Deep Sea News has some neat pictures of a rhizocephalan for you to look at. Rhizocephalans are definitely one of the stranger parasitic crustaceans, with an almost fungal-looking structure that spreads through their crustacean host, and its machiavellian hijacking of the host's reproductive system for its own ends. Seriously, take a look, though you may want to wait a little if you've just had breakfast.
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.
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.
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