		wildsingapore homepage sitemap to the online guide search \| glossary coral rubble index

coral rubble

Ascidians
Class Ascidiacea

Ascidians are found mostly on hard surfaces such as rocks, jetty pilings and coral rubble. They also grow on seagrasses and other vegetation in the seagrass lagoon.

'Askidion' comes from the Greek word for 'bladder'.

An ascidian is a complex animal, It usually has a circulatory system, a digestive system, a heart and other organs. It generates a one-way current through its body and a part of the gut is modified to filter out plankton from this water flow. The entire animal is encased in a little bag. In fact, 'Ascidiacea' means 'a little bag'.

Thick skinned: Some ascidians have a sturdy outer coat called the tunic. Thus, they are sometimes called tunicates. The tunic supports and protects them.

Cross-section of
a solitary ascidian
Place your mouse over the
diagram for details on the different parts the ascidian

As the animal grows bigger, the tunic also grows with it. Unlike other creatures with a tough outer coat, tunicates don't have to moult to get bigger!

Some ascidians form as solitary animals, sometimes called simple ascidians. These range from 1mm to more than 10cm.

Squirty surprise: Some solitary ascidians have bands of muscles along their body. When these muscles constrict, water squirts out of the animal. So they are sometimes also called sea squirts. They may do this to get rid of something in them, or when they are disturbed.

Colonial ascidians: Other ascidians may form colonies with many tiny individual animals connected to one another. They may sometimes also be called compound ascidians. The colony can range from a few centimeters in diameter to a meter or more, and up to several centimeters thick. On Chek Jawa, colonial ascidians grow as slimy layers and blobs on rocks, jetty pilings and other hard surfaces.

Sponge or ascidian? Ascidians are often mistaken for sponges. However, while sponges are simple animals without specialised organs, ascidians are more complex animals. While ascidians tend to be smooth and slimy, sponges tend to be rough and are usually not slimy. For more about sponges.

Ascidian babies: Almost all ascidians are hermaphrodites, having both male and female organs. Most avoid self-fertilisation by developing either eggs or sperm at any one time. Most solitary ascidians release their eggs and sperm into the water for external fertilisation. Colonial ascidians usually retain and brood their eggs. Colonial ascidians can also multiply by budding off.

Ascidian babies are like us! Ascidians are actually closely related to vertebrates like us! Their free-swimming larvae look like and are called tadpoles. These have a stiff notochord (a primitive spinal cord). The subphylum they belong to 'Urochordata' means 'tail string'. Some also have an eye spot.

The free-swimming stage can last for 36 hours or as little as a few minutes! The tadpoles do not feed. When the larva decides to settle down, the tail, notocord and eyespot are absorbed as the larva sticks itself, usually headfirst, onto a hard surface. The larvae then undergos metamorphosis and matures into the adult form. Here are really cool close-up photos of ascidian larvae: on Image Quest 3-D Marine Library; and Sea squirts: our distant cousins by Wim van Egmond and Jan Parmentier on the Microscopy UK website.

Role in the habitat: Ascidians are eaten by some creatures such as nudibranchs. Some sponge crsabs make their living disguises out of ascidians instead of sponges.

Human uses: As ascidians are closely related to vertebrates, studying them helps us better understand the ancestry of vertebrates and our own biology. The study of some sea squirts (Botryllus sp.), for example, is helping to shed light on how the AIDs virus functions.

Sea squirt
Polycarpa sp.

Colonial ascidian
This one looks like melted chocolate
on the rocks!

Colonial ascidian
Family Polyclinidae

Colonial ascidians
growing on jetty pilings

A closer look at the ascidians on the pilings

Colonial ascidians of the seagrass lagoon

quick facts
Jelly-like blobs or layers on rocks and other hard surfaces on the rocky shore, seagrass lagoon and coral rubble area.

Classification:
Class Ascidiacea
Subphylum Urochordata
Phylum Chordata

See also ...
Sponges are often mistaken for ascidians and visa versa.

Links
Ascideacea on the Canada's Aquatic Environments webpage on the University of Guelph website: an easy introduction to the more technical aspects of their morphology, metabolism, reproduction, ecology with an interesting section on their idiosyncracies and photos.
Exotic Tunicates on the Move on the University of Washington wesbite: a brief introduction to ascidians plus interesting ideas and projects to do with ascidians. Lots of photos and diagrams too.
Sea Squirts on Zoo Lab on the University of Wisconsin-La Crosse website: a brief introduction with a photo of a larva showing the body parts.
Tunicates extraordinaire by Jean-Marie Cavanihac on the Microscopy UK website: fact sheet on tunicates with lots of lovely super close look at tunicates and their larvae.
Tunicates With Salad On The Side by Richard L. Howey on the Microscopy UK website: lots of information on tunicates and the creatures that live on them.
Ascidians: Sea Squirts, Tunicates on the Sea Slug Forum by Dr Bill Rudman: an introduction with close-up photos.
Sea Squirts on Canada's Arctic site at the University of Guelph: a brief intro to sea squirts with examples of those from the Arctic!
Ascidians on Life on Australian Seashores by Keith Davey on the Marine Education Society of Australia website: a fabulous goldmine of a site which covers various kinds of intertidal habitats and their inhabitants. Fact sheet on ascidians with lovely diagrams showing the changes that the larva goes through as it develops into an adult, and details on a species found in Australia.
Sea squirt immunity: the AID connection on the Marine Biological Laboratory, Woods Hole website: how the study of sea squirts help us better understand AIDs.
Chordata (Ascidicea) from Bio 136 on the University of California, Santa Cruz website: labelled diagram of an ascidian.
Sea squirts: our distant cousins by Wim van Egmond and Jan Parmentier on the Microscopy UK website: lots of lovely super close look at the fascinating larvae of sea squirts.

Other references

Barnes, Robert D. & Ruppert, Edward E., 1996. Invertebrate Zoology. Harcourt College Publishers. 6th Edition. pp. 1056, G-1-16, I-1-30.
Pechenik, Jan A., 2000. Biology of the Invertebrates. McGraw-Hill Book Co., Singapore. 578 pp.
Tan, Leo W. H. & Ng, Peter K. L., 1988. A Guide to Seashore Life. The Singapore Science Centre, Singapore. 160 pp. online version
Lim, S., P. Ng, L. Tan, & W. Y. Chin, 1994. Rhythm of the Sea: The Life and Times of Labrador Beach. Division of Biology, School of Science, Nanyang Technological University & Department of Zoology, the National University of Singapore. 160 pp.
Wood, Elizabeth & Michael Aw, 2002. Reef Fishes, Corals and Invertebrates of Malaysia and the South China Sea. New Holland Publishers, UK. 144 pp.

a companion website to the chek jawa guidebook website©ria tan 2003 www.wildsingapore.com

a companion website to the chek jawa guidebook
website©ria tan 2003 www.wildsingapore.com