Ascidians are members of the phylum Chordata, the phylum that includes vertebrates, or animals with a backbone. They have extremely different adult and larval stages in both appearance and function. To be recognised a chordate, an animal must at some stage throughout its life cycle possess a nerve chord, a notochord and gill slits. As larvae, the ascidian resembles a free-swimming larval fish which is adapted primarily for dispersal, at this stage they do not feed. Upon finding a suitable habitat to spend the rest of their adult life, they attach by their head, and metamorphose by absorbing their tail and expanding their upper body becoming a sedentary, water filtering animal.
The body of an ascidian is characterised by a large perforated pharynx, similar to that of a sieve, for filter feeding and is covered by a protective external layer of cellulose –like material, this is known as the tunic. They possess an inhalant siphon, which pumps water carrying microscopic food into the pharynx and an exhalent siphon through which waste water is expelled from the body. The filtration system of ascidians is extremely efficient, removing particles as small as bacteria.
Similar to the blue-throated ascidian, the orange-throated ascidian occurs in densely-packed colonies that may extend up to 25 centimetres across. Individual zooids emerge from a common base on a short, wide stalk. They have a translucent, rounded, gelatinous test with raised orange inhalant and exhalent siphons, through which the muscular bands of the pharynx are visible. Colonies increase in size by budding off new individuals, while new habitats can be colonised through sexual reproduction.
Occurrence at the Busselton Jetty:
Not as abundant as the blue-throated ascidian, there are several small colonies of the orange-throated ascidian visible from the underwater observatory.
Image by: R. Austin