Symbion

Symbion is the name of a relatively recently-discovered animal genus, unique enough to have merited its own phylum. It was originally represented by a single species, pandora, but related species including americanus have since been described.

Many commentaries on Symbion refer to the original work by Symbion pandora's co-discoverers Peter Funch and Reinhardt Kristensen, which Funch followed up in a more detailed description of some of S. pandora's bizarre life cycle. The observations made by Funch and Kristensen involved both live specimens under a stereo microscope and fixed specimens in a scanning electron microscope.

Taxonomy

Funch and Kristensen, being the first to elucidate the morphology of the genus, proposed the following taxonomy for S. pandora : kingdom Animalia, phylum Cycliophora, class Eucycliophora, order Symbiida, family Symbiidae, genus Symbion, species pandora.

The genus name refers to the species' symbiotic, commensal habitatation on the mouthparts of the lobster, on whose leftovers it feeds. The name of the first-discovered species, "pandora", was chosen to describe the plethora of alloforms borne by the species in its feeding stage.

Thus far Symbion is the only genus assigned to the phylum Cycliophora, which is the 36th phylum to be included in the animal kingdom. Funch has suggested that Cycliophora is sufficiently related to several other phyla, particularly Entoprocta, to justify their amalgamation into a new superphylum; other related phylum is Entoprocta. Genetic analysis of Symbion appears to indicate a closer relation to Gnathifera.

Life Cycle

A brief summary of Symbion's life cycle follows. The dominant generation of Symbion is an asexual feeding (or "sessile") stage, during which the organisms cling to their host lobster with an adhesive disc. In the case of Symbion pandora, the host crustacean is the Norwegian lobster (Nephrops norvegicus); other hosts include the American lobster (Homarus americanus), host to Symbion americanus, and the European lobster (Homarus gammarus). The saclike Symbion reaches its greatest size in this stage, approximately 0.5 mm in length. Accompanied by a regeneration of the feeding stage's inner organs is a series of interior buddings that results in the birth of Symbion larvae, the first of three motile and non-feeding stages. The larvae are released with a new feeding stage already growing inside, enabling Symbion to continue its asexual cycle and to fully populate its host's mouthparts.

The sexual cycle of the species begins when the moulting period of the host lobster nears completion. The feeding stage of Symbion, perhaps signaled by hormones from the lobster, now begins to produce two alternate motile stages, dwarf males and females. The mature, sperm-filled dwarf males attach themselves to feeding stages inside which a female and its oocyte are developing. A zygote results in the female, which escapes from the feeding stage only to return and settle back onto the lobster host. While the female dies and then degenerates, leaving only a cyst of cuticle, its embryo differentiates into a chordoid larva. The larva, equipped with cilia for locomotion, hatches and settles onto a new lobster host. There the chordoid larva metamorphoses into another asexual feeding stage<ref name=Funch1995>, and Symbion's entire life cycle is repeated.

Bibliography

• Irish KE, Norse EA (1996) Scant emphasis on marine biodiversity. Conservation Biology 10: 680.
• Obst M, Funch P, Giribet G (2005) Hidden diversity and host specificity in cycliophorans: a phylogeographic analysis along the North Atlantic and Mediterranean Sea. Molecular Ecology 14: 4427–4440.