Arthrotardigrada

Batillipes sp., copyright C. Schulze & A. Schmidt-Rhaesa.

Belongs within: Tardigrada.
Contains: Styraconyxinae.

The Arthrotardigrada are a primarily marine group of tardigrades with a median cephalic cirrus, three pairs of lateral cephalic cirri and two or three pairs of clavae (Kristensen & Higgins 1984).

The ways of water bears
Published 6 August 2007
Renaudarctus psammocryptus, from Kristensen & Higgins (1984).

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. 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 whereas 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).

Archechiniscus: distinctively indifferent

Published 20 October 2010

Archechiniscus marci, 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).

Systematics of Arthrotardigrada
<==Arthrotardigrada
    |--Archechiniscus Schulz 1953 [Archechiniscidae]R-M86
    |    `--A. marci Schulz 1953R-M86
    |--Tetrakentron [Tetrakentronidae]BK77
    |    `--T. synaptaeBK77
    |--StygarctidaeR-M86
    |    |--Pseudostygarctus McKirdy et al. 1976R-M86
    |    |    `--P. triungulatus McKirdy et al. 1976R-M86
    |    |--Mesostygarctus Renaud-Mornant 1979R-M86
    |    |    `--M. intermedius Renaud-Mornant 1979R-M86
    |    |--Parastygarctus Renaud-Debyser 1965R-M86
    |    |    |--P. higginsi Renaud-Debyser 1965R-M86
    |    |    `--P. sterreri Renaud-Mornant 1970R-M86
    |    |--Megastygarctides McKirdy et al. 1976R-M86
    |    |    |--M. isounguis Renaud-Mornant 1981R-M86
    |    |    `--M. orbiculatus McKirdy et al. 1976R-M86
    |    `--Stygarctus Schulz 1951R-M86
    |         |--S. abornatus McKirdy et al. 1976R-M86
    |         |--S. bradypus Schulz 1951R-M86
    |         |--S. gourbaultae Renaud-Mornant 1981R-M86
    |         `--S. granulatus Pollock 1970R-M86
    |--Batillipes Richters 1909 [Batillipedidae]R-M86
    |    |--B. acaudatus Pollock 1971R-M86
    |    |--B. adriaticus de Zio et al. 1979R=M86
    |    |--B. annulatus de Zio 1962R-M86
    |    |--B. bullacaudatus McGinty & Higgins 1968R-M86
    |    |--B. carnonensis Fize 1957R-M86
    |    |--B. dicrocercus Pollock 1970R-M86
    |    |--B. friaufi Riggin 1962R-M86
    |    |--B. gilmartini McGinty 1969R-M86
    |    |--B. littoralis Renaud-Debyser 1969R-M86
    |    |    |--B. l. littoralisR-M86
    |    |    `--B. l. submersus d’Hondt 1970R-M86
    |    |--B. mirus Richters 1908R-M86
    |    |--B. noerrevangi Kristensen 1978R-M86
    |    |--B. pennaki Marcus 1946R-M86
    |    |--B. phreaticus Renaud-Debyser 1959R-M86
    |    |--B. roscoffensis Kristensen 1978R-M86
    |    |--B. similis Schulz 1955R-M86
    |    `--B. tubernatis Pollock 1971R-M86
    `--HalechiniscidaeR-M86
         |--StyraconyxinaeR-M86
         |--Florarctus Delamare Deboutteville & Renaud-Mornant 1965 [Florarctinae]R-M86
         |    |--F. antillensis van der Land 1968R-M86
         |    |--F. cinctus Renaud-Mornant 1976R-M86
         |    |--F. heimi Delamare Deboutteville & Renaud-Mornant 1965R-M86
         |    |--F. hulingsi Renaud-Mornant 1976R-M86
         |    `--F. salvati Delamare Deboutteville & Renaud-Mornant 1965R-M86
         |--ClavarctinaeB92
         |    |--Euclavarctus Renaud-Mornant 1975B92
         |    |--Clavarctus Renaud-Mornant 1983B92
         |    |--Exoclavarctus Renaud-Mornant 1983B92
         |    `--Moebjergarctus Bussau 1992B92
         |         `--*M. manganis Bussau 1992B92
         |--HalechiniscinaeR-M86
         |    |--Orzeliscus du Bois-Reymond Marcus 1952R-M86
         |    |    `--O. belopus du Bois-Reymond Marcus 1952R-M86
         |    `--Halechiniscus Richters 1908R-M86
         |         |--H. guiteli Richters 1908R-M86
         |         |--H. perfectus Schulz 1955R-M86
         |         |--H. remanei Schulz 1955R-M86
         |         |--H. subterraneus Renaud-Debyser 1959R-M86
         |         `--H. tuleari Renaud-Mornant 1978R-M86
         `--TanarctinaeR-M86
              |--Actinarctus Schulz 1935R-M86
              |    |--A. doryphorusBK77
              |    `--A. lyrophorus Renaud-Mornant 1978R-M86
              `--Tanarctus Renaud-Mornant 1980R-M86
                   |--T. arborspinosus Lindgren 1971R-M86
                   |--T. heterodactylus Renaud-Mornant 1980R-M86
                   |--T. ramazzottii Renaud-Mornant 1975R-M86
                   |--T. tauricus Renaud-Debyser 1959R-M86
                   `--T. velatus McKirdy et al. 1976R-M86

*Type species of generic name indicated

References

[BK77] Barel, C. D. N., & P. G. N. Kramers. 1977. A survey of the echinoderm associates of the north-east Atlantic area. Zoologische Verhandelingen 156: 1–159.

[B92] Bussau, C. 1992. New deep-sea Tardigrada (Arthrotardigrada, Halechiniscidae) from a manganese nodule area of the eastern South Pacific. Zoologica Scripta 21 (1): 79–91.

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.

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.

Pollock, L. W. 1976. Marine Flora and Fauna of the Northeastern United States. Tardigrada. NOAA: Seattle.

[R-M86] Renaud-Mornant, J. 1986. Tardigrada. In: Botosaneanu, L. (ed.) Stygofauna Mundi: A Faunistic, Distributional, and Ecological Synthesis of the World Fauna inhabiting Subterranean Waters (including the Marine Interstitial) pp. 254–262. E. J. Brill/Dr W. Backhuys: Leiden.

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