Greenlandic water bears reveal a new morphological trait of external head sensory organs

Tardigrades are microscopic panarthropods renowned for their ability to undergo cryptobiosis. While integrative taxonomy of tardigrades has been intensively applied in the description of tardigrade species over the past two decades, many details of their external morphology remain poorly recognized and under-described. This limitation is largely due to their small size and the limited morphological features useful for classical taxonomy. In particular, compared to heterotardigrades, the external sensory structures of eutardigrades are less studied. In this study, we present a detailed morphological analysis using FE-SEM of two Greenlandic apochelan species, Milnesium grandicupula sp. nov. and M. cf. longiungue. We identified several previously unobserved anatomical structures in milnesiids, including minute sensory structures in the cephalic region. Detailed microphotographs revealed that apochelan tardigrades also possess a set of external sensory structures on the head sensory fields, similar to those observed in other major evolutionary lineages within tardigrades. This finding corroborates the hypothesis of the homology of head sensory organs between eutardigrades and heterotardigrades. Notably, these small sensory structures exhibit a distinctive pattern: a relatively large central organ surrounded by smaller pores, which may be comparable to the sensory dorsal organ (SDO) of crustaceans and the cephalic median organ (CMO) of trilobites.