Symbion is the name of a genus of aquatic animals, less than 0.5 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.[1] This was the first new phylum of multicelled organism to be discovered since the Loricifera in 1983.

Scientific classification
Kingdom: Animalia
Superphylum: Lophotrochozoa
Phylum: Cycliophora
Funch & Kristensen, 1995
Class: Eucycliophora
Funch & Kristensen, 1995
Order: Symbiida
Funch & Kristensen, 1995
Family: Symbiidae
Funch & Kristensen, 1995
Genus: Symbion
Funch & Kristensen, 1995
  • Symbion americanus Obst, Funch & Kristensen, 2005
  • Symbion pandora Funch & Kristensen, 1995
  • and at least one other


Symbion was discovered in 1995 by Reinhardt Kristensen and Peter Funch[2] on the mouthparts of the Norway lobster (Nephrops norvegicus), and other, related, species have since been discovered on:

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.[5]

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.[6] 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.
  • 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.[2]


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 larval stage may be unscientifically referred to as sea worms. The sexual reproductive cycle is triggered when the host crustacean molts its skin in order to grow.[7]


  1. Marshall, Michael (28 April 2010). "Zoologger: The most bizarre life story on Earth?". New Scientist. Retrieved 19 November 2018. ... In 1995, Peter Funch and Reinhardt Møbjerg Kristensen, both then at the University of Copenhagen, Denmark, discovered an animal so unlike any other that a new phylum – Cycliophora – had to be created just for it. ...
  2. P. Funch & R. M. Kristensen (1995). "Cycliophora is a new phylum with affinities to Entoprocta and Ectoprocta". Nature. 378 (6558): 711–714. doi:10.1038/378711a0.
  3. 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 (14): 4427–4440. doi:10.1111/j.1365-294X.2005.02752.x. PMID 16313603.
  4. Neves RC, Kristensen RM, Wanninger A (March 2009). "Three-dimensional reconstruction of the musculature of various life cycle stages of the cycliophoran Symbion americanus". J. Morphol. 270 (3): 257–70. doi:10.1002/jmor.10681. PMID 18937332.
  5. P. Funch; P. Thor & M. Obst (2008). "Symbiotic relations and feeding biology of Symbion pandora (Cycliophora) and Triticella flava (Bryozoa)". Vie et Milieu. 58: 185–188.
  6. "Cycliophorans - Cycliophora - Details - Encyclopedia of Life". Encyclopedia of Life. Retrieved 2017-09-02.
  7. Piper, Ross (2007), Extraordinary Animals: An Encyclopedia of Curious and Unusual Animals, Greenwood Press.
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