Sexual dimorphism, allometric morphological changes, and wild diet of the endemic Antarctic tardigrade Milnesium rastrum.

BACKGROUND: The Antarctic continent harbors unique microfauna that evolved independently under extreme conditions. The tardigrade Milnesium rastrum, discovered in East Antarctica, is characterized by atypical, aberrant claw configurations (CC) and pronounced sexual dimorphism. However, its detailed morphology and ecological roles have remained obscure due to an extreme scarcity of specimens. During the 67th Japanese Antarctic Research Expedition (JARE67, 2025-2026), we discovered a moss colony where M. rastrum was abundant near Langhovde, providing a rare opportunity for comprehensive morphometric and ecological analyses. RESULTS: Out of 33 collected individuals, 17 were males, 14 were adult females, and two were juveniles, indicating a balanced sex ratio of approximately 1:1. Significant sexual dimorphism was observed: females were larger (up to 742 μm) than males (up to 549 μm), and males exhibited a more slender posterior body. On leg I, males possessed enlarged, hook-like secondary branches. Males consistently exhibited a {4-4} CC, with a {2-2} pattern on leg I, whereas female body length showed a positive correlation with the number of points on the secondary branches, with larger individuals showing as many as seven points. Intestine content analysis revealed remnants of Hypsibiidae claws and Diphascon-like long buccal tubes. This provides direct evidence that M. rastrum preys on the sympatric tardigrade Diphascon langhovdense in its natural habitat. CONCLUSIONS: This study provides the first comprehensive characterization of sexual dimorphism, growth dynamics, and the wild diet of M. rastrum. Our findings clarify distinct morphological variations between sexes and suggest different developmental strategies regarding molting. Additionally, confirming D. langhovdense as a natural prey source clarifies the specific ecological role of M. rastrum as a predator in the Langhovde microfauna community. Overall, these insights fundamentally advance our understanding of the morphological adaptations and ecological interactions of endemic tardigrades in extreme Antarctic environments.