Occurrence at the Busselton Jetty:
There are several red-lipped ascidians positioned close to the windows of the underwater observatory, which are intriguing to onlookers as they look somewhat similar to a “heart”. Occasionally the large siphons will close if an unsuspecting fish ventures too close. It is believed that a single ascidian can filter through over 5000 litres of water per day.
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.
The red-lipped ascidian is a large, solitary species growing to a maximum height of 20 centimetres. The body is an opaque, pinkish colour with two large siphons that may be red, or have alternate red and white segments. Often this colour is not visible, as most solitary ascidians are overgrown by encrusting organisms or other colonies of ascidians, sponges and algae. Small individuals have a more rounded appearance, with a translucent test. Characteristic of species within the family Pyuridae, the red-lipped ascidian bears cilia, or tiny hairs, at the base of the inhalant siphon and around the edges of the gill slits which beat vigorously to draw water down into the pharynx. Here, oxygen is absorbed by the gills from the water, while trapped nutrients are moved by the cilia to the mouth and intestines for digestion.
The red-lipped ascidian is one of the most commonly encountered species of ascidian in southern Australia, however is found from Broome, WA to Lizard Island, Qld and around Tasmania. They are also widespread overseas. Small individuals can be found growing on seagrass, adults inhabit reefs in depths of up to 100 metres.
Other common names include: Solitary ascidian, red-throated ascidian, brown sea squirt
Briedahl, Harry. (1997) Australia’s Southern Shores. Environment Australia, Victoria
Edgar, G. (1997) Australian Marine Life: The Plants and Animals of Temperate Waters, 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