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 is...how 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.