Rhizocephalans are actually fairly close relatives of barnacles, both of them belonging to a group of crustaceans called Thecostraca. Though the different thecostracan subgroups are very different in adult morphology, they are united by their similar larval morphology. As well as the standard crustacean nauplius larva, thecostracans have an additional larval stage known as a cypris larva, a motile stage with specialised sensory structures that the larva uses to seek out a suitable host or substrate to attach to and develop into the adult. As well as barnacles and rhizocephalans, the Thecostraca includes another few small crustacean groups, the Ascothoracida, the Acrothoracica, and the Facetotecta. Acrothoracica or burrowing barnacles burrow into hard substrates such as mollusc shells, other free-living barnacles, corals or limestone. Ascothoracida are minute parasites of molluscs and other marine animals. But I thought I'd reply to Deep Sea News' post by writing something on the last group, the Facetotecta.
The infraclass (or subclass, or whatever you want to call it) Facetotecta contains a single genus, Hansenocaris. Despite being discovered well over a hundred years ago, Hansenocaris remains, in many regards, very little known. The main thing we don't know about Hansenocaris is what it actually looks like. So far, this group of thecostracans is known only from distinctive larvae referred to as "y-larvae"* - the adult form is a complete mystery. The most distinctive feature of the y-larva is its large univalved head shield, whose faceted nature is the source of the name "Facetotecta". Both nauplius and cypris stages have been collected and well-studied, but that's as far as it goes (the photo above, from Høeg & Kolbasov, 2002, shows an SEM of a y-cypris). Certain features of the cypris larva's morphology suggest that, like most other thecostracan subgroups, Hansenocaris becomes parasitic at maturity (as I noted in a comment at Deep Sea News, the non-parasitic barnacles are actually the odd ones out here), but no-one knows on what.
*Why "y-larvae"? That I couldn't tell you, but according to Ponomarenko (2006) the name dates back to their original description by Hansen in 1899.
One suggestion that has been made is that y-larvae may fit into the little-known sexual phase of the life cycle of the Tantulocarida (Ponomarenko, 2006). Tantulocarids are another group of ectoparasitic crustaceans (living on other crustaceans) that are believed to be the sister group of the thecostracans, and are another group of animals that contend for the title of "almost too stupidly bizarre to be believable". The tantulocarid life cycle is unique in lacking the usual moulting stages of all other crustaceans - instead, the mature adult actually develops within the attached parasitic tantulus larva (Boxshall & Lincoln, 1997). Check out the diagram below from Boxshall & Lincoln (1997) showing this process. Personally, I have a hard time thinking of this as development from larva to adult (despite all the papers describing it as such) - it looks more like a reproductive process where the adult develops asexually by a sort of internal budding from the tantulus. Reproduction in tantulocarids is either asexual or sexual (Huys et al., 1993) - in the asexual phase, the tantulus larva swells into a sac filled with developing eggs that are believed to be retained to hatch out into fully-developed tantulus larvae. In the sexual phase, as shown, the larva gives rise to a single mature adult. What happens once the mature male is released into the world is unknown, but as the male lacks functional mouthparts it undoubtedly dies after finding a mate. The sexual female described by Huys et al. (1993) was still attached to the host via the umbilicus, the nutrient-delivering tube you can see in fig. 26 below, and it seems possible that it remains so attached for all or most of its life, continuing to draw nutrients from the host to nurse its developing eggs. Huys et al.'s female, unfortunately, was carrying only immature eggs, so whether the eggs are retained to the tantulus stage as in the parthenogenetic phase, or hatch into more standard nauplius larvae, remains unknown.
This period of ignorance does give a window for identifying y-larvae with tantulocaridans. The idea is also not without phylogenetic merit - tantulocaridans are regarded as the sister group to Thecostraca, while most phylogenetic analyses place Facetotecta as fairly basal thecostracans. However, the identification does not seem to be well-accepted*. Most significantly, the tantulus larva of Tantulocarida has no sign of lattice organs. Lattice organs are specialised sensory organs on the carapaces of thecostracan cypris larvae, including y-larvae, that are believed to function in helping the cypris find a suitable substrate for attachment** (Høeg & Kolbasov, 2002). It would be surprising if sexually-produced dispersing larvae of tantulocaridans were to possess lattice organs but asexually-produced dispersing larvae (that surely would have just as much use for them) did not.
*Google Books can be very frustrating. The Google Book preview for Scholtz (2004) allows me to read that "there are strong arguments against a tantulocaridan-facetotectan relation" at the end of page 209, but then page 210, where I would have undoubtedly been told about said arguments, is not part of the preview. Gah!
**However, Lange & Schram (2002) identified apparent sensory structures on the carapace of fossil thylacocephalans as possibly homologous to lattice organs, despite the believed free-living nature of thylacocephalans. Mind you, the strange morphology of Thylacocephala was so highly derived that it's pretty much anyone's guess just what they were up to.
Which brings us firmly back to square one. The adult form of Hansenocaris remains one more example of just how little we know of the marine environment. It's out there somewhere - the question is where.
REFERENCES
Boxshall, G. A., & R. J. Lincoln. 1987. The life cycle of the Tantulocarida (Crustacea). Philosophical Transactions of the Royal Society of London Series B - Biological Sciences 315 (1173): 267-303.
Høeg, J. T., & G. A. Kolbasov. 2002. Lattice organs in y-cyprids of the Facetotecta and their significance in the phylogeny of the Crustacea Thecostraca. Acta Zoologica 83: 67-79.
Huys, R., G. A. Boxshall & R. J. Lincoln. 1993. The tantulocaridan life cycle: the circle closed? Journal of Crustacean Biology 13 (3): 432-442.
Lange, S., & F. R. Schram. 2002. Possible lattice organs in Cretaceous Thylacocephala. Contributions to Zoology 71 (4): 159-169.
Ponomarenko, E. A. 2006. Facetotecta - unsolved mystery of marine biology. Russian Journal of Marine Biology 32 (Suppl 1): S1-S10.
Scholtz, G. 2004. Evolutionary Developmental Biology of Crustacea. CRC Press.
an almost fungal-looking structure that spreads through their crustacean host
ReplyDeleteNature has been reading too much Lovecraft. (I swear, that's almost verbatim from The Whisperer In the Dark.)
It is really, really unpleasant reading long stretches of white text on a black background. Especially the after image that happens when tabbing back to the rest of the Net, which is mostly black text on white. Unfortunately, your RSS feed is also truncated. If it were a full feed, I could just read your otherwise excellent posts with my own style sheet. As it is, I have to give up on this blog for now. It is really just too uncomfortable to read.
ReplyDeleteUnfortunately, your RSS feed is also truncated.
ReplyDeleteThat's odd - I subscribe to the feed myself through Google Reader so I can see what it looks like to other readers, and I always get the full post. I've got my settings on full feed. Is there anything amiss with your personal reader settings (what reader do you use, for that matter)? Does anyone else have this problem?
Personally, I don't notice any difficulty with reading white on black - indeed, I prefer the look of white on black, that's why I use it. I also don't notice the after-image you complain about. Would anyone else like to comment?
what do we know about the evolutionary history of Thecostraca? It seems too coincidental to me that adult members of this group are all non-free-living - they're either found attached to some other critter as a parasite or attached to some abiotic substrate. I wonder if parasitism could pave the way for barnacle attachment. Or vice versa? There seems to be a lot of head-to-substrate specialization going on in this group.
ReplyDeleteThe phylogeny in Høeg & Kolbasov (2002) seems to be fairly typical. The Rhizocephala and Thoracica (barnacles proper) are sister groups, followed by the Acrothoracica (burrowing barnacles). Those three groups together form the Cirripedia, by the way. It is unclear whether Hansenocaris or Ascothoracida is the basalmost branch in the Thecostraca.
ReplyDeleteConsidering the relatively nested position of Thoracica among parasites, plus the fact that the Thecostraca's closest relatives in the Tantulocarida are also parasites, I find it intriguingly suggestive that barnacles may have once had a parasitic ancestry, or at least an epizoic ancestry like Acrothoracica. While Acrothoracica burrow into other animals such as corals, I haven't found any indication that they actually feed directly on their hosts.
Awesome post christopher!
ReplyDeleteI get the full feed in my Google Reader too. I have had comments on my blog too from a couple readers who described themselves as older that the white on dark blue was hard to see, one these I'll give my blog a make over.
Jim, there is a brand spanking new phylogeny of thoracican barnacles, blogged over at yours truly last month! Of course this excludes the Acrothoracica, can't have it all!
I used to not be disturbed by white text on black either, but that was when I had a 20-inch CRT monitor. Now I have a 15-inch LCD, and suddenly white-on-black has become impossible! Maybe this is a factor in the disagreements over whether the color scheme is legible or not?
ReplyDeleteFor more than a century, crustaceans called facetotectans appeared to live in a biological Neverland: nobody had ever seen an adult member of the species, until now.
ReplyDeletehttp://blogs.discovermagazine.com/discoblog/2008/05/21/i-dont-want-to-grow-up-im-a-parasitic-crustacean/