Field of Science

Do giant deep sea isopods have protist endosymbionts?

Apparently not as of this [admittedly slim] description of intestinal microbes from 1982. Seems to be mostly bacteria and nematodes - though one wonders if there are some cool gregarine or something inside one of those.

On the topic of isopod gut endosymbionts (though not of the deep sea), there's a recent PNAS paper starring a wood-boring isopod devoid of cellulose-digesting gut microbes, capable of lignocellulose digestion all on its own! (King et al. 2010 PNAS "Molecular insight into lignocellulose digestion by a marine isopod in the absence of gut microbes") Limnoria quadripunctata, quite amazingly, actually lacks gut microbiota entirely (unlike the bivalve woodboring 'shipworms', which do have a flourishing gut culture) and as animals are known for failing at lignocellulose digestion on their own, raised some interesting questions about how they do it.

Turns out, Limnoria not only has unusually high glycosyl hydrolase (GH) expression levels in its transcriptome, but also the first described case of endogenously produced GH7 in metazoa. This was then followed up by finding GH7 expression in the Daphnia and Gammarus(both also crustacea) ESTs. Seems like the GH7 domain is key to self-sufficient cellulose digestion, in animals anyway. Another question did it get there? LGT from somewhere? The authors consider recent LGT unlikely as the metazoan sequences are quite distant from anything else in tree (and don't fall in the midst of another clade; though there's arguably too few sequences to judge at this point).

Thus, it seems that what most strikingly enables the gastromicrobially deprived Limnoria to digest wood on their own is GH7 (and potentially hemocyanins). Curiously, the most abundant cellulases in the termite gut, produced mostly by its protist symbionts, are GH7.

This little adventure, like most others on this blog, was completely unplanned and sporadic. Someone brought up deep sea isopods, thus I couldn't just ignore that very important question of ultra high priority. Mostly because giant isopods are fucking cool. Oh, and giant. Would make an adorable, wonderful pet, if it weren't for the whole deep sea thing.

Source: Ross Gwynn on Reddit. 2.5 feet long, or roughly 75cm, though that may be exaggerated.


  1. I've only briefly skimmed the paper, but I'm bothered by the fact that the authors didn't appear to do any controls to test for microbial DNA in their cDNA library. The original paper which concluded that the digestive tract of Limnoria was free of microbes is dated 1978. Thus, it's unlikely that they used DNA probes in their study. I didn't see if any papers have been released more recently to confirm that result. Still, I would have expected the authors of this paper to at least test for microbial contamination.

  2. Yeah I had some doubts about it too, but it was like 3am or something and the whole expanded GH7 transcriptome argument sounded acceptable at the time. Thinking about it again, it seems fishy -- the GH7 may well come from some cryptic endosymbiont, and it being found in a confined clade of crustaceans does not mean it is of metazoan origin -- may well be a case of endosymbionts cospeciating with the metazoan lineages and forming a clade that appears to belong to metazoans. The group of hypothetical cryptic endosymbionts may be extremely derived from their relatives and thus not branch with anything, but their trees are poorly sampled, in my completely amateur opinion. I'm not saying I totally disbelieve them, but perhaps the first reported case of an autonomously cellulose-digesting metazoan should be a bit better supported than it is there...

    Why do papers so often look a lot worse on a second read? ^^

  3. It's from PNAS, that should raise a warning flag straight off. Also, you had to look in their supplemental data to find their methods for RNA extraction and cDNA synthesis, which really bugs me, because it's critical information that the reader needs to evaluate the validity of the paper.


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