Premaxillary movements in cyprinodontiform fishes: an unusual protrusion mechanism facilitates "picking" prey capture

Premaxillary protrusion is hypothesized to confer a number of feeding advantages to teleost fishes; however, most proposed advantages relate to enhanced stealth or suction production during prey capture. Cyprinodontiformes exhibit an unusual form of premaxillary protrusion where the descending process of the premaxilla does not rotate anteriorly to occlude the sides of the open mouth during prey capture. Instead, the premaxilla is protruded such that it gives the impression of a beak during prey capture. We quantified premaxillary kinematics during feeding in four cyprinodontiform taxa and compared them with three percomorph taxa to identify any performance consequences of this protrusion mechanism. Individual prey capture events were recorded using digital high-speed video at 250-500 frames per second (n4 individuals, 4 strikes per individual). Species differed in the timing of movement and the maximum displacement of the premaxilla during the gape cycle and in the contribution of the premaxilla to jaw closing. Cyprinodontiform taxa produced less premaxillary protrusion than the percomorph taxa, and were consistently slower in the time to maximum gape. Further, it appears cyprinodontiforms can alter the contribution of the premaxilla to mouth closure on an event-specific basis. We were able to demonstrate that, within at least one species, this variability is associated with the location of the prey (bottom vs. water column). Cyprinodontiform upper jaw movements likely reflect increased dexterity associated with a foraging ecology where prey items are "picked" from a variety of locations: the bottom, water column, or surface. We postulate that dexterity requires slow, precisely controlled jaw movements; thus, may be traded off for some aspects of suction-feeding performance, such as protrusion distance and speed.     

Functional morphology of prey capture in the sturgeon,Scaphirhynchus albus

Acipenseriformes (sturgeon and paddlefish) are basal actinopterygians with a highly derived cranial morphology that is characterized by an anatomical independence of the jaws from the neurocranium. We examined the morphological and kinematic basis of prey capture in the Acipenseriform fish Scaphirhynchus albus, the pallid sturgeon. Feeding pallid sturgeon were filmed in lateral and ventral views and movement of cranial elements was measured from video sequences. Sturgeon feed by creating an anterior to posterior wave of cranial expansion resulting in prey movement through the mouth. The kinematics of S. albus resemble those of other aquatic vertebrates: maximum hyoid depression follows maximum gape by an average of 15 ms and maximum opercular abduction follows maximum hyoid depression by an average of 57 ms. Neurocranial rotation was not a part of prey capture kinematics in S. albus, but was observed in another sturgeon species, Acipenser medirostris. Acipenseriformes have a novel jaw protrusion mechanism, which converts rostral rotation of the hyomandibula into ventral protrusion of the jaw joint. The relationship between jaw protrusion and jaw opening in sturgeon typically resembles that of elasmobranchs, with peak upper jaw protrusion occurring after peak gape.     

Comparative kinematics of cypriniform premaxillary protrusion

Premaxillary protrusion has evolved multiple times within teleosts, and has been implicated as contributing to the evolutionary success of clades bearing this adaptation. Cypriniform fishes protrude the jaws via the kinethmoid, a median sesamoid bone that is a synapomorphy for the order. Using five cypriniform species, we provide the first comparative kinematic study of jaw protrusion in this speciose order. Our goals were to compare jaw protrusion in cypriniforms to that in other clades that independently evolved upper jaw protrusion, assess the variation in feeding kinematics among members of the order, and test if variation in the shape of the kinethmoid has an effect on either jaw kinematics or the degree of suction or ram used during a feeding event. We also examined the coordination in the relative timings of upper and lower jaw movements to gain insight on the cypriniform protrusile mechanism. Overall, speed of protrusion in cypriniforms is slower than in other teleosts. Protrusion speed differed significantly among cypriniforms but this is likely not due to kinethmoid shape alone; rather, it may be a result of both kinethmoid shape and branching patterns of the A1 division of the adductor mandibulae. In the benthic cypriniforms investigated here, upper jaw protrusion contributed up to 60% of overall ram of the strikes and interestingly, these species also produced the most suction. There is relatively little coordination of upper and lower jaw movements in cypriniforms, suggesting that previous hypotheses of premaxillary protrusion via lower jaw depression are not supported within Cypriniformes. Significant variation in kinematics suggests that cypriniforms may have the ability to modulate feeding, which could be an advantage if presented with the challenge of feeding on different types of prey.     

Using zebrafish to investigate cypriniform evolutionary novelties: functional development and evolutionary diversification of the kinethmoid

Although the zebrafish has become a popular model organism for biomedical studies, we propose that the wealth of morphological novelties that characterize this cypriniform fish makes it well suited for investigating the development of evolutionary innovations. Morphological novelties associated with feeding in cypriniform fishes include: a unique structure of the pharyngeal jaws in which the lower pharyngeal jaws are enlarged and opposed to a pad on the basioccipital process; a palatal organ found on the roof of the buccal chamber that is thought to help process detrital food within the buccal chamber; and, the kinethmoid, a novel ossification that effects a unique means of premaxillary protrusion. We present new morphological and developmental data and review functional data regarding the role of the kinethmoid in premaxillary protrusion in the zebrafish. Premaxillary protrusion plays an important role in effective prey acquisition in teleosts and the evolution of a unique means of premaxillary protrusion within Cypriniformes may have led to a number of trophic radiations within this clade. Ontogenetic data from zebrafish show that substantial premaxillary protrusion is not seen until these fish have undergone metamorphosis at which point the adductor mandibulae musculature becomes divided and all ligamentous attachments become established. A comparative study of families within Cypriniformes shows diverse morphologies of the kinethmoid. The morphological diversification that characterizes the kinethmoid suggests that this feeding structure has played a role in trophic radiations within Cypriniformes, since the morphology of this feature is correlated with feeding habits.     

Morphology of a picky eater: a novel mechanism underlies premaxillary protrusion and retraction within cyprinodontiforms

Upper jaw protrusion is hypothesized to improve feeding performance in teleost fishes by enhancing suction production and stealth of the feeding event. However, many cyprinodontiform fishes (mid-water feeders, such as mosquitofish, killifish, swordtails, mollies and pupfish) use upper jaw protrusion for "picking" prey out of the water column or off the substrate; this feeding mode may require improved jaw dexterity, but does not necessarily require increased stealth and/or suction production. We describe functional aspects of the bones, muscles and ligaments of the anterior jaws in three cyprinodontiform genera: Fundulus (Fundulidae), Gambusia and Poecilia (Poeciliidae). All three genera possess a premaxillomandibular ligament that connects the premaxilla of the upper jaw to the mandible. The architecture of this ligament is markedly different from the upper-lower jaw connections previously described for basal atherinomorphs or other teleosts, and this loose ligamentous connection allows for more pronounced premaxillary protrusion in this group relative to closely related outgroup taxa. Within poeciliids, a novel insertion of the second division of the adductor mandibulae (A2) onto the premaxilla has also evolved, which allows this jaw adductor to actively retract the premaxilla during mouth closing. This movement is in contrast with most other teleosts, where the upper jaw is retracted passively via pressure applied by the adduction of the lower jaw. We postulate that this mechanism of premaxillary protrusion mediates the cyprinodontiforms' ability to selectively pick specific food items from the water column, surface or bottom, as a picking-based feeding mechanism requires controlled and coordinated "forceps-like" movements of the upper and lower jaws. This mechanism is further refined in some poeciliids, where direct muscular control of the premaxillae may facilitate picking and/or scraping material from the substrate.     

Suction among pickers: jaw mechanics,dietary breadth and feeding behaviour in beach‐spawning Leuresthes spp. compared with their relatives

Jaw mechanics and dietary breadth in California grunion Leuresthes tenuis and Gulf grunion Leuresthes sardina were compared with three other members of the tribe Atherinopsini to test whether these two species have evolved a novel jaw protrusion that might be associated with feeding narrowly on abundant prey near spawning beaches. Quantitative comparison of cleared‐and‐stained specimens of five members of the atherinopsine clade showed that, compared with false grunion Colpichthys regis, topsmelt Atherinops affinis and jacksmelt Atherinopsis californiensis, L. tenuis and L. sardina have longer, more downwardly directed premaxillary protrusion, expanded dentary and premaxillary bones, greater lower jaw rotation and larger premaxilla–vomer separation. Leuresthes tenuis showed greater differences than L. sardina in these features. Comparison of the gut contents of L. tenuis and A. affinis with zooplankton samples collected simultaneously with these fishes in the water column within 1 km of shore showed that, as predicted, L. tenuis fed predominantly on mysid crustaceans and had a narrower diet than A. affinis. High‐speed video analysis showed that L. tenuis exhibits a mean time to maximum jaw protrusion c. 2·5 times shorter than that of A. affinis. The grunion sister species, especially L. tenuis, have evolved suction feeding that may allow efficient feeding on common, evasive prey near spawning sites. The morphological traits seen in both species of Leuresthes signify a marked difference from their closest relatives in prey capture and suggest a type of jaw protrusion not yet seen in cyprinodontiforms or perciforms.     

Feeding behavior and kinematics of the lesser electric ray, Narcine brasiliensis (Elasmobranchii: Batoidea)

Jaw protrusion is a major functional motif in fish feeding and can occur during mouth opening or closing. This temporal variation impacts the role that jaw protrusion plays in prey apprehension and processing. The lesser electric ray Narcine brasiliensis is a benthic elasmobranch (Batoidea: Torpediniformes) with an extreme and unique method of prey capture. The feeding kinematics of this species were investigated using high-speed videography and pressure transduction. The ray captures its food by protruding its jaws up to 100% of head length (approximately 20% of disc width) beneath the substrate and generating negative oral pressures (< or = 31 kPa) to suck worms into its mouth. Food is further winnowed from ingested sediment by repeated, often asymmetrical protrusions of the jaws (> 70 degrees deviation from the midline) while sand is expelled from the spiracles, gills and mouth. The pronounced ram contribution of capture (jaw protrusion) brings the mouth close enough to the food to allow suction feeding. Due to the anatomical coupling of the jaws, upper jaw protrusion occurs in the expansive phase (unlike most elasmobranchs and similar to bony fishes), and also exhibits a biphasic (slow-open, fast-open) movement similar to tetrapod feeding. The morphological restrictions that permit this unique protrusion mechanism, including coupled jaws and a narrow gape, may increase suction performance, but also likely strongly constrain dietary breadth.     

Development of the cypriniform protrusible jaw complex in Danio rerio: Constructional insights for evolution

Studies on the evolution of complex biological systems are difficult because the construction of these traits cannot be observed during the course of evolution. Complex traits are defined as consisting of multiple elements, often of differing embryological origins, with multiple linkages integrated to form a single functional unit. An example of a complex system is the cypriniform oral jaw apparatus. Cypriniform fishes possess an upper jaw characterized by premaxillary protrusion during feeding. Cypriniforms effect protrusion via the kinethmoid, a synapomorphy for the order. The kinethmoid is a sesamoid ossification suspended by ligaments attaching to the premaxillae, maxillae, palatines, and neurocranium. Upon mouth opening, the kinethmoid rotates as the premaxillae move anteriorly. Along with bony and ligamentous elements, there are three divisions of the adductor mandibulae that render this system functional. It is unclear how cypriniform jaws evolved because although the evolution of sesamoid elements is common, the incorporation of the kinethmoid into the protrusible jaw results in a function that is atypical for sesamoids. Developmental studies can show how biological systems are assembled within individuals and offer clues about how traits might have been constructed during evolution. We investigated the development of the protrusible upper jaw in zebrafish to generate hypotheses regarding the evolution of this character. Early in development, the adductor mandibulae arises as a single unit. The muscle divides after ossification of the maxillae, on which the A1 division will ultimately insert. A cartilaginous kinethmoid first develops within the intermaxillary ligament; it later ossifies at points of ligamentous attachment. We combine our structural developmental data with published kinematic data at key developmental stages and discuss potential functional advantages in possessing even the earliest stages of a system for protrusion. J. Morphol. 2010. © 2010 Wiley‐Liss, Inc.     

Co‐evolution of the premaxilla and jaw protrusion in cichlid fishes (Heroine: Cichlidae)

The ability of Perciform fishes to protrude their jaw has likely been critical to the trophic diversification of this group, which includes approximately 20% of all vertebrates. The length of the ascending process of the premaxilla is thought to influence the maximum extent that cichlids and other Perciforms protrude their oral jaw. Using a combination of morphometrics, kinematics, and new phylogenetic hypotheses for 20 Heroine cichlid species, we tested the evolutionary relationship between the length of the premaxillary ascending process and maximum jaw protrusion. In this clade, the length of the ascending process of the premaxilla ranged from 11.6–32.7% with respect to standard length whereas maximum jaw protrusion ranged from 3.5–23.4% with respect to standard length. The evolutionary relationships among the Heroine cichlids obtained from the genetic partitions cytochrome b, S7, and RAG1 showed limited concordance. However, correlations between the length of the ascending process and maximum jaw protrusion were highly significant when examined as independent contrasts using all three topologies. Evolutionary change in the length of the ascending process of the premaxilla is likely critical for determining the amount of jaw protrusion in Perciform groups such as cichlid fishes. © 2010 The Linnean Society of London, Biological Journal of the Linnean Society, 2010, 100 , 619–629.     

Dental Histology of a Characoid Fish from the Plio-Pleistocene of Acre, Brazil

The histology of premaxillary teeth of Colossoma sp. from the Solimöes Formation of Northern Brazil is described, in comparison with Recent material. Analysis by light and scanning electron microscopy shows that they share with those of the serrasalminids, histological (though not anatomical) features of carnivorous fishes. The enameloid in both groups displays surface-parallel images of calcified fibres along the periphery of the teeth while internally, these images present a random distribution. Such similarities suggest that both the serrasalminids and myleinids may have evolved from carnivorous ancestors. The serrasalminids remain carnivorous, while the herbivorous habit of the myleinids may have been acquired secondarily. This change of diet is probably related to changes in the shape of the teeth which are used for crushing fruits, seeds and leaves. The teeth in adult Colossoma are unicuspid, though not conical. The molar or incisive form of these teeth may have evolved independently from the conical-shaped primitive dentition of the characoids, without passing through any stage of fusion of dental papillae, which is observed in the ontogenetic development of other characoids.     

Micro- and macroevolutionary decoupling of cichlid jaws: a test of Liem's key innovation hypothesis

The extent to which elements of functional systems can change independently (modularity) likely influences the diversification of lineages. Major innovations in organismal design, like the pharyngeal jaw in cichlid fishes, may be key to a group's success when they relax constraints on diversification by increasing phenotypic modularity. In cichlid fishes, pharyngeal jaw modifications that enhanced the ability to breakdown prey may have freed their oral jaws from serving their ancestral dual role as a site of both prey capture and prey processing. This functional decoupling that allowed the oral jaws to become devoted solely to prey capture has been hypothesized to have permitted the two sets of cichlid jaws to evolve independently. We tested the hypothesis that oral and pharyngeal jaw mechanics are evolutionarily decoupled both within and among Neotropical Heroine cichlids. In the trophically polymorphic species Herichthys minckleyi, molariforms that exhibit enlarged molarlike pharyngeal jaw teeth were found to have approximately 400% greater lower jaw mass compared to H. minckleyi with the alternative papilliform pharyngeal morphology. However, oral jaw gape, lower jaw velocity ratios, anterior jaw linkage mechanics, and jaw protrusion did not differ between the morphotypes. In 40 other Heroine species, there was a weak correlation between oral jaw mechanics and pharyngeal jaw mass when phylogenetic history was ignored. Yet, after expansion of the cytochrome b phylogeny for Heroines, change in oral jaw mechanics was found to be independent of evolutionary change in pharyngeal jaw mass based on independent contrasts. Evolutionary decoupling of oral and pharyngeal jaw mechanics has likely played a critical role in the unparalleled trophic diversification of cichlid fishes.     

Structures associated with feeding in three broad-mouthed, benthic fish groups

Synopsis The flatheads, toadfishes, and goosefishes discussed here hold certain features in common. All are bottom-living forms with depressed head areas and broad gapes, and all eat large food items: fishes and/or crabs. All have developed structural specializations in association with this diet. The three groups are at most distantly related, and their feeding specializations are different and have evolved from different bases. In flatheads the combination of large food items and depressed head regions seems to have led to the separation of the two halves of the pelvic girdle, a feature in which they differ from their scorpaenoid relatives. Toadfish peculiarities associated with feeding are various but most notable in those that pass crabs they eat through the gape and into the mouth. Goosefish feeding is centered around the use of a lure to attract prey to within striking distance. The three fish groups are discussed separately, but their feeding structures are compared to one another in the final section of the paper.     

Hair cell heterogeneity and ultrasonic hearing: recent advances in understanding fish hearing

The past decade has seen a wealth of new data on the auditory capabilities and mechanisms of fishes. We now have a significantly better appreciation of the structure and function of the auditory system in fishes with regard to their peripheral and central anatomy, physiology, behaviour, sound source localization and hearing capabilities. This paper deals with two of the newest of these findings, hair cell heterogeneity and the detection of ultrasound. As a result of this recent work, we now know that fishes have several different types of sensory hair cells in both the ear and lateral line and there is a growing body of evidence to suggest that these hair cell types arose very early in the evolution of the octavolateralis system. There is also some evidence to suggest that the differences in the hair cell types have functional implications for the way the ear and lateral line of fishes detect and process stimuli. Behavioural studies have shown that, whereas most fishes can only detect sound to 1-3 kHz, several species of the genus Alosa (Clupeiformes, i.e. herrings and their relatives) can detect sounds up to 180 kHz (or even higher). It is suggested that this capability evolved so that these fishes can detect one of their major predators, echolocating dolphins. The mechanism for ultrasound detection remains obscure, though it is hypothesized that the highly derived utricle of the inner ear in these species is involved.     

Jaw structures and movements in higher teleostean fishes

All of the diverse jaw structures in higher teleosts appear to be modifications of a single basal type and are treated as such. Only some of the principal variants are discussed. Though the two jaws act as a coordinated unit during feeding, their movements are different. The upper and lower jaws are discussed separately. In the upper jaw the principal concern is with the various types of premaxillary protrusion and with the secondary development in some groups of a rocking premaxilla. For the lower jaw most of the account is devoted to the repeated differentiation of movements in its anterior and posterior sections. The paper concludes with comments on the jaw apparatus as a functional unit and its evolution in higher teleosts.     

Structure and function of the hyolingual system in Hynobius and its bearing on the evolution of prey capture in terrestrial salamanders

The Hynobiidae is generally regarded as the most phylogenetically basal and least derived extant family of terrestrial salamanders. As in the other families of terrestrial salamanders, prey capture in the Hynobiidae is accomplished by lingual prehension. In Hynobius, the prey capture system appears to be a mosaic of derived and primitive features. This, in conjunction with previous studies, suggests that the hyolingual systems of all families of terrestrial salamanders have evolved various degrees of specialization since the appearance of the common ancestral condition. We propose that the generalized feeding system for the extant terrestrial salamanders includes a hyolingual skeleton comprised of one basibranchial, one pair of radial or radial-like structures, two pairs of ceratobranchials, two pairs of epibranchials, one pair of ceratohyals, and one urohyal arranged in a configuration similar to that of Hynobius; a simple, sac-like secondary tongue pad; a lift and thrust system of tongue projection; a four-part gape cycle; and a forward head and body surge. Modifications to this general plan, previously described for the disparate families, include various changes in the size, shape, and definition of the tongue pad, changes in the specific types of structures and configurations in the anterior hyolingual skeleton, secondary ossification in the posterior hyolingual skeleton, the appearance of various protrusion, projection, and flipping systems for tongue protraction, simplification of the kinematic gape profile, and loss of the forward head and body surge. The evolutionary trends in these modifications have provided a rich data set from which much phylogenetic information has been inferred. © 1996 Wiley-Liss, Inc.     

Evolution of levers and linkages in the feeding mechanisms of fishes

The evolution of feeding mechanisms in the ray-finned fishes(Actinopterygii) is a compelling example of transformation ina musculoskeletal complex involving multiple skeletal elementsand numerous muscles that power skull motion. Biomechanicalmodels of jaw force and skull kinetics aid our understandingof these complex systems and enable broad comparison of feedingmechanics across taxa. Mechanical models characterize how musclesmove skeletal elements by pulling bones around points of rotationin lever mechanisms, or by transmitting force through skeletalelements connected in a linkage. Previous work has focused onthe feeding biomechanics of several lineages of fishes, buta broader survey of skull function in the context of quantitativemodels has not been attempted. This study begins such a surveyby examining the diversity of mechanical design of the oraljaws in 35 species of ray-finned fishes with three main objectives:(1) analyze lower jaw lever models in a broad phylogenetic rangeof taxa, (2) identify the origin and evolutionary patterns ofchange in the linkage systems that power maxillary rotationand upper jaw protrusion, and (3) analyze patterns of changein feeding design in the context of actinopterygian phylogeny.The mandibular lever is present in virtually all actinopterygians,and the diversity in lower jaw closing force transmission capacity,with mechanical advantage ranging from 0.04 to 0.68, has importantfunctional consequences. A four-bar linkage for maxillary rotationarose in the Amiiformes and persists in various forms in manyteleost species. Novel mechanisms for upper jaw protrusion basedon this linkage for maxillary rotation have evolved independentlyat least five times in teleosts. The widespread anterior jawslinkage for jaw protrusion in percomorph fishes arose initiallyin Zeiformes and subsequently radiated into a wide range ofpremaxillary protrusion capabilities.     

Evolution of novel jaw joints promote trophic diversity in coral reef fishes

We investigated the functional morphology and ecology of biting among the squamipinnes, an assemblage of nine successful and distinctive reef fish families. We demonstrate that an intramandibular joint (IMJ) may have evolved at least three and possibly five times in this assemblage and discuss the impact of this recurring innovation in facilitating prey-capture by biting. Using character mapping on a supertree for the squamipinnes, we reveal up to seven gains or losses of intramandibular flexion, all associated with trophic transitions between free-living and attached prey utilization. IMJs are basal in six of the studied families whereas the origin of intramandibular flexion in the Chaetodontidae (butterflyfishes) coincides with a transition from ram-suction feeding to benthic coral feeding, with flexion magnitude reaching its peak (49 ± 2.7°) in the coral scraping subgenus Citharoedus . Although IMJs generally function to augment vertical gape expansion during biting behaviours to remove small invertebrates, algae or coral from the reef, the functional ecology of IMJs in the Pomacanthidae (angelfishes) stands in contrast. Pomacanthid IMJs exhibit over 35° of flexion, permitting gape closure when the jaws are fully protruded. We demonstrate the widespread IMJ occurrence among extant biters to result from a complex convergent evolutionary history, indicating that the IMJ is a major functional innovation that enhances biting strategies in several prominent reef fish groups.  © 2008 The Linnean Society of London, Biological Journal of the Linnean Society , 2008, 93 , 545–555.     

The locomotory system of pearlfish Carapus acus: What morphological features are characteristic for highly flexible fishes?

The body curvature displayed by fishes differs remarkably between species. Some nonmuscular features (e.g., number of vertebrae) are known to influence axial flexibility, but we have poor knowledge of the influence of the musculotendinous system (myosepta and muscles). Whereas this system has been described in stiff‐bodied fishes, we have little data on flexible fishes. In this study, we present new data on the musculotendinous system of a highly flexible fish and compare them to existing data on rigid fishes. We use microdissections with polarized light microscopy to study the three‐dimensional anatomy of myoseptal tendons, histology and immunohistology to study the insertion of muscle fiber types into tendons, and μ‐CT scans to study skeletal anatomy. Results are compared with published data from stiff‐bodied fishes. We identify four important morphological differences between stiff‐bodied fishes and Carapus acus: (1) Carapus bears short tendons in the horizontal septum, whereas rigid fishes have elongated tendons. (2) Carapus bears short lateral tendons in its myosepta, whereas stiff‐bodied fishes bear elongated tendons. Because of its short myoseptal tendons, Carapus retains high axial flexibility. In contrast, elongated tendons restrict axial flexibility in rigid fishes but are able to transmit anteriorly generated muscle forces through long tendons down to the tail. (3) Carapus bears distinct epineural and epipleural tendons in its myosepta, whereas these tendons are weak or absent in rigid fishes. As these tendons firmly connect vertebral axis and skin in Carapus, we consider them to constrain lateral displacement of the vertebral axis during extreme body flexures. (4) Ossifications of myoseptal tendons are only present in C. acus and other more flexible fishes but are absent in rigid fishes. The functional reasons for this remain unexplained. J. Morphol., 2012. © 2011 Wiley Periodicals, Inc.     

Predicting patterns of prey use from morphology of fishes

Synopsis Ecomorphological analyses that search for patterns of association between morphological and prey-use data sets will have a greater chance of understanding the causal relationships between form and diet if the morphological variables used have known consequences for feeding performance. We explore the utility of fish body size, mouth gape and jaw-lever mechanics in predicting patterns of prey use in two very different communities of fishes, Caribbean coral reef fishes, and species of the Centrarchidae that live in Lake Opinicon, Ontario. In spite of major differences in the spectrum of potential prey available, the centrarchids of Lake Opinicon show dietary transitions during ontogeny that are very similar to those seen among and within species of Caribbean groupers (Serranidae). The transition from small zooplankton to intermediate sized invertebrates and ultimately to fishes appears to be very general in ram-suction feeding fishes and is probably driven largely by the constraints of mouth size on prey capture ability. The jaw-lever systems for mouth opening and closing represent direct trade-offs for speed and force of jaw movement. The ratio of in-lever to out-lever in the opening system changes during ontogeny in bluegill, indicating that the mechanics and kinematics of jaw movement may change as well. Among 34 species of Caribbean reef fishes, biting species had jaw-closing ratios that favored force translation, while species that employ rapid-strike ram-suction had closing ratios that enhanced speed of closing and mouth opening ratios that favored a more rapid expansion of the mouth during the strike. We suggest that when prey are categorized into functional groups, reflecting the specific performance features that are important in capturing and handling them, and the differences among habitats in the available prey resource are taken into account, general patterns can be found in morphology-diet relations that cross phylogenetic boundaries.     

Morphological evidence supporting the monophyly of the family Polynemidae (Teleostei: Perciformes) and its sister relationship with Sciaenidae

The monophyly of Polynemidae was evaluated and its sister relationship with Sciaenidae discussed, based on osteological and myological characters from 24 polynemid species in eight genera, with comparisons with acanthomorph fishes from literature and 86 species in 8 orders and 63 families examined. Polynemidae was inferred as a monophyletic group, strongly supported by 19 synapomorphies, including four unique characters (unnamed bone present on cephalic sensory canal extending from supratemporal, third actinost not supporting pectoral-fin rays, section A1 comprising lateral and medial elements, and division of obliquus inferioris present between lower postcleithrum and rod-like process on coracoid) in percoids. In addition, seven pectoral girdle characters were recognized, with the girdle possessing filament-like sensory rays, an adaptation to benthic life in muddy water. The sister relationship of Polynemidae and Sciaenidae was supported by six synapomorphies, including two rather rare (a single branchiostegal ray suspended by epihyal and posterior portions of pelvic bones on both sides interdigitated) and two unique characters (metapterygoid and quadrate interdigitated medially and anterior extension of the nasal canal).