Friday, May 10, 2013

Ascidians & Other Tunicates (Phylum Chordata: Subphylum Tunicata) of Singapore

Tunicates (phylum Chordata, subphylum Tunicata) are close relatives of the vertebrates. They possess a notochord (usually only in the larval stages), a dorsal nerve cord (only in the larval stages), and a U-shaped gut. Most species lose the notochord as they mature, whereas in the vertebrates the notochord in the embryo eventually develops into the vertebral column (also known as the backbone) as they grow.

Adult tunicates are characterised by having a cellulose-like outer covering or "tunic" enclosing their body. They normally have two siphons - an incurrent siphon which takes in water, and an excurrent siphon which expels the water. Being filter feeders, they feed on the plankton in the water that enters the incurrent siphon.

Most tunicates are hermaphroditic with both male and female reproductive organs. In some species, fertilisation takes place externally as they release both eggs and sperm into the water. Others release the sperm into the water but brood the eggs internally, fertilising them with the sperm brought in by the siphon. Several species are also able to reproduce asexually by budding.

Here are some of the tunicates that I have seen in local waters, but unfortunately, no paper on the identification of tunicates in local waters has been published yet, and hence they are mostly unidentified. Hopefully someone who drops by my blog can help to identify them!


Members of this order are commonly referred to as salps, though sometimes people only use this term to specifically refer to those of the order Salpida. They are pelagic tunicates that float either alone or in colonies, sometimes forming long chains up to several metres long. The two siphons are located at opposite ends, and hence also provide the thrust for movement as water enters from one end and gets expelled from another. Salps are generally transparent and jelly-like, and are sometimes mistaken for jellyfish. Like other tunicates, they are filter feeders.

The above photo shows two transparent unidentified salps.


Ascidians are sessile tunicates that are usually attached to the substrate or other structures. They can be solitary or colonial, though solitary species sometimes occur in clumps. For the colonial (or compound) species, many individuals (or zooids) live together in a common gelatinous matrix.

Colonial ascidians are often mistaken for sponges. To differentiate them from the latter, take a closer look to spot the individual zooids, as shown in the above photo. Unlike the salps, the two siphons of ascidians are usually oriented such that they face the same side (usually upwards). Ascidians are also commonly called sea squirts as they tend to eject water from their siphons when disturbed.

Polycarpa sp.
The solitary sea squirt in the above photo is probably a Polycarpa sp. It is commonly seen in coastal areas, growing to about 10cm tall.

solitary sea squirt
This solitary sea squirt is usually found in areas with fast flowing water, attached to rocks or coral rubble.

solitary sea squirt
This solitary sea squirt is commonly seen under overhanging rocks.

solitary sea squirt
This smooth, translucent sea squirt is usually seen under rocks.

solitary sea squirt
This translucent sea squirt has a slightly rougher texture, and is also found under rocks.

This unknown compound ascidian is occasionally seen attached to rocks and coral rubble in coastal areas.

Aplidium sp.
This pale orange colonial ascidian is probably an Aplidium sp. It can be seen on reeflats.

colonial ascidians
Many of these colourful blob-like colonial ascidians can be seen attached to rocks and jetty pillars on some of Singapore's shores.

colonial ascidians
These pink colonial ascidians are usually seen near coral reefs.

colonial ascidians
These orange blob-like colonial ascidians are usually seen attached to rocks and pillars of jetties.

colonial ascidians
These pale orange colonial ascidians are usually seen attached to seagrasses.

colonial ascidians
These pink colonial ascidians are usually found under rocks.

Diplosoma sp.
These blue and green colonial ascidians are from the family Didemnidae, most probably a Diplosoma sp. They are commonly seen growing on seagrass blades, seaweed and sometimes just on the bottom.

Diplosoma sp.
These greenish colonial ascidians with whitish margins are from the family Didemnidae, likely to be a Diplosoma sp. as well. They can be seen growing on the ground, or on seagrasses and seaweed.

These green colonial ascidians are also from the family Didemnidae, and are usually found on the sides of rocks.

colonial ascidian
This enrusting colonial ascidian is usually found under rocks and coral rubble.

colonial ascidian
This encrusting colonial ascidian is often mistaken for sponges. It can be found on rocks and coral rubble.

colonial ascidian
These bright orange colonial ascidians are usually found attached to seagrasses or seaweed.

colonial ascidian
These pale colonial ascidians with black margins are usually found under rocks or on the pillars of jetties.

  • Allen, G. & R. Steene. 1999. Indo-Pacific Coral Reef Field Guide. Singapore. Tropical Reef Research. 378 pp.
  • Encyclopedia of Life. Retrieved May 10, 2013, from
  • Gittenberger, A. The Dutch Ascidians Homepage. Retrieved May 10, 2013, from
  • Ruppert, E.E. and R.D. Barnes. 1991. Invertebrate Zoology (International Edition). Saunders College Publishing. U.S.A. 1056 pp.
  • World Register of Marine Species. 2012. Retrieved May 10, 2013, from

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