Dominique Adriaens

Background Syngnathids are a highly derived and diverse fish clade comprising the pipefishes, pipe‐horses, and seahorses. They are characterized by a plethora of iconic traits that increasingly capture the attention of biologists, including geneticists, ecologists, and developmental biologists. The current understanding of the origins of their derived body plan is, however, hampered by incomplete and limited descriptions of the early syngnathid ontogeny. Results We provide a comprehensive description of the development of Nerophis ophidion , Syngnathus typhle , and Hippocampus erectus from early cleavage stages to release from the male brooding organ and beyond, including juvenile development. We comparatively describe skeletogenesis with a particular focus on dermal bony plates, the snout‐like jaw morphology, and appendages. Conclusions This most comprehensive and detailed account of syngnathid development to date suggests that convergent phenotypes (e.g., reduction and loss of the caudal fins), likely arose by distinct ontogenetic means in pipefishes and seahorses. Comparison of the ontogenetic trajectories of S. typhle and H. erectus provides indications that characteristic features of the seahorse body plan result from developmental truncation. Altogether, this work provides a valuable resource and framework for future research to understand the evolution of the outlandish syngnathid morphology from a developmental perspective.

In limbless fossorial vertebrates such as caecilians (Gymnophiona), head‐first burrowing imposes severe constraints on the morphology and overall size of the head. As such, caecilians developed a unique jaw‐closing system involving the large and well‐developed m. interhyoideus posterior , which is positioned in such a way that it does not significantly increase head diameter. Caecilians also possess unique muscles among amphibians. Understanding the diversity in the architecture and size of the cranial muscles may provide insights into how a typical amphibian system was adapted for a head‐first burrowing lifestyle. In this study, we use dissection and non‐destructive contrast‐enhanced micro‐computed tomography (μCT) scanning to describe and compare the cranial musculature of 13 species of caecilians. Our results show that the general organization of the head musculature is rather constant across extant caecilians. However, the early‐diverging Rhinatrema bivittatum mainly relies on the ‘ancestral’ amphibian jaw‐closing mechanism dominated by the m. adductores mandibulae , whereas other caecilians switched to the use of the derived dual jaw‐closing mechanism involving the additional recruitment of the m. interhyoideus posterior . Additionally, the aquatic Typhlonectes show a greater investment in hyoid musculature than terrestrial caecilians, which is likely related to greater demands for ventilating their large lungs, and perhaps also an increased use of suction feeding. In addition to three‐dimensional interactive models, our study provides the required quantitative data to permit the generation of accurate biomechanical models allowing the testing of further functional hypotheses.