Jaw muscle fiber type distribution in Hawaiian gobioid stream fishes: histochemical correlations with feeding ecology and behavior

Kinetic comparisons of jaw opening, jaw closing and locomotor muscles

Understanding contraction dynamics of skeletal muscle is critically important to appreciate performance capabilities of skeletal structures, especially for structures responsible for feeding and/or locomotion. Furthermore, it is important to understand how temperature can impact contraction dynamics in vertebrates that are regularly exposed to fluctuations in temperature. We aimed to address differences between jaw opening (sternhyoideus), jaw closing (adductor mandibulae) and locomotor (abductor superficialis) muscle contraction dynamics in a labrid fish. Additionally, we aimed to understand how temperature changes impact contraction kinetics in these muscles. To address these aims we collected cunner (Tautogolabrus adspersus) from Long Island Sound and removed their primary locomotor and jaw opening/closing muscles. Following dissection, the muscles were placed on a muscle ergometer that controlled length, stimulus duration and stimulation intensity. Muscles were exposed to 7, 15 and 22 °C for all experiments. We found that the swimming muscle was slower to contract and relax than both feeding muscles. Swimming muscle produced more power than feeding muscles when tested at 8 Hz and 15 °C. Jaw closing muscle produced more force than the other two muscle types when tested at22 °C. Despite these differences, muscle contraction kinetics were remarkably similar across the three muscle types and temperatures. To our knowledge this is the first study to measure in vitro contraction dynamics of fish jaw opening and closing muscle. This advances the understanding of the physiological capabilities of these muscles. Additionally, differences in contraction dynamics can further our understanding of the physiological limits temperature impart on whole muscle contraction.

Lip service: Histological phenotypes correlate with diet and feeding ecology in herbivorous pacus

Complex prey processing requires the repositioning of food between the teeth, as modulated by a soft tissue appendage like a tongue or lips. In this study, we trace the evolution of lips and ligaments, which are used during prey capture and prey processing in an herbivorous group of fishes. Pacus (Serrasalmidae) are Neotropical freshwater fishes that feed on leaves, fruits, and seeds. These prey are hard or tough, require high forces to fracture, contain abrasive or caustic elements, or deform considerably before failure. Pacus are gape-limited and do not have the pharyngeal jaws many bony fishes use to dismantle and/or transport prey. Despite their gape limitation, pacus feed on prey larger than their mouths, relying on robust teeth and a hypertrophied lower lip for manipulation and breakdown of food. We used histology to compare the lip morphology across 14 species of pacus and piranhas to better understand this soft tissue. We found that frugivorous pacus have larger, more complex lips which are innervated and folded at their surface, while grazing species have callused, mucus-covered lips. Unlike mammalian lips or tongues, pacu lips lack any intrinsic skeletal or smooth muscle. This implies that pacu lips lack dexterity; however, we found a novel connection to the primordial ligament which suggests that the lips are actuated by the jaw adductors. We propose that pacus combine hydraulic repositioning of prey inside the buccal cavity with direct oral manipulation, the latter using a combination of a morphologically heterodont dentition and compliant lips for reorienting food.

Sucker Shapes, Skeletons and Bioinspiration: How Hard and Soft Tissue Morphology Generates Adhesive Performance in Waterfall Climbing Goby Fishes

Many teleost fishes, such as gobies, have fused their paired pelvic fins into an adhesive disc. Gobies can use their pelvic suckers to generate passive adhesive forces (as in engineered suction cups) and different species exhibit a range of adhesive performance, with some even able to climb waterfalls. Previous studies have documented that, in the Hawaiian Islands, species capable of climbing higher waterfalls produce the highest passive pull-off forces, and species found at higher elevation sites are likely to have more rounded suction discs than those found in the lowest stream segments. Morphology of the pelvic girdle also varies between species, with more robust skeletons in taxa with superior passive adhesion. To investigate what factors impact the passive adhesive performance of waterfall climbing gobies, we tested biomimetic suction cups designed with a range of shapes and embedded bioinspired "skeletons" based on micro-CT scans of goby pelvic girdles. We found that while the presence of an internal skeleton may provide some support against failure, the performance of suction cups may be more strongly affected by their external shape. Nonetheless, factors besides external shape and skeletal morphology may still have a stronger influence on sucker tenacity. Our results suggest that the relationship between suction disc morphology and adhesive performance may be influenced by a variety of physical factors, and live animal performance likely is further complicated by muscle activation and climbing behavior. These results have implications for the evolution of suction disc shape in adhesive fishes and for improving the design of biomimetic suction cups.

From yellow to silver: Transforming cranial morphology in European eel (Anguilla anguilla)

The European eel (Anguilla anguilla) has been extensively studied, especially because of its highly specialized migratory behaviour associated with substantial phenotypic transformations. During this migration, one of those transformations the eel undergoes is from yellow to silver eel, a process known as silvering. Although the cranial morphology during the earlier glass, elver and yellow eel stages are well studied, little is known about actual morphological changes during the transformation process from the yellow to the silver eel stage. Yet, literature suggests drastic changes in musculoskeletal anatomy. Here, we investigated the cranial musculoskeletal morphology of 11 male European eels at different stages during silvering, resulting both from natural and artificial maturation. Using 3D-reconstructed µCT data of the head, the skull and cranial muscles associated with jaw closing and respiration were studied. Eye size was used as a proxy for the silvering stage. Size-adjusted jaw muscle volumes increased during silvering, although insignificantly. Accordingly, a near-significant increase in bite force was observed. Respiratory muscles size did increase significantly during silvering, however. Considering the eel's long migration, which often includes deep and thus potentially oxygen-poor environments, having a better performing respiratory system may facilitate efficient migration. Both overall skull dimensions and specifically orbit size increased with eye index, suggesting they play a role in accommodating the enlarging eyes during silvering. Finally, artificially matured eels had a wider and taller skull, as well as larger jaw muscles than wild silver eels. This could be caused (a) by different conditions experienced during the yellow eel stage, which are maintained in the silver eel stage, (b) by side effects of hormonal injections or (c) be part of the maturation process as artificially induced silver eels had a higher eye index than the wild silver eels.

Functional correlations of axial muscle fiber type proportions in the waterfall-climbing Hawaiian stream fish Sicyopterus stimpsoni

Assessing the factors that contribute to successful locomotor performance can provide critical insight into how animals survive in challenging habitats. Locomotion is powered by muscles, so that differences in the relative proportions of red (slow-oxidative) vs. white (fast-glycolytic) fibers can have significant implications for locomotor performance. We compared the relative proportions of axial red muscle fibers between groups of juveniles of the amphidromous gobiid fish, Sicyopterus stimpsoni, from the Hawaiian Islands. Juveniles of this species migrate from the ocean into freshwater streams, navigating through a gauntlet of predators that require rapid escape responses, before reaching waterfalls which must be climbed (using a slow, inching behavior) to reach adult breeding habitats. We found that fish from Kaua'i have a smaller proportion of red fibers in their tail muscles than fish from Hawai'i, matching expectations based on the longer pre-waterfall stream reaches of Kaua'i that could increase exposure to predators, making reduction of red muscle and increases in white muscle advantageous. However, no difference in red muscle proportions was identified between fish that were either successful or unsuccessful in scaling model waterfalls during laboratory climbing trials, suggesting that proportions of red muscle are near a localized fitness peak among Hawaiian individuals.

Dimorphism throughout the European eel's life cycle: are ontogenetic changes in head shape related to dietary differences?

A well-known link exists between an organism's ecology and morphology. In the European eel, a dimorphic head has been linked to differences in feeding ecology, with broad-headed eels consuming harder prey items than narrow-headed ones. Consequently, we hypothesized that broad-heads should exhibit a cranial musculoskeletal system that increases bite force and facilitates the consumption of harder prey. Using 3D-reconstructions and a bite model, we tested this hypothesis in two life stages: the sub-adult yellow eel stage and its predecessor, the elver eel stage. This allowed us to test whether broad- and narrow-headed phenotypes show similar trait differences in both life stages and whether the dimorphism becomes more pronounced during ontogeny. We show that broad-headed eels in both stages have larger jaw muscles and a taller coronoid, which are associated with higher bite forces. This increased bite force together with the elongated upper and lower jaws in broad-headed eels can also improve grip during spinning behavior, which is used to manipulate hard prey. Head shape variation in European eel is therefore associated with musculoskeletal variation that can be linked to feeding ecology. However, although differences in muscle volume become more pronounced during ontogeny, this was not the case for skeletal features.

Stairway to heaven: evaluating levels of biological organization correlated with the successful ascent of natural waterfalls in the Hawaiian stream goby Sicyopterus stimpsoni

Selective pressures generated by locomotor challenges act at the level of the individual. However, phenotypic variation among individuals that might convey a selective advantage may occur across any of multiple levels of biological organization. In this study, we test for differences in external morphology, muscle mechanical advantage, muscle fiber type and protein expression among individuals of the waterfall climbing Hawaiian fish Sicyopterus stimpsoni collected from sequential pools increasing in elevation within a single freshwater stream. Despite predictions from previous laboratory studies of morphological selection, few directional morphometric changes in body shape were observed at successively higher elevations. Similarly, lever arm ratios associated with the main pelvic sucker, central to climbing ability in this species, did not differ between elevations. However, among climbing muscles, the adductor pelvicus complex (largely responsible for generating pelvic suction during climbing) contained a significantly greater red muscle fiber content at upstream sites. A proteomic analysis of the adductor pelvicus revealed two-fold increases in expression levels for two respiratory chain proteins (NADH:ubiquinone reductase and cytochrome b) that are essential for aerobic respiration among individuals from successively higher elevations. Assessed collectively, these evaluations reveal phenotypic differences at some, but not all levels of biological organization that are likely the result of selective pressures experienced during climbing.