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Symbion is a genus of peculiar microscopic animals, with no obvious close relatives, and which was therefore given its own phylum, called Cycliophora.

Symbion was discovered in 1995 by Reinhardt Kristensen and Peter Funch on the mouthparts of the Norway lobster (Nephrops norvegicus), and other, related, species have since been discovered on the American lobster (Homarus americanus, host to Symbion americanus) and the European lobster (Homarus gammarus, host to an as yet un-named species of Symbion). The genus is so named because of its commensal relationship with the lobster (a form of symbiosis) -- it feeds on the leftovers from the lobster's own feeding.

The phylogenetic position of Symbion remains unclear: originally the phyla Ectoprocta and Entoprocta were considered possible relatives of Symbion, based on structural similarities. However, genetic studies suggest that Symbion may be more closely related to Gnathifera.

Symbion pandora has a bilateral, sac-like body with no coelom. There are three basic life stages:

• Asexual Feeding Stage - At this stage, S. pandora is neither male nor female. It has a length of 347 μm and a width of 113 μm. On the posterior end of the sac-like body is a stalk with an adhesive disc, which attaches itself to the host. On the anterior end is a ciliated funnel (mouth) and an anus.

• Female - S. pandora is the same size as the male in this stage. It does, however, have a digestive system which collapses and reconstitutes itself as a larva. (Funch & Kristensen, 1995)

• Male - S. pandora has a length of 84 μm and a width of 42 μm during this stage. It has no mouth or anus, which signifies the absence of a digestive system. It also has two reproductive organs.

Symbion can reproduce both asexually by budding and sexually. In sexual reproduction the male attaches to a feeding stage and impregnates a budding female. The female then separates from the feeding stage and attaches herself to another host, where the larva in her develops. The female dies, and the larva escapes.

• A lobster's microscopic friend
• Cycliophora (Wheel Wearers)

• P. Funch & R. M. Christensen (1995). Cycliophora is a new phylum with affinities to Entoprocta and Ectoprocta. Nature 378: 711–714. doi:10.1038/378711a0. 
• M. Obst, P. Funch & G. Giribet (2005). Hidden diversity and host specificity in cycliophorans: a phylogeographic analysis along the North Atlantic and Mediterranean Sea. Molecular Ecology 14: 4427–4440. doi:10.1111/j.1365-294X.2005.02752.x.