An integrative description of a new species of marine water bear (Heterotardigrada: Echiniscoididae) from the tidal zone of the east coast of Korea

Tardigrades, or water bears, are microscopic invertebrates renowned for their extraordinary resilience and cryptic diversity. Their microscopic size, paucity of taxonomically distinctive morphological characters and limited molecular data have caused significant taxonomic deflation. Echiniscoides, the most speciose genus in the family Echiniscoididae, also harbours multiple undescribed species. In this study, we describe a new marine tardigrade species, Echiniscoides testudolapis sp. nov. collected from barnacles inhabiting the intertidal zone of Uljin, Korea. The new species was characterized using an integrative taxonomic approach combining detailed morphological, morphometric and molecular analyses, including DIC microscopy, SEM microscopy and molecular phylogenetic reconstructions based on mitochondrial COI sequences. Morphologically, E. testudolapis sp. nov. is distinguished by its dorsal cuticular sculpturing in the form of circular discs with central punctations (weakly elevated processes), the presence of sensory organs on all legs (papilliform on legs I–II, IV and spike-like on legs III) and a dominant claw formula of 9,9,9,8. Notably, our COI analyses revealed substantial intraspecific genetic variability, with 20 haplotypes identified among 28 individuals (p-distances = 0.1–2.8%). In contrast, the examined nuclear markers exhibited low to no genetic variation (p-distances are 0.05%, 0%, 0.2–0.5% and 0.2–0.4% for 18S, 28S, ITS1 and ITS2, respectively). Single nucleotide polymorphism (SNP) analyses indicated that the observed COI polymorphisms are likely functionally neutral. These findings highlight the value of an integrative taxonomic approach in revealing cryptic diversity and provide new insights into the evolutionary processes and biogeographic patterns shaping marine tardigrade diversity.