Picture credit : Tim Long

Tunicates, also know as Urochordata or Ascidians are commonly called sea squirts. Tunicates are among the most common marine invertebrates with around 3,000 species. Most tunicates live attached to a hard surface on the ocean floor. It often comes as a surprise to learn that they are actually more closely related to vertebrates like ourselves than to most other invertebrate animals.

Most tunicates are hermaphrodites. They avoid self fertilisation by either having the eggs and sperm reject each other, or by having the eggs and sperm mature at different times. Sperm are released into the sea but the eggs are retained within the body where they are fertilised by sperm brought in with incoming water. The eggs are brooded within the body until they hatch.

Tunicates are filter feeders, feeding by drawing often hundreds of litres of water each day through the inhalant siphon. This water passes through the pharynx where small particles are filtered out before the water is expelled through the exhalent siphon. The water current is caused by beating cilia. Water can also be pushed out of the atrial cavity by muscular contractions of the tunic if the tunicate is threatened. The small particles of plankton, etc, are trapped on a continually moving layer of mucous. This mucous is released by special cells and is moved across the surface of the pharynx by the beating of many small cilia, until it is passes into the digestive system where the food particles and mucous are digested.

On Koh Tao, we have blooms of Tunicates over some of our natural dive sites and also our artificial dive sites. These fast growing organisms often compete for space with the coral transplants that we have in these areas. We are currently conducting some research with another local dive school to study them further.

If you would like to help with our research or learn more about Tunicates around Koh Tao then visit our Eco Internships page.