ABSTRACT An animal's morphology directly influences its ability to perform fundamental tasks, which can be vital to its survival and reproduction. This is especially true in harsh or extreme environments, like the shifting‐sand desert of the Sahara. Additionally, sexes can differ in their morphology, performance, or in the relationship between morphology and performance, with implications for behaviors such as habitat selection, predator avoidance, and reproduction. Despite being common across a broad geographic range, little is known of the ecomorphology and sexual dimorphism of the sandfish skink ( Scincus scincus ). To investigate this, we first validated the uses of molecular sexing protocols and morphology (hemipene eversion) to identify the sex of our animals. We then tested for differences in body size, the multivariate morphological phenotype, and coloration between the sexes. While patterns generally suggested male bias in both body size and limb dimension in concordance with previous studies, these differences were not statistically significant. Additionally, we found several exceptions to the standard color‐based sex assignment of these animals, especially for females. We then conducted an experiment to test how body size and dimensions affect performance at optimum temperatures. We quantified three different types of movement: sprinting on top of sand (used in hunting behavior), swimming under sand (an undulatory form of locomotion used in thermoregulation), and diving into sand (an important escape behavior). Overall, skinks with larger body sizes sprinted faster and swam faster through sand. Female skinks sprinted faster than male skinks even after accounting for differences in body size. Larger heads, wider bodies, and longer limbs were negatively correlated with swim speed. Diving performance exhibited the lowest among‐individual variation, did not differ between the sexes, and was not dependent on body size or shape. Finally, we found no evidence of among‐individual correlations in performance measures, suggesting each performance is independent of the others. Overall, this experiment provides insight into morphology–performance relationships across a number of ecologically important aspects of locomotion for a species inhabiting an extreme desert environment.
ABSTRACT Complex life cycles characterized by distinct life phases connected by a metamorphosis raise the question whether traits of these distinct life phases can evolve independently or remain evolutionarily coupled. Because metamorphosis can include trait reorganization between life phases, the adaptive decoupling hypothesis proposes that larval and adult traits may evolve largely independent of each other. In anurans, the cranial system undergoes a major reorganization during metamorphosis, transitioning from a distinct cartilaginous larval cranium to an ossified adult cranium, yet this transformation has never been quantitatively characterized across species. We applied Anatomical Network Analysis (AnNA) to the cranial musculoskeletal systems of larval and adult anurans with different feeding and developmental modes. AnNA describes the topological organization of the cranium without the need for homologous landmarks, allowing comparisons between morphologically disparate systems. We calculated topological parameters describing integration and modularity, as well as connectivity modules (Q‐modules) within each cranial network. As part of that analysis, expanded morphological descriptions of the cranial musculoskeletal system of larval and adult Smilisca phaeota were made. Larval and embryonic crania are topologically highly integrated and less modular, whereas adult crania are less integrated and organized into distinct modules. As a result, larval and adult life phases occupy distinct regions within a topological morphospace, whereby larvae and embryos of endotrophic anurans, despite lacking a free‐living larval stage, cluster consistently within the larval topological space. Together, these findings support the adaptive decoupling hypothesis for the cranial system across anuran life phases.
ABSTRACT The axial complex (proboscis complex or heart‐kidney) is well‐studied for the shallow‐living enteropneusts that burrow into the sediment and use the muscular proboscis as the primary locomotor organ. However, the deep‐sea enteropneusts in the family Torquaratoridae are epibenthic surface‐dwellers with a dispersed, poorly developed proboscis musculature. There is little data on the structure of their axial complex (AC). We have studied in detail the microscopic anatomy of the AC of three specimens of torquaratorid Quatuoralisia malakhovi Ezhova et Lukinykh, 2022. The proboscis of Q. malakhovi is widely fused with the collar along the entire dorsal side rather than being connected by a narrow stalk. The AC includes the buccal diverticulum (stomochord), pericardium, glomerulus (axial organ) and associated blood vessels, perihaemal coeloms, and proboscis skeletal elements. The latter are two separate symmetrical plates supporting the buccal diverticulum laterally. The buccal diverticulum is elongated in transverse direction and packed with large, vacuolated cells without a lumen. It is completely separated from the main course of the gut. The pericardium lies on the dorsal side of the buccal diverticulum. The heart is absent. The glomerulus consists of two symmetrical extensions separated by the confluence of the dorsal blood vessel. The latter is accompanied by the perihaemal coeloms and splits into several branches resembling the mouth of a river, which then flow into the glomerulus. Two buccal blood vessels with folded walls run from the sides of the glomerulus to the ventral blood vessel. A protocoelomic coelomopore is absent, however we found a pair of mesocoelomic coelomoducts and coelomopores. The torquaratorid AC demonstrates varying degrees of reduction. None of the previously studied species of Torquaratoridae possesses a complete set of AC components characteristic of shallow‐living enteropneusts. Torquaratoridae are derived in terms of structure and composition of the AC compared to non‐torquaratorid hemichordates.
ABSTRACT While long bones are mostly utlised for skeletochronology of extant and extinct vertebrates, among crocodilians, dermal bones have also been studied. However, there is a lack of a systematic and comprehensive study of the variation in osteohistology and skeletochronology of limb, axial, and dermal bones of extant crocodilians. Here, we undertake a comprehensive assessment of the histology of multiple postcranial elements of a single, captive adult Crocodylus niloticus (with approximately known age) to assess their utility for skeletochronology. We document the intraskeletal histological variability in the skeleton and show that despite the gastralia recording the highest number of growth rings, the radius, ulna, and fibula are more reliable for skeletochronology. We show substantial variation in the histology of the osteoderms depending on the plane of sectioning, as well as their location on the body of the crocodile. Additionally, we report that anteriorly sectioned dorsal osteoderms preserved the highest number of growth marks. Our study provides important information regarding intraskeletal variation in osteohistology and presents key information regarding the choice of skeletal elements for reliable life history reconstruction of C. niloticus .
ABSTRACT Supramedullary neurons (SMNs) are located on the dorsal surface of the hindbrain and spinal cord in many adults of teleost fishes. Comparative morphological studies have revealed differences in SMN size, number and distribution among fishes. However, little information exists on the projection patterns of SMN neurites within the central nervous system. We focus on a single species of adult teleost fish, the cunner, Tautogolabrus adspersus , a fish that has 25–46 SMN somata aligned dorsally along the midline in the hindbrain and rostral third of the spinal cord. Light and transmission electron microscopic techniques provide new information on the general morphology of SMNs and Lucifer yellow filled SMNs reveal that these neurons are not morphologically homogeneous. Rather, four different morphotypes can be identified based on projection patterns of major neurites in the hindbrain and spinal cord wholemounts. The morphological variations suggest that some SMNs exit the brain exclusively through the first dorsal root while others exit the central nervous system through a dorsal root and the trigeminal nerve. Despite the remarkable size and accessibility of SMNs, their function(s) remains unknown. One proposed role of these cells is eliciting mucous secretion in response to skin abrasion from predation. Variation in projection patterns in the cunner provides a morphological framework from which to design electrophysiological studies to help reveal SMN function.
