Symbion americanus Obst, Funch and Kristensen, 2006
With its formal description in 2006, Symbion americanus, found living on the mouthparts of the American Lobster (Homarus americanus), became the second species described from the phylum Cycliophora--which was itself first described only a decade earlier (Funch and Kristensen 1995; Obst et al. 2006). (An apparently distinct third cycliophoran species lives on the mouthparts of the European Lobster [Homarus gammarus]; Obst et al. 2005). Available evidence suggests that S. americanus may in fact include several cryptic species (Obst et al. 2005, 2006; Baker and Giribet 2007; Baker et al. 2007).
All three known hosts of Cycliophora are members of the lobster family Nephropidae. Reports of cycliophorans on nematodes and non-nephropid crustaceans (e.g., copepods) are apparently all in error and instead are based on observations of chonotrich ciliates. Examination by transmission electron microscopy is required to see that, in contrast to a cycliophoran, the ciliate consists of just a single cell with several nuclei. (Kristensen 2002)
Cycliophorans have a very complex life cycle that alternates between sexual and asexual phases. The most prominent stage is the asexual and sessile feeding stage, which lives attached to the setae of the host lobster's mouthparts and filters small food particles from the water. For a detailed description of the complex life cycle of Symbion pandora, see General Description on the S. pandora page; for a whimsical but informative account, check out the CreatureCast podcast on this topic.
A growing body of evidence suggests that the sister group to the Cycliophora is the Entoprocta (=Kamptozoa) (Fuchs et al. 2010 and references therein), consistent with the suggestion made by Funch and Kristensen (1995) in their original description, although cycliophorans share many similarities with the Rotifera and some molecular analyses have indicated a close relationship between these two groups (e.g., Winnepenninckx et al. 1998).
The phylum name Cycliophora is derived from Greek roots meaning "wheel bearing", referring to the circular mouth ring. The genus name Symbion is derived from Greek roots meaning "living together", referring to this animal's intimate association with its lobster host. The specific epithet americanus is a reference to its host, the American Lobster.