It has been a very upsetting morning. I have been planning for some months now to attend the International Conference of Arachnology in Brazil, booked my plane tickets ages ago, had my vaccinations... and discovered this morning that I have spent the last two months wandering about with the wrong leaving date in my head, and that I was due to leave a week earlier than I thought. So I have missed my plane flight, missed the conference.... There is one word for this. It begins with f and rhymes with 'duck'.
Anyway, if there's any readers who have been paying attention to what goes on here, you may have noticed that there was no 'Taxon of the Week' last week. That was due to things being rather hectic as I tried to organise things for the conference that isn't going to happen in my existence. So I'm introducing last week's taxon today, and the subject of today is the Arthrotardigrada.
Tardigrades are microscopic invertebrates commonly referred to as 'water bears'. I have elsewhere referred to tardigrades as possibly the cutest of all invertebrates, and I see no reason to retract that statement. Some forms put me in mind of nothing so much as little eight-legged versions of Winnie-the-Pooh, bumbling their way through an aquatic Hundred-Acre Wood. The image above shows Renaudarctus psammocryptus Kristensen & Higgins, 1984, and is from the original description. Tardigrades are divided into two main classes, the Heterotardigrada and Eutardigrada - the Heterotardigrada possess cephalic appendages which are lacking in the Eutardigrada, and lack the Malpighian tubules present in eutardigrades (Nelson, 2002). Heterotardigrades have a separate gonopore and anus, while eutardigrades have a single cloacal opening. Most Heterotardigrada also have ventral and dorsal segmental plates, though these have been lost in some families. A third class, Mesotardigrada, has been named for a single species which has unfortunately not been found since its original discovery. In a comment that just makes the reader beg for the back-story, Jørgensen & Kristensen (2004) note that "Mesotardigrades were described from a hot spring in Nagasaki, Japan... however the monotypic class has never been recovered since its original description, and the type locality disappeared just after the Second World War". 'Holotype lost' is an unfortunately not uncommon complaint in taxonomy, but 'type locality disappeared' is definitely unusual.
Arthrotardigrada is one of the two orders of Heterotardigrada (the other is the Echiniscoidea). They possess a median cephalic cirrus, three pairs of lateral cephalic cirri and two or three pairs of clavae (club-shaped appendages on the head) (Kristensen & Higgins, 1984). The suggestion has been made that Heterotardigrada may be paraphyletic with regards to Eutardigrada, and Arthrotardigrada may be paraphyletic within Heterotardigrada (making it the basal assemblage of all Tardigrada), but this remains uncertain. The molecular analysis of Jørgensen & Kristensen (2004) found weak support for a monophyletic Heterotardigrada, but this was not significantly statistically superior to a paraphyletic Heterotardigrada and the authors were only able to test a small number of species. Morphological analysis by Nichols et al. (2006) found a monophyletic Heterotardigrada, but not Arthrotardigrada. Almost all Arthrotardigrada are marine - only a single species is known from freshwater, Styraconyx hallsi (Nelson, 2002).
Perhaps one of the most remarkable features of tardigrades is the ability to enter cryptobiosis, which is a dormant state in which they can sometimes survive long periods of unfavourable conditions. Dormant tardigrades often form a shrivelled cyst called a "tun". All sorts of hyperbolic claims can be found on the internet for the survival abilities of a tardigrade tun, but I haven't yet been able to find a proper source for any of these claims, so they should probably be taken with the contents of a small Siberian salt mine (to steal a phrase from Alan Kazlev).
Jørgensen, A., & R. M. Kristensen. 2004. Molecular phylogeny of Tardigrada—investigation of the monophyly of Heterotardigrada. Molecular Phylogenetics and Evolution 32 (2): 666-670.
Kristensen, R. M., & R. P. Higgins. 1984. A new family of Arthrotardigrada (Tardigrada: Heterotardigrada) from the Atlantic coast of Florida, U.S.A. Transactions of the American Microscopical Society 103 (3): 295-311.
Nelson, D. R. 2002. Current status of the Tardigrada: evolution and ecology. Integrative and Comparative Biology 42 (3): 652-659.
Nichols, P. B., D. R. Nelson & J. R. Garey. 2006. A family level analysis of tardigrade phylogeny. Hydrobiologia 558 (1): 53-60.