''Didemnum molle'' is a

species In biology, a species is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of the appropriate s ...

colonial Colonial or The Colonial may refer to: * Colonial, of, relating to, or characteristic of a colony or colony (biology) Architecture * American colonial architecture * French Colonial * Spanish Colonial architecture Automobiles * Colonial (1920 au ...

tunicate A tunicate is a marine invertebrate animal, a member of the subphylum Tunicata (). It is part of the Chordata, a phylum which includes all animals with dorsal nerve cords and notochords (including vertebrates). The subphylum was at one time ...

in the

family Family (from la, familia) is a group of people related either by consanguinity (by recognized birth) or affinity (by marriage or other relationship). The purpose of the family is to maintain the well-being of its members and of society. Idea ...

Didemnidae Didemnidae, or Didemnidæ, is a family of colonial tunicates in the order Enterogona. These marine animals are found in shallow water on the seabed. Members of this family have small zooids that form encrusting colonies. The body of each zooid ...

. It is commonly known as the tall urn ascidian, the green barrel sea squirt or the green reef sea-squirt. It is native to the

Red Sea The Red Sea ( ar, البحر الأحمر - بحر القلزم, translit=Modern: al-Baḥr al-ʾAḥmar, Medieval: Baḥr al-Qulzum; or ; Coptic: ⲫⲓⲟⲙ ⲛ̀ϩⲁϩ ''Phiom Enhah'' or ⲫⲓⲟⲙ ⲛ̀ϣⲁⲣⲓ ''Phiom ǹšari''; ...

and the

tropical The tropics are the regions of Earth surrounding the Equator. They are defined in latitude by the Tropic of Cancer in the Northern Hemisphere at N and the Tropic of Capricorn in the Southern Hemisphere at S. The tropics are also referred to ...

waters of the Indo-Pacific region.

Description

A colony of ''Didemnum molle'' is composed of a number of linked

zooid A zooid or zoöid is a single animal that is part of a colonial animal. This lifestyle has been adopted by animals from separate unrelated taxa. Zooids are multicellular; their structure is similar to that of other solitary animals. The zoo ...

s adhering to the substrate. Each individual zooid has a plump, urn-shaped body with a diameter of up to . The tunic (body wall) is made of a

cellulose Cellulose is an organic compound with the formula , a polysaccharide consisting of a linear chain of several hundred to many thousands of β(1→4) linked D-glucose units. Cellulose is an important structural component of the primary cell w ...

-like substance that is strong but flexible, and is perforated by numerous small holes, the buccal siphons, through which water is drawn into the body. At the top of the body is the large atrial siphon through which water flows out. The tunic is strengthened by the presence of many spherical calcareous spicules which are particularly concentrated round the neck of the urn. The colony is white, brown or green, or some combination of these colours. This hue is due to the symbiotic

blue-green algae Cyanobacteria (), also known as Cyanophyta, are a phylum of gram-negative bacteria that obtain energy via photosynthesis. The name ''cyanobacteria'' refers to their color (), which similarly forms the basis of cyanobacteria's common name, bl ...

of the genus ''

Prochloron ''Prochloron'' (from the Greek ''pro'' (before) and the Greek ''chloros'' (green) ) is a genus of unicellular oxygenic photosynthetic prokaryotes commonly found as an extracellular symbiont on coral reefs, particularly in didemnid ascidians (sea ...

'' present in the tissues. The tunic contains mycosporins which filter out ultra-violet radiation and protect the symbiotic cyanobacteria from damage in strong sunlight. In shallow water the colonies are small, the colour is more intense and there are more spicules present making the external surface bright white. At greater depths the colonies are larger, there are fewer spicules and the colour is more greyish or brownish. The tunic exudes large quantities of

mucus Mucus ( ) is a slippery aqueous secretion produced by, and covering, mucous membranes. It is typically produced from cells found in mucous glands, although it may also originate from mixed glands, which contain both serous and mucous cells. It ...

which makes this species sticky to the touch. It is very similar to '' Atriolum robustum'' in appearance, especially young colonies, but can be distinguished by the mucus secretion and the network of canals connecting the thorax internally to the atrial siphon.

Distribution

''Didemnum molle'' is native to the western Indo-Pacific region where it is common and widely distributed. It grows on rock or dead coral on shallow reefs. In New Caledonia there is a green variant that grows direct onto sand.

Biology

''Didemnum molle'' is a suspension feeder. Water is drawn into the body of each zooid through the numerous buccal siphons, phytoplankton,

zooplankton Zooplankton are the animal component of the planktonic community ("zoo" comes from the Greek word for ''animal''). Plankton are aquatic organisms that are unable to swim effectively against currents, and consequently drift or are carried along by ...

and fine organic particles are filtered out and the water is exhaled through the atrial siphon. Asexual reproduction takes place by budding. A new bud will form and begin to actively feed while the zooid from which it emerged gradually regresses and is eventually re-adsorbed. Sexual reproduction also takes place. Each zooid is hermaphrodite and fertilisation takes place in the body cavity when a clutch of eggs is fertilised by sperm drawn in with the water stream. The embryos are brooded for a few days before the tadpole-like

larva A larva (; plural larvae ) is a distinct juvenile form many animals undergo before metamorphosis into adults. Animals with indirect development such as insects, amphibians, or cnidarians typically have a larval phase of their life cycle. ...

e are expelled into the

water column A water column is a conceptual column of water from the surface of a sea, river or lake to the bottom sediment.Munson, B.H., Axler, R., Hagley C., Host G., Merrick G., Richards C. (2004).Glossary. ''Water on the Web''. University of Minnesota-D ...

. These quickly settle and cement themselves to suitable surfaces and start new colonies.

References

External links

* {{Taxonbar, from=Q1551985 Enterogona Fauna of the Indian Ocean Fauna of the Pacific Ocean Fauna of the Red Sea Animals described in 1886 Taxa named by William Abbott Herdman