Sea cucumbers are Echinoderms, like starfish and sea urchins. There are 1,250 known species of Sea Cucumbers, with the largest species reaching up to 3.3 metres in length. Here on Koh Tao the largest ones we have seen ones have been approximately 8 feet long.
The average Sea Cucumber grows between 3.9 to 12”. Sea cucumbers has elongated, cylindrical bodies which are red, black, blue, green or brown in color. They have soft bodies even though they have some sort of skeleton below the surface of their body. The skeleton usually consists of plates and spines. These structures can be exposed and used against predators (sea cucumbers are not tasty meal when their bodies suddenly harden).
Sea cucumbers use an interesting technique when they are faced with danger. They are able to expel internal organs along with toxic substance (called holothurin) towards the predator and all expelled/missing organs regenerated within 1.5 to 5 weeks. They have five rows of paired tentacles on the surface of the body which are used for locomotion and for feeding. They do not have brain. Their nervous system consists of a ring and several nerves and their senses are poorly developed.
Sea cucumbers breathe via their anus. Branched network of hollow tubules absorb oxygen from the water that circulates through the anus. They can modulate the shape of their body when needed, so they can pass through tight spaces.
Their diet consists of decaying organic matter (from the sea floor) and plankton. They play an important role in cleaning the ecosystem by removing remains of dead animals.
Sea cucumbers are nocturnal creatures (active during the night). They are able for self-reproduction (they can produce offspring without partner). However, they prefer to reproduce by mixing with other sea cucumbers. Fertilisation occurs in the water after they release of huge amount of eggs and sperm cells. Fertilised eggs undergo several larval stages before they turn into adult sea cucumbers. Their lifespan, depending on species is between 5 and 10 years.
Some of the most common species here on Koh Tao are the Pinkfish, Black sea cucumber, Marbled sea cucumber, Orange spiked sea cucumber and on occasions, the Giant Synaptid Cucumber. All play an important role as decomposers, clearing algae from the substrate and help keep nutrient levels in the water low.
Would you like to learn more about the ocean and its marine life? Then sign up for the Reef Check Eco Diver Course.