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Functional limitations on feeding ability were investigated in the mollusc-crushing Caribbean hogfish, Lachnolaimus maximus (Labridae). Two constraints were proposed to limit the maximum size prey L. maximus can eat: pharyngeal jaw gape and crushing force. These factors yield diflferent quantitative expectations for the relationship between fish size and maximum prey size. Their relative importance for predation on a frequently consumed gastropod (Cerithium litteratum) was investigated in laboratory performance tests designed to determine the largest snails fish could eat. Cerithium predation was found to be force limited rather than gape limited. The importance of this functional constraint in determining the largest Cerithium consumed by wild fish was examined by comparing hogfish feeding capability, as determined by the performance tests, to the maximum size snails found in the stomach contents of field-collected fish. Crushing ability appears to limit Cerithium predation in natural fish populations. The utility of performance testing for determining the functional and ecological importance of morphology is discussed.

Functional and structural patterns in the pharyngeal jaw apparatus ofeuteleostean fishes are described and analysed as a case study of the transformation of a complex biological design. The sequential acquisition ofstructural novelties in the pharyngeal apparatus is cnnsidered in relation to both current hypotheses of eutrlrostran phylogeny and patterns of pharyngeal jaw function. Several euteleostean cladrs ai-e corroborated as being monophyletic, and morphologically consenzative features of the pharyngeal jaw apparatus are rccognimd. Functional analysis, using cinematography and electromyography, reveals four distinct patterns of muscle activity during feeding in primitive euteleosts (oil dcsian in vertebrates awaits fiirther information on the acquisition of both structural and function;il novelties at successive hicrarchical levels within monophyletic clades. This is suggcrted as a key goal of fiiture research in functional and evolutionary morphology.

SYNOPSIS. The actinopterygian fishes are an exemplary clade for the study of structural and functional evolutionary patterns. With over half of all vertebrate species, ray-finned fishes have diversified into a wide variety of habitats, and considerable progress has been made over the last fifteen years in understanding the genealogical relationships of actinopterygians. This symposium has contributed to our understanding of phylogenetic patterns in actinopterygians and to knowledge of the major structural and functional patterns in locomotor, auditory, trophic, and neural systems. A number of key areas for future research have been identified. (1) The relationships of “palaeonisciform“ fishes, (2) the study of trends in feeding and locomotor systems within a phylogenetic context, (3) the identification of primitive patterns of pharyngeal jaw movement and steady and unsteady locomotor patterns in actinopterygians, (4) the homologies, identification, and functional significance of neural pathways in the telencephalon, and (5) the comparative study of form-function relations in the auditory system. The study of teleost fish biology has proceeded at the expense of data on primitive actinopterygians ( e.g., Polypterus, Polyodon, Aapenser, Lepisosteus, Amia ) which are especially important in the analysis of structural and functional patterns in ray-finned fishes.

Several morphologically defined species of cichlid fishes ( Cichlasoma ) endemic to the Cuatro Cienegas basin of Mexico and differing in tooth structure, body shape, and diet are allelically identical at 27 gene loci. The presence of only one Mendelian population in each of three drainage systems studied and the occurrence of two of the morphotypes in the same broods indicate that the supposed species are morphs. That trophic radiation in the Cuatro Cienegas cichlids has been achieved through ecological polymorphism rather than speciation raises questions regarding the genetic basis for the extensive intralacustrine radiation of cichlids in Africa and elsewhere.

Cichlid fishes in African rift lakes have undergone rapid speciation, resulting in "species flocks" with more than 300 endemic species in some of the lakes. Most researchers assume that there is little phenotypic variation in cichlid fishes. I report here extensive phenotypic plasticity in a Neotropical cichlid species. I examined the influence of diet on trophic morphology during ontogeny in Cichlasonia managuense. Two groups of full siblings were fed two different diets for eight months after the onset of feeding; thereafter both groups were fed a common diet. Phenotypes that differed significantly at 8.5 months converged almost completely at 16.5 months. If feeding on two different diets is continued after 8.5 months, the phenotypes remain distinct. Differences in diet and possibly in feeding mode are believed to have caused these phenotypic changes. Phenotypic plasticity is described in terms of a qualitative model of heterochrony in which phenotypic change in morphology is explained as retardation of the normal developmental rate. If phenotypic expression of morphology is equally plastic in African cichlid species as it may be in the American cichlids, as exemplified by C. managuense, then taxonomic, ecological, and evolutionary analyses of "species flocks" may be in need of revision. However, Old World cichlids may be less phenotypically plastic than New World cichlids, and this may contribute to the observed differences in speciation rate and degree of endemism.

Established empirical cases of sympatric speciation are scarce, although there is an increasing consensus that sympatric speciation might be more common than previously thought. Midas cichlid fish are one of the few substantiated cases of sympatric speciation, and they formed repeated radiations in crater lakes. In contrast, in the same environment, such radiation patterns have not been observed in other species of cichlids and other families of fish. We analyze morphological and genetic variation in a cichlid species (Archocentrus centrarchus) that co-inhabits several crater lakes with the Midas species complex. In particular, we analyze variation in body and pharyngeal jaw shape (two ecologically important traits in sympatrically divergent Midas cichlids) and relate that to genetic variation in mitochondrial control region and microsatellites. Using these four datasets, we analyze variation between and within two Nicaraguan lakes: a crater lake where multiple Midas cichlids have been described and a lake where the source population lives. We do not observe any within-lake clustering consistent across morphological traits and genetic markers, suggesting the absence of sympatric divergence in A. centrarchus. Genetic differentiation between lakes was low and morphological divergence absent. Such morphological similarity between lakes is found not only in average morphology, but also when analyzing covariation between traits and degree of morphospace occupation. A combined analysis of the mitochondrial control region in A. centrarchus and Midas cichlids suggests that a difference between lineages in the timing of crater lake colonization cannot be invoked as an explanation for the difference in their levels of diversification. In light of our results, A. centrarchus represents the ideal candidate to study the genomic differences between these two lineages that might explain why some lineages are more likely to speciate and diverge in sympatry than others.

Background The oral and pharyngeal jaw of cichlid fishes are a classic example of evolutionary modularity as their functional decoupling boosted trophic diversification and contributed to the success of cichlid adaptive radiations. Most studies until now have focused on the functional, morphological, or genetic aspects of cichlid jaw modularity. Here we extend this concept to include transcriptional modularity by sequencing whole transcriptomes of the two jaws and comparing their gene coexpression networks. Results We show that transcriptional decoupling of gene expression underlies the functional decoupling of cichlid oral and pharyngeal jaw apparatus and the two units are evolving independently in recently diverged cichlid species from Lake Tanganyika. Oral and pharyngeal jaw coexpression networks reflect the common origin of the jaw regulatory program as there is high preservation of gene coexpression modules between the two sets of jaws. However, there is substantial rewiring of genetic architecture within those modules. We define a global jaw coexpression network and highlight jaw-specific and species-specific modules within it. Furthermore, we annotate a comprehensive in silico gene regulatory network linking the Wnt and AHR signalling pathways to jaw morphogenesis and response to environmental cues, respectively. Components of these pathways are significantly differentially expressed between the oral and pharyngeal jaw apparatus. Conclusion This study describes the concerted expression of many genes in cichlid oral and pharyngeal jaw apparatus at the onset of the independent life of cichlid fishes. Our findings suggest that – on the basis of an ancestral gill arch network—transcriptional rewiring may have driven the modular evolution of the oral and pharyngeal jaws, highlighting the evolutionary significance of gene network reuse. The gene coexpression and in silico regulatory networks presented here are intended as resource for future studies on the genetics of vertebrate jaw morphogenesis and trophic adaptation.

The independent evolution of the two toothed jaws of cichlid fishes is thought to have promoted their unparalleled ecological divergence and species richness. However, dental divergence in cichlids could exhibit substantial genetic covariance and this could dictate how traits like tooth numbers evolve in different African Lakes and on their two jaws. To test this hypothesis, we used a hybrid mapping cross of two trophically divergent Lake Victoria species (Haplochromis chilotes × Haplochromis nyererei) to examine genomic regions associated with cichlid tooth diversity. Surprisingly, a similar genomic region was found to be associated with oral jaw tooth numbers in cichlids from both Lake Malawi and Lake Victoria. Likewise, this same genomic location was associated with variation in pharyngeal jaw tooth numbers. Similar relationships between tooth numbers on the two jaws in both our Victoria hybrid population and across the phylogenetic diversity of Malawi cichlids additionally suggests that tooth numbers on the two jaws of haplochromine cichlids might generally coevolve owing to shared genetic underpinnings. Integrated, rather than independent, genomic architectures could be key to the incomparable evolutionary divergence and convergence in cichlid tooth numbers.

A common pattern of adaptive diversification in freshwater fishes is the repeated evolution of elongated open water (limnetic) species and high-bodied shore (benthic) species from generalist ancestors. Studies on phenotype-diet correlations have suggested that population-wide individual specialization occurs at an early evolutionary and ecological stage of divergence and niche partitioning. This variable restricted niche use across individuals can provide the raw material for earliest stages of sympatric divergence. We investigated variation in morphology and diet as well as their correlations along the benthiclimnetic axis in an extremely young Midas cichlid species, Amphilophus tolteca, endemic to the Nicaraguan crater lake Asososca Managua. We found that A. tolteca varied continuously in ecologically relevant traits such as body shape and lower pharyngeal jaw morphology. The correlation of these phenotypes with niche suggested that individuals are specialized along the benthic-limnetic axis. No genetic differentiation within the crater lake was detected based on genotypes from 13 microsatellite loci. Overall, we found that individual specialization in this young crater lake species encompasses the limnetic- as well as the benthic macro-habitat. Yet there is no evidence for any diversification within the species, making this a candidate system for studying what might be the early stages preceding sympatric divergence.

Liem, Karel F. (Museum of Comparative Zoology, Harvard University, Cambridge, Mass. 02138) 1974. Evolutionary strategies and morphological innovations: Cichlid pharyngeal jaws. Syst. Zool. 22:425-441.-The percoid fish family Cichlidae possesses a phenomenal ability to colonize lakes and to diversify to an extent unmatched by any other vertebrate family in the presence of predator pressure and strong competition. The invading cichlids successfully occupy contiguous and occasionally overlapping adaptive zones and specialize progressively into diversified subzones, ramifying prodigiously and covering a breadth of total adaptation that would have been entirely unpredictable if we were aware only of the rudiments of the evolutionary process. This evolutionary avalanche can be attributed to the cooccurrence of a wide range of prospective adaptive zones in the lacustrine environment, and the presence of a unique morphological key innovation of maximum versatility. The new adaptive complex has been revealed in this study by electromyographic analysis synchronized with cineradiography of the cichlid pharyngeal jaw apparatus. The morphological novelty characterizing the family Cichlidae involves the development of: a synarthrosis between the lower pharyngeal jaws, a strategic shift of insertion of the two fourth levator externi muscles, and synovial joints between upper pharyngeal jaws and basicranium. This specialized, highly integrated key innovation enables the cichlids not only to transport (deglutination) but also to prepare food, freeing the premaxillary and mandibular jaws to evolve numerous specializations dealing with the collection of dramatically diverse foods. The functional integration of the innovation is so basic and its potential adaptive versatility so rich that it is maintained throughout the adaptive radiation even though numerous nondisruptive evolutionary changes do take place, providing prodigious opportunities for explosive evolution during the exploitation of rich resources of food in the lacustrine environment. The conversion of the preexisting elements into a new and significantly improved cichlid adaptive complex of high selective value may have evolved by rapid steps under influence of strong selection pressure acting on the minor reconstruction of the -genotype which is involved in evolutionary changes of the pertinent ontogenetic mechanisms. Such relatively simple evolutionary processes are probably the cause for the general phenomenon that only slight reconstructions of existing structures are necessary for successful and rapid adaptation to drastic shifts of adaptive zones.