Archechiniscus: Distinctively Indifferent

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).

REFERENCES

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.

Return of the Water Bears (Taxon of the Week: Tardigrada)

False colour SEM image of two tardigrades, from here.

If you're a long-time reader of this site, you're probably already aware of the existence of tardigrades or water bears, microscopic stumpy-legged invertebrates. Previous posts on Tardigrada have given an overview of the main subgroups of tardigrades, and suggested how you might find your own specimens. The next logical step, I suppose, would be to say a few things about tardigrade ecology, and for that I shall draw heavily from the excellent reviews of Nelson & Marley (2000) and Nelson (2002).

Tardigrades may live in salt water, fresh water or terrestrially among mosses and leaf litter. However, because all tardigrades require at least a film of water to live in, the boundary between freshwater and terrestrial species is a trifle blurry and many species can be found in both. Tardigrades feed on plants and algae; their mouthparts have a piercing stylus through which they suck the cytoplasm out of cells. Different techniques are used for collecting marine and limno-terrestrial species, and I mention that solely because it gives me an opportunity to note that one of the methods for collecting marine tardigrades (and other sand-dwelling meiofauna) involves sieving material through a fine mesh net referred to as "Higgins' mermaid bra" (or, depending on author, "Gwen's mermaid bra", as it was Mrs Higgins who invented the tool used by her husband).


Close-up of the head of the tardigrade Macrobiotus. The stylet apparatus is visible inside the head; the stylets are everted when the animal is feeding. Photograph by Martin Mach.

In one of my earlier posts, I referred to the well-known ability of tardigrades to form resistant tuns when exposed to unfavorable conditions, a process called cryptobiosis. What I did not explain at that time was that five different types of cryptobiosis have been identified in tardigrades: encystment (production of a dormant phase without significant water loss), anoxybiosis (resistance to low oxygen levels), cryobiosis (resistance to freezing temperatures), osmobiosis (resistance to elevated salinity) and anhydrobiosis (resistance to desiccation). Not all tardigrades share all five resistances - for instance, anhydrobiosis (the best-known form) is only found among terrestrial tardigrades - and different species will have different degrees of resistance. Much has been made of the resilience of at least some tardigrade tuns, such as their ability to survive immersion for up to eight hours in liquid helium at -272°C (Rebecchi et al., 2007; for comparison, absolute zero is calculated to be -273.15°C) and even to survive exposure to the vacuum of space (Jönsson et al., 2008). However, the often-repeated claim that tardigrade tuns can survive for more than one hundred years seems to be unsupported (Jönsson & Bertolani, 2001, reviewed the 1948 report generally cited in support of this claim and found that the tuns tested in that report in fact failed to revive); tuns have not yet been definitely shown to survive for more than ten years.



Cryobiosis, the ability to withstand freezing, allows tardigrades to inhabit cryoconite holes like the one shown above in a photo from here. Cryoconite holes develop when darkly-coloured dust accumulates in patches on a sheet of ice; the increased heat absorption by the dark dust melts the surrounding ice, forming a small patch of liquid water. This water may then become home to bacteria, algae and other microscopic organisms released by the melting ice - a self-contained microscopic ecosystem where a nematode may be the most fearsome predator in town. The cryoconite hole may freeze up again when the winter comes, of course, but its inhabitants can wait in the ice for the sun to come again.

REFERENCES

Jönsson, K. I., & R. Bertolani. 2001. Facts and fiction about long-term survival in tardigrades. Journal of Zoology 255 (1): 121-123.

Jönsson, K. I., E. Rabbow, R. O. Schill, M. Harms-Ringdahl & P. Rettberg. 2008. Tardigrades survive exposure to space in low Earth orbit. Current Biology 18 (17): R729-R731.

Nelson, D. R. 2002. Current status of the Tardigrada: evolution and ecology. Integrative and Comparative Biology 42 (3): 652-659.

Nelson, D. R., & N. J. Marley. 2000. The biology and ecology of lotic Tardigrada. Freshwater Biology 44 (1): 93-108.

Rebecchi, L., T. Altiero & R. Guidetti. 2007. Anhydrobiosis: the extreme limit of desiccation tolerance. Invertebr. Survival J. 4: 65-81.

My First Tardigrades

Well, the lab course I've been tutoring reached the point yesterday that I'd been waiting for ever since I found out I'd be tutoring it - I've now seen my first tardigrades! I covered the generalities of tardigrades last year, but as I've noted before, there is nothing to compare with seeing an organism that you've previously only known from the literature in real life. And I have to say - tardigrades are just as adorable as I'd always imagined them to be. If not more so.


Many tardigrades store their eggs underneath the cuticle, then shed them along with the cuticle when moulting. This photo by William West, from here, shows a specimen of Milnesium tardigradum in the act of doing so.

While I couldn't tell you exactly what species they were, but they looked much like the photo (from here) at the top of this post. Little sausage-shaped animals with four pairs of little stumpy legs, on which they bumbled about in a decidedly endearing manner. I wasn't the only person impressed, either - I heard more than one student exclaiming aloud how cute they were when they looked down the microscope. When you've been trying a week previously to encourage enthusiasm in students about nematodes, it's certainly nice to have an organism that pretty much sells itself.



If you'd like to see your own tardigrades, collect some moss or lichen and sit it in a petri dish with some water (use distilled water or rainwater, not tap water). Leave it there for enough time for the tardigrades to become active and crawl out of the moss. (While they may be found in terrestrial habitats, tardigrades require at least a film of water to move in. If they dry out, they either die or enter a dormant resistant stage known as a tun.) After 3-24 hours, pipette some water out of the base of the dish (tardigrades don't swim, so they settle to the bottom) into another dish or watchglass. Place your pipetted sample under a stereo microscope, and you should be able to see the tardigrades crawling around on the bottom! (As well as other small organisms such as rotifers, nematodes and possibly protozoa.) If you want a closer look, you can transfer a tardigrade onto a slide using a micropipette. A drop of alcohol added to the slide will knock the tardigrade out. Take a look at the Microbial Life page for more details.

However, in the interests of safety, I feel I must draw your attention to the warning given by www.tardigrades.com (from whence comes the Victorian-style illustration above):

Please note the following mental and health risks: in some case addictive behaviour towards tardigrades has been noted. And, even worse, young people showed an increased interest in non-commercial, zoological and even philosophical topics. As a rule excited readers can be successfully calmed down by means of scholarly biology lectures, e.g. featuring the properties of allium cepa or the difference between mitosis and meiosis. Please note that it might be unwise to mention tardigrades in presence of those biology teachers who have never heard of them. We do not want to be held responsible for nervous breakdowns or any other possible consequences that might be caused by tardigrade abuse.

Oh #$%^& Me - It's the taxon of the (last) week

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).

REFERENCES

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.