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Symbion is the only genus in the phylum Cycliophora, which was recognized only in 1995 with the description of Symbion pandora from specimens living on the mouthparts of Norwegian Lobsters (Nephrops norvegicus) in the North Atlantic (Funch and Kristensen 1995). Interestingly, the sessile feeding stage of S. pandora has been known since the 1960s, but was not described until 1995 (Funch and Kristensen 1997, cited in Kristensen 2002).
The cycliophoran body is divided into an anterior buccal funnel, an oval trunk, and a posterior acellular stalk and adhesive disc by which the animal attaches itself to setae (flexible hair-like projections) on the host's mouthparts. Females are about 350 µm long and 100 µm wide. They are suspension feeders, obtaining food by creating water currents with dense cilia around the open end of the buccal funnel. The U-shaped gut is ciliated along its entire length, ending with an anus located near the base of the buccal funnel. Circulation and gas exchange are presumably accomplished by simple diffusion. Obst and Funch (2003) reported S. pandora population densities ranging from fewer than 100 to more than 500 feeding stages per mouthpart.
Symbion (and the phylum Cycliophora) currently includes just two described species: Symbion pandora Funch and Kristensen, 1995 from the mouthparts of the Norwegian Lobster (Nephrops norvegicus) and S. americanus Obst, Funch, and Kristensen, 2006 from the mouthparts of the American Lobster (Homarus americanus) (an apparently distinct third species lives on the mouthparts of the European Lobster [Homarus gammarus]; Obst et al. 2005) . For at least S. americanus, there is evidence suggesting that this nominal species 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 Symbion 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; 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 pandora is a reference to the feeding stage, which contains both an inner bud and a Pandora larva with a miniature feeding stage inside, reminding the authors of Pandora's Box of Greek mythology. The specific epithet americanus is a reference to the host of S. americanus, the American Lobster.(Funch and Kristensen 1995)