symbiont

Symbion is the name of a genus of aquatic animals, less than ½ mm wide, found living attached to the bodies 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, new phylum Cycliophora shortly after they were discovered in 1995. This was the first new phylum of multicelled organism to be discovered since the Loricifera in 1983.

Scientific classification

Symbion was discovered in 1995 by Reinhardt Kristensen and Peter Funch^[1] 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)^{[2] [3]}
• 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 meals.

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

Physical Description

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
  • 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.^[1]
  • 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.

Reproduction

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. The sexual reproductive cycle is triggered when the host crustacean molts its skin in order to grow.^[4]

External links

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

References