Myoanatomy of three aberrant kinorhynch species: similar but different?

Abstract

Aberrant kinorhynchs show several modifications deviating from the typical kinorhynch body plan, including a modified introvert with very elongated and flexible scalids, a weakly developed neck, and a slender trunk with less distinct segmentation. How these aberrant external features are reflected in the inner anatomy and how their aberrant body plan evolved is not understood. Here, we provide a comprehensive and comparative myoanatomical study of three putatively, distant related worm-like species: Cateria styx, Franciscideres kalenesos and Zelinkaderes yong. Despite the weak external segmentation of the trunk, the studied species show a distinct segmental arrangement of the musculature. However, this arrangement is shifted posteriorly with respect to the external segmentation, because the extremely thin and flexible cuticle is lacking the apodeme-like cuticular thickenings (pachycycli) where the longitudinal muscles usually attach. The muscular arrangement in the three species is overall similar, yet, C. styx show most resemblance to the allomalorhagid F. kalenesos, whereas the cyclorhagid Z. yong differs in several ways. This suggests a closer relationship of C. styx to Allomalorhagida. Whereas most kinorhynchs prefer muddy sediments, both the allomalorhagid and cyclorhagid worm-like kinorhynchs are mainly found in sandy environments, suggesting that a flexible, slender body evolved at least twice independently as an adaptation to the interstitial environment.

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

“…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 ].…”

“…Aberrant kinorhynchs are distantly related (cyclorhagid and allomalorhagid) genera (see Sørensen et al [ 40 ]) that share several modifications from the typical kinorhynch body architecture (see [ 27 , 41 ]) most conspicuously having a more elongated trunk, a flexible thin cuticle that folds during movement, and less differentiated trunk segments. Studies of these external divergences and how they relate to the organization of the subjacent organ systems will offer a deeper insight into the evolution of segmentation in kinorhynchs.…”

“…Studies of these external divergences and how they relate to the organization of the subjacent organ systems will offer a deeper insight into the evolution of segmentation in kinorhynchs. Myoanatomical studies on three aberrant genera and species Cateria styx Gerlach, 1956; Franciscideres kalenesos Dal Zotto, Di Domenico, Garraffoni & Sørensen, 2013; and Zelinkaderes yong Altenburger, Rho, Chang & Sørensen, 2015 have recently been carried out by Herranz et al [ 27 ]. Results showed that the trunk muscles follow a segmental arrangement in the three studied species; however, the longitudinal muscles showed a posterior shift in their position in respect to the segmental organization of the tegumental plates.…”

“…This was interpreted to be a consequence of the absence of cuticular thickenings (pachycycli) normally used for muscle attachment in non-aberrant kinorhynchs. Additionally, the elongation of the trunk, the thin cuticle, and less distinct external segmentation was interpreted as an adaptation to the interstitial environment [ 27 ].…”

“…Here, we study the neuroanatomy of three aberrant kinorhynchs with an elongated, worm-like body plan and less distinct segmentation, Cateria styx , Franciscideres kalenesos and Zelinkaderes yong . The myoanatomy of these species was reconstructed by Herranz et al [ 27 ] and the patterns will be compared to their neuroanatomy here examined through immunohistochemistry and CLSM with a standard selection of molecular markers (antibodies against acetylated α-tubulin, FMRFamides, and serotonin). Our aims are furthermore to investigate: (1) whether the nervous system of aberrant kinorhynchs shows a segmental pattern congruent with the external segmentation as in remaining kinorhynchs, (2) or whether there is any posterior shift of the neural components along an anterior-posterior axis mirroring their aberrant myoanatomy, (3) revisit and discuss the spatial alignment of segmental patterns in the cuticle, nervous system and musculature across all studied kinorhynchs, (4) compare and discuss segmentation patterning between kinorhynchs and other segmented phyla.…”