Belongs within: Euarthropoda.
Contains: Rhizocephala, Thoracica.
The Thecostraca are a crustacean clade containing barnacles (Thoracica) and their close relatives. Within the clade, Ascothoracida and Rhizocephala are parasitic lineages, while Acrothoracica are sessile filter-feeders that burrow into the surface of an animal host. Hansenocaris is known only from juvenile specimens; adults are unknown but are perhaps also parasitic. Despite their divergent appearance as adults, members of the Thecostraca are united by characters of the larvae. The Acrothoracica, Rhizocephala and Thoracica are further united in a clade Cirripedia by features including modification of the antennules in the cypris larva for use in exploratory walking while seeking a settlement site, and in permanent cementation using glandular secretions. In contrast, cypris larvae of the Ascothoracida attach to their intended host by means of direct grasping with the antennules.
The secret of Y-larvae
Published 23 February 2008
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. Certain features of the cypris larva’s morphology suggest that, like most other thecostracan subgroups, Hansenocaris becomes parasitic at maturity 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 develops from the attached parasitic tantulus larva (Boxshall & Lincoln 1987). 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 below, 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.
*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.
Forcing out the secret
Published 22 May 2008
Though environmental collections of facetotectans have so far yielded on nauplii and cyprids, a significant step closer to their adult identity was taken by 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 whereas 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.
Systematics of Thecostraca
Synapomorphies (from Grygier 1987): Nauplii lacking post-maxillular limb buds; compound eyes with less than four crystalline cones per ommatidium; strongly developed frontal filaments associated with eyes.
<==Thecostraca | i. s.: RhamphoverritorLSE13 |--Hansenocaris [Facetotecta]G87 | |--H. acutifronsG87 | |--H. hanseni [=Proteolepas hanseni]G87 | |--H. pacificaG87 | `--H. rostrataG87 |--Cirripedia [Cirrhopoda]G87 | | i. s.: MitellaG20 | | |--M. pollicepsG20 | | `--M. spinosaO81 | | Epopella plicataHS01 | | Notomegabalanus decorusHS01 | | Lepadites Blumenbach 1803TMNZ64 | | Pentalasmis vitreaH04 | | ‘Amalthea’ coxi Sowerby 1908H08 | | Hesperibalanus hesperiusO10 | | Tubicinella balaenaeG20 | | Branta auritaG20 | | Arcoscalpellum novaezelandiaePG98 | |--RhizocephalaLS02 | |--ThoracicaLS02 | `--AcrothoracicaLS02 | |--Trypetesa Norman 1903H75 [Apygophora, TrypetesidaeMD01] | `--PygophoraMD01 | |--CryptophialidaeMD01 | `--LithoglyptidaeMD01 `--AscothoracidaG87 | i. s.: Ulophysema oeresundense Brattström 1936BK77, H75 | Waginella sandersiG87 | ParascothoraxG87 | Gorgonolaureus muzikaeG87 | Endosacculus Voigt 1959H75 | `--*E. moltkiae Voigt 1959H75 |--LauridaMD01 | |--PetrarcidaeMD01 | |--LauraA99 [LauridaeMD01] | | `--L. bicornutaA99 | `--Synagoga Norman 1888LS02 [SynagogidaeMD01] | |--S. miraA99 | `--S. sandersiA99 `--DendrogastridaMD01 |--Ctenosculum Heath 1910BR05 [CtenosculidaeMD01] | `--*C. hawaiiense Heath 1910BR17 |--AscothoraxBK77 [AscothoracidaeMD01] | `--A. ophioctenisBK77 `--DendrogasterBK77 [DendrogastridaeMD01] |--D. astericolaBK77 |--D. dichotomusBK77 |--D. dogieliBK77 |--D. murmanensis [=Myriocladus murmanensis]BK77 `--D. rimskykorsakowiBK77 Boring: Trypetesa caveata Tomlinson 1963H75
*Type species of generic name indicated
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