Symbion is a genus of commensal aquatic animals, less than 0.5 mm wide, found living attached to the mouthparts of cold-water lobsters. They have sac-like bodies, and three distinctly different forms in different parts of their two-stage life-cycle. They appear so different from other animals that they were assigned their own phylum, Cycliophora, also called wheel wearers, shortly after being discovered in 1995. Cycliophora was the first new phylum of multicelled organism to be discovered since the Loricifera in 1983.
Taxonomy
Symbion pandora was discovered in 1995 by Reinhardt Kristensen and Peter Funch on the mouthparts of the Norway lobster (Nephrops norvegicus). Other, related, species have since been discovered on:
- the American lobster (Homarus americanus, host to Symbion americanus)
- the European lobster (Homarus gammarus, host to an as yet unnamed 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 foods.
They are peculiar microscopic animals, with no obvious close relatives, which were therefore given their own phylum, called Cycliophora. The phylogenetic position of Symbion is still not finally settled. It is currently placed in the clade Polyzoa along with the phyla Ectoprocta and Entoprocta, based on genetic analysis.
Description
thumb|Diagram
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.
- Sexual Stage
- 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.
- 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.
The larval stage is unscientifically referred to as "sea worms".