Archechiniscus marci. Figure from Pollock (1976).
Archechiniscus is a genus of three species of marine tardigrade found in littoral habitats. They can be readily distinguished from other marine tardigrades by their unique arrangement of claws: two pairs, with the internal pair on the end of a long pair of toes but the external pair set directly onto the foot. Most of the other distinguishing features of Archechiniscus are more negative: they lack conspicuous segmentation or ornamentation. The presence of cephalic appendages marks Archechiniscus as belonging to the heterotardigrades rather than the eutardigrades; within the Heterotardigrada, it belongs to the paraphyletic 'arthrotardigrade' group. Opinions have differed as to whether it should be placed in the family Halechiniscidae or in its own separate family; Jørgensen et al. (2010) identified the broad Halechiniscidae as polyphyletic and plumped for placing Archechiniscus in its own family (though potentially as the sister group of their more restricted Halechiniscidae).
As for most marine tardigrades, there doesn't appear to be a great deal of info about the lifestyle of Archechiniscus. Archechiniscus symbalanus got its name due to being collected in association with barnacles (Chang & Rho, 1998) but I don't know what it was doing there. As littoral inhabitants, Archechiniscus are resistant to a higher degree of desiccation than other marine tardigrades (Jönsson & Järemo, 2003) but do not show the extremes of resistance found in some other tardigrades (remember, not all tardigrades are resistant to adverse conditions, and not all tardigrades are resistant to the same adverse conditions).
Chang, C.-Y., & H.-S. Rho. 1998. Three new tardigrade species associated with barnacles from the Thai coast of Andaman Sea. Korean Journal of Biological Sciences 2: 323-331.
Jönsson, K. I., & J. Järemo. 2003. A model on the evolution of cryptobiosis. Annales Zoologici Fennici 40: 331-340.
Jørgensen, A., S. Faurby, J. G. Hansen, N. Møbjerg & R. M. Kristensen. 2010. Molecular phylogeny of Arthrotardigrada (Tardigrada). Molecular Phylogenetics and Evolution 54 (3): 1006-1015.
Pollock, L. W. 1976. Marine Flora and Fauna of the Northeastern United States. Tardigrada. NOAA: Seattle.
I'm mildly chuffed that I got the order right, because now I can reveal that I don't rightly know what a 'digit' in a claw is. Is it just a spur on the claw (i.e. nonmovable?). I am in doubt because I don't know if taxonomists use 'digit' in the same way for other arthropods. For example, in spiders with comb-like structures on the tarsal claws, what do you call the individual spines of the comb?
For example, in spiders with comb-like structures on the tarsal claws, what do you call the individual spines of the comb?
Teeth, normally. Even if there's only one tooth. However, the tardigrade papers that I read over while researching this post referred to the ventral 'tooth' on the outer claws of this genus as a 'spur'.
Ah - I have realised my error - it came from the wording of the overview paper I was reading - "The class Eutardigrada includes the unarmored orders Apochela (terrestrial) and Parachela (primarily terrestrial and freshwater, with a few marine species); their legs terminate in claws without digits". So I assumed there were 'claws with digits' and claws without. In fact, legs terminate either with claws, or with digits + claws. As for getting the order right... that was a fluke!