If you were brought up right, you have been down in rock pools as a kid annoying sea squirts In rock pools, kids can give sea squirts a poke. The sea squirts release a small jet of water with surprising speed and energy. Pretty thrilling if you are under ten [I like doing it too]. The most commonly seen sea squirts, (also called ascideans or tunicates), are these tough inter-tidal sea squirts that are a favourite with children. They are also commonly found on jetties, or in deeper water. They might seem boring compared to coral, but tunicates as a group come in a variety of amazing forms and colours. There are 3000 ascidean species world wide, 717 known species in Australia and at least 249 just from Tasmania. Every year more get discovered. Where to find them During Coastal activities Tough, brown sea squirts are really common in the intertidal zone and on jetties where there is a bit of wave movement. In NSW, pyura sea squirts dominate a belt of agitated water in the inter-tidal zone, but they are less numerous on Victoria’s Bass Strait coast and in Tasmania. There are many others to be found as well, but most are usually hidden under rocks and boulders near the low tide mark. The more delicate and most colourful species, are more commonly seen by divers. Divers and snorkelers Other tunicates can be seen everywhere in deeper waters, even in the cover of the kelp forest where a lot of other bottomdwelling invertebrates are not usually seen. Ascideans are particularly common near shady overhangs, crevices, or caves. Why are they Important? In the sub-tidal zone sea squirts can give barren areas a bit of structure and will provide a lot of hiding places for small fish and invertebrates. Mollusc can burrow inside their flesh and live as parasites inside them. Most other animals just use it for shelter including little amphipods, shrimps and crabs. Fish will sometimes use them as an attachment points for their eggs. What eats them? Sea squirts occasional get eaten by stingrays and starfish. Smaller species also get attacked by nudibranch sea slugs, 15 chitons, shelled molluscs and some fish. In Chile, humans also eat some sea squirt species. sea squirts provide structure on the bottom for other animals to live and breed on, including this annoying nudibranch laying its eggs What do sea squirts eat? Sea squirts are filter feeders. The mouth opening leads into a sack which is perforated with thousands of gill slits. Sea water is drawn in at the mouth and food particles are filtered out. The gills also extract oxygen from the sea water. Pyura sea squirts suck in mostly tiny phytoplankton (plant) cells. They don’t appear to have any helpful bacteria or fancy enzymes to break down complex food like we do. They are basically totally vegetarian, sitting there waiting for large booms in the supply of ‘greens’ in the water. The water left after the filtering has been done leaves through the “squirt” opening. The exhaust opening is always smaller. The smaller exhaust means that the water comes out forcefully and it ‘jets’ away from the sea squirt. This feature is to stop the sea squirt from wasting its energy re-processing the same water. They can filter a large amount of water. A tiny 2cm wide sea squirt can filter one litre of water an hour. Crazy Blood Squirts have a small v-shaped heart and blood. The heart can alter its direction of flow for reasons that aren’t understood. Some species have high levels of the rare metal vanadium in the blood rather than iron, and others have slightly acidic blood. So they are a bit like the “Alien” of movie fame. Breeding When the water goes a nice green shade with a bumper crop of new plant food, such as the algal blooms that follow the Spring winds, they can build up energy for breeding. Sea squirts don’t understand the male/female thing and have both male and female parts (they are “hermaphrodites”). Their reproductive organs mature at different times, that way they don’t fertilise themselves when they release into the water. The larvae of pyura sea squirts look a bit like tiny tadpoles and have a spinal chord but no backbone. They also have a rudimentary brain and eye spots. After a brief life of a few hours in the plankton the little “tadpole” attaches itself by the head to something solid in shallow water. It then transforms. 16 Common Species Australia In the tide pools and shallows Pyura Stolonifera or “Cunjevoi” These sea squirts occur in reefs and on sand in 0-12m depth. They are found from Shark Bay (W.A.) to Noosa (Qld.), including Tasmania. They are also found throughout the southern hemisphere. They can grow to 300mm in height. Pyura can be distinguished from other species of tunicate by its thick leathery tunic, a flat upper surface surrounded by a ridge, and two siphons that lie close together and project slightly above the flat surface. They are often used by fisherman as bait or eaten as food in Chile. P.stolonifera loves to hang out in big clumps. That way as they breed they have more chance of being fertilised by a neighbour before the current rips the reproductive products away. They don’t disperse far, which is one reason why populations of sea squirts seem to be patchy. In deeper water Colonial ascideans Botrylloides magnicoelum, courtesy ncmg.org.au Some species of tunicates are colonial meaning that they group together and share some body parts. These often colourful colonies of sea squirts can also reproduce in clusters by budding off new tunicates to more rapidly expand the colony. Sycozoa A common tunicate in Southern estuaries is Sycozoa pulchra, a tunicate that attaches to the bottom with a long stalk. They are an important breeding habitat for juvenile fish. The stalk allows it to ‘rise above’ other filter feeders 17 and take advantage of the slightly faster and less disturbed current that flows away from the bottom. The stalk also sways in the current in a way that always presents an open mouth to the prevailing current. Rare spotted handfish use these stalks as breeding structures in the Derwent River. This tunicate literally loses its head in summer. Sycozoa sea squirts store up their food inside the stalk in good times, ready for the approaching famine. In early summer the ‘head’ of the tunicate slowly dies and the remaining stalk stays dormant through the summer and autumn. In winter, the ‘head’ regenerates from energy stored in the stalk, ready for the next spring bonanza of food. Red-mouthed solitary ascidean This is a very noticeable tunicate on southern reefs. It use to be called Herdmania momus, but recently this species has been split into 5 new species (including two new Australian species). The commonest and biggest species is the one found in Tassie, H. grandis. Herdmania fimbriae, a new species found from southern to north-eastern Australia is different again. It is a big job to work out the differences for yourself unless you are dissecting it (after first getting a science degree). Stalked sea tulip This sea squirt is sub-tidal but likes rough and shallow reefs with strong wave action. The Sea Tulip comes in a variety of colours such as orange, purple, yellow or pink. These bright colours aren’t from the Sea Tulip but an encrusting sponge, Halisarca australiensis, which covers its surface. The Sea Tulip gains protection from predators by using the sponge’s chemical defences, and the sponge benefits by having a surface on which to grow Brain Scycozoa Not all sycozoa come in stalks, Sycozoa cerebriformis consists of folded colony of colonial sea squirts of vertical zooids in double rows. The siphons are positioned along the top edge of the twisted colony structure. Colonies of this species start as small flattened fan shapes. As they grow and expand, the colonies fold and bend which results in the brain-like appearance. They are common to southern and eastern Australia.