Functional morphology of the feeding apparatus, feeding constraints, and suction performance in the nurse sharkGinglymostoma cirratum

Morphological Variability and Function of Labial Cartilages in Sharks (Chondrichthyes, Elasmobranchii)

(1) Background: Labial cartilages (LCs), as their name suggests, lie in the folds of the connective tissue, the lips, framing the gape of elasmobranch chondrichthyans. As such, these cartilages lie laterally to the jaws and marginal teeth. They are considered to influence the ability of creating suction during the feeding process. As past studies have shown, LCs in sharks are as diverse as their varied feeding techniques and differ between species in number, size, shape, and position. This allows establishing parameters for inferring the feeding and hunting behaviors in these ecologically important fishes. (2) Methods: We present a study of LCs based on the CT scans of more than 100 extant shark species and, therefore, represent at least one member of every living family within the Euselachii, excluding batoids. (3) Results: Accordingly, sharks without labial cartilages or that have only small remnants are ram feeders or use pure biting and mainly occupy higher trophic levels (tertiary and quaternary consumers), whereas suction-feeding sharks have higher numbers (up to five pairs) of well-developed LCs and occupy slightly lower trophic levels (mainly secondary consumers). Species with unique feeding strategies, like the cookie-cutter shark (Isistius brasiliensis, an ectoparasite), display distinct shapes of LCs, while generalist species, conversely, exhibit a simpler arrangement of LCs. (4) Conclusions: We propose a dichotomous identification key to classify single LCs into different morphotypes and propose combinations of morphotypes that result in suction feeding differing in strength and, therefore, different hunting and feeding strategies. The conclusions of this study allow to infer information about feeding strategies not only in extant less-known sharks but also extinct sharks.

Location, Location, Location! Evaluating Space Use of Captive Aquatic Species—A Case Study with Elasmobranchs

The space use of captive animals has been reliably used as a tool to measure animal welfare in recent years. However, most analyses of space use focus primarily on terrestrial animals, with very little emphasis placed on the space use of aquatic animals. By comparing the space use of these animals to their natural histories and what would be expected of them physiologically, a general assessment of their overall welfare can be obtained. Using the Zoomonitor program, this study investigated the space use of five elasmobranch species housed in a captive aquatic environment: a blacktip reef shark (Carcharhinus melanopterus), a nurse shark (Ginglymostoma cirratum), a smooth dogfish (Musteluscanis), a bonnethead shark (Sphyrna tiburo), and a blacknose shark (Carcharhinus acronotus). The exhibit was delineated into five different zones: three represented the animal locations along the X/Y axis (‘Exhibit Use’), and two zones were related to the Z-axis (‘Depth Use’). The location of each individual on both the X/Y and Z axes was recorded during each observation. Heat maps generated from the Zoomonitor program were used in conjunction with the Spread of Participation Index (SPI) to interpret the data. It was found that while all the individuals used their given space differently, the Exhibit Use was relatively even overall (the SPI values ranged from 0.0378 to 0.367), while the Depth Use was more uneven (the SPI ranged from 0.679 to 0.922). These results mostly reflected what would be expected based on the species’ natural histories. However, for the smooth dogfish, the observed Exhibit Use and activity patterns revealed a mismatch between the anticipated and the actual results, leading to further interventions. As demonstrated here, space use results can be utilized to make positive changes to husbandry routines and enclosure designs for aquatic individuals; they are thus an important additional welfare measure to consider for aquatic species.

Cranial morphology of the orectolobiform shark, Chiloscyllium punctatum Müller & Henle, 1838

Elasmobranchs, comprising sharks, skates, and rays, have a long evolutionary history extending back into the Palaeozoic. They are characterized by various unique traits including a predominantly cartilaginous skeleton, superficial prismatic phosphatic layer, and permanent tooth replacement. Moreover, they exhibit a more or less marked sexual dimorphism. Especially the morphology of the chondrocranium and the elements of the whole cranial region of extant and extinct chondrichthyans can provide valuable information about corresponding functions, e.g. the feeding apparatus might reflect the diet of the animals. However, studies on sexual dimorphisms are lacking in orectolobiform sharks, therefore, little is known about possible sexual dimorphic characters in the cranial region in this group. For this reason, we present in this study a comprehensive morphological description of the cranial region of the brownbanded bamboo shark Chiloscyllium punctatum Müller & Henle, 1838, with a special focus on its sexual dimorphic characters. Our results reveal clear morphological differences in both sexes of the examined C. punctatum specimens, particularly in the chondrocranium and the mandibular arch. The female specimen shows a comparatively more robust and compact morphology of the chondrocranium. This pattern is also evident in the mandibular arch, especially in the palatoquadrate. The present study is the first to describe the morphology of an orectolobiform shark species in detail using both manual dissection and micro-CT data. The resulting data furthermore provide a starting point for pending studies and are intended to be a first step in a series of comparative studies on the morphology of the cranial region of orectolobiform sharks, including the determination of possible sexual dimorphic characteristics.

The morphology and evolution of chondrichthyan cranial muscles: A digital dissection of the elephantfish Callorhinchus milii and the catshark Scyliorhinus canicula

The anatomy of sharks, rays, and chimaeras (chondrichthyans) is crucial to understanding the evolution of the cranial system in vertebrates due to their position as the sister group to bony fishes (osteichthyans). Strikingly different arrangements of the head in the two constituent chondrichthyan groups-holocephalans and elasmobranchs-have played a pivotal role in the formation of evolutionary hypotheses targeting major cranial structures such as the jaws and pharynx. However, despite the advent of digital dissections as a means of easily visualizing and sharing the results of anatomical studies in three dimensions, information on the musculoskeletal systems of the chondrichthyan head remains largely limited to traditional accounts, many of which are at least a century old. Here, we use synchrotron tomographic data to carry out a digital dissection of a holocephalan and an elasmobranch widely used as model species: the elephantfish, Callorhinchus milii, and the small-spotted catshark, Scyliorhinus canicula. We describe and figure the skeletal anatomy of the head, labial, mandibular, hyoid, and branchial cartilages in both taxa as well as the muscles of the head and pharynx. In Callorhinchus, we make several new observations regarding the branchial musculature, revealing several previously unreported or ambiguously characterized muscles, likely homologous to their counterparts in the elasmobranch pharynx. We also identify a previously unreported structure linking the pharyngohyal of Callorhinchus to the neurocranium. Finally, we review what is known about the evolution of chondrichthyan cranial muscles from their fossil record and discuss the implications for muscle homology and evolution, broadly concluding that the holocephalan pharynx is likely derived from a more elasmobranch-like form which is plesiomorphic for the chondrichthyan crown group. This dataset has great potential as a resource, particularly for researchers using these model species for zoological research, functional morphologists requiring models of musculature and skeletons, as well as for palaeontologists seeking comparative models for extinct taxa.

Bladderworts, the smallest known suction feeders, generate inertia-dominated flows to capture prey

Aquatic bladderworts (Utricularia gibba and U. australis) capture zooplankton in mechanically triggered underwater traps. With characteristic dimensions less than 1 mm, the trapping structures are among the smallest known to capture prey by suction, a mechanism that is not effective in the creeping-flow regime where viscous forces prevent the generation of fast and energy-efficient suction flows. To understand what makes suction feeding possible on the small scale of bladderwort traps, we characterised their suction flows experimentally (using particle image velocimetry) and mathematically (using computational fluid dynamics and analytical mathematical models). We show that bladderwort traps avoid the adverse effects of creeping flow by generating strong, fast-onset suction pressures. Our findings suggest that traps use three morphological adaptations: the trap walls' fast release of elastic energy ensures strong and constant suction pressure; the trap door's fast opening ensures effectively instantaneous onset of suction; the short channel leading into the trap ensures undeveloped flow, which maintains a wide effective channel diameter. Bladderwort traps generate much stronger suction flows than larval fish with similar gape sizes because of the traps' considerably stronger suction pressures. However, bladderworts' ability to generate strong suction flows comes at considerable energetic expense.

Suction Flows Generated by the Carnivorous Bladderwort Utricularia—Comparing Experiments with Mechanical and Mathematical Models

Suction feeding is a well-understood feeding mode among macroscopic aquatic organisms. The little we know about small suction feeders from larval fish suggests that small suction feeders are not effective. Yet bladderworts, an aquatic carnivorous plant with microscopic underwater traps, have strong suction performances despite having the same mouth size as that of fish larvae. Previous experimental studies of bladderwort suction feeding have focused on the solid mechanics of the trap door’s opening mechanism rather than the mechanics of fluid flow. As flows are difficult to study in small suction feeders due to their small size and brief event durations, we combine flow visualization on bladderwort traps with measurements on a mechanical, dynamically scaled model of a suction feeder. We find that bladderwort traps generate flows that are more similar to the inertia-dominated flows of adult fish than the viscosity-dominated flows of larval fish. Our data further suggest that axial flow transects through suction flow fields, often used in biological studies to characterize suction flows, are less diagnostic of the relative contribution of inertia versus viscosity than transverse transects.

High-performance suction feeding in an early elasmobranch

High-performance suction feeding is often presented as a classic innovation of ray-finned fishes, likely contributing to their remarkable evolutionary success, whereas sharks, with seemingly less sophisticated jaws, are generally portrayed as morphologically conservative throughout their history. Here, using a combination of computational modeling, physical modeling, and quantitative three-dimensional motion simulation, we analyze the cranial skeleton of one of the earliest known stem elasmobranchs, Tristychius arcuatus from the Middle Mississippian of Scotland. The feeding apparatus is revealed as highly derived, capable of substantial oral expansion, and with clear potential for high-performance suction feeding some 50 million years before the earliest osteichthyan equivalent. This exceptional jaw performance is not apparent from standard measures of ecomorphospace using two-dimensional data. Tristychius signals the emergence of entirely new chondrichthyan ecomorphologies in the aftermath of the end-Devonian extinction and highlights sharks as significant innovators in the early radiation of the modern vertebrate biota.

Skeletal kinematics of the hyoid arch in the suction-feeding shark Chiloscyllium plagiosum

White-spotted bamboo sharks, Chiloscyllium plagiosum, generate strong suction-feeding pressures that rival the highest levels measured in ray-finned fishes. However, the hyostylic jaw suspension of these sharks is fundamentally different from the actinopterygian mechanism, including more mobile hyomandibulae, with the jaws and ceratohyal suspended from the hyomandibulae. Prior studies have proposed skeletal kinematics during feeding in orectolobid sharks from indirect measurements. Here, we tested these hypotheses using XROMM to measure cartilage motions directly. In agreement with prior hypotheses, we found extremely large retraction and depression of the ceratohyal, facilitated by large protraction and depression of the hyomandibula. Somewhat unexpectedly, XROMM also showed tremendous long-axis rotation (LAR) of both the ceratohyal and hyomandibula. This LAR likely increases the range of motion for the hyoid arch by keeping the elements properly articulated through their large arcs of motion. XROMM also confirmed that upper jaw protraction occurs before peak gape, similarly to actinopterygian suction feeders, but different from most other sharks in which jaw protrusion serves primarily to close the mouth. Early jaw protraction results from decoupling the rotations of the hyomandibula, with much of protraction occurring before peak gape with the other rotations lagging behind. In addition, the magnitudes of retraction and protraction of the hyoid elements are independent of the magnitude of depression, varying the shape of the mouth among feeding strikes. Hence, the large variation in suction-feeding behavior and performance may contribute to the wide dietary breadth of bamboo sharks.

Dual function of the pectoral girdle for feeding and locomotion in white-spotted bamboo sharks

Positioned at the intersection of the head, body and forelimb, the pectoral girdle has the potential to function in both feeding and locomotor behaviours-although the latter has been studied far more. In ray-finned fishes, the pectoral girdle attaches directly to the skull and is retracted during suction feeding, enabling the ventral body muscles to power rapid mouth expansion. However, in sharks, the pectoral girdle is displaced caudally and entirely separate from the skull (as in tetrapods), raising the question of whether it is mobile during suction feeding and contributing to suction expansion. We measured three-dimensional kinematics of the pectoral girdle in white-spotted bamboo sharks during suction feeding with X-ray reconstruction of moving morphology, and found the pectoral girdle consistently retracted about 11° by rotating caudoventrally about the dorsal scapular processes. This motion occurred mostly after peak gape, so it likely contributed more to accelerating captured prey through the oral cavity and pharynx, than to prey capture as in ray-finned fishes. Our results emphasize the multiple roles of the pectoral girdle in feeding and locomotion, both of which should be considered in studying the functional and evolutionary morphology of this structure.

Function of the hypobranchial muscles and hyoidiomandibular ligament during suction capture and bite processing in white-spotted bamboo sharks, Chiloscyllium plagiosum

Suction feeding in teleost fish is a power-dependent behavior, requiring rapid and forceful expansion of the orobranchial cavity by the hypobranchial and trunk muscles. To increase power production for expansion, many species employ in-series tendons and catch mechanisms to store and release elastic strain energy. Suction feeding sharks such as Chiloscyllium plagiosum lack large in-series tendons on the hypobranchials, yet two of the hypobranchials, the coracohyoideus and coracoarcualis (CH and CA; hyoid depressors), are arranged in-series, and run deep and parallel to a third muscle, the coracomandibularis (CM, jaw depressor). The arrangement of the CH and CA suggests that C. plagiosum is using the CH muscle rather than a tendon to store and release elastic strain energy. Here we describe the anatomy of the feeding apparatus, and present data on hyoid and jaw kinematics and fascicle shortening in the CM, CH and CA quantified using sonomicrometry, with muscle activity and buccal pressure recorded simultaneously. Results from prey capture show that prior to jaw and hyoid depression the CH is actively lengthened by shortening of the in-series CA. The active lengthening of the CH and pre-activation of the CH and CA suggest that the CH is functioning to store and release elastic energy during prey capture. Catch mechanisms are proposed involving a dynamic moment arm and four-bar linkage between the hyoidiomandibular ligament (LHML), jaws and ceratohyals that is influenced by the CM. Furthermore, the LHML may be temporarily disengaged during behaviors such as bite processing to release linkage constraints.

Teeth penetration force of the tiger shark Galeocerdo cuvier and sandbar shark Carcharhinus plumbeus

This study examined the minimum force required of functional teeth and replacement teeth in the tiger shark Galeocerdo cuvier and the sandbar shark Carcharhinus plumbeus to penetrate the scales and muscle of sheepshead Archosargus probatocephalus and pigfish Orthopristis chrysoptera. Penetration force ranged from 7·7-41·9 and 3·2-26·3 N to penetrate A. probatocephalus and O. chrysoptera, respectively. Replacement teeth required significantly less force to penetrate O. chrysoptera for both shark species, most probably due to microscopic wear of the tooth surfaces supporting the theory shark teeth are replaced regularly to ensure sharp teeth that are efficient for prey capture.

X-ray computed tomography library of shark anatomy and lower jaw surface models

The cranial diversity of sharks reflects disparate biomechanical adaptations to feeding. In order to be able to investigate and better understand the ecomorphology of extant shark feeding systems, we created a x-ray computed tomography (CT) library of shark cranial anatomy with three-dimensional (3D) lower jaw reconstructions. This is used to examine and quantify lower jaw disparity in extant shark species in a separate study. The library is divided in a dataset comprised of medical CT scans of 122 sharks (Selachimorpha, Chondrichthyes) representing 73 extant species, including digitized morphology of entire shark specimens. This CT dataset and additional data provided by other researchers was used to reconstruct a second dataset containing 3D models of the left lower jaw for 153 individuals representing 94 extant shark species. These datasets form an extensive anatomical record of shark skeletal anatomy, necessary for comparative morphological, biomechanical, ecological and phylogenetic studies.

Durophagous biting in sea otters (Enhydra lutris) differs kinematically from raptorial biting of other marine mammals

Sea otters represent an interesting model for studies of mammalian feeding evolution. Although they are marine mammals, sea otters returned to the sea relatively recently and feed at the surface. Therefore, they represent a transitional stage of aquatic adaptation. Currently no feeding performance studies of sea otters have been conducted. The main objective of this study was to characterize the feeding kinematic profile in sea otters. It was hypothesized that sea otters would exhibit a terrestrial feeding behavior and that they forcefully crush hard prey at large gapes. As a result, biting kinematics would be congruent with biting behavior reported for their terrestrial ancestors, thus providing additional evidence that raptorial biting is a conserved behavior even in recently aquatic mammals. Sea otters consistently used a durophagous raptorial biting mode characterized by large gapes, large gape angles, and lack of lateral gape occlusion. The shorter skulls and mandibles of sea otters, along with increased mechanical advantages of the masseter and increased bite force, form a repertoire of functional traits for durophagy. Here we consider durophagy to be a specialized raptorial biting feeding mode. A comparison of feeding kinematics of wild vs captive sea otters showed no significant differences in lateral kinematic profiles and only minor differences in three frontal kinematic profiles, which included a slower maximum opening gape velocity, a slower maximum gape opening velocity, and a slower maximum closing gape velocity in captive sea otters. Data indicate functional innovations for producing large bite forces at wide gape and gape angles.

Modulation of shark prey capture kinematics in response to sensory deprivation

The ability of predators to modulate prey capture in response to the size, location, and behavior of prey is critical to successful feeding on a variety of prey types. Modulating in response to changes in sensory information may be critical to successful foraging in a variety of environments. Three shark species with different feeding morphologies and behaviors were filmed using high-speed videography while capturing live prey: the ram-feeding blacktip shark, the ram-biting bonnethead, and the suction-feeding nurse shark. Sharks were examined intact and after sensory information was blocked (olfaction, vision, mechanoreception, and electroreception, alone and in combination), to elucidate the contribution of the senses to the kinematics of prey capture. In response to sensory deprivation, the blacktip shark demonstrated the greatest amount of modulation, followed by the nurse shark. In the absence of olfaction, blacktip sharks open the jaws slowly, suggestive of less motivation. Without lateral line cues, blacktip sharks capture prey from greater horizontal angles using increased ram. When visual cues are absent, blacktip sharks elevate the head earlier and to a greater degree, allowing them to overcome imprecise position of the prey relative to the mouth, and capture prey using decreased ram, while suction remains unchanged. When visual cues are absent, nurse sharks open the mouth wider, extend the labial cartilages further, and increase suction while simultaneously decreasing ram. Unlike some bony fish, neither species switches feeding modalities (i.e. from ram to suction or vice versa). Bonnetheads failed to open the mouth when electrosensory cues were blocked, but otherwise little to no modulation was found in this species. These results suggest that prey capture may be less plastic in elasmobranchs than in bony fishes, possibly due to anatomical differences, and that the ability to modulate feeding kinematics in response to available sensory information varies by species, rather than by feeding modality.

Body ram, not suction, is the primary axis of suction feeding diversity in spiny-rayed fishes

Suction feeding fishes exhibit diverse prey capture strategies that vary in their relative use of suction and predator approach (ram), which is often referred to as the ram-suction continuum. Previous research has found that ram varies more than suction distances among species, such that ram accounts for most differences in prey capture behaviors. To determine whether these findings hold at broad evolutionary scales, we collected high-speed videos of 40 species of spiny-rayed fishes (Acanthomorpha) feeding on live prey. For each strike, we calculated the contributions of suction, body ram (swimming), and jaw ram (mouth movement relative to the body) to closing the distance between predator and prey. We confirm that the contribution of suction distance is limited even in this phylogenetically and ecologically broad sample of species, with the extreme suction area of prey capture space conspicuously unoccupied. Instead of a continuum from suction to ram, we find that variation in body ram is the major factor underlying the diversity of prey-capture strategies among suction-feeding fishes. Independent measurement of the contribution of jaw ram revealed that it is an important component of diversity among spiny-rayed fishes, with a number of ecomorphologies relying heavily on jaw ram, including pivot feeding in syngnathiforms, extreme jaw protruders, and benthic sit-and-wait ambush predators. A combination of morphological and behavioral innovations have allowed fish to invade the extreme jaw ram area of prey capture space. We caution that while two-species comparisons may support a ram-suction trade-off, these patterns do not speak to broader patterns across spiny-rayed fishes

Multisensory integration and behavioral plasticity in sharks from different ecological niches

The underwater sensory world and the sensory systems of aquatic animals have become better understood in recent decades, but typically have been studied one sense at a time. A comprehensive analysis of multisensory interactions during complex behavioral tasks has remained a subject of discussion without experimental evidence. We set out to generate a general model of multisensory information extraction by aquatic animals. For our model we chose to analyze the hierarchical, integrative, and sometimes alternate use of various sensory systems during the feeding sequence in three species of sharks that differ in sensory anatomy and behavioral ecology. By blocking senses in different combinations, we show that when some of their normal sensory cues were unavailable, sharks were often still capable of successfully detecting, tracking and capturing prey by switching to alternate sensory modalities. While there were significant species differences, odor was generally the first signal detected, leading to upstream swimming and wake tracking. Closer to the prey, as more sensory cues became available, the preferred sensory modalities varied among species, with vision, hydrodynamic imaging, electroreception, and touch being important for orienting to, striking at, and capturing the prey. Experimental deprivation of senses showed how sharks exploit the many signals that comprise their sensory world, each sense coming into play as they provide more accurate information during the behavioral sequence of hunting. The results may be applicable to aquatic hunting in general and, with appropriate modification, to other types of animal behavior.

Absence of suction feeding ichthyosaurs and its implications for triassic mesopelagic paleoecology

Mesozoic marine reptiles and modern marine mammals are often considered ecological analogs, but the extent of their similarity is largely unknown. Particularly important is the presence/absence of deep-diving suction feeders among Mesozoic marine reptiles because this would indicate the establishment of mesopelagic cephalopod and fish communities in the Mesozoic. A recent study suggested that diverse suction feeders, resembling the extant beaked whales, evolved among ichthyosaurs in the Triassic. However, this hypothesis has not been tested quantitatively. We examined four osteological features of jawed vertebrates that are closely linked to the mechanism of suction feeding, namely hyoid corpus ossification/calcification, hyobranchial apparatus robustness, mandibular bluntness, and mandibular pressure concentration index. Measurements were taken from 18 species of Triassic and Early Jurassic ichthyosaurs, including the presumed suction feeders. Statistical comparisons with extant sharks and marine mammals of known diets suggest that ichthyosaurian hyobranchial bones are significantly more slender than in suction-feeding sharks or cetaceans but similar to those of ram-feeding sharks. Most importantly, an ossified hyoid corpus to which hyoid retractor muscles attach is unknown in all but one ichthyosaur, whereas a strong integration of the ossified corpus and cornua of the hyobranchial apparatus has been identified in the literature as an important feature of suction feeders. Also, ichthyosaurian mandibles do not narrow rapidly to allow high suction pressure concentration within the oral cavity, unlike in beaked whales or sperm whales. In conclusion, it is most likely that Triassic and Early Jurassic ichthyosaurs were 'ram-feeders', without any beaked-whale-like suction feeder among them. When combined with the inferred inability for dim-light vision in relevant Triassic ichthyosaurs, the fossil record of ichthyosaurs does not suggest the establishment of modern-style mesopelagic animal communities in the Triassic. This new interpretation matches the fossil record of coleoids, which indicates the absence of soft-bodied deepwater species in the Triassic.

Feeding of the megamouth shark (Pisces: Lamniformes: Megachasmidae) predicted by its hyoid arch: a biomechanical approach

Studies of the megamouth shark, one of three planktivorous sharks, can provide information about their evolutionary history. Megamouth shark feeding has never been observed in life animals, but two alternative hypotheses on biomechanics suggest either feeding, i.e., ram feeding or suction feeding. In this study, the second moment of area of the ceratohyal cartilages, which is an indicator of the flexural stiffness of the cartilages, is calculated for 21 species of ram- and suction-feeding sharks using computed tomography. The results indicate that suction-feeding sharks have ceratohyal cartilages with a larger second moment of area than ram-feeding sharks. The result also indicates that the ram-suction index, which is an indicator of relative contribution of ram and suction behavior, is also correlated with the second moment of area of the ceratohyal. Considering that large bending stresses are expected to be applied to the ceratohyal cartilage during suction, the larger second moment of area of the ceratohyal of suction-feeding sharks can be interpreted as an adaptation for suction feeding. Based on the small second moment of area of the ceratohyal cartilage of the megamouth shark, the feeding mode of the megamouth shark is considered to be ram feeding, similar to the planktivorous basking shark. From these results, an evolutionary scenario of feeding mechanics of three species of planktivorous sharks can be suggested. In this scenario, the planktivorous whale shark evolved ram feeding from a benthic suction-feeding ancestor. Ram feeding in the planktivorous megamouth shark and the basking shark evolved from ram feeding swimming-type ancestors and that both developed their unique filtering system to capture small-sized prey.

Biology meets engineering: the structural mechanics of fossil and extant shark teeth

The majority of studies on the evolution and function of feeding in sharks have focused primarily on the movement of cranial components and muscle function, with little integration of tooth properties or function. As teeth are subjected to sometimes extreme loads during feeding, they undergo stress, strain, and potential failure. As attributes related to structural strength such as material properties and overall shape may be subjected to natural selection, both prey processing ability and structural parameters must be considered to understand the evolution of shark teeth. In this study, finite element analysis was used to visualize stress distributions of fossil and extant shark teeth during puncture, unidirectional draw (cutting), and holding. Under the loading and boundary conditions here, which are consistent with bite forces of large sharks, shark teeth are structurally strong. Teeth loaded in puncture have localized stress concentrations at the cusp apex that diminish rapidly away from the apex. When loaded in draw and holding, the majority of the teeth show stress concentrations consistent with well designed cantilever beams. Notches result in stress concentration during draw and may serve as a weak point; however they are functionally important for cutting prey during lateral head shaking behavior. As shark teeth are replaced regularly, it is proposed that the frequency of tooth replacement in sharks is driven by tooth wear, not tooth failure. As the tooth tip and cutting edges are worn, the surface areas of these features increase, decreasing the amount of stress produced by the tooth. While this wear will not affect the general structural strength of the tooth, tooth replacement may also serve to keep ahead of damage caused by fatigue that may lead to eventual tooth failure.