DIDEMNIDAE Giard 1872

Kott, Patricia, 2009, Taxonomic revision of Ascidiacea (Tunicata) from the upper continental slope off north-western Australia, Journal of Natural History 43 (31 - 32), pp. 1947-1986 : 1964-1965

Family DIDEMNIDAE Giard 1872

Type genus: Didemnum Savigny, 1816 .

The family is the most speciose and diverse in the Ascidiacea, although it is not common in deep or even intermediate waters, being recorded mostly from relatively shallow locations. It displays one of the most prolific patterns of replication known, budding from the oesophageal neck of the zooids and resulting in colonies containing many hundreds of small zooids arranged in elaborate common cloacal systems in colonies that range from rapidly spreading thin, encrusting ones supported by hard substrates to massive, self-supporting three- dimensional colonies. The family usually contains minute calcareous spicules in the test that often are crowded together to form a skeletal like internal support (usually found in the more massive upright colonies). Zooids have only three or four rows of stigmata. Muscle bands are on the thorax, sometimes continuing into the test, free of the zooid, as a retractor muscle. The gut loop is a short, usually curved loop with a smooth more or less spherical stomach. The testis is entire or divided into two, three, or more follicles and the vas deferens (sometimes acting as a seminal vesicle) is wound around the testis or is straight. The ovary is small with only a single egg fertilized at a time. Embryos are incubated in the test in the base of the colony, in due course being liberated from the surface of the colony or through the large common cloacal canal,

Colonies in this family gain their nutriment from the combined contributions of the large numbers of small zooids with associated small organs. It is possible that the pressure to find an alternative to filter feeding could not be accommodated in these very small zooids. Indeed, a group of shallow water tropical didemnid species ( Kott 2001) have reinforced their filter-feeding capacity (sometimes becoming completely autotrophic) by developing relationships with chlorophyll-containing symbionts – a solution not available to organisms in deep water habitats.

Only one specimen of this family is in the present collection.