Fishes navigating complex aquatic environments rely on various sensory systems, primarily the lateral line system and vision, to guide their movements. One interesting example is the Mexican blind cavefish (Astyanax mexicanus). This fish relies on the lateral line system as it navigates through the environment without the aid of sight. It is unclear, however, how they might navigate through a novel environment when the lateral line is not functional. In this study, we used high-speed videography to quantify whether naïve blind cavefish alter locomotor behavior, navigation patterns, and the use of body and fins to explore a novel environment with obstacles when the lateral line is ablated. Blind cavefish with an intact lateral line demonstrated deliberate slower exploratory movements and navigated around obstacles with fewer touching events. Conversely, fish with ablated lateral line exhibited increased speed to potentially improve flow sensing. Fish with an ablated lateral line also touched obstacles more often, suggesting a reliance on fin and snout mechanoreception for navigation. These results show the blind cavefish have compensatory sensory mechanisms to navigate novel environments when their major sensory system is not functioning.

Batoids (skates and rays) are the most speciose group of cartilaginous fishes with a diverse array of ecological adaptations and swimming modes. Early skeletal fossil remains and recent phylogenetic analyses suggest that convergence among batoids has occurred independently multiple times. The drivers for such disparity patterns and possible association with modularity and phenotypic integration among batoids are not fully understood. Here we employed geometric morphometrics and phylogenetic comparative methods to characterize the evolutionary trends in the basal fin skeleton of extinct and extant batoids and dorsoventrally flattened sharks. We found that the most speciose orders of batoids, Myliobatiformes and Rajiformes, display the lowest levels of morphological disparity, while Torpediniformes and Rhinopristitiformes have the highest disparity. Differences in evolutionary rates by habitat indicate that both reef and freshwater species evolved faster than deep-sea and shelf-distributed species. We further explored the differences based on swimming modes and found that species with oscillatory swimming exhibit higher evolutionary rates on their coracoid bar. We found that specific groups underwent different rates of evolution on each element of the pectoral fin. This was corroborated by the modularity and integration analyses, which indicate differences in the covariation between structures among the analyzed groups. The convergence analysis does not support the resemblance between flattened sharks and batoids; however we found convergence between extinct batoids and modern guitarfishes. Our findings suggest that habitat and swimming mode have shaped the pectoral fin evolution among batoids.

Sexual selection and sexual conflict often result in the evolution of morphological traits that function to improve reproductive success, often termed sexual weapons and ornaments. Sexual weapons serve to increase the reproductive success of the ardent sex (typically males in dioecious taxa) by force, whereas sexual ornaments are considered ‘desirable’ by the opposite sex, or may exploit pre-existing sensory bias. Cartilaginous fishes (Chondrichthyes: sharks, skates, rays, and chimaeras) exhibit a complex spectrum of reproductive modes and marked variation in the prevalence of genetic polyandry and multiple mating. For these reasons, Chondrichthyes represent an ideal group to study sexual selection, sexual conflict, and their evolutionary consequences. In this review, we summarise existing knowledge regarding the function of several putative ‘weapons of sexual conflict’ (sexual weaponry used to coerce or force females to mate) and ornaments possessed by cartilaginous fishes. Subsequently, we discuss what chondrichthyans and these traits can tell us about sexual selection more broadly, and we highlight major knowledge gaps in the field. A lack of observational data impedes our ability to make robust claims about the function of several traits. However, there is reason to suggest that weaponry resulting from sexually antagonistic selection is abundant in chondrichthyan taxa, whilst only one potential case of sexual ornamentation is known.

Swimming ability is critical for navigating complex benthic habitats, yet the biomechanical strategies demersal sharks employ to modulate body and fin movements across varying speeds remain largely unexplored. This study examines speed-dependent kinematic patterns in the small-spotted catshark (Scyliorhinus canicula), a benthic species with limited endurance for sustained swimming. Using high-speed videography in a flow tank, we quantified adjustments in tail beat frequency, body angle, wave speed and curvature across a range of speeds (0.5–6 body lengths per second). Our results reveal that S. canicula exhibits distinct kinematic shifts as speed increases, adopting a more streamlined posture and increasing tail beat frequency to accommodate higher flow rates. Principal component analysis identified swimming speed as the primary factor influencing kinematic variation, with higher speeds necessitating more consistent body alignment and tail movement. Strouhal numbers within the optimal range for propulsive efficiency (0.2–0.4) at intermediate speeds (1–2 BL s⁻¹) suggest that S. canicula maximizes energetic efficiency within this range, although further research is required to elucidate the metabolic implications. This study establishes a foundational framework for understanding the biomechanics of steady swimming in a demersal shark, providing insights into the ecological and evolutionary pressures shaping locomotor adaptations in benthic species.

The spotted ratfish Hydrolagus colliei is the most common holocephalan species exhibited in aquaria worldwide for introducing deep-sea environments and raising awareness of their conservation. However, little is known about the biology of H. colliei. Current practices in aquaria allow long-term survival of sexually mature H. colliei specimens; however, this species struggles to complete a reproductive cycle in captivity mostly because embryos do not reach the hatchling stage. The aquarists of Planet Ocean Montpellier (POM, France) have bred H. colliei for 15 years and recorded parameters suitable for this species' successful embryonic and post-embryonic development. POM aquarists now regularly record egg-laying events of H. colliei and use four tanks to incubate eggs and raise neonates, late hatchlings, early and intermediate juveniles, subadults, and sexually mature specimens. In this work we provide the first long-term biometric data on H. colliei from the hatchling to the subadult stage. We also report the biotic and abiotic parameters sufficient to breed H. colliei in aquaria. We finally describe the methods used to facilitate individual monitoring of specimens along the ontogeny and several pathologies identified in this species, their putative causes, and the corresponding treatments. This work highlights the importance of ex situ research and points to the valuable outcomes of collaborative efforts between aquaria and academia in deciphering the biology of species whose study in the wild remains challenging.

Paleontologists must confront the challenge of studying the forms and functions of extinct species for which data from preserved fossils are extremely limited, yielding only a fragmented picture of life in deep time. In response to this hurdle, we describe the nascent field of paleoinspired robotics, an innovative method that builds upon established techniques in bioinspired robotics, enabling the exploration of the biology of ancient organisms and their evolutionary trajectories. This Review presents ways in which robotic platforms can fill gaps in existing research using the exemplars of notable transitions in vertebrate locomotion. We examine recent case studies in experimental paleontology, highlighting substantial contributions made by engineering and robotics techniques, and further assess how the efficient application of robotic technologies in close collaboration with paleontologists and biologists can offer additional insights into the study of evolution that were previously unattainable.

Shoaling behavior is known to increase survival rates during attacks from predators, minimize foraging time, favor mating, and potentially increase locomotor efficiency. The onset of shoaling typically occurs during the larval phase, but it is unclear how it may improve across ontogenetic stages in forage fishes. Warming is known to increase metabolic rates during locomotion in solitary fish, and shoaling species may adjust their collective behavior to offset the elevated costs of swimming at higher temperatures. In this study, we quantified the effects of warming on shoaling performance across the ontogeny of a small forage fish, zebrafish (Danio rerio) at different speeds. Shoals of larval, juvenile, and adult zebrafish were acclimated at two temperatures (28°C and 32°C), and metabolic rates were quantified prior to and following nonexhaustive exercise at high speed. Shoals of five individuals were filmed in a flow tank to analyze the kinematics of collective movement. We found that zebrafish improve shoaling swimming performance from larvae to juveniles to adults. In particular, shoals become more cohesive, and both tail beat frequency (TBF) and head-to-tail amplitude decrease with ontogeny. Early life stages have higher thermal sensitivity in metabolic rates and TBF especially at high speeds, when compared to adults. Our study shows that shoaling behavior and thermal sensitivity improve as zebrafish shift from larval to juvenile to adult stages.