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SUMMARYOwing to its high degree of complexity and plasticity, the cichlid pharyngeal jaw apparatus has often been described as a key evolutionary innovation. The majority of studies investigating pharyngeal muscle behavior and function have done so in the context of feeding. Analysis of enzyme activities (citrate synthase, 3-hydroxyacyl-CoA dehydrogenase and L-lactate dehydrogenase) of pharyngeal muscles in the Lake Malawi cichlid Tramitichromis intermedius revealed differences between pharyngeal jaw muscles and between males and females. Therefore, these muscles have different performance characteristics, resulting in different functional characteristics of the muscles within the complex. Furthermore, the differences between muscles of males and females represent fundamental differences in muscular metabolic processes between sexes. This study is the first to demonstrate that the pharyngeal anatomy is not only used for food processing but is possibly responsible for sound production, in turn influencing sexual selection in cichlid fish.

The cellulose-rich walls that protect plant cells are difficult to digest, and therefore mechanical food processing is a key aspect of herbivory across vertebrates. Cell walls are typically broken down by translation of flattened teeth in the occlusal plane (i.e. grinding) as part of a complex, rhythmic chewing stroke. The grass carp, Ctenopharyngodon idella, is a voracious, invasive herbivorous fish that relies solely on its pharyngeal teeth, located in the back of the throat, for mechanical processing of plant material. Here, we describe the musculoskeletal anatomy of the pharyngeal jaws of grass carp and use XROMM to quantify chewing kinematics and muscle strain. The pharyngeal jaws are suspended in a sling of 11 muscles and maintain no bony articulation with any other skeletal elements in the head. The jaws bear long, serrated teeth that are worn during use into flattened tooth cusps. Our kinematic data show that this wear is the result of the teeth being elevated into occlusion against the basioccipital process and keratinous chewing pad, not tooth-on-tooth occlusion. Pharyngeal jaw elevation results from large strains in the jaw elevator muscle, the levator arcus branchialis V, to drive a pulley-like mechanism that rotates the jaws about a pivot point at the symphysis between the left and right pharyngeal jaws. These complex, rhythmic jaw rotations translate the teeth laterally across the chewing surface throughout the occlusion phase. The grass carp chewing system is strikingly similar in gross morphology and masticatory function to herbivorous chewing strategies in other vertebrates.

Teleost fishes typically capture prey with the oral jaws and perform most types of prey-processing behavior with the pharyngeal jaw apparatus. In these fishes, the motor patterns associated with the different stages of feeding are quite distinct, and fish can modify muscle activity patterns when feeding on different prey. We examined motor pattern variation in the queen triggerfish, Balistes vetula, a versatile predator that both captures and processes prey with its oral jaws. During feeding on three prey that differed in hardness and elusiveness, three distinct patterns of behavior could be identified on the basis of patterns of muscle activity: prey capture, buccal manipulation and blowing. During prey capture by suction feeding, the retractor arcus palatini muscle (RAP) commenced activity before the levator operculi muscle (LOP). In both buccal manipulation and blowing, the RAP began activity well after the onset of activity in the LOP. Both prey capture and buccal manipulation motor patterns varied when fish fed on different prey. When capturing hard-shelled and non-elusive prey, B. vetula did not employ suction feeding but, instead, the fish directly bit parts of its prey. The motor pattern exhibited during direct biting to capture prey was different from that during suction feeding, but was indistinguishable from the pattern seen during the repeated cycles of buccal manipulation. Harder prey elicited significantly longer bursts of activity in the jaw adductor muscles than did soft prey. In spite of the involvement of the oral jaws in virtually all stages of feeding, B. vetula shows levels of variation between patterns of behavior and types of prey characteristic of previously studied teleost fishes. Thus, the coupling of capture and processing behavior patterns in the repertoire of the oral jaws does not appear to constrain the behavioral versatility of this species.

This study examines patterns of variation in 15 electromyographic (EMG) variables measured from recordings of pharyngeal jaw muscle activity during prey processing in four species of the perciform fish family Haemulidae. Two questions were of primary interest. (1) Are motor patterns conserved across the four species? (2) Do the fishes alter (modulate) muscle activity patterns when feeding on different prey types? The experimental design used allowed the partitioning of variance in EMG variables among species, among individuals within species, among days within individuals, among feedings within days, and among prey types. Only one variable exhibited a significant species effect, indicating that the four species used virtually the same motor pattern during prey processing. In response to three prey types differing in hardness, all four species demonstrated an ability to modulate several EMG variables that characterized the intensity of electrical activity. However, variables characterizing the relative timing of muscle activities were not influenced by prey type. A significant variance component was found among recording days and, together with the possibility of variation among experimental preparations, this raises questions about the extent of previously reported inter-individual variation in EMGs. These results support a growing data base on aquatic feeding in lower vertebrates which finds that: (1) motor patterns tend to be highly conserved among closely related taxa; (2) the ability to modulate motor patterns in response to different prey types appears to be a general property of teleost fish feeding mechanisms; and (3) variation in experimental EMG data is ubiquitous and, when unaccounted for, confounds comparisons among treatment groups.