Functional morphology of the pharyngeal jaw apparatus in perciform fishes: An experimental analysis of the haemulidae

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.     

Pharyngeal biting mechanics in centrarchild and cichlid fishes: insights into a key evolutionary innovation

This study compares the pharyngeal biting mechanism of the Cichlidae, a family of perciform fishes that is characterized by many anatomical specializations, with that of the Centrarchidae, a family that possesses the generalized perciform anatomy. Our objective was to trace the key structural and functional changes in the pharyngeal jaw apparatus that have arisen in the evolution from the generalized to derived (cichlid) perciform condition. We propose a mechanical model of pharyngeal biting in the Centrarchidae and compare this with an already existing model for pharyngeal biting in the family Cichlidae. Central to our centrarchid model is a structural coupling between the upper and lower pharyngeal jaws. This coupling severely limits independent movement of the pharyngeal jaws, in contrast to the situation in the speciose Cichlidae, in which the upper and lower pharyngeal jaw movements are to a large extent independent. We tested both models by electrically stimulating nine muscles of the branchial and hyoid apparatuses in three centrarchild and three cichlid species. The results confirmed the coupled movement of the upper and lower pharyngeal jaws in the Centrarchidae and the independence of these movements in the Cichlidae. We suggest that the key structural innovation in the development of the functionally versatile cichlid (labroid) pharyngeal jaw apparatus was the decoupling of epibranchials 4 from the upper pharyngeal jaws. This structural decoupling implies the decoupling of the movements of the upper and lower pharyngeal jaws and leads to a cichlid (labroid) type of pharyngeal bite. The initial decoupling facilitated a cascade of changes, each leading to improved biting effectiveness and/or to increased mobility and mechanical flexibility of the pharyngeal jaws. The shift of insertion of the m. levator externus 4 which has been considered the primary innovation in the transformation probably arose secondarily. The transformation of the pharyngeal biting mechanism in the perciforms is an excellent example of decoupling of structures associated with diversification of form and function and with increased speciation rates.     

Functional design and evolution of the pharyngeal jaw apparatus in euteleostean fishes

Functional and structural patterns in the pharyngeal jaw apparatus of euteleostean fishes are described and analysed as a case study of the transformation of a complex biological design. The sequential acquisition of structural novelties in the pharyngeal apparatus is considered in relation to both current hypotheses of euteleostean phylogeny and patterns of pharyngeal jaw function. Several euteleostean clades are corroborated as being monophyletic, and morphologically conservative features of the pharyngeal jaw apparatus are recognized. Functional analysis, using cinematography and electromyography, reveals four distinct patterns of muscle activity during feeding in primitive euteleosts (Esox) and in derived euteleostean fishes(Perca, Micropterus, Ambloplites, Pomoxis). The initial strike, buccal manipulation, pharyngeal manipulation, and the pharyngeal transport of prey into the oesophagus all involve unique muscle activity patterns that must be distinguished in analyses of pharyngeal jaw function. During pharyngeal transport, the upper and lower pharyngeal jaws are simultaneously protracted and retracted by the action of dorsal and ventral musculoskeletal gill arch couplings. The levator externus four and retractor dorsalis muscles, anatomical antagonists, overlap for 70–90°of their activity period. Levatores externi one and two are the main protractors of the upper pharyngeal jaws in the acanthopterygian fishes studied. The major features of pharyngeal jaw movement in primitive euteleosts are retained in many derived clades in spite of a dramatic structural reorganization of the pharyngeal region. Homologous muscles have radically changed their relative activity periods while pharyngeal jaw kinematics have been modified relatively little. Patterns of transformation of activity may thus bear little direct relationship to the sequence of structural modification in the evolution of complex designs. Overall function of a structural system may be maintained, however, through co-ordinated modifications of the timing of muscle activity and anatomical reorientation of the musculoskeletal system. Deeper understanding of the principles underlying the origin and transformation of functional design in vertebrates awaits further information on the acquisition of both structural and functional novelties at successive hierarchical levels within monophyietic clades. This is suggested as a key goal of future research in functional and evolutionary morphology.     

Morphology of the Parrotfish Pharyngeal Jaw Apparatus

SYNOPSIS. Analysis of the anatomy of the pharyngeal apparatusof parrotfish demonstrates extraordinary specialization of thegrinding jaws. The epibranchials have lost their gill-bearingfunction. The first epibranchial is the structural element ofthe pharyngeal valve that is operated by the first levator externus,first branchial adductor and part one of the transversus dorsalismuscles. Five pairs of muscles (fourth levator externus, levatorposterior lateralis and medialis, fifth branchial adductor,part two of the transversus ventralis) are positioned to adductthe lower pharyngeal. The retractor dorsalis and fourth obliquusdorsalis are positioned to retract the upper pharyngeal. Thethird levator internus and transversus dorsalis posterior protractthe upper pharyngeal. The fourth levator externus, both partsof the levator posterior and the fifth adductor are massiveand pinnate. Deep fossae for the attachment of the fourth levatorexternus and levator posterior muscles are sculpted out of theneurocranium. A ventral spike process of the prootic and expandedhemal postzygapophyses of the first three vertebrae are skeletalfeatures associated with the elaborated musculature of the pharynx.Synovial joints are present between the basicranium and upperpharyngeals, between the upper pharyngeals and fourth epibranchialsand between the lower pharyngeal and cleithrum. The upper pharyngealsact as a single unit bound by cruciate ligaments. The fourthepibranchial is a key element in the pharyngeal apparatus andserves to direct forces generated by the transversus ventralis,fifth adductor, levator posterior lateralis, transversus dorsalisposterior and fourth obliquus dorsalis.     

A functional morphospace for the skull of labrid fishes: patterns of diversity in a complex biomechanical system

The Labridae (including wrasses, the Odacidae and the Scaridae) is a species‐rich group of perciform fishes whose members are prominent inhabitants of warm‐temperate and tropical reefs worldwide. We analyse functionally relevant morphometrics for the feeding apparatus of 130 labrid species found on the Great Barrier Reef and use these data to explore the morphological and mechanical basis of trophic diversity found in this assemblage. Morphological measurements were made that characterize the functional and mechanical properties of the oral jaws that are used in prey capture and handling, the hyoid apparatus that is used in expanding the buccal cavity during suction feeding, and the pharyngeal jaw apparatus that is used in breaking through the defences of shelled prey, winnowing edible matter from sand and other debris, and pulverizing the algae, detritus and rock mixture eaten by scarids (parrotfishes). A Principal Components Analysis on the correlation matrix of a reduced set of ten variables revealed complete separation of scarids from wrasses on the basis of the former having a small mouth with limited jaw protrusion, high mechanical advantage in jaw closing, and a small sternohyoideus muscle and high kinematic transmission in the hyoid four‐bar linkage. Some scarids also exhibit a novel four‐bar linkage conformation in the oral jaw apparatus. Within wrasses a striking lack of strong associations was found among the mechanical elements of the feeding apparatus. These weak associations resulted in a highly diverse system in which functional properties occur in many different combinations and reflect variation in feeding ecology. Among putatively monophyletic groups of labrids, the cheilines showed the highest functional diversity and scarids were moderately diverse, in spite of their reputation for being trophically monomorphic and specialized. We hypothesize that the functional and ecological diversity of labrids is due in part to a history of decoupled evolution of major components of the feeding system (i.e. oral jaws, hyoid and pharyngeal jaw apparatus) as well as among the muscular and skeletal elements of each component. © 2004 The Linnean Society of London, Biological Journal of the Linnean Society, 2004, 82 , 1–25.     

Trophic ecomorphology in eastern Pacific blennioid fishes: character transformation of oral jaws and associated change of their biological roles

Synopsis The ecomorphological relationships between the oral jaws and food spectra were highlighted in 34 species of Gulf of California blennioid fishes (5 Tripterygiidae, 13 Labrisomidae, 11 Chaenopsidae and 5 Blenniidae). Twenty-nine species are microcarnivorous, two are omnivorous browsers, two are algae grazers and one was an ‘ectoparasite’ feeder. The spectrum of oral (as opposed to pharyngeal) jaw (OJA) morphology ranges from plesiomorphic, suction-feeding (relatively large, protrusible jaws, with many coniform-caniniform teeth) to apomorphic, biting (relatively small, non protrusible jaws, with a single row of incisiform teeth). As species with similar morphology may widely differ in food, it is concluded, that morphology is not a reliable predictor for ecology in this case. With the exception of a few specialists, species with apomorphic, biting OJA utilize sessile items in addition to mobile categories and thus show a higher food diversity as compared to species with plesiomorphic OJA. Thus in the present case morphological differentiation goes along with ecological generalization. Only three blenniid species with the most apomorphic OJA may be considered as specialized also with regard to food resource utilization. Transformation of morphological characters and the ecological role of the OJA of blennioids may serve as a model to illustrate the steps required to achieve a biting-browsing and grazing feeding apparatus in many taxa of modern acanthopterygian reef fishes.     

Does evolutionary innovation in pharyngeal jaws lead to rapid lineage diversification in labrid fishes?

Background  

Major modifications to the pharyngeal jaw apparatus are widely regarded as a recurring evolutionary key innovation that has enabled adaptive radiation in many species-rich clades of percomorph fishes. However one of the central predictions of this hypothesis, that the acquisition of a modified pharyngeal jaw apparatus will be positively correlated with explosive lineage diversification, has never been tested. We applied comparative methods to a new time-calibrated phylogeny of labrid fishes to test whether diversification rates shifted at two scales where major pharyngeal jaw innovations have evolved: across all of Labridae and within the subclade of parrotfishes.     

Independent evolution of the specialized pharyngeal jaw apparatus in cichlid and labrid fishes

Background  

Fishes in the families Cichlidae and Labridae provide good probable examples of vertebrate adaptive radiations. Their spectacular trophic radiations have been widely assumed to be due to structural key innovation in pharyngeal jaw apparatus (PJA), but this idea has never been tested based on a reliable phylogeny. For the first step of evaluating the hypothesis, we investigated the phylogenetic positions of the components of the suborder Labroidei (including Pomacentridae and Embiotocidae in addition to Cichlidae and Labridae) within the Percomorpha, the most diversified (> 15,000 spp) crown clade of teleosts. We examined those based on 78 whole mitochondrial genome sequences (including 12 newly determined sequences) through partitioned Bayesian analyses with concatenated sequences (13,933 bp).     

Ecomorphological relationships among Caribbean tetraodontiform fishes

The anatomy of the oral jaw apparatus, lever-arm mechanics and the diet of six species of Caribbean fishes in the order Tetraodontiformes were investigated to explore the relationships between trophic morphology and feeding habit in these fishes. Tetraodontiforms use their oral jaw apparatus to capture and reduce a broad range of prey types such as plankton, polychaete worms, holothuroids, sea urchins, crabs, molluscs, gorgonians and algae. The different feeding habits of tetraodontiforms are reflected by differences in the morphological and biomechanical features of their oral jaw apparatus that appear to enhance their abilities to feed on hard prey organisms. Species that bite and crush hard, benthic prey organisms had more massive bones and muscles, longer jaw-opening in-levers, and higher jaw-closing lever ratios than the planktivorous, suction-feeding species. Masses of the jaw and suspensorium bones and lower jaw adductor muscles as well as the jaw-opening in-levers and jaw-closing lever ratios of crushers were greater than those of biters. In contrast, the mass of the adductor muscle of the upper jaw did not vary among species with different diets, indicating that this muscle may not be central to the factors that determine patterns of prey use in these fishes. The diversity of feeding behaviours and the wide range of feeding habits among fishes in the order Tetraodontiformes illustrate the versatility of the oral jaw apparatus as a single functional feeding system in fishes.     

Kinematics of the pharyngeal jaws during feeding in Oreochromis niloticus (Pisces,Perciformes)

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.     

Anatomy of the feeding apparatus of the lemon shark,Negaprion brevirostris

The anatomy of the feeding apparatus of the lemon shark, Negaprion brevirostris, is investigated by gross dissection, computer axial tomography, and histological staining. The muscles and ligaments of the head associated with feeding are described. The upper and lower jaws are suspended by the hyoid arch, which in turn is braced against the chondrocranium by a complex series of ligaments. In addition, various muscles and the integument contribute to the suspension and stability of the jaws. The dual jaw joint is comprised of lateral and medial quadratomandibular joints that resist lateral movement of the upper and lower jaws on one another. This is important during feeding involving vigorous head shaking. An elastic ethmoplatine ligament that unites the anterior portion of the upper jaw to the neurocranium is involved with upper jaw retraction. The quadratomandibularis muscle is divided into four divisions with a bipinnate fiber arrangement of the two large superficial divisions. This arrangement would permit a relatively greater force per unit volume and reduce muscle bulging of the jaw adductor muscle in the spatially confined cheek region. Regions of relatively diffuse integumental ligaments overlying the adductor mandibulae complex and the levator palatoquadrati muscle, interspersed with localized regions of longer tendonlike attachments between the skin and the underlying muscle, permit greater musculoskeletal movement relative to the skin. The nomenclature of the hypobranchial muscles is discussed. In this shark they are comprised of the unsegmented coracomandibularis and coracohyoideus, and the segmented coracoarcualis. © 1995 Wiley-Liss, Inc.     

Molecular phylogeny and speciation of the surfperches (Embiotocidae, Perciformes).

Labroid fishes include a variety of families, such as wrasses (Labridae), odacids (Odacidae), damselfishes (Pomacentridae), parrotfishes (Scaridae), cichlids (Cichlidae), and surfperches (Embiotocidae). With only 23 species, the small embiotocid family exhibits a remarkably low species diversity compared to the large species diversity of the Cichlidae. Using mitochondrial DNA sequences of all 14 extant embiotocid genera, we established a molecular phylogeny of the family and compared it with a previously proposed morphological phylogeny. Genetic differentiation among embiotocids was compared to that among cichlids. Although species numbers are extremely different between these two families, the degrees of genetic differentiation within each family was found to be very similar.     

Patterns of Evolution in the Feeding Mechanism of Actinopterygian Fishes

SYNOPSIS. Structural and functional patterns in the evolutionof the actinopterygian feeding mechanism are discussed in thecontext of the major monophyletic lineages of ray-finned fishes.A tripartite adductor mandibulae contained in a maxillary-palatoquadratechamber and a single mechanism of mandibular depression mediatedby the obliquus inferioris, sternohyoideus, and hyoid apparatusare primitive features of the Actinopterygii. Halecostome fishesare characterized by having an additional mechanism of mandibulardepression, the levator operculi—opercular series coupling,and a maxilla which swings anteriorly during prey capture. Theseinnovations provide the basis for feeding by inertial suctionwhich is the dominant mode of prey capture throughout the halecostomeradiation. A remarkably consistent kinematic profile occursin all suction-feeding halecostomes. Teleost fishes possessa number of specializations in the front jaws including a geniohyoideusmuscle, loss of the primitive suborbital adductor component,and a mobile premaxilla. Structural innovations in teleost pharyngealjaws include fusion of the dermal tooth plates with endoskeletalgill arch elements, the occurrence of a pharyngeal retractormuscle, and a shift in the origin of the pharyngohyoideus. Thesespecializations relate to increased functional versatility ofthe pharyngeal jaw apparatus as demonstrated by an electromyographicstudy of pharyngeal muscle activity in Esox and Ambloplites.The major feature of the evolution of the actinopterygian feedingmechanism is the increase in structural complexity in both thepharyngeal and front jaws. Structural diversification is a functionof the number of independent biomechanical pathways governingmovement.     

Terrestrial feeding in the Mudskipper Periophthalmus (Pisces: Teleostei): A cineradiographic analysis

Mudskipping gobies (Periophthalminae) are among the most terrestrial of amphibious fishes. Specializations associated with terrestrial prey capture and deglutition have been studied in Periophthalmus koelreuteri by light and X-ray cinematography which permits direct visualization of pharyngeal jaw movement during deglutition. Anatomical specializations of the pharyngeal jaws are described and include depressible teeth, a large ventral process on ceratobranchial five, and muscular modifications.
Multiple terrestrial feedings occur by Periophthalmus without a return to the water, and cineradiography reveals that the buccal cavity is often filled with air during terrestrial excursions in contrast to some previous hypotheses. Transport of the prey into the oesophagus occurs primarily by anteroposterior movement of the upper pharyngeal jaw. The lower pharyngeal jaw plays a limited role in food transport and may serve primarily to hold and position prey. The bite between upper and lower pharyngeal jaws occurs between the anterior teeth, and both jaws are protracted together during raking of food into the oesophagus. Functional specializations correlated with terrestrial feeding include obligatory use of pharyngeal jaws for swallowing even small prey items and positioning of the prey in the pharynx by pharyngeal jaw and hyoid movements alone.
This analysis of terrestrial feeding allows hypotheses of design constraints imposed by the aquatic medium on fishes to be raised and tested.     

Muscle fiber types and functional demands in feeding mechanisms of fishes

Profiles of muscle fiber types and pharyngeal jaw dentition vary in accordance with trophic demands and skeletal organization in teleost fishes. Carnivorous, omnivorous, and molluscivorous members of the ecologically analogous Cichlidae and Centrarchidae were compared in terms of their pharyngeal jaw anatomy and branchial muscle histochemistry. The two families differed greatly in patterns of tooth form, wear, and replacement. Four muscle fiber type patterns were discoverd: (1) single fiber, (2) zoned, (3) mosaic, and (4) zoned-mosaic. Multiple fiber type muscles were more prevalent in fishes that masticate tough foods with their pharyngeal jaws. Such muscles were also more prevalent in cichlids than in centrarchids. It appears that muscles with multiple fiber types in lower vertebrates are, as a rule, compartmentalized, whereas in higher vertebrates, multiple fiber type muscles are a musaic matrix. The occurrence of mosaic patterns in some fish branchial muscles, however, suggests that mosaic muscles are initially single fiber type muscles exposed to complex functional demands, such as food preparation. Furthermore, it is plausible that the evolutionary replacement of the lower vertebrate zoning pattern by the higher vertebrate mosaic matrix is directly related to the effects of gravity, a force more influential on terrestrial than on aquatic organisms.     

Functional morphology of the pharyngeal jaw apparatus in moray eels

Moray eels (Muraenidae) are a relatively large group of anguilliform fishes that are notable for their crevice-dwelling lifestyle and renowned for their ability to consume large prey. Morays apprehend their prey by biting and then transport prey by extreme protraction and retraction of their pharyngeal jaw apparatus. Here, we present a detailed interpretation of the mechanisms of pharyngeal jaw transport based on work with Muraena retifera. We also review what is known of the moray pharyngeal jaw apparatus from the literature and provide comparative data on the pharyngeal jaw elements and kinematics for other moray species to determine whether interspecific differences in morphology and behavior are present. Rather than comprising broad upper and lower processing tooth plates, the pharyngeal jaws of muraenine and uropterygiine morays, are long and thin and possess large, recurved teeth. Compared with the muraenines, the pharyngobranchials of the uropterygiines do not possess a horn-shaped process and their connection to the fourth epibranchial is dorsal rather than medial. In addition, the lower tooth plates do not exhibit a lateral groove that serves as a site of muscle attachment for the pharyngocleitheralis and the ventral rather than the lateral side of the lower tooth plate attaches to the fourth ceratobranchial. In all morays, the muscles positioned for protraction and retraction of the pharyngeal apparatus have undergone elongation, while maintaining the generalized attachment sites on the bones of the skull and axial skeleton. Uropterygiines lack a dorsal retractor muscle and we presume that retraction of the pharyngeal jaws is achieved by the pharyngocleitheralis and the esophagus. The fifth branchial adductor is greatly hypertrophied in all species examined, suggesting that morays can strongly adduct the pharyngeal jaws during prey transport. The kinematics of biting behavior during prey capture and transport resulted in similar magnitudes of cranial movements although the timing of kinematic events was significantly different and the duration of transport was twice as long as prey capture. We speculate that morays have evolved this alternative prey transport strategy as a means of overcoming gape constraints, while hunting in the confines of coral reefs.     

Ecological and evolutionary consequences of the trophic polymorphism in Cichlasoma citrinellum (Pisces: Cichlidae)

The neotropical cichlid fish Cichlasoma citrinellum is polymorphic in the structure of its pharyngeal jaw apparatus and external morphology. The pharyngeal jaws are either gracile and bear slender, pointed teeth (papilliform) or robust with strong, rounded teeth (molariform). Molariform morphs have a ‘benthic’, and papilliform morphs a ‘limnetic’ body form. Furthermore, this species is also polychromatic, with yellow and black morphs. The molariform morphology of the pharyngeal jaw apparatus adapts the fish for cracking and feeding on snails. Based on analysis of stomach contents, 94% of the molariform morph ate snails whereas only 19%, of the papilliform morph did so. This result suggests that the morphs occupy different ecological niches. The morphology of the pharyngeal jaw apparatus does not correlate significantly with sex, but it does with body colouration (P<0.005). Cichlasoma citrinellum mate assortatively with their own colour; therefore a mating preference for colour may lead to genetic isolation of trophic morphs. The frequency of the molariform morph differs strikingly among populations of five Nicaraguan lakes and its abundance is correlated with the abundance of snails, the fishes' principal prey item. Among populations the frequency of molariform morphs decreases in the dry season. Morphology possibly changes reversibly within particular individuals between seasons. These results suggest that phenotypic plasticity and polymorphisms may be an adaptive characteristic of cichlid fishes. Patterns of intraspecific morphological variation match patterns of interspecific morphological diversification which suggests that universal developmental mechanisms canalize the possible expressions of morphology. The ability to respond morphologically to environmental shifts, in conjunction with genetically determined trophic polymorphisms and sexual selection via mate choice, could be the basis for speciation through intermediate stages of polymorphism of the impressive adaptive radiation of cichlid fishes.     

Ecological relations in the evolution of acanthopterygian fishes in warm-temperate communities of the northeastern Pacific

Synopsis The species composition of acanthopterygian fishes in warm-temperate communities of the northeastern Pacific reflects the influence of ecological relations on teleostean evolution. The species are of either temperate or tropical derivation, with the temperate derivatives (e.g., scorpaeniforms, pleuronectiforms and zoarcoid perciforms) being mostly generalized carnivores, and the tropical derivatives (almost all of them perciforms) ranging from generalized carnivores to a diversity of specialized carnivores and herbivores. The tropical group dominates, with species of the labroid families Pomacentridae, Embiotocidae and Labridae being especially prominent, based mainly on specialized abilities to feed on sessile invertebrates and zooplankters. Other perciforms of tropical stock that do well here include kyphosids, which are herbivores. These trophic capabilities have been inherited from tropical ancestors and are poorly developed among the temperate derivatives. Despite their successes in warm-temperate habitats, few tropical derivatives have extended their distributions into the cold-temperate region; similarly, temperate derivatives have been to a large extent limited in spreading southward into the warm-temperate region. These limits to distribution cannot be attributed to problems with food resources, but are readily explained by effects of surface currents on early life-history stages in this coastal upwelling system.     

Specializations and limitations in the utilization of food resources by the carp,Cyprinus carpio: a study of oral food processing

Synopsis The wide variety of aquatic food is considered to be instrumental for the diversification in fish species. Yet their abilities and inabilities of handling food are poorly known. For these reasons the food processing and feeding repertoire of the adult carp, Cyprinus carpio, fed on a variety of food types, were analyzed by light and X-ray cinematography of the head parts and by electromyography of the head and body muscles during feeding. Nine stereotyped movement patterns (particulate intake, gulping, rinsing, spitting, selective retention of food, transport, crushing, grinding and deglutition) compose the feeding process, their sequence and frequency were adjusted to the type of food. Following quantitative morphological analysis at macroscopic, light- and electronmicroscopical level, the relations between the functioning and architecture of the feeding apparatus were established. The structure and dimensions of the mouth opening, the protrusible upper jaw, the slit-shaped pharyngeal cavity, the palatal and postlingual organ, the branchial sieve, the pharyngeal masticatory apparatus and the distribution of taste buds, mucous cells and muscle fibers along the oropharyngeal surface were the directive structural characters used for estimating the abilities in food processing. The specializations for utilizing food items and its limitations, derived from structural and functional data, are compared with diet data found in the literature in order to evaluate the relative position of the carp in competition for food in the aquatic environment. It is established that the ‘omnivorous’ carp is specialized in effective handling of several categories of aquatic food, even when these are mixed with non-food (bottom invertebrates <4% SL in diameter) since the palatal organ enables the carp to separate food from non-food. This includes very hard-skinned food items, processed with the powerful pharyngeal jaws of the fish, and to a lesser extent zooplankton (>250 μm). The carp is at the same time very limited in processing long and struggling prey (e.g. fish) as well as vegetable matter, due to the lack of oral teeth and the specialized morphology of its pharyngeal chewing apparatus. These feeding abilities agree with diet data from literature. The reported herbivorism of carp illustrates its opportunism in feeding behaviour. Specialization in feeding is discussed and the necessity to take into account the total series of post-capture feeding actions for a more complete view on trophic specialization. Food intake and the intra-oral food processing of carp are bound to the structures of its sensory, central processing and effector apparatus and to the plasticity in their functioning. These together determine its feeding efficiency in exploiting the available aquatic food resources. Next to ethological and ecological studies functional morphology is another important tool to explain the trophic interactions of fish.