A few months ago, I wrote a post on the mysterious y-larvae or Hansenocaris (Facetotecta), distinctive crustaceans known only from their larval form and of unknown adult morphology. I've just been informed of a significant step that has been taken towards solving this mystery (Glenner et al., 2008).
One way to discover the adult form of Hansenocaris would be to rear larvae through to adulthood. However, so far it has not been possible to rear y-larvae past the cypris stage (y-larvae belong to a group of crustaceans called Thecostraca, also including barnacles, that hatch out as a nauplius larva, which eventually transforms into a cypris larva, followed by other larval stages or adulthood). Glenner et al. have broken that barrier by exposing cypris y-larvae to the moulting hormone 20-hydroxyecdysone (20-HE). Exposure to this hormone induced the cyprids to moult through to the next stage in the life cycle.
The appearance of this next stage certainly goes some way to explaining why adult facetotectans have not yet been recognised. Gone are the swimming appendages and arthropod segmentation of the cypris. Instead, the y-larvae moults into a limbless, worm-like organism that wriggles vigorously. This worm-like creature is without a properly developed digestive system or other such extravagances, and about the only feature suggestive of its arthropodan nature are the disorganised and degenerate remnants of a pair of compound eyes. Overall, the emerged organism (dubbed an "ypsigon" by Glenner et al.) bears a significant resemblance to the vermigon larva described for some members of another group of the Thecostraca, the parasitic Rhizocephala. Ypsigons kept in culture for 24 hours underwent another moult (remaining as an ypsigon), but no specimens were kept alive beyond that stage. I am inclined to wonder whether the laboratory-induced form is truly the same as what would emerge in the wild or whether the growth hormone adversely affected the larvae's development, but Glenner et al. do indicate that rhizocephalan vermigons induced in the lab are comparable to those occurring naturally.
Loss of derived arthropod characters is not uncommon among endoparasitic crustaceans - the Rhizocephala and Pentastomida are two particularly extreme examples, and certain features of the y-larvae cypris had already lead researchers to suspect that the adult might be parasitic. Unfortunately, it is still unknown what the host of that parasitic adult might be. It is also worth stressing that, contrary to what has been implied on news releases, the ypsigon is not the adult, but an additional larval stage, probably (by analogy with the rhizocephalan vermigon) representing the stage at which the y-larva enters its host. Identification of the full adult form (which may or may not resemble the vermigon) will probably require the identification of that host. Despite the similarities between the facetotectan ypsigon and the rhizocephalan vermigon, the two groups are not believed to be each other's closest relatives within the Thecostraca (Facetotecta are a basal branch, while Rhizocephala are closer to barnacles - Høeg & Kolbasov, 2002), suggesting that this derived parasitic form has developed independently in the two groups. We eagerly await any discovery that will reveal the final clue to this mystery and present with a fully adult facetotectan in all its slimy glory.
Glenner, H., J. T. Hoeg, M. J. Grygier & Y. Fujita. 2008. Induced metamorphosis in crustacean y-larvae: Towards a solution to a 100-year-old riddle. BMC Biology 6: 21.
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.