Intraspecific variation in feeding mechanics and bite force in durophagous stingrays

Cranial musculature of batoids: A standardized nomenclature

Batoids (rays and skates) are cartilaginous fishes whose jaws are not articulated directly to the neurocranium. The only point of contact between them are the hyomandibular cartilages, resulting in a unique mandibular suspension called euhyostyly. Due to this decoupling of the jaws from the skull, muscles play an essential role in modulating mandibular movements during the feeding process, especially during mandibular protrusion. The main objectives of our study were: (1) to examine the mandibular musculature of eight batoid species from different orders in the Batoidea and (2) establish a standardized musclulature terminology for future comparative myological studies in batoids. For each muscle bundle, the general characteristics of each cranial muscle were described and their origin and insertions were identified. The number of muscle bundles differed intraspecifically. On the dorsal surface, we reported the first evidence of the presence of the precranial muscle (PCM) in U. halleri, as well as the ethmoideo-parethmoidalis muscle (ETM) in R. velezi, P. glaugostigma and Z. exasperata; in addition, the insertion of the spiracularis muscle (SP) extended to the ventral surface of the oropharyngeal tract in myliobatiforms. On the ventral surface of the head, both N. entemedor and M. californica exhibited additional muscles in the mandibular area. These muscles were renamed as part of the standardization of mandibular terminology: the depressor mandibularis minor (DMM) in N. entemedor and the adductor mandibulae profundus (AMP) in M. californica. The standardization of terminology is essential for futures studies of the mandibular apparatus in batoids, to facilitate the morphological description of muscles in species without anatomical accounts and for continuity in broader comparative analyses.

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.

Comparative cranio-mandibular myology of three species of Batoidea from the Southern Gulf of California, Mexico

The mandibular apparatus of batoids (skates, electric rays, guitarfishes, stingrays, and sawfishes) is composed of a few skeletal elements to which the muscular bundles, responsible for all movements involved in the feeding mechanism, are inserted. The description of the different mandibular morphologies can help to understand the different feeding guilds in this group. In this study, we examined the cranio-mandibular myology of adult Rostroraja velezi, Narcine entemedor, and Zapteryx exasperata, three species of rays that coexist in the Southern Gulf of California, Mexico. This study described the muscles on the ventral and the dorsal surfaces for each species, identified the origins and insertions of these muscles, as well as the general characteristics of muscle morphology. There were 17 and 18 muscle bundles attached to the feeding apparatus, including five on the dorsal surface. Only the levator rostri, which elevates the rostrum during feeding, showed considerable differences in shape and size among species. The muscles of the adductor complex showed the greatest differences in size among the three species. N. entemedor presented the exclusive muscle X in the lower mandibular area and the extreme reduction of the coracohyoideus in the pharyngeal area derived from the absence of the basihyal cartilage. The information generated in our study supports the morphological specialization of electric rays (N. entemedor) for an almost exclusive suction feeding strategy.

The Lesser-Known Transitions: Organismal Form and Function Across Abiotic Gradients

From minute-to-minute changes, or across daily, seasonal, or geological timescales, animals are forced to navigate dynamic surroundings. Their abiotic environment is continually changing. These changes could include alterations to the substrates animals locomote on, flow dynamics of the microhabitats they feed in, or even altitudinal shifts over migration routes. The only constancy in any organism's day-to-day existence is the heterogeneity of the habitats they move through and the gradients in the physical media (e.g., air, water) they live in. We explored a broad range of organismal transitions across abiotic gradients and investigated how these organisms modify their form, function, and behavior to accommodate their surrounding media. We asked the following questions: (1) What are some challenges common to animals in changing media or moving between media? (2) What are common solutions to these recurring problems? (3) How often are these common solutions instances of either convergence or parallelism? Our symposium speakers explored these questions through critical analysis of numerous datasets spanning multiple taxa, timescales, and levels of analysis. After discussions with our speakers, we suggest that the role of physical principles (e.g., drag, gravity, buoyancy, viscosity) in constraining morphology and shaping the realized niche has been underappreciated. We recommend that investigations of these transitions and corresponding adaptations should include comparisons at multiple levels of biological organization and timescale. Relatedly, studies of organisms that undergo habitat and substrate changes over ontogeny would be worthwhile to include in comparisons. Future researchers should ideally complement lab-based morphological and kinematic studies with observational and experimental approaches in the field. Synthesis of the findings of our speakers across multiple study systems, timescales, and transitional habitats suggests that behavioral modification and exaptation of morphology play key roles in modulating novel transitions between substrates.

Ecological and Phenotypic Diversification after A Continental Invasion in Neotropical Freshwater Stingrays

Habitat transitions are key potential explanations for why some lineages have diversified and others have not - from Anolis lizards to Darwin's finches. The ecological ramifications of marine-to-freshwater transitions for fishes suggest evolutionary contingency: some lineages maintain their ancestral niches in novel habitats (niche conservatism), whereas others alter their ecological role. However, few studies have considered phenotypic, ecological, and lineage diversification concurrently to explore this issue. Here, we investigated the macroevolutionary history of the taxonomically and ecologically diverse Neotropical freshwater river rays (subfamily Potamotrygoninae), which invaded and diversified in the Amazon and other South American rivers during the late Oligocene to early Miocene. We generated a time-calibrated, multi-gene phylogeny for Potamotrygoninae and reconstructed evolutionary patterns of diet specialization. We measured functional morphological traits relevant for feeding and used comparative phylogenetic methods to examine how feeding morphology diversified over time. Potamotrygonine trophic and phenotypic diversity are evenly partitioned (non-overlapping) among internal clades for most of their history, until 20-16 mya, when more recent diversification suggests increasing overlap among phenotypes. Specialized piscivores (Heliotrygon and Paratrygon) evolved early in the history of freshwater stingrays, while later trophic specialization (molluscivory, insectivory, and crustacivory) evolved in the genus Potamotrygon. Potamotrygonins demonstrate ecological niche lability in diets and feeding apparatus; however, diversification has mostly been a gradual process through time. We suggest that competition is unlikely to have limited the potamotrygonine invasion and diversification in South America.

Tooth structure, mechanical properties, and diet specialization of Piranha and Pacu (Serrasalmidae): A comparative study

The relationship between diet, bite performance, and tooth structure is a topic of common interest for ecologists, biologists, materials scientists, and engineers. The highly specialized group of biters found in Serrasalmidae offers a unique opportunity to explore their functional diversity. Surprisingly, the piranha, whose teeth have a predominantly cutting function and whose main diet is soft flesh, is capable of exerting a greater bite force than a similarly sized pacu, who feeds on a hard durophagous diet. Herein, we expand our understanding of diet specialization in the Serrasalmidae family by investigating the influence of elemental composition and hierarchical structure on the local mechanical properties, stress distribution, and deformation mechanics of teeth from piranha (Pygocentrus nattereri) and pacu (Colossoma macropomum). Microscopic and spectroscopic analyses combined with nanoindentation and finite element simulations are used to probe the hierarchical features to uncover the structure-property relationships in piranha and pacu teeth. We show that the pacu teeth support a durophagous diet through its broad cusped-shaped teeth, thicker-irregular enameloid, interlocking interface of the dentin-enameloid junction, and increased hardness of the cuticle layer due to the larger concentrations of iron present. Comparatively, the piranha teeth are well suited for piercing due to their conical-shape which we report as having the greatest stiffness at the tip and evenly distributed enameloid. STATEMENT OF SIGNIFICANCE: The hierarchical structure and local mechanical properties of the piranha and pacu teeth are characterized and related to their feeding habits. Finite element models of the anterior teeth are generated to map local stress distribution under compressive loading. Bioinspired designs from the DEJ interface are developed and 3D printed. The pacu teeth are hierarchically structured and have local mechanical properties more suitable to a durophagous diet than the piranha. The findings here can provide insight into the design and fabrication of layered materials with suture interfaces for applications that require compressive loading conditions.

Marine Protection Induces Morphological Variation in the California Moray, Gymnothorax mordax

The effectiveness of marine protected areas (MPAs) on the general health and conservation of species, habitats, and community interactions is of great interest to researchers, managers, and recreationalists. However, the ecological and behavioral diversity of vertebrate predators of southern California kelp forests limits our ability to make general conclusions about MPA effectiveness across a variety of species. Identifying and studying species with extreme feeding habits or prey-capture strategies may offer greater insight into predator-prey relationships and reveal the trophic importance of an animal in the larger community. Moray eels (family Muraenidae) have been shown to have morphological and behavioral adaptations that allow them to consume large prey whole, identifying them as important predators. From 2015 to 2018, we studied the health and feeding behavior of a long-lived, elusive, and benthic kelp forest predator, the California moray eel (Gymnothorax mordax). We trapped eels inside and outside of Blue Cavern Onshore State Marine Conservation Area, an MPA on the northwest side of Santa Catalina Island, CA which prohibits the take of any species. Over 4 years, we captured 1736 eels. Overall, we found that morays were longer, older, heavier, had higher body condition, and were found in greater abundance within the MPA. Although fish comprised the majority of their summer diet, morays outside of the MPA were consuming a more diverse set of fish, while kelp bass comprised more than half of the diet for morays inhabiting the MPA. Additionally, we found that morays within the MPA had larger relative vertical gape distances (VGDs) and narrower heads. Our recapture data support the high site fidelity of morays, indicating that their diet and morphology are influenced by their local community. While the majority of morays are thriving in the MPA, as suggested by their robust sizes and longevity, high abundance appears to result in higher frequencies of cannibalism, the presence of an undescribed disease, and lower growth rates. Our results suggest that the MPA affects the life history of morays and may select for an alternative feeding strategy in which eels develop larger VGDs, smaller adductor muscles, and a specialized diet which is presumably influenced by the local environment. In addition, observations of cannibalistic behavior and species-specific disease provide us with important insight into natural factors that may still regulate populations removed from anthropogenic disturbances such as fishing.

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.