# Didemnidae
## Lead

Didemnidae, or Didemnidæ, is the largest family of colonial tunicates in the order Aplousobranchia.
These marine animals are found in shallow water on the seabed. Members of this family have reduced zooids that form highly integrated and encrusting sheet-like colonies. The body of each zooid is divided into a thorax and an abdomen. They are highly speciose with 578 named species representing a fifth of the species in the class Ascidiacea and members of this family can be found in marine waters across the globe. Didemnidae is the only genera within Chordata to form obligate photo symbiotic relationships which are made with cyanobacteria form the genus Prochloron.

## Systematics

The World Register of Marine Species lists the following genera:
* Atriolum Kott, 1983
* Clitella Kott, 2001
* Coelocormus Herdman, 1886
* Didemnum Savigny, 1816
* Diplosoma Macdonald, 1859
* Leptoclinides Bjerkan, 1905

## Description

Didemnids small zooids can range 5mm to under 1mm in length and display an elongated digestive track and coiling of sperm ducts. They grow in sheet like colonies that can be coloured in shades of black, brown, white, violet, or pink. However, colour cannot be used in species identification as there is much variation within species and overlap between different groups. In symbiotic genera their colour is mainly determined by the composition of their bacterial photo symbionts. Didemnid colonies can grow meters long by the process of fusion and some species can move within their colonies. Their body wall is supported by aragonite spicules except in the genus Diplosoma where they have been secondarily lost. In the remaining genera the spicules have a star shaped polyaxon form and are loosely arranged in a pentagonal pattern. These spicules can be as large as 0.24mm in diameter but usually remain between 0.1 and 0.05mm. Some members have multiple sets of spicules with different forms. These spicule forms can be diagnostic of the species.

## Habitat

Didemnids can be found in tropical to temperate marine waters throughout the world but have the highest diversity along the boundary between the Indian and Pacific Oceans. They can be commonly found in intertidal settings where their flat colonies help resist wave action and survive out of water for two hours at temperatures of up to 35 degrees. Members of this family can also occur well below sea level with Lissoclinum perforatum existing at depths of 150m.

## Evolution

Molecular analysis supports Didemnidae and each genus within it as a monophyletic group which diverged from Polycitoridea. The earliest known didemnid fossils occur in the Paris Basin late in the Ypresian stage.
However, the obligate photo symbiont genera do not form a monophyletic group indicating that Prochloron evolved for a single genus before adapting to similar genera that had already diverged, supported by it's molecular similarities, but the mechanism for how the obligate symbiont was transferred between genera is unknown.

## Ecology

Didemnid species vary greatly in the extent to which they rely on photo symbiosis. The genera Clitella and Coelocormusremain exclusively active suspension feeders while Atriolum, Leptoclinides, and Polysyncraton can engage in facultative photo symbiosis. In these cases the symbiont colonies grow externally on the test. The genera Didemnum, Diplosoma, Trididemnum, and Lissoclinum are completely dependant on their symbiosis with internal colonies either in the common cloacal cavity or the tunic.
The primary photobionts are cyanobacteria from the genus Prochloron which has not been found without a Didemnid host. Prochloron contains chlorophyl a and b which preferentially absorb light between the 420-460 and 640-680 nm wavelengths respectively. Although, all photosymbiotic Didemnids use Prochloron, some will also facultatively associate with red cyanobacteria from the genera Synechocystis or Oscillatoria. Didemnum etiolum is the only species thought to be dependent on red cyanobacteria. These symbionts widen the spectrum of light the didemnid can use by producing phycoerythrin pigments that absorb light between the 470 to 620 nm range. In cases were a Didemnid uses red cyanobacteria they will be restricted to the tunic while Prochloron is grown in the common cloacal cavity except in Trididemnum clinides which can cultivate both types of bacteria in the tunic.
In exchange for the nutrients the Didemnid receives Prochloron is provided an exclusive environment that is protected from predation. Prochloron can also use the Didemnid test as a way to filter out damaging UV radiation by releasing Mycosporine like amino acids into the test. Prochloron may also contribute to defense by producing cytotoxins. Analysis of Trididemnum solidum found that it contained the toxins didemnin B and tamandarin A that were demonstrated to be distasteful a variety of fresh and salt water with species. Although, they can still be parasitized by copepods within the cloacal cavity from the family Notodelphyidae which have evolved species specific relationships.

## Gallery

* Atriolum robustum
* Didemnum molle
* Diplosoma spongiforme
* Lissoclinum patellum
* Polysyncraton bilobatum