Insights into mud dragon morphology (Kinorhyncha, Allomalorhagida): myoanatomy and neuroanatomy of Dracoderes abei and Pycnophyes ilyocryptus

Abstract

Comprehensive morphological investigations covering the diversity of metazoan lineages are needed to obtain a complete picture of organ system evolution. Despite the increased amount of studies on lesser-known phyla during the last decades, the gap in knowledge for these lineages is still remarkable. This is the case for kinorhynchs, or mud dragons, where only a few genera, mainly belonging to the major clade Cyclorhagida, have undergone detailed morphological examinations. Here, we explore the neuroanatomy and myoanatomy in the other major clade, Allomalorhagida, through studies of Dracoderes abei and Pycnophyes ilyocryptus by immunochemistry, CLSM, and computational 3D reconstruction. The current phylogenetic position of Dracoderes as a sister group to all the remaining allomalorhagids makes it a key taxon for understanding the evolution of organ systems within Kinorhyncha. Clear segmental arrangement of muscles and nerves was found in the trunk of D. abei and P. ilyocryptus, excluding modifications observed in the posteriormost segments. When comparing current and previous studies of allomalorhagids and cyclorhagids, the nervous system shows a conserved pattern across kinorhynchs, while the musculature shows significant variation among genera. Segmentation in kinorhynchs is restricted to the trunk, including mesoderm (muscles) and ectoderm derivated structures (nerves and glands). The nervous system shows a segmental arrangement in the first eight trunk segments, whereas the last three segments show some deviating arrangements most likely related to a post-hatching development of these segments. The presence of non-segmental trunk muscles is interpreted as adaptations for increased introvert motility and trunk flexibility.