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 lesserknown 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.

Revisiting kinorhynch segmentation: variation of segmental patterns in the nervous system of three aberrant species

“…Adult kinorhynchs exhibit a conserved body plan with a radial head bearing a terminal mouth, a neck, and a trunk composed of eleven articulated segments [ 6 , 7 ]. Within the trunk, the musculature, nervous system, epidermal glands and sensory organs are segmentally arranged [ 6 – 10 ]. Yet, the development of the segments pre- and post-hatching is poorly studied (e.g., [ 11 – 13 ] summarized in [ 7 ]).…”

“…However, no new morphological evidence was produced to support this. Thus, interpretations of kinorhynchs as unsegmented, “superficially segmented”, “pseudo-segmented” or “truly segmented” were mostly based on speculations, old ultrastructural data from a restricted number of ‘model species’, and ‘whatever fits the current tree’, until more recent studies finally produced new evidence using immunohistochemistry (e.g., [ 8 – 10 , 26 – 30 ]).…”

“…Over the last two decades, an increasing number of morphological studies based on immunohistochemistry, confocal laser microscopy (CLSM) and three-dimensional rendering have addressed kinorhynch internal anatomy in a variety of genera complementing traditional transmission electron microscopy-based investigations (e.g., [ 9 , 10 , 28 – 30 ]). Comprehensive myoanatomical studies in kinorhynchs have so far been achieved in representatives of eight genera ( Antygomonas , Cateria , Echinoderes , Franciscideres, Dracoderes , Pycnophyes , Setaphyes and Zelinkaderes ) where all of them show a segmental pattern of at least dorsoventral and longitudinal muscle sets [ 8 , 10 , 27 , 29 , 30 , 38 ]. Neuroanatomical studies using immunolabeling have been carried out for five genera ( Antygomonas , Echinoderes , Dracoderes , Pycnophyes and Setaphyes ) [ 9 , 10 , 26 , 30 ].…”

“…Comprehensive myoanatomical studies in kinorhynchs have so far been achieved in representatives of eight genera ( Antygomonas , Cateria , Echinoderes , Franciscideres, Dracoderes , Pycnophyes , Setaphyes and Zelinkaderes ) where all of them show a segmental pattern of at least dorsoventral and longitudinal muscle sets [ 8 , 10 , 27 , 29 , 30 , 38 ]. Neuroanatomical studies using immunolabeling have been carried out for five genera ( Antygomonas , Echinoderes , Dracoderes , Pycnophyes and Setaphyes ) [ 9 , 10 , 26 , 30 ]. These studies show a similar nervous system architecture that mirrors the anterior-posterior pattern of repeated units shown by the tegumental plates and the musculature in most of the segments [ 8 – 10 , 30 , 39 ].…”

“…Neuroanatomical studies using immunolabeling have been carried out for five genera ( Antygomonas , Echinoderes , Dracoderes , Pycnophyes and Setaphyes ) [ 9 , 10 , 26 , 30 ]. These studies show a similar nervous system architecture that mirrors the anterior-posterior pattern of repeated units shown by the tegumental plates and the musculature in most of the segments [ 8 – 10 , 30 , 39 ].…”