Sea Squirts

Sea squirts

Sea squirts date back at least 500 to 600 million years. They have very advanced body structures and belong to the same Phylum as humans (Chordata). Their larval stage looks very similar to a tadpole, complete with rod cells (notochords) running down their backs just like those of embryonic vertebrates.

Sea squirts are characterised by:

• having a hollow body with two openings
• being found either alone (solitary) or in colonies
• being attached to a reef
• having no head

The role of sea squirts on the Reef

Sea squirts, or ascidians as they are also known, fill two roles on the reef. First, they filter the waters of the reef helping to keep them clear. A three-centimetre sea squirt can filter up to one litre of water each hour. Secondly, they strain minute plants and animals from the water for food and release the resulting nutrients as concentrated wastes that can be used by other organisms.

Feeding

Sea squirts get their name from their feeding habits. They have two openings, called siphons. Water is drawn through one opening. As the water passes through the body of the sea squirt, food particles are filtered by a filter basket before being passed out through the other siphon.

Protection

Sea squirts have a thick outer coat (or test) that protects them from most predators. The test has a molecular structure similar to the cellulose found in plants.

Reproduction

Sea squirts are hermaphrodites, possessing both male and female sexual organs. In some species the eggs are held within the body and the sperm is drawn from the water through the siphon to fertilise them. All solitary species release eggs and sperm into the water where fertilisation occurs.

After spawning, a tadpole-like larvae is produced. Like other chordates such as fish, reptiles and mammals, this larval stage has rod cells down its back. After a short free-swimming existence, the larvae settle and transform into the adult stage. They loose the notochord as they develop.

Sea squirts and humans

In southern waters, sea squirts are used as bait for fishing. They are also of economic importance as they foul ships and other marine structures.