A recently described phylum in the animal kingdom. Only one microscopic species, Symbion pandora, has been described. Its sessile stage is approximately 0.3 mm in length and 0.1 mm wide. All known occurrences are from the mouth limbs of the Norwegian lobster, which can be totally encrusted with thousands of the sessile stage of the animal (see illustration).
Scanning electron micrograph of the Norwegian lobster's mouthpart with numerous feeding stages of Symbion pandora. (Courtesy of P. Funch and R. M. Kristensen)
The sessile animal has a circular mouth surrounded by a ring of cilia, which is used for filtering small food particles, such as bacteria or algae. The anatomy is relatively simple, with a U-shaped gut, similar to what is found in bryozoans or some sessile rotiferans. It is the extremely complex life cycle of S. pandora that makes the phylum unique. The life cycle consists of an asexual and a sexual generation with no less than two types of free-swimming larva, dwarf male and female; two stages of sessile feeding animals, which brood the male and female; and one type of larva (Pandora larva) inside a brooding chamber (called a marsupium). Furthermore, the sessile animal has internal budding, whereby it grows by loss of the head itself and replacement of the old gut and feeding system with a new bud coming from embryonic cells in the basal part of the body.
