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.

Pennate myofibrils of the rat temporal muscle

BACKGROUND

The force a muscle exerts is partly determined by anatomical parameters, such as its physiological cross-section. The temporal muscle is structurally heterogeneous. To the authors' knowledge, the ultrastructure of this muscle has been poorly specifically studied.

METHODS

Five adult Wistar rats weighting 350-400g were used as temporal muscle donors. Tissues were specifically processed and studied under transmission electron microscope.

RESULTS

On ultrathin cuts, the general ultrastructural pattern of striated muscles was observed. Moreover, pennate sarcomeres were identified, sharing a one-end insertion on the same Z-disc. Bipennate morphologies resulted when two neighbor sarcomeres, attached on different neighbor Z-discs and separated at that end by a triad, converged to the same Z-disc at the opposite ends, thus building a thicker myofibril distinctively flanked by triads. Tripennate morphologies were identified when sarcomeres from three different Z-discs converged to the same Z-disc at the opposite ends.

CONCLUSIONS

These results support recent evidence of sarcomeres branching gathered in mice. Adequate identification of the sites of excitation-contraction coupling should be on both sides of a myofibril, on bidimensional ultrathin cuts, to avoid false positive results due to putative longitudinal folds of myofibrils.

The rise of biting during the Cenozoic fueled reef fish body shape diversification

We demonstrate that the stunning trophic diversity of modern reef fishes is a relatively recent state driven by a dramatic transformation in representation of major feeding modes. Since the Early Cenozoic, when over 95% of teleost lineages were suction feeders, there has been a steady increase in direct biting feeding modes. A variety of novelties and jaw modifications permitted reef fishes to feed on substrate-bound prey using direct biting and grazing behaviors and opened this rich adaptive zone, which we show elevated rates of body shape evolution. Taken together, our results indicate that recent diversification of the feeding mechanism played a major role in ecologically and phenotypically shaping the modern fauna of reef fishes.

Diversity of feeding mechanisms is a hallmark of reef fishes, but the history of this variation is not fully understood. Here, we explore the emergence and proliferation of a biting mode of feeding, which enables fishes to feed on attached benthic prey. We find that feeding modes other than suction, including biting, ram biting, and an intermediate group that uses both biting and suction, were nearly absent among the lineages of teleost fishes inhabiting reefs prior to the end-Cretaceous mass extinction, but benthic biting has rapidly increased in frequency since then, accounting for about 40% of reef species today. Further, we measured the impact of feeding mode on body shape diversification in reef fishes. We fit a model of multivariate character evolution to a dataset comprising three-dimensional body shape of 1,530 species of teleost reef fishes across 111 families. Dedicated biters have accumulated over half of the body shape variation that suction feeders have in just 18% of the evolutionary time by evolving body shape ∼1.7 times faster than suction feeders. As a possible response to the ecological and functional diversity of attached prey, biters have dynamically evolved both into shapes that resemble suction feeders as well as novel body forms characterized by lateral compression and small jaws. The ascendance of species that use biting mechanisms to feed on attached prey reshaped modern reef fish assemblages and has been a major contributor to their ecological and phenotypic diversification.

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.

Does abiotic noise promote segregation of functional diversity in Neotropical anuran assemblages?

The abiotic noise of streams can mask the acoustic signals of anurans with a large body size calling at low frequencies, but not the signals emitted by anurans with a small body size calling at high frequencies. As a consequence, the body size of species in assemblages alongside streams is, on average, lower and less variable than that of assemblages away from streams. Given that the body size in anurans is frequently related to life-history traits, it is expected that functional diversity (FD) will be lower in anuran assemblages alongside streams than in assemblages away from streams. We calculated and compared FD, based on six functional traits, for anuran species in seven localities in different biogeographical regions in the Neotropics. In five lowland localities, FD was lower in assemblages alongside streams than in assemblages away from streams. However, the reverse trend was found in two Andean localities. Noise from streams, acting as an environmental filter, could promote low FD because taxa whose phenotype differs from an optimal type (high call frequency, small body size and associated traits) are excluded from riparian places. However, such habitat filtering could be stronger and affect more anurans in lowland assemblages than in those at medium elevation.

Bite force mechanics and allometry of piranha (Serrasalmidae)

The bite force of the piranha (Serrasalmidae) has drawn considerable attention due to its ability to effectively capture and masticate prey. Herein, we analyze theoretical anterior bite forces using a lever approach and compare them to in-vivo maximum bite forces. We provide a mechanics analysis that explains the scaling allometry of the bite force (F_output) with the length of the fish (l), F_outputαl².

Use of bioacoustics in species identification: Piranhas from genus Pygocentrus (Teleostei: Serrasalmidae) as a case study

The genus Pygocentrus contains three valid piranha species (P. cariba, P. nattereri and P. piraya) that are allopatric in tropical and subtropical freshwater environments of South America. This study uses acoustic features to differentiate the three species. Sounds were recorded in P. cariba, two populations of P. nattereri (red- and yellow-bellied) and P. piraya; providing sound description for the first time in P. cariba and P. piraya. Calls of P. cariba were distinct from all the other studied populations. Red- and yellow-bellied P. nattereri calls were different from each other but yellow-bellied P. nattereri calls were similar to those of P. piraya. These observations can be explained by considering that the studied specimens of yellow-bellied P. nattereri have been wrongly identified and are actually a sub-population of P. piraya. Morphological examinations and recent fish field recordings in the Araguari River strongly support our hypothesis. This study shows for the first time that sounds can be used to discover identification errors in the teleost taxa.

Phylogenomics of Piranhas and Pacus (Serrasalmidae) Uncovers How Dietary Convergence and Parallelism Obfuscate Traditional Morphological Taxonomy

The Amazon and neighboring South American river basins harbor the world's most diverse assemblages of freshwater fishes. One of the most prominent South American fish families is the Serrasalmidae (pacus and piranhas), found in nearly every continental basin. Serrasalmids are keystone ecological taxa, being some of the top riverine predators as well as the primary seed dispersers in the flooded forest. Despite their widespread occurrence and notable ecologies, serrasalmid evolutionary history and systematics are controversial. For example, the sister taxon to serrasalmids is contentious, the relationships of major clades within the family are inconsistent across different methodologies, and half of the extant serrasalmid genera are suggested to be non-monophyletic. We analyzed exon capture to reexamine the evolutionary relationships among 63 (of 99) species across all 16 serrasalmid genera and their nearest outgroups, including multiple individuals per species to account for cryptic lineages. To reconstruct the timeline of serrasalmid diversification, we time-calibrated this phylogeny using two different fossil-calibration schemes to account for uncertainty in taxonomy with respect to fossil teeth. Finally, we analyzed diet evolution across the family and comment on associated changes in dentition, highlighting the ecomorphological diversity within serrasalmids. We document widespread non-monophyly of genera within Myleinae, as well as between Serrasalmus and Pristobrycon, and propose that reliance on traits like teeth to distinguish among genera is confounded by ecological homoplasy, especially among herbivorous and omnivorous taxa. We clarify the relationships among all serrasalmid genera, propose new subfamily affiliations, and support hemiodontids as the sister taxon to Serrasalmidae. [Characiformes; exon capture; ichthyochory; molecular time-calibration; piscivory.].

Origin and evolution of sound production in Serrasalmidae

Among piranhas, sound production is known in carnivorous species, whereas herbivorous species were thought to be mute. Given that these carnivorous sonic species have a complex sonic apparatus, we hypothesize that intermediate forms could be found in other serrasalmid species. The results highlight the evolutionary transition from a simple sound-producing mechanism without specialized sonic structures to a sonic mechanism involving large, fast-contracting sonic muscles. Hypaxial muscles in basal herbivores primarily serve locomotion, but some fibres cause sound production during swimming accelerations, meaning that these muscles have gained a dual function. Sound production therefore seems to have been acquired through exaptation, i.e. the development of a new function (sound production) in existing structures initially shaped for a different purpose (locomotion). In more derived species (Catoprion and Pygopristis), some fibres are distinguishable from typical hypaxial muscles and insert directly on the swimbladder. At this stage, the primary function (locomotion) is lost in favour of the secondary function (sound production). In the last stage, the muscles and insertion sites are larger and the innervation involves more spinal nerves, improving calling abilities. In serrasalmids, the evolution of acoustic communication is characterized initially by exaptation followed by adaptive evolution.