Milnesium (Tardigrada: Apochela) having elongated buccal tube from Mt. Yukikuradake (Toyama and Niigata prefecture, Japan)

The order Apochela (Tardigrada: Eutardigrada) includes Milnesium and three monospecific genera Limmenius, Milnesioides, and Bergtrollus. These three genera have elongated buccal tubes and are found from limited areas in the world: Australia, New Zealand, and the Lyngen Alps in Norway, respectively. The existence of such milnesiids having elongated buccal tube in Japan was once reported at a conference but it has not been described further.

We found the undescribed species again in summer 2023. From a moss sample on the summit of Mt. Yukikuradake, we obtained milnesiid specimens having a narrow, elongated buccal tube. Although the morphological data corresponded to the genus Bergtrollus, molecular phylogeny clearly indicated that the species belonged to the genus Milnesium; therefore, we describe it as Milnesium utsugii sp. nov. Molecular phylogenetic analysis also showed that Milnesioides spp. and M. utsugii sp. nov. were clearly delimited, suggesting that the elongated-mouth evolved at least two times in the family Milnesiidae. Hatchlings of the new species had higher buccal tube width/length ratio compared to the adults, indicating a developmental change to a longer and narrower shape.

This is the fourth species of the genus Milnesium in Japan. Interestingly, the moss sample at the type locality of the new species housed another undescribed Milnesium which was placed within a clade with an undescribed Norwegian species, implying a relationship in zoogeographic histories between M. utsugii sp. nov., Bergtrollus, and the undescribed Milnesium.