Homology of the head sensory structures between Heterotardigrada and Eutardigrada supported in a new species of water bears (Ramazzottidae: Ramazzottius)

Abstract

Phylum Tardigrada is represented by microscopic eight-legged panarthropods that inhabit both terrestrial and marine environments. Although tardigrades are emerging model animals for the research including physiology, evolutionary biology and astrobiology, knowledge of their external morphology is still insufficient. For instance, the homologies between marine and terrestrial relatives largely remain unexplored. In this study we provide detailed pictures of the head sensory organs present in a new tardigrade, Ramazzottius groenlandensis sp. nov. The specimens were collected from a mixed moss and lichen sample on Ella Island, East Greenland. The new species differs from other congeneric species in having polygonal sculpturing on the dorsal cuticle, which is accentuated in the posterior region of the body, a lateral papilla on the leg IV, and distinctive egg morphology. A Bayesian phylogenetic analysis (18S rRNA + 28S rRNA + COI) confidently placed the new species within the genus Ramazzottius. Interestingly, the new species shows a full set of well-developed cephalic organs, which correspond to all sensory fields found in eutardigrades. Details on the full set of head organs were present only for heterotardigrades. The surface of these organs is covered with small pores, presumably playing a sensory role. This discovery suggests the homology of head sensory structures between heterotardigrades and eutardigrades, implying that the distinctive arrangement and positioning of sensory organs on the head was a plesiomorphic feature of tardigrades. Moreover, we found that the Ramazzottius oberhaeuseri morphotype forms a morphogroup, not a monophyletic species complex.