The star ascidian is a colonial sea-squirt (2). The individual members of the colony (known as zooids) are embedded in a jelly-like coating called a 'test' in groups of 3-12 around a common opening; this arrangement gives rise to the star-like patterns referred to by the common name (3). Sea-squirts earn their common name because they expel a jet of water when disturbed (3). They are also known as 'tunicates' due to the tunic-like test of many species. The colour of colonies is variable, but includes blue, brown and yellow; furthermore the colour of the zooids often contrasts with that of the test (3). Sea-squirts belong to the same phylum as humans, the 'chordates'. The free-swimming larval stage is a 'tadpole larva', which possesses a stiff chord known as a 'notochord' as well as a nerve chord, which pass along the flexible tail; the possession of both a notochord and a nerve chord at some point in the lifecycle of an animal is a crucial defining characteristic shared by all chordates (3).
All sea-squirts filter suspended particles from the water. They maintain a current that passes through their body by beating tiny hair-like structures known as 'cilia'; the water enters through an opening called the 'inhalant siphon', and passes through the pharynx, where plankton and detritus become trapped in mucus and are passed to the stomach. The water then passes out through a second opening called the 'exhalent siphon'. In the star ascidian, the exhalent siphons of all the members of the colony open into a shared chamber, known as the cloaca, water then passes out of a shared exhalent siphon at the centre of the colony (3).
In the star ascidian, the zooids are hermaphroditic(3); after fertilisation, eggs are retained until the tadpole larvae have formed (2). The larvae are then released through the exhalent siphon, and live in the water column for about 36 hours, before settling and forming new colonies (2). Asexual reproduction can also take place through budding(3). Colonies may live for up to one and a half years (3). Star ascidians are predated upon by cowries (members of the genus Trivia), which are gastropod molluscs; cowries also lay their eggs into holes made in the star-ascidian test (3).
Very common and widespread around the coasts of Britain. It also occurs around Ireland and in mainland Europe from the Faeroe Islands and Norway, reaching as far south as the Mediterranean. It is also known from the western Atlantic along parts of the coast of North America, where it is thought to have been introduced on the hulls of ships (2).
Typically occurs on the lower shore and in shallow depths, but has been recorded at depths of several hundred meters (2). It seems to fare particularly well in sheltered sites, such as docks, and grows on a wide range of stable substrates including rock, algae and artificial surfaces (2).
Reproduction that does not involve the formation of sex cells (‘gametes’). In many species, asexual reproduction can occur by fission (or in plants ‘vegetative reproduction’); part of the organism breaks away and develops into a separate individual. Some animals, including vertebrates can develop from unfertilised eggs, this process, known as parthenogenesis gives rise to offspring that are genetically identical to the parent.
Type of asexual reproduction (reproduction that does not involve the formation of sex cells), in which new individuals develop from the parent organism, forming a swelling similar in appearance to a bud. The ‘bud’ slowly separates from the parent as it grows.
Relating to or belonging to a colony (a group of organisms living together in a group).
Possessing both male and female sex organs.
Of the stage in an animal's lifecycle after it hatches from the egg. Larvae are typically very different in appearance to adults; they are able to feed and move around but usually are unable to reproduce.
The tube connecting the mouth to the internal body cavity where digestion occurs. In vertebrates, the part of the gut between the mouth and the oesophagus.
Aquatic organisms that drift with water movements; may be either phytoplankton (plants), or zooplankton (animals).
An individual colony member of colonial invertebrates, such as bryozoans.
Hiscock, K., (2001) Botryllus schlosseri. Star ascidian. Marine Life Information Network: Biology and Sensitivity Key Information Sub-programme [on-line]. Plymouth:Marine Biological Association of the United Kingdom. [cited 27/11/02]. http://www.marlin.ac.uk/species/Botryllusschlosseri.htm
Fish, J. D. & Fish, S. (1996) A student's guide to the seashore. Second Edition. Cambridge University Press, Cambridge.
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