction of Moving Morphology (XROMM) and fluoromicrometry show great promise for revealing the 3D form–function relationships of cranial and cervical musculoskeletal structures. XROMM has been applied to measure 3D jaw kinematics and tooth occlusion in mammalian mastication and 3D pharyngeal jaw mechanics in fishes. The form–function relationships of the temporomandibular and other cranial joints are being explored with XROMM, including cranial kinesis in squamates, birds, and fishes. Muscle strain can be measured with fluoromicrometry by implanting small radio-opaque beads and tracking them with biplanar fluoroscopy. Fluoromicrometry has been used to measure muscle strain in muscles with complex architectures, such as the axial muscles during suction feeding in fishes and tongue deformation during swallowing in mammals. XROMM, fluoromicrometry, and buccal pressure measurements together have been used to measure the instantaneous power required for suction feeding and relate required power to the available muscle power. In the future, some systems that may benefit greatly from XROMM and fluoromicrometry are tooth–food interactions during mastication, food transport and swallowing, and the form–function relationships of feeding muscles with complex muscle-tendon architecture, such as the mammalian muscles of mastication and the multi-part adductor mandibulae of fishes.

In this paper, we develop a gripper by mimicking the pharyngeal jaw of a moray eel to perform assembly tasks. A moray eel has two sets of jaws called the oral jaw (the main jaw) and the pharyngeal jaw (the smaller jaw). The pharyngeal jaw is mobile, which means it is able to go forward or backward relative to the oral jaw. By likening each jaw as a two-finger parallel gripper, we develop a bio-inspired robotic hand with similar structure, which we call a double jaw hand. The two parallel grippers in the double jaw hand are connected by a prismatic joint on a common approaching vector. Similar to moray eels, the double jaw hand is able to hold objects using the outer gripper and hand over them to the inner gripper (inspired by how a moray eel captures and swallows objects). Also, it is able to assemble and disassemble industrial parts (inspired by how a moray eel cuts its prey). Several experiments are performed to demonstrate the design and the abilities.

Cyprinids constitute the largest fish family and are characterized by their pharyngeal teeth. The masticatory mechanism is still poorly understood. The complex of structures that determine the movements of pharyngeal teeth and chewing pad in the carp (Cyprinus carpio L.) is analyzed. Activities in 16 head muscles of a free-swimming carp were recorded. X-ray cinerecordings, synchronized with electromyograms, were made of the intake, transport, mastication, and deglutition of radiopaque food pellets. Metal markers allowed a detailed movement analysis. Masticatory cycles are bilaterally synchronous and show distinct crushing and grinding patterns. Direct masticatory muscles that suspend and connect the pharyngeal bones steer and stabilize the masticatory movements. Baudelot's ligament, between skull and pectoral girdle, is applied as fulcrum, effects a crucial shift of the rotation axis of the pharyngeal jaw, and transforms crushing into grinding; simultaneous abduction lengthens the grinding stroke. Body muscles supply indirectly the power for mastication; they also appear to be regulated more distantly. The epaxial muscles lift the skull and thereby the levators of the pharyngeal bones, thus transmitting high forces to the teeth. They also stretch the levator of the bone as soon as occlusion is reached and thus optimize its production of forces during grinding. The hypaxial muscles retract the pharyngeal bones indirectly during grinding and power the teeth in sliding. The chewing pad, previously assumed to be motionless, rotates rostroventrad with the skull and intensifies grinding. Respiration and mastication are mutually related. The extensive movements of the pharyngeal bones are permitted only by the simultaneous expansion of the buccopharynx and a slide-coupling in the branchial floor. Muscular pads that line the pharynx are shown to transport food toward the teeth. The constrictor pharyngis effects deglutition. Natural food, intestinal contents, and feces of the carp were analyzed with respect to the capacity for distinct masticatory operations. During the experiments pellets, barley, and worms were fed. The carp is specialized for polyphagy and this appears to be based on the profiles of the heterodont teeth rather than on drastic changes in the two preprogrammed activity patterns. Comparison of the pharyngeal jaw system in the carp and higher teleosts emphasizes the structural design for the application of large forces in this cyprinid.

The gross morphology and electrical activity of the muscles of the pharyngeal apparatus of centrarchid sunfishes (Lepomis) are analyzed within a monophyletic clade containing species specialized for snail-eating. Outgroup comparisons of both structure and activity patterns of muscles permit examination of the relationship between specialized diet and function of the trophic apparatus. In most sunfish species, electrical activity in the pharyngocleithralis internus muscle significantly overlaps that in the retractor dorsalis muscle during pharyngeal transport, indicating that the upper and lower pharyngeal jaws retract together. Activity in the pharyngohyoideus, levatores externi, and levator posterior also significantly overlaps activity of the retractor dorsalis. Snail-eating is associated with derived morphological, behavioral, and functional features. The shell is crushed before pharyngeal transport, correlated with extensive overlap in activity periods of muscles. One species, Lepomis microlophus, possesses a highly stereotyped neuromuscular repertoire that does not vary with prey type. All prey, even fish and worms, are subjected to crushing. Lepomis gibbosus exhibits the crushing pattern of muscle activity only when feeding on snails. L. microlophus has a hypertrophied levator posterior muscle, but the lines of action of the pharyngeal muscles are similar to the primitive condition. Pharyngeal transport in this species is unique in that activity of the pharyngocleithralis internus alternates with that of the retractor dorsalis. In sunfishes, alterations in the central control of peripheral structures have produced major changes in the sequence in which homologous components of the structural network are activated.

Among the acanthopterygian fishes, the Labridae possess the most highly integrated and specialized pharyngeal jaw apparatus. The integrated feature involves many osteological components and aspects of muscle form, architecture, composition, and function. The upper jaw articulates by means of a true diarthrosis with the pharyngeal process of the parasphenoid, whereas the lower jaw has established physical contact with the cleithrum. Complex muscle fusions have contributed significantly in the development of a double muscle sling operating the lower jaw. The original levator externus 4 fuses with the central head of the obliquus posterior, whereas the original levator posterior combines with the lateral head of the obliquus posterior as well as with the adductor branchialis 5. During the masticatory cycle, both upper and lower jaws undergo complex movement orbits resulting in shearing and crushing functions. Shearing occurs as the forward moving upper jaw collides with the dorsally held lower jaw. Crushing is effected by an extreme posterodorsal movement of the lower jaw against the retracted upper jaw, thereby establishing full occlusion of the teeth. The specialized morphological and functional design of the labrid pharyngeal jaw apparatus is similar to that found in cichlids. In sharp contrast to primitive acanthopterygian fishes, the Labridae and Cichlidae exhibit a spectacular morphological diversity that parallels their ecological diversification. Our combined functional and historical analysis has established a correlation between the complex integration of the pharyngeal jaw apparatus and morphological and ecological diversity in the Labridae and Cichlidae.

A new mechanical model for function of the pharyngeal jaw apparatus in generalized perciform fishes is developed from work with the family Haemulidae. The model is based on anatomical observations, patterns of muscle activity during feeding (electromyography), and the actions of directly stimulated muscles. The primary working stroke of the pharyngeal apparatus involves simultaneous upper jaw depression and retraction against a stabilized and elevating lower jaw. The working stroke is characterized by overlapping activity in most branchial muscles and is resolved into three phases. Four muscles (obliquus dorsalis 3, levator posterior, levator externus 3/4, and obliquus posterior) that act to depress the upper jaws become active in the first phase. Next, the retractor dorsalis, the only upper jaw retracting muscle, becomes active. Finally, there is activity in several muscles (transversus ventrales, pharyngocleithralis externus, pharyngohyoideus, and protractor pectoralis) that attach to the lower jaws. The combined effect of these muscles is to elevate and stabilize the lower jaws against the depressing and retracting upper jaws. The model identifies a novel mechanism of upper jaw depression, here proposed to be the primary component of the perciform pharyngeal jaw bite. The key to this mechanism is the joint between the epibranchial and toothed pharyngobranchial of arches 3 and 4. Dorsal rotation of epibranchials 3 and 4 about the insertion of the obliquus posterior depresses the lateral border of pharyngobranchials 3 and 4 (upper jaw). The obliquus dorsalis 3 muscle crosses the epibranchial-pharyngo-branchial joint in arches 3 and 4, and several additional muscles effect epibranchial rotation. Five upper jaw muscles cause upper jaw depression upon electrical stimulation: the obliquus dorsalis 3, levator posterior, levator externus 3/4, obliquus posterior, and transversus dorsalis. This result directly contradicts previous interpretations of function for the first three muscles. The presence of strong depression of the upper pharyngeal jaws explains the ability of many generalized perciform fishes to crush hard prey in their pharyngeal apparatus.

Osteoglossomorph fishes are characterized by having three sets of jaws: a mandibular jaw apparatus (MJA) anteriorly, a pharyngeal jaw apparatus (PJA) posteriorly, and a tongue-bite apparatus (TBA) associated with basihyal and parasphenoid teeth. The TBA is a novel complex feature of the head that characterizes osteoglossomorph fishes and provides a case study in the origin of novel functions and roles in the vertebrate musculoskeletal system. The function of the tongue-bite in the osteoglossomorph fish Notopterus was characterized by using high-speed cinematography and electromyography. The tongue-bite is used during intraoral prey processing to shred and disable prey. Two distinct uses of the TBA were defined on the basis of kinematic and electromyographic profiles: raking and open-mouth chewing. During raking behavior, the prey is held fixed in the MJA, the neurocranium is elevated, and the pectoral girdle is retracted. The adductor mandibulae, hypaxialis, epaxialis, and posterior intermandibularis muscles are all highly active, but only very low activity is observed in the sternohyoideus muscle. During open-mouth chewing behavior, the prey is located within the oral cavity, posterior pectoral girdle rotation is less than during raking, and the levator operculi muscle shows relatively high activity. We propose that a shearing action of the basihyal (moved anteroposteriorly by the posterior intermandibularis and hypaxial muscles) with respect to the neurocranium (elevated by epaxial muscles) is the critical aspect of the tongue-bite in Notopterus. The body muscles (epaxials and hypaxials) provide the main power for the tongue-bite. We hypothesize that lack of sternohyoideus activity during intraoral prey processing, posterior pectoral girdle rotation, and a long-fibered posterior intermandibularis muscle are novel structures associated with the tongue-bite apparatus within osteoglossomorph fishes.

Cichlids possess a complex pharyngeal jaw apparatus, the osteological components of which are two upper pharyngeal jaws, articulating with the neurocranial base, and a single lower pharyngeal jaw. Quantitative cinera-diography revealed that pharyngeal food processing in Oreochromis niloticus involves transport, mastication, and swallowing, effected by cyclical pharyngeal jaw movements. Transport and swallowing occur by simultaneous retractions of both upper pharyngeal jaws. Food reduction (mastication) is effected by lower jaw elevation (compression) and protraction (shear) during upper jaw retraction. Each movement cycle contains a transport, reduction, and swallowing component, although their relative importance may vary within a feeding sequence. The upper and lower pharyngeal jaws show opposite anteroposterior movements during most of the cycle. Variations in the amplitudes and the durations of the different movement components reflect the consistency and the size of the food.

Hypophthalmichthys molitrix, silver carp, is an invasive Asian carp that has become increasingly widespread and ecologically destructive within the upper Mississippi River Basin. Its complex trophic anatomy may help explain the apparent efficiency with which they consume phytoplankton, outcompeting native filter feeders. This cypriniform species is characterized by trophic synapomorphies that include a palatal organ, loss of upper pharyngeal jaws, and a hypertrophied lower pharyngeal jaw. However, in silver carp these structures have become greatly modified and diverge from the more basal condition that characterizes species such as goldfish. The trophic apparatus of silver carp is composed of discrete structures that are functionally coupled: filtering plates, paired epibranchial organs (EBO), a modified palatal organ composed of large muscular folds that interdigitate with the filtering plates, and hypertrophied lower pharyngeal jaws and teeth. The filtering plates fill a significant portion of the buccal cavity, especially since the distal parts of these filtering plates make up a key component of the EBOs. EBOs, food aggregating structures found in many teleosts, are thought to have independently evolved at least six times. Ranging in complexity from small slits on the dorsal wall of the pharyngeal cavity to exceedingly intricate spiraling structures, EBOs are morphologically diverse among filter-feeding fishes. Despite this morphological diversity and broad taxonomic distribution, little is known regarding the functional anatomy of the EBO. Moreover, the EBO in silver carp is distinct from the organs previously described in other species, being created by four independent pharyngeal involutions (instead of the more typical one or two) that form spiral-shaped pharyngeal tubes surrounded by circumferential muscle. On each side of the head greatly hypertrophied hyomandibulae and opercles are connected to the anterior cartilaginous caps of the bilateral EBOs via enlarged muscles. Given that these fish are pump filter feeders we hypothesize that the opercula may compress and expand the EBOs during pumping causing food to be moved posteriorly toward the pharyngeal jaws.

Innovations can facilitate bursts of diversification by increasing access to novel resources and the attainment of novel functional designs. Pharyngognathy, exhibited by highly diverse groups such as wrasses and cichlid fishes, is hypothesized to increase foraging capacity and efficiency. Here, I test the hypothesis that pharyngeal jaw shape and tooth morphology are adaptive in an ecologically diverse radiation of Neotropical cichlid fishes that spans North, Central and South America. I partitioned species into generalized trophic guilds using published stomach content analyses and quantified shape variation of the lower pharyngeal jaw (LPJ) using geometric morphometrics. Additionally, I tested for convergence in LPJ shape and trophic guild by mapping the phylogeny onto the principal components and testing for shifts towards similar evolutionary regimes. Major LPJ shape variation included the length and orientation (i.e. narrow or wide) of the lateral processes and length of the medial process, which varied based on the proportion of fishes and plants consumed. Pharyngeal tooth number, diversity and the frequency of tooth types were not evenly distributed among trophic guilds. There were seven distinct evolutionary regimes that converged upon four optima. Pharyngeal jaw diversification is associated with the exploitation of novel resources among Neotropical cichlids such that pharyngeal specialization has increased access to otherwise poorly accessible resources, such as resources that are difficult to crush (e.g. hard-shelled organisms) and assimilate (e.g. algae).

The anatomy of the feeding apparatus in fishes, including both oral and pharyngeal jaw elements, is closely related to the ecology of a species. During ontogeny, the oral and pharyngeal jaws undergo dramatic changes. To better understand how such ontogenetic changes occur and relate to the feeding ecology of a species, ontogenetic series of four closely related members of the family Sciaenidae (Cynoscion nebulosus, Cynoscion regalis, Micropogonias undulatus, and Leiostomus xanthurus) were examined. Sciaenids were selected because as adults they exhibit considerable specialization of the feeding apparatus correlated with differences in foraging habitats. However, it is not clear when during ontogeny the structural specializations of the feeding apparatus develop, and thereby enable early life history stage (ELHS) sciaenids to partition their foraging habitats. A regression tree was recovered from the analysis and three divergences were identified during ontogeny. There are no measurable differences in elements of the feeding apparatus until the first divergence at 8.4 mm head length (HL), which was attributed to differences in average gill filament length on the second ceratobranchial. The second divergence occurred at 14.1 mm HL and was associated with premaxilla length. The final divergence occurred at 19.8 mm HL and was associated with differences in the toothed area of the fifth certatobranchial. These morphological divergences suggest that ELHS sciaenids may be structurally able to partition their foraging habitats as early as 8.4 mm HL.

The black carp, Mylopharyngodon piceus (Osteichthyes: Cyprinidae), crushes its snail and other molluscan prey with robust pharyngeal jaws and strong bite forces. Using gross morphology, histological sectioning, and X-ray reconstruction of moving morphology (XROMM), we investigated structural, behavioral, and mechanical aspects of pharyngeal jaw function in black carp. Strut-like trabeculae in their pharyngeal jaws support large, molariform teeth. The teeth occlude with a hypertrophied basioccipital process that is also reinforced with stout trabeculae. A keratinous chewing pad is firmly connected to the basioccipital process by a series of small bony projections from the base of the pedestal. The pharyngeal jaws have no bony articulations with the skull, and their position is controlled by five paired muscles and one unpaired median muscle. Black carp can crush large molluscs, so we used XROMM to compare pharyngeal jaw postures as fish crushed ceramic tubes of increasing sizes. We found that black carp increase pharyngeal jaw gape primarily by ventral translation of the jaws, with ventral rotation and lateral flaring of the jaws also increasing the space available to accommodate large prey items. A stout, robust ligament connects left and right jaws together firmly, but allows some rotation of the jaws relative to each other. Contrasting with the pharyngeal jaw mechanism of durophagous perciforms with fused left and right lower pharyngeal jaws, we hypothesize that this ligamentous connection may serve to decouple tensile and compressive forces, with the tensile forces borne by the ligament and the compressive forces transferred to the prey.

Sexual dimorphism in ecologically relevant traits is ubiquitous in animals. However, other types of intraspecific phenotypic divergence, such as trophic polymorphism, are less common. Because linkage to sex should often lead to balancing selection, understanding the association between sex and phenotypic divergence could help explain why particular species show high morphological variability. To determine if sexual dimorphism could be helping to maintain ecomorphological variation in a classic case of intraspecific trophic polymorphism, we examined the association between sex and morphological divergence in the cichlid Herichthys minckleyi. Although H. minckleyi with enlarged molariform teeth on their pharyngeal jaws have been reported to more commonly be male, we did not find an association between sex and pharyngeal morphotype. Sex was associated with divergence in body size (as measured through standard length). But, sex was not associated with any of the other trophic traits examined. However, pharyngeal morphotype did show an association with gut length, gape, and tooth number. Sexual dimorphism is not playing a central role in enhancing trophic diversity within H. minckleyi.

The pharyngeal jaw apparatus is a key innovation hypothesised to increase foraging efficiency and facilitate utilisation of novel resources among teleost fishes. Here, we tested whether dietary characteristics could predict pharyngeal jaw morphology among eight species of Neotropical Crenicichla. Additionally, we tested the hypothesis that pharyngeal jaws may impose a functional constraint on piscivory via pharyngeal gape. We quantified the shape of the lower pharyngeal jaw (LPJ) using linear and geometric morphometrics and quantified diet using the relative volumetric proportions of prey items. We used principal component analysis to describe major axes of variation in LPJ shape and dietary patterns. The major axis of dietary variation significantly predicted LPJ morphology, which was driven by a significant relationship between LPJ shape and rates of piscivory. We also found that rates of piscivory predicted size-corrected LPJ depth. Size-corrected pharyngeal gape also significantly predicted rates of piscivory such that pharyngeal jaws may constrain piscivory by limiting pharyngeal gape. Strong form-to-function linkage between pharyngeal morphology and trophic patterns suggests an adaptive quality of the pharyngeal apparatus but may also impose functional constraints when consumers must switch prey or when prey availability is temporally or spatially unpredictable.

The dentition of cyprinid fishes is restricted to the right and left fifth ceratobranchials, which form the lower pharyngeal jaw (LPJ). The African members of the cyprinid subfamily Danioninae are nested within the tribe Chedrini. We compare the pharyngeal dentitions of African and of Asian Chedrini (Danioninae), including Danio rerio (Danionini), considering the following morphological characters: the pattern of dentition, the number of pharyngeal tooth rows, the curving and inclination of the tooth crowns, and the ratio of the tooth-bearing area to the length of the LPJ. Similar to a previous genetic based study a close relationship is proposed among the African genera Engraulicypris, Mesobola, and Rastrineobola, and among Leptocypris, Neobola, and Chelaethiops, all of which were formerly assigned as ‘Neobolines’. Species of Rastrineobola, Engraulicypris, Mesobola, and Neobola develop an additional (fourth) row of pharyngeal teeth. The dentition of Rastrineobola argentea (Pellegrin, 1904) and the shape of the LPJ of Engraulicypris sardella (Gunther, 1868) characterize these two species as specialized within the Chedrini. An additional pharyngeal tooth row also occurs infrequently in the zebrafish D. rerio (Hamilton, 1822). The ‘fourth’ row of this fish species is possibly the result of a different process for acquiring four rows of pharyngeal teeth, likely by the splitting of one tooth row.