Behaviour

The red-throated ascidian is a solitary, self-sterile hermaphrodite. Adults of the Great Barrier Reef usually live on the underside of coral boulders but in other locations can be found attached to vertical substrates such as rock walls. The surface of the body wall is attached to the substrate via the side or base of the test with the animal usually orientating itself so that it's siphons project horizontally into the water column; this may contribute to efficient feeding.

Feeding

Adult H. momus, like other ascidians, are extremely efficient filter feeders. On the anterior (head) end of the ascidian are the buccal and atrial siphons. These siphons are used to constantly filter seawater which travels through the buccal siphon, into the branchial basket and is expelled via the atrial siphon. Water current is produced by tiny hairs, called cilia, that lie on the margins of the animals gill slits. As water is pumped through the neck region (called the pharynx) plankton and other suspended food particles are trapped by a sticky mucous lining. This mucous forms a remarkably efficient net of about 0.5 microns (really small) which allows the removal of even the tiniest plankton (e.g. bacteria) from the seawater (Ruppert et al. 2004). The net is secreted by the endostyle, which is a small groove running along the length of the pharynx. Cilia then convey this sticky mucous net and the food it has trapped to the base of the pharynx. Here it is rolled into a  thread and transported to the oesophagus.

Food particles travel down the oesophagus, are digested by enzymes secreted in the stomach and are transported through the intestines to the rectum. Here the faeces is formed which is then discharged through the anus and out the atrial siphon. Filter feeding organisms like these need to filter an enormous quantitiy of water per day to provide adequate food. On average, this requires an entire body's worth of volume in water is filtered per second (Ruppert et al. 2004). A specimen of just a few centimetres in length will pump nearly 200 liters of water through its body in just one day! As part of the filtering system, individuals will periodically contract their muscles, compressing the body and forcing water jets from the siphons before they close. Such behaviour often occurs in response to disturbance and acts to remove unwanted particles (see video below). Periodic squirting of water from the siphons is characteristic to all ascidians and is the reason for the common name “sea squirt”.