Occurrence at the Busselton Jetty:
There are many growing clusters of blue-throated ascidians visible from the windows of the underwater observatory. One of the first animals to colonise jetty piles, the blue-throated ascidian starts off as an individual. Over several years a colony of up to thousands of individuals is created as they divide and fan outward.
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.
Blue-throated ascidians are a colonial species, whereby individual animals known as zooids join to a common base in distinctive grape-like clusters. Zooids are a transparent light blue colour, with three blue spots in between the two siphons and visible circular bands of muscle within the pharynx. They grow to a maximum height of 6 centimetres, and expand to thousands of individuals within a single colony. Colonies increase in size by budding off new individuals (asexual reproduction), while new habitats can be colonised by sexual reproduction. Blue-throated ascidians are hermaphrodites, which mean they possess both male and female sexual organs. To avoid self-fertilisation sperm and eggs are produced at different times. The egg is retained in the body, in a parental oviduct, while sperm from another individual is then drawn in through the inhalant siphon to fertilise the egg. After only several hours the fertilised egg develops and releases free-swimming larvae into the water column, where they have a far greater chance of survival than individual eggs that float among the plankton.
Blue-throated ascidians are found along reef ledges from 3 to 12 metres depth around the Australian mainland and throughout the Indo-West Pacific region.
Briedahl, Harry. (1997) Australia’s Southern Shores. Environment Australia, Victoria
Edgar, G. (1997) Australian Marine Life: The Plants and Animals of Temperate Waters, New Holland Publishers, Australia
Morrison, S., Storrie, A. (1999) Wonders of Western Waters. Department of Conservation and Land Management, Western Australia
Morrison, S and P., Storrie, A. (2003) Beneath Busselton Jetty. Department of Conservation and Land Management, Western Australia
Image: Sue Barstow