Scaling of feeding biomechanics in the horn shark Heterodontus francisci: ontogenetic constraints on durophagy

Four-bar Geometry is Shared among Ecologically DivergentFish Species

Understanding the factors that influence morphological evolution is a major goal in biology. One such factor is the ability to acquire and process prey. Prey hardness and evasiveness are important properties that can impact evolution of the jaws. Similar diets and biomechanical systems have repeatedly evolved among fish lineages, providing an opportunity to test for shared patterns of evolution across distantly related organisms. Four-bar linkages are structures often used by animals to transmit force and motion during feeding and that provide an excellent system to understand the impact of diet on morphological and biomechanical evolution. Here, we tested how diet influences the evolutionary dynamics of the oral four-bar linkage system in wrasses (Family: Labridae) and cichlids (Family: Cichlidae). We found that shifts in prey hardness/evasiveness are associated with limited modifications in four-bar geometry across these two distantly related fish lineages. Wrasse and cichlid four-bar systems largely exhibit many-to-one mapping in response to dietary shifts. Across two iconic adaptive radiations of fish, an optimal four-bar geometry has largely been co-opted for different dietary functions during their extensive ecological diversification. Given the exceptional jaw diversity of both lineages, many-to-one mapping of morphology to mechanical properties may be a core feature of fish adaptive radiation.

Diversity in the internal functional feeding elements of sympatric morphs of Arctic charr (Salvelinus alpinus)

The diversity of functional feeding anatomy is particularly impressive in fishes and correlates with various interspecific ecological specializations. Intraspecific polymorphism can manifest in divergent feeding morphology and ecology, often along a benthic-pelagic axis. Arctic charr (Salvelinus alpinus) is a freshwater salmonid known for morphological variation and sympatric polymorphism and in Lake Þingvallavatn, Iceland, four morphs of charr coexist that differ in preferred prey, behaviour, habitat use, and external feeding morphology. We studied variation in six upper and lower jaw bones in adults of these four morphs using geometric morphometrics and univariate statistics. We tested for allometric differences in bone size and shape among morphs, morph effects on bone size and shape, and divergence along the benthic-pelagic axis. We also examined the degree of integration between bone pairs. We found differences in bone size between pelagic and benthic morphs for two bones (dentary and premaxilla). There was clear bone shape divergence along a benthic-pelagic axis in four bones (dentary, articular-angular, premaxilla and maxilla), as well as allometric shape differences between morphs in the dentary. Notably for the dentary, morph explained more shape variation than bone size. Comparatively, benthic morphs possess a compact and taller dentary, with shorter dentary palate, consistent with visible (but less prominent) differences in external morphology. As these morphs emerged in the last 10,000 years, these results indicate rapid functional evolution of specific feeding structures in arctic charr. This sets the stage for studies of the genetics and development of rapid and parallel craniofacial evolution.

How to Survive a (Juvenile) Piranha Attack: An Integrative Approach to Evaluating Predator Performance

Figures

Cory cat panel figureDrawing of bite force measuring equipment and indentation rig Pygocentrus nattereri jaw muscle morphology and skull anatomyBox plot grid of number of Pygocentrus nattereri bites before puncture along different body regions of Corydoras trilineatus during feeding trials resultsDrawing of color-coded Corydoras trilineatus with attack frequencies and average bites until puncture by Pygocentrus nattereriBox plot of average voluntary juvenile Pygocentrus nattereri bite forces to standard lengthPanel of linear ordinary least-squares regressions of Pygocentrus nattereri bite force to adductor mandibulae mass, standard length, and body massOrdinary least-squares regressions of voluntary bites to restrained bites of Pygocentrus nattereriPanel of indentation tests for intact and removed Corydoras trilineatus scutesPanel of indentation tests for Corydoras trilineatus body region.

Synopsis

There is an evolutionary arms race between predators and prey. In aquatic environments, predatory fishes often use sharp teeth, powerful bites, and/or streamlined bodies to help capture their prey quickly and efficiently. Conversely, prey are often equipped with antipredator adaptations including: scaly armor, sharp spines, and/or toxic secretions. This study focused on the predator-prey interactions between the armored threestripe cory catfish (Corydoras trilineatus) and juvenile red-bellied piranha (Pygocentrus nattereri). Specifically, we investigated how resistant cory catfish armor is to a range of natural and theoretical piranha bite forces and how often this protection translated to survival from predator attacks by Corydoras. We measured the bite force and jaw functional morphology of P. nattereri, the puncture resistance of defensive scutes in C. trilineatus, and the in situ predatory interactions between the two. The adductor mandibulae muscle in juvenile P. nattereri is robust and delivers an average bite force of 1.03 N and maximum bite force of 9.71 N, yet its prey, C. trilineatus, survived 37% of confirmed bites without any damage. The C. trilineatus armor withstood an average of nine bites before puncture by P. nattereri. Predation was successful only when piranhas bit unarmored areas of the body, at the opercular opening and at the caudal peduncle. This study used an integrative approach to understand the outcomes of predator-prey interactions by evaluating the link between morphology and feeding behavior. We found that juvenile P. nattereri rarely used a maximal bite force and displayed a net predation success rate on par with other adult vertebrates. Conversely, C. trilineatus successfully avoided predation by orienting predator attacks toward their resilient, axial armor and behavioral strategies that reduced the predator's ability to bite in less armored regions of the body.

The allometric scaling of oxygen supply and demand in the California horn shark, Heterodontus francisci

The gill surface area of aquatic ectotherms is thought to be closely linked to the ontogenetic scaling of metabolic rate, a relationship that is often used to explain and predict ecological patterns across species. However, there are surprisingly few within-species tests of whether metabolic rate and gill area scale similarly. We examined the relationship between oxygen supply (gill area) and demand (metabolic rate) by making paired estimates of gill area with resting and maximum metabolic rates across ontogeny in the relatively inactive California horn shark, Heterodontus francisci. We found that the allometric slope of resting metabolic rate was 0.966±0.058 (±95% CI), whereas that of maximum metabolic rate was somewhat steeper (1.073±0.040). We also discovered that the scaling of gill area shifted with ontogeny: the allometric slope of gill area was shallower in individuals <0.203 kg in body mass (0.564±0.261), but increased to 1.012±0.113 later in life. This appears to reflect changes in demand for gill-oxygen uptake during egg case development and immediately post hatch, whereas for most of ontogeny, gill area scales in between that of resting and maximum metabolic rate. These relationships differ from predictions of the gill oxygen limitation theory, which argues that the allometric scaling of gill area constrains metabolic processes. Thus, for the California horn shark, metabolic rate does not appear limited by theoretical surface-area-to-volume ratio constraints of gill area. These results highlight the importance of data from paired and size-matched individuals when comparing physiological scaling relationships.

Developing elastic mechanisms: ultrafast motion and cavitation emerge at the millimeter scale in juvenile snapping shrimp

Organisms such as jumping froghopper insects and punching mantis shrimp use spring-based propulsion to achieve fast motion. Studies of elastic mechanisms have primarily focused on fully developed and functional mechanisms in adult organisms. However, the ontogeny and development of these mechanisms can provide important insights into the lower size limits of spring-based propulsion, the ecological or behavioral relevance of ultrafast movement, and the scaling of ultrafast movement. Here, we examined the development of the spring-latch mechanism in the bigclaw snapping shrimp, Alpheus heterochaelis (Alpheidae). Adult snapping shrimp use an enlarged claw to produce high-speed strikes that generate cavitation bubbles. However, until now, it was unclear when the elastic mechanism emerges during development and whether juvenile snapping shrimp can generate cavitation at this size. We reared A. heterochaelis from eggs, through their larval and postlarval stages. Starting 1 month after hatching, the snapping shrimp snapping claw gradually developed a spring-actuated mechanism and began snapping. We used high-speed videography (300,000 frames s-1) to measure juvenile snaps. We discovered that juvenile snapping shrimp generate the highest recorded accelerations (5.8×105±3.3×105 m s-2) for repeated-use, underwater motion and are capable of producing cavitation at the millimeter scale. The angular velocity of snaps did not change as juveniles grew; however, juvenile snapping shrimp with larger claws produced faster linear speeds and generated larger, longer-lasting cavitation bubbles. These findings establish the development of the elastic mechanism and cavitation in snapping shrimp and provide insights into early life-history transitions in spring-actuated mechanisms.

Ontogenetic changes in the tooth morphology of bull sharks (Carcharhinus leucas)

Teeth are an integral component of feeding ecology, with a clear link between tooth morphology and diet, as without suitable dentition prey cannot be captured nor broken down for consumption. Bull sharks, Carcharhinus leucas, undergo an ontogenetic niche shift from freshwater to marine habitats, which raises the question: does tooth morphology change with ontogeny? Tooth shape, surface area and thickness were measured using both morphometrics and elliptic Fourier analysis to determine if morphology varied with position in the jaw and if there was an ontogenetic change concordant with this niche shift. Significant ontogenetic differences in tooth morphology as a function of position in the jaw and shark total length were found, with upper and lower jaws of bull sharks presenting two different tooth morphologies. Tooth shape and thickness fell into two groupings, anterior and posterior, in both the upper and lower jaws. Tooth surface area, however, indicated three groupings, mesial, intermediate and distal, in both the upper and lower jaws. While tooth morphology changed significantly with size, showing an inflection at sharks of 135 cm total length, each morphological aspect retained the same tooth groupings throughout. These ontogenetic differences in tooth morphologies reflect tooth strength, prey handling and heterodonty.

2D and 3D visualizations of archosaur jaw muscle mechanics, ontogeny and phylogeny using ternary diagrams and 3D modeling

Comparing patterns of performance and kinematics across behavior, development and phylogeny is crucial to understand the evolution of complex musculoskeletal systems such as the feeding apparatus. However, conveying 3D spatial data of muscle orientation throughout a feeding cycle, ontogenetic pathway or phylogenetic lineage is essential to understanding the function and evolution of the skull in vertebrates. Here, we detail the use of ternary plots for displaying and comparing the 3D orientation of muscle data. First, we illustrate changes in 3D jaw muscle resultants during jaw closing taxa the American alligator (Alligator mississippiensis). Second, we show changes in 3D muscle resultants of jaw muscles across an ontogenetic series of alligators. Third, we compare 3D resultants of jaw muscles of avian-line dinosaurs, including extant (Struthio camelus, Gallus gallus, Psittacus erithacus) and extinct (Tyrannosaurus rex) species to outline the reorganization of jaw muscles that occurred along the line to modern birds. Finally, we compare 3D resultants of jaw muscles of the hard-biting species in our sample (A. mississippiensis, T. rex, P. erithacus) to illustrate how disparate jaw muscle resultants are employed in convergent behaviors in archosaurs. Our findings show that these visualizations of 3D components of jaw muscles are immensely helpful towards identifying patterns of cranial performance, growth and diversity. These tools will prove useful for testing other hypotheses in functional morphology, comparative biomechanics, ecomorphology and organismal evolution.

Early shape divergence of developmental trajectories in the jaw of galeomorph sharks

Background

The onset of morphological differences between related groups can be tracked at early stages during embryological development. This is expressed in functional traits that start with minor variations, but eventually diverge to defined specific morphologies. Several processes during this period, like proliferation, remodelling, and apoptosis for instance, can account for the variability observed between related groups. Morphological divergence through development is often associated with the hourglass model, in which early stages display higher variability and reach a conserved point with reduced variability from which divergence occurs again to the final phenotype.

Results

Here we explored the patterns of developmental shape changes in the lower jaw of two shark species, the bamboo shark (Chiloscyllium punctatum) and the catshark (Scyliorhinus canicula). These two species present marked differences in their foraging behaviour, which is reflected in their adult jaw morphology. By tracing the developmental sequence of the cartilage condensation, we identified the onset of cartilage for both species at around stage 31. Other structures that developed later without a noticeable anlage were the labial cartilages, which appear at around stage 33. We observed that the lower jaw displays striking differences in shape from the earliest moments, without any overlap in shape through the compared stages.

Conclusions

The differences observed are also reflected in the functional variation in feeding mechanism between both species. Likewise, the trajectory analysis shows that the main differences are in the magnitude of the shape change through time. Both species follow a unique trajectory, which is explained by the timing between stages.

Applied Functional Biology: Linking Ecological Morphology to Conservation and Management

Many researchers work at the interface of organisms and environment. Too often, the insights that organismal, or functional, biologists can bring to the understanding of natural history, ecology, and conservation of species are overlooked. Likewise, natural resource managers are frequently focused on the management of populations and communities, while ignoring key functional traits that might explain variation in abundance and shifts in species composition at these ecological levels. Our intention for this symposium is two-fold: (1) to bring to light current and future research in functional and ecological morphology applicable to concerns and goals of wildlife management and conservation and (2) to show how such studies can result in measurable benchmarks useful to regulatory agencies. Symposium topics reveal past, present, and future collaborations between functional morphologists/biomechanists and conservation/wildlife biologists. During the SICB 2020 Annual Meeting, symposium participants demonstrated how data gathered to address fundamental questions regarding the causes and consequences of organismal form and function can also help address issues of conservation and wildlife management. Here we review how these, and other, studies of functional morphology, biomechanics, ecological development morphology and performance can inform wildlife conservation and management, principally by identifying candidate functional traits that have clear fitness consequences and population level implications.

Correlation of skull morphology and bite force in a bird-eating bat (Ia io; Vespertilionidae)

Background

Genetic and ecological factors influence morphology, and morphology is compatible with function. The morphology and bite performance of skulls of bats show a number of characteristic feeding adaptations. The great evening bat, Ia io (Thomas, 1902), eats both insects and birds (Thabah et al. J Mammal 88: 728-735, 2007), and as such, it is considered to represent a case of dietary niche expansion from insects to birds. How the skull morphology or bite force in I. io are related to the expanded diet (that is, birds) remains unknown. We used three-dimensional (3D) geometry of the skulls and measurements of bite force and diets from I. io and 13 other species of sympatric or closely related bat species to investigate the characteristics and the correlation of skull morphology and bite force to diets.

Results

Significant differences in skull morphology and bite force among species and diets were observed in this study. Similar to the carnivorous bats, bird-eaters (I. io) differed significantly from insectivorous bats; I. io had a larger skull size, taller crania, wider zygomatic arches, shorter but robust mandibles, and larger bite force than the insectivores. The skull morphology of bats was significantly associated with bite force whether controlling for phylogeny or not, but no significant correlations were found between diets and the skulls, or between diets and residual bite force, after controlling for phylogeny.

Conclusions

These results indicated that skull morphology was independent of diet, and phylogeny had a greater impact on skull morphology than diet in these species. The changes in skull size and morphology have led to variation in bite force, and finally different bat species feeding on different foods. In conclusion, I. io has a larger skull size, robust mandibles, shortened dentitions, longer coronoid processes, expanded angular processes, low condyles, and taller cranial sagittal crests, and wider zygomatic arches that provide this species with mechanical advantages; their greater bite force may help them use larger and hard-bodied birds as a dietary component.

Feeding habits of the cockfish, Callorhinchus callorynchus (Holocephali: Callorhinchidae) from off northern Argentina

ABSTRACT The feeding habits of Callorhinchus callorynchus were investigated in coastal waters off northern Argentina. The effect of body size, seasons and regions was evaluated on female diet composition using a multiple-hypothesis modelling approach. Callorhinchus callorynchus fed mainly on bivalves (55.61% PSIRI), followed by brachyuran crabs (10.62% PSIRI) and isopods (10.13% PSIRI). Callorhinchus callorynchus females showed changes in the diet composition with increasing body size and also between seasons and regions. Further, this species is able to consume larger bivalves as it grows. Trophic level was 3.15, characterizing it as a secondary consumer. We conclude that C. callorynchus showed a behavior of crushing hard prey, mainly on bivalves, brachyuran, gastropods and anomuran crabs. Females of this species shift their diet with increasing body size and in response to seasonal and regional changes in prey abundance or distribution.

Killing them softly: Ontogeny of jaw mechanics and stiffness in mollusk-feeding freshwater stingrays

Durophagous predators consume hard-shelled prey such as bivalves, gastropods, and large crustaceans, typically by crushing the mineralized exoskeleton. This is costly from the point of view of the bite forces involved, handling times, and the stresses inflicted on the predator's skeleton. It is not uncommon for durophagous taxa to display an ontogenetic shift from softer to harder prey items, implying that it is relatively difficult for smaller animals to consume shelled prey. Batoid fishes (rays, skates, sawfishes, and guitarfishes) have independently evolved durophagy multiple times, despite the challenges associated with crushing prey harder than their own cartilaginous skeleton. Potamotrygon leopoldi is a durophagous freshwater ray endemic to the Xingu River in Brazil, with a jaw morphology superficially similar to its distant durophagous marine relatives, eagle rays (e.g., Aetomylaeus, Aetobatus). We used second moment of area as a proxy for the ability to resist bending and analyzed the arrangement of the mineralized skeleton of the jaw of P. leopoldi over ontogeny using data from computed tomography (CT) scans. The jaws of P. leopoldi do not resist bending nearly as well as other durophagous elasmobranchs, and the jaws are stiffest nearest the joints rather than beneath the dentition. While second moment has similar material distribution over ontogeny, mineralization of the jaws under the teeth increases with age. Neonate rays have low jaw stiffness and poor mineralization, suggesting that P. leopoldi may not feed on hard-shelled prey early in life. These differences in the shape, stiffness and mineralization of the jaws of P. leopoldi compared to its durophagous relatives show there are several solutions to the problem of crushing shelled prey with a compliant skeleton.

Broader head, stronger bite: In vivo bite forces in European eel Anguilla anguilla

This work examined three different phenotypes of the yellow-eel stage of the European eel Anguilla anguilla, broad-heads, narrow-heads and eels with an intermediate head shape. The aim was to see whether broad-headed A. anguilla, which generally consume harder, larger prey, such as crustaceans and fish, exerted greater bite force than the narrow-headed variant, which mainly consume soft, small prey such as chironomid larvae. It was found that in 99 yellow A. anguilla, in vivo bite force of broad-heads are higher compared with narrow-heads and intermediates.

Built to bite? Differences in cranial morphology and bite performance between narrow- and broad-headed European glass eels

The presence of two phenotypes in a single species is a widespread phenomenon, also observed in European eel (Anguilla anguilla). This dimorphism has been related to dietary differences in the subadult elver and yellow eel stages, with broad-heads generally feeding on harder and/or larger-bodied prey items than narrow-heads. Nevertheless, both broad- and narrow-headed phenotypes can already be found among glass eels, the stage preceding the elver eel stage. As these glass eels are considered nonfeeding, we investigate here to what degree the observed variation in head width is reflected in variation in the musculoskeletal feeding system, as well as whether this reflects the same variation observed in the older, dimorphic yellow eels. Additionally, we investigate whether musculoskeletal differences between broad- and narrow-headed glass eels have implications on their feeding performance and could thus impact prey preference when eels start feeding. Therefore, we compared the cranial musculoskeletal system of five broad- and narrow-headed glass eels using 3D-reconstructions and simulated the glass eel's bite force using the data of the muscle reconstructions. We found that the variation in the musculoskeletal system of glass eels indeed reflects that of the yellow eels. Broader heads were related to larger jaw muscles, responsible for mouth closure. Accordingly, broad-heads could generate higher bite forces than narrow-headed glass eels. In addition, broader heads were associated with higher coronoid processes and shorter hyomandibulae, beneficial for dealing with higher mechanical loadings and consequently, harder prey. We, thus, show that head width variation in glass eels is related to musculoskeletal differences which, in turn, can affect feeding performance. As such, differences in prey preference can already take place the moment the eels start feeding, potentially leading to the dimorphism observed in the elver and yellow eel stage.

The tooth, the whole tooth and nothing but the tooth: tooth shape and ontogenetic shift dynamics in the white shark Carcharodon carcharias

Results from this study of the white shark Carcharodon carcharias include measurements obtained using a novel photographic method that reveal significant differences between the sexes in the relationship between tooth cuspidity and shark total length, and a novel ontogenetic change in male tooth shape. Males exhibit broader upper first teeth and increased distal inclination of upper third teeth with increasing length, while females do not present a consistent morphological change. Substantial individual variation, with implications for pace of life syndrome, was present in males and tooth polymorphism was suggested in females. Sexual differences and individual variation may play major roles in ontogenetic changes in tooth morphology in C. carcharias, with potential implications for their foraging biology. Such individual and sexual differences should be included in studies of ontogenetic shift dynamics in other species and systems.

Comparative lever analysis and ontogenetic scaling in esocid fishes: Functional demands and constraints in feeding biomechanics

When animals grow, the functional demands that they experience often change as a consequence of their increasing body size. In this study, we examined the feeding biomechanics in esocid species that represent different size classes (small, Esox americanus; intermediate, Esox niger; large, Esox lucius), and how their bite forces and associated functional variables change as they grow. In order to evaluate bite performance through ontogeny, we dissected and measured dimensions of the feeding apparatus and the adductor mandibulae muscle complex with its segmentum facialis subdivisions such as the ricto-malaris, stegalis and endoricto-malaris across a wide range of body sizes. The collected morphological data was used as input variables for a published anatomical model to simulate jaw function in these fish species. Maximum bite forces for both anterior bite and posterior bite increased in isometry in E. americanus and E. niger. The posterior bite of E. lucius also increases in isometry, however, the anterior bite increases in positive allometry. Intraspecific comparison within E. lucius indicated the increase of bite forces in more developed individuals accelerated after the fish grew out of fingerling stage. In addition, our analysis indicated functional differentiation between subdivisions of the adductor mandibulae segmentum facialis, as well as interspecific differences in the pattern of contribution to the bite performance by these subdivisions. Our study provides insights into not only the musculoskeletal basis of the jaw function of esocid species, but also the feeding capacity of this species in relation to the functional demands it faces as one of the top predators in lake and river systems. J. Morphol. 277:1447-1458, 2016. © 2016 Wiley Periodicals, Inc.

Feeding biomechanics of the cownose ray, Rhinoptera bonasus, over ontogeny

Growth affects the performance of structure, so the pattern of growth must influence the role of a structure and an organism. Because animal performance is linked to morphological specialization, ontogenetic change in size may influence an organism's biological role. High bite force generation is presumably selected for in durophagous taxa. Therefore, these animals provide an excellent study system for investigating biomechanical consequences of growth on performance. An ontogenetic series of 27 cownose rays (Rhinoptera bonasus) were dissected in order to develop a biomechanical model of the feeding mechanism, which was then compared with bite forces measured from live rays. Mechanical advantage of the feeding apparatus was generally conserved throughout ontogeny, while an increase in the mass and cross-sectional area of the jaw adductors resulted in allometric gains in bite force generation. Of primary importance to forceful biting in this taxon is the use of a fibrocartilaginous tendon associated with the insertion of the primary jaw adductor division. This tendon may serve to redirect muscle forces anteriorly, transmitting them within the plane of biting. Measured bite forces obtained through electrostimulation of the jaw adductors in live rays were higher than predicted, possibly due to differences in specific tension of actual batoid muscle and that used in the model. Mass-specific bite forces in these rays are the highest recorded for elasmobranchs. Cownose rays exemplify a species that, through allometric growth of bite performance and morphological novelties, have expanded their ecological performance over ontogeny.

Are cranial biomechanical simulation data linked to known diets in extant taxa? A method for applying diet-biomechanics linkage models to infer feeding capability of extinct species

Performance of the masticatory system directly influences feeding and survival, so adaptive hypotheses often are proposed to explain craniodental evolution via functional morphology changes. However, the prevalence of "many-to-one" association of cranial forms and functions in vertebrates suggests a complex interplay of ecological and evolutionary histories, resulting in redundant morphology-diet linkages. Here we examine the link between cranial biomechanical properties for taxa with different dietary preferences in crown clade Carnivora, the most diverse clade of carnivorous mammals. We test whether hypercarnivores and generalists can be distinguished based on cranial mechanical simulation models, and how such diet-biomechanics linkages relate to morphology. Comparative finite element and geometric morphometrics analyses document that predicted bite force is positively allometric relative to skull strain energy; this is achieved in part by increased stiffness in larger skull models and shape changes that resist deformation and displacement. Size-standardized strain energy levels do not reflect feeding preferences; instead, caniform models have higher strain energy than feliform models. This caniform-feliform split is reinforced by a sensitivity analysis using published models for six additional taxa. Nevertheless, combined bite force-strain energy curves distinguish hypercarnivorous versus generalist feeders. These findings indicate that the link between cranial biomechanical properties and carnivoran feeding preference can be clearly defined and characterized, despite phylogenetic and allometric effects. Application of this diet-biomechanics linkage model to an analysis of an extinct stem carnivoramorphan and an outgroup creodont species provides biomechanical evidence for the evolution of taxa into distinct hypercarnivorous and generalist feeding styles prior to the appearance of crown carnivoran clades with similar feeding preferences.

Feeding performance of king Mackerel, Scomberomorus cavalla

Feeding performance is an organism's ability to capture and handle prey. Although bite force is a commonly used metric of feeding performance, other factors such as bite pressure and strike speed are also likely to affect prey capture. Therefore, this study investigated static bite force, dynamic speeds, and predator and prey forces resulting from ram strikes, as well as bite pressure of the king mackerel, Scomberomorus cavalla, in order to examine their relative contributions to overall feeding performance. Theoretical posterior bite force ranged from 14.0-318.7 N. Ram speed, recorded with a rod and reel incorporated with a line counter and video camera, ranged from 3.3-15.8B L/s. Impact forces on the prey ranged from 0.1-1.9 N. Bite pressure, estimated using theoretical bite forces at three gape angles and tooth cross-sectional areas, ranged from 1.7-56.9 MPa. Mass-specific bite force for king mackerel is relatively low in comparison with other bony fishes and sharks, with relatively little impact force applied to the prey during the strike. This suggests that king mackerel rely on high velocity chases and high bite pressure generated via sharp, laterally compressed teeth to maximize feeding performance.

Morphology does not predict performance: jaw curvature and prey crushing in durophagous stingrays

All stingrays in the family Myliobatidae are durophagous, consuming bivalves and gastropods, as well as decapod crustaceans. Durophagous rays have rigid jaws, flat teeth that interlock to form pavement-like tooth plates, and large muscles which generate bite forces capable of fracturing stiff biological composites (e.g., mollusk shell). The relative proportion of different prey types in the diet of durophagous rays varies between genera with some stingray species specializing on particular mollusk taxa, while others are generalists. The tooth plate module provides a curved occlusal surface on which prey is crushed, and this curvature differs significantly among myliobatids. We measured the effect of jaw curvature on prey-crushing success in durophagous stingrays. We milled aluminum replica jaws rendered from computed tomography scans, and crushed live mollusks, 3D printed gastropod shells, and ceramic tubes with these fabricated jaws. Our analysis of prey items indicate that gastropods were consistently more difficult to crush than bivalves (i.e. were stiffer), but that mussels require the greatest work-to-fracture. We found that replica shells can provide an important proxy for investigations of failure mechanics. We also found little difference in crushing performance between jaw shapes, suggesting that disparate jaws are equally suited for processing different types of shelled prey. Thus, durophagous stingrays exhibit a many-to-one mapping of jaw morphology to mollusk crushing performance.

Mega-Bites: extreme jaw forces of living and extinct piranhas (Serrasalmidae)

Here, we document in-vivo bite forces recorded from wild piranhas. Integrating this empirical data with allometry, bite simulations, and FEA, we have reconstructed the bite capabilities and potential feeding ecology of the extinct giant Miocene piranha, Megapiranha paranensis. An anterior bite force of 320 N from the black piranha, Serrasalmus rhombeus, is the strongest bite force recorded for any bony fish to date. Results indicate M. paranensis' bite force conservatively ranged from 1240–4749 N and reveal its novel dentition was capable of resisting high bite stresses and crushing vertebrate bone. Comparisons of body size-scaled bite forces to other apex predators reveal S. rhombeus and M. paranensis have among the most powerful bites estimated in carnivorous vertebrates. Our results functionally demonstrate the extraordinary bite of serrasalmid piranhas and provide a mechanistic rationale for their predatory dominance among past and present Amazonian ichthyofaunas.

Ontogenetic changes in jaw-muscle architecture facilitate durophagy in the turtle Sternotherus minor

Differential scaling of musculoskeletal traits leads to differences in performance across ontogeny and ultimately determines patterns of resource use during development. Because musculoskeletal growth of the feeding system facilitates high bite-force generation necessary to overcome the physical constraints of consuming more durable prey, durophagous taxa are well suited for investigations of the scaling relationships between musculoskeletal growth, bite-force generation and dietary ontogeny. To elucidate which biomechanical factors are responsible for allometric changes in bite force and durophagy, we developed and experimentally tested a static model of bite-force generation throughout development in the durophagous turtle Sternotherus minor. Moreover, we quantified the fracture properties of snails found in the diet to evaluate the relationship between bite force and the forces required to process durable prey. We found that (1) the static bite-force model accurately predicts the ontogenetic scaling of bite forces, (2) bite-force positive allometry is accomplished by augmenting muscle size and muscle pennation, and (3) the rupture forces of snails found in the diet show a similar scaling pattern to bite force across ontogeny. These results indicate the importance of muscle pennation for generating high bite forces while maintaining muscle size and provide empirical evidence that the allometric patterns of musculoskeletal growth in S. minor are strongly linked to the structural properties of their primary prey.