Segmentations in fins enable large morphing amplitudes combined with high flexural stiffness for fish-inspired robotic materials

Fish fins do not contain muscles, yet fish can change their shape with high precision and speed to produce large and complex hydrodynamic forces—a combination of high morphing efficiency and high flexural stiffness that is rare in modern morphing and robotic materials. These “flexo-morphing” capabilities are rare in modern morphing and robotic materials. The thin rays that stiffen the fins and transmit actuation include mineral segments, a prominent feature whose mechanics and function are not fully understood. Here, we use mechanical modeling and mechanical testing on 3D-printed ray models to show that the function of the segmentation is to provide combinations of high flexural stiffness and high morphing amplitude that are critical to the performance of the fins and would not be possible with rays made of a continuous material. Fish fin–inspired designs that combine very soft materials and very stiff segments can provide robotic materials with large morphing amplitudes and strong grasping forces.

longfin causes cis-ectopic expression of the kcnh2a ether-a-go-go K+ channel to autonomously prolong fin outgrowth

Organs stop growing to achieve a characteristic size and shape in scale with the animal's body. Likewise, regenerating organs sense injury extents to instruct appropriate replacement growth. Fish fins exemplify both phenomena through their tremendous diversity of form and remarkably robust regeneration. The classic zebrafish mutant longfint2 develops and regenerates dramatically elongated fins and underlying ray skeleton. We show longfint2 chromosome 2 overexpresses the ether-a-go-go-related voltage-gated potassium channel kcnh2a. Genetic disruption of kcnh2a in cis rescues longfint2, indicating longfint2 is a regulatory kcnh2a allele. We find longfint2 fin overgrowth originates from prolonged outgrowth periods including by showing Kcnh2a chemical inhibition during late stage regeneration fully suppresses overgrowth. Cell transplantations demonstrate longfint2-ectopic kcnh2a acts tissue autonomously within the fin intra-ray mesenchymal lineage. Temporal inhibition of the Ca2+-dependent phosphatase calcineurin indicates it likewise entirely acts late in regeneration to attenuate fin outgrowth. Epistasis experiments suggest longfint2-expressed Kcnh2a inhibits calcineurin output to supersede growth cessation signals. We conclude ion signaling within the growth-determining mesenchyme lineage controls fin size by tuning outgrowth periods rather than altering positional information or cell-level growth potency.

The Fish Family Poeciliidae as a Model to Study the Evolution and Diversification of Regenerative Capacity in Vertebrates

The capacity of regenerating a new structure after losing an old one is a major challenge in the animal kingdom. Fish have emerged as an interesting model to study regeneration due to their high and diverse regenerative capacity. To date, most efforts have focused on revealing the mechanisms underlying fin regeneration, but information on why and how this capacity evolves remains incomplete. Here, we propose the livebearing fish family Poeciliidae as a promising new model system to study the evolution of fin regeneration. First, we review the current state of knowledge on the evolution of regeneration in the animal kingdom, with a special emphasis on fish fins. Second, we summarize recent advances in our understanding of the mechanisms behind fin regeneration in fish. Third, we discuss potential evolutionary pressures that may modulate the regenerative capacity of fish fins and propose three new theories for how natural and sexual selection can lead to the evolution of fin regeneration: (1) signaling-driven fin regeneration, (2) predation-driven fin regeneration, and (3) matrotrophy-suppressed fin regeneration. Finally, we argue that fish from the family Poeciliidae are an excellent model system to test these theories, because they comprise of a large variety of species in a well-defined phylogenetic framework that inhabit very different environments and display remarkable variation in reproductive traits, allowing for comparative studies of fin regeneration among closely related species, among populations within species or among individuals within populations. This new model system has the potential to shed new light on the underlying genetic and molecular mechanisms driving the evolution and diversification of regeneration in vertebrates.

Effects of Group Behavior in The Predatory Raid on Damselfish Nests By The False Cleanerfish Aspidontus Taeniatus

The benefits of group behavior have been reported in a variety of animals. The false cleanerfish Aspidontus taeniatus, which resembles the bluestreak cleaner wrasse Labroides dimidiatus, is the best-known example of mimicry in vertebrates. This mimicry system has been viewed as an aggressive mimicry to bite fish fins. However, recent field studies have reported that large individuals of the false cleanerfish form groups and jointly raid fish nests to eat eggs that are guarded by their parents. Since the cleaner wrasse does not form such groups or specialize in egg-eating, the feeding groups of the false cleanerfish is assumed to reduce the effectiveness of mimicry. Here, we conducted field observations to clarify the functions of group behavior in egg-eating in the false cleanerfish. The false cleanerfish formed groups of 2–12 individuals when they raided breeding nests of 13 damselfish (Pomacentridae) and one triggerfish (Balistidae). The results showed that the group behavior has two effects: a dilution effect, which reduces the risk of being attacked by egg-guarding fish, and an increase in foraging efficiency. We conclude that the false cleanerfish need to form cooperative foraging groups during egg-eating because the egg-guarding parents could see through the mimicry.

Developmental hourglass and heterochronic shifts in fin and limb development

How genetic changes are linked to morphological novelties and developmental constraints remains elusive. Here we investigate genetic apparatuses that distinguish fish fins from tetrapod limbs by analyzing transcriptomes and open chromatin regions (OCRs). Specifically, we compared mouse forelimb buds with the pectoral fin buds of an elasmobranch, the brown-banded bamboo shark (Chiloscyllium punctatum). A transcriptomic comparison with an accurate orthology map revealed both a mass heterochrony and hourglass-shaped conservation of gene expression between fins and limbs. Furthermore, open-chromatin analysis suggested that access to conserved regulatory sequences is transiently increased during mid-stage limb development. During this stage, stage-specific and tissue-specific OCRs were also enriched. Together, early and late stages of fin/limb development are more permissive to mutations than middle stages, which may have contributed to major morphological changes during the fin-to-limb evolution. We hypothesize that the middle stages are constrained by regulatory complexity that results from dynamic and tissue-specific transcriptional controls.

Effects of Tissue Preservation on Carbon and Nitrogen Stable Isotope Signatures in Syngnathid Fishes and Prey

Isotopic stable analysis (SIA) is a powerful tool in the assessment of different types of ecological and physiological studies. For that, different preservation methods for sampled materials are commonly used prior to isotopic analysis. The effects of various preservation methods (freezing, ethanol and formaldehyde) were analyzed for C:N, and δ13C and δ15N signals on a variety of tissues including dorsal fins (three seahorse and two pipefish species), seahorse newborns (three seahorses species), and prey (copepods and different stages of Artemia) commonly used to feed the fishes under rearing conditions. The aims of the study were: (i) to evaluate isotopic effects of chemical preservation methods across different types of organisms and tissues, using frozen samples as controls, and (ii) to construct the first conversion models available in syngnathid fishes. The chemical preservation in ethanol and, to a lesser extent, in formaldehyde significantly affected δ13C values, whereas the effects on δ15N signatures were negligible. Due to their low lipid content, the isotopic signals in fish fins was almost unaffected, supporting the suitability of dorsal fins as the most convenient material in isotopic studies on vulnerable species such as syngnathids. The regression equations provided resulted convenient for the successful conversion of δ13C between preservation treatments. Our results indicate that the normalization of δ15N signatures in preserved samples is unnecessary. The conversion models should be applicable in isotopic field studies, laboratory experiments, and specimens of historical collections.

Thyroid hormone regulates proximodistal identity in the fin skeleton

Across the ∼30,000 species of ray-finned fish, fins show incredible diversity in overall shape and in the patterning of the supportive bony rays. Fin length mutant zebrafish have provided critical insights into the developmental pathways that regulate relative fin size. However, the processes that govern skeletal patterning along the proximodistal axis of the fin have remained less well understood. Here, we show that thyroid hormone regulates proximodistal identity of fin rays, distalizing gene expression profiles, morphogenetic processes during outgrowth, and ultimate morphology of the fin. This role for thyroid hormone in specifying proximodistal identity appears conserved between development and regeneration, in all the fins, and between species. We demonstrate that proximodistal identity is regulated independently from pathways that determine size, and we show that modulating proximodistal patterning relative to growth can recapitulate the spectrum of fin ray diversity found in nature.

Effects of Tissue Preservation on Carbon and Nitrogen Stable Isotope Signatures in Syngnathid Fishes and Prey

Isotopic stable analysis (SIA) is a powerful tool in the assessment of different types of ecological and physiological studies. For that, different preservative methods for the samples are commonly used prior to isotopic analysis. The effects of various preservative methods (drying, freezing, ethanol and formaldehyde) have been analyzed for C:N ratio, δ13C and δ15N on a variety of tissues including dorsal fins (three seahorse and two pipefish species), seahorse newborns (three seahorses species), and prey (copepods and different stages of Artemia) commonly used to feed the fishes in rearing conditions. The aims of the study were to: (i) evaluate isotopic effects of preservation methods across tissues; and (ii) construct the first conversion models available in syngnathid fishes. The preservation in ethanol and to a lesser extend in formaldehyde significantly affected δ13C values, whereas δ15N signatures were not affected significantly. Due to their low lipid content, the isotopic signals in fish fins were almost unaffected, supporting the suitability of dorsal fins as a convenient tool in isotopic studies on vulnerable species such as syngnathids. The regression equations provided permit the successful conversion of δ13C and δ15N values between preservative treatments. The conversion models can be applied to isotopic studies in the field and in the laboratory.