Monday, April 02, 2012

Sea Cucumbers (Phylum Echinodermata: Class Holothuroidea) of Singapore

Sea cucumbers (class Holothuroidea) are elongated and somewhat cylindrical animals with distinct front and rear ends. More than 23 species of sea cucumbers have been recorded in Singapore.

They belong to a bigger group of animals called the spiny-skinned animals (phylum Echinodermata - "echino" roughly means "spiny"; "derma" roughly means "skin"). Examples of other echinoderms include sea stars, brittle stars, and sea urchins.

Like other echinoderms, sea cucumbers have a five-part body plan with radial symmetry (i.e. pentaradial symmetry), at least in some stage of life. In other words, you can divide a sea cucumber into 5 equal parts. This feature may not be as obvious compared to other echinoderms like sea stars and sea urchins. Just try to imagine a sea cucumber as a very tall but skinny sea urchin without spines - so tall that it has to lie down sideways and move around like a worm. In other words, the mouth of a sea cucumber is not facing down and the anus up, but rather, they are facing the sides like the 2 ends of a cylinder lying sideways.

Stichopus ocellatus
Echinoderms are brainless, but despite that, they can still perform their daily functions - they can move, they can eat, they can shit, and they can reproduce - all these without a brain! Also, instead of blood vessels, echinoderms have a water vascular system. This system is essentially a network of water-filled vessels used for internal transportation of oxygen, food and waste. By adjusting the water pressure, the sea cucumber is able extend and retract its tube feet for moving and extend its oral tentacles for feeding. Most of them are detritus feeders, swallowing sand, mud or silt and digest any organic matter within, while others are suspension feeders and gather organic matter with their tentacles in the water. Meanwhile on the other end, the anus is not only used for excreting waste but also for breathing as well - the breathing organs, called respiratory trees, are located near the anus.

Holothuria fuscocinerea
To protect themselves, some sea cucumbers are able to eject white, sticky threads (cuverian tubules) or even their internal organs to confuse and deter predators too. Like other echinoderms, they are able to regenerate the lost organs, but in the meantime, they will not be able to feed. Made of a special tissue (catch connective tissue), the sea cucumber can also harden itself when harassed by a predator, or become soft to creep into cracks and crevices. Most species also produces a poison called holothurin, and hence sea cucumbers must be properly treated to remove the toxins before they can be consumed by humans.

Sea cucumbers are usually positively identified by extracting their internal skeletons, which are made up of microscopic calcified structure called ossicles, joined by the connective tissue. Many of the species in local waters, however, can be identified based on their physical appearances.

Family Holothuriidae

This is one of the biggest sea cucumber families, and many of the members are harvested for food. They generally have thick, muscular bodies covered with papillae (bumpy projections), and they have tube feet for moving around. Most are deposit feeders, using their feeding tentacles to pick up detritus (tiny decaying particles). Many of them can also eject cuverian tubules or internal organs when threatened.

sandfish sea cucumber (Holothuria scabra)
The Sandfish Sea Cucumber (Holothuria scabra) is one of the most commercially valuable species from this family found in local waters. Often eaten as a delicacy in Chinese restaurants, they are sometimes over-harvested in the areas where they can be found. The body is somewhat flattened ventrally, and the colour is usually brownish grey. This species burrows into the sand, using their oral tentacles to gather tiny food particles in the sand. Their sand-like coloration allows them to camouflage with the surrounding sand. It can grow to about 50cm long.

sandfish sea cucumber (Holothuria scabra)
Black coloured variations of the Sandfish Sea Cucumber are sometimes seen in local waters.

stonefish sea cucumber (Actinopyga lecanora)
The Stonefish Sea Cucumber (Actinopyga lecanora) is another edible species. It is usually found among rocks in the coral rubble and coral reefs, appearing like a piece of stone, hence the commercial name. It has a very smooth body, and the colour varies from a uniform brown, to light brown with darker blotches. The area around the anus is usually whitish, and five anal teeth can be seen in the anus. It was recently found that extracts from this sea cucumber can be used to treat leishmaniasis, a tropical parasitic disease affecting 12 million people in 88 countries. It can grow to about 30cm long.

Actinopyga sp.
A smaller and reddish Actinopyga species has been seen in local waters. It is not yet sure if this is a juvenile stonefish sea cucumber or a different species.

brown sandfish sea cucumber (Bohadschia vitiensis
The Brown Sandfish Sea Cucumber (Bohadschia vitiensis) is also collected for food, though it is not as popular as the sandfish. It can eject white, sticky cuverian tubules when disturbed. On its body are numerous dark spots on a yellowish background. It usually burrows into the sand in the day. It can grow to over 30cm long.

black sea cucumber (Holothuria leucospilota)
The Black Sea Cucumber (Holothuria leucospilota) is commonly found under big rocks or coral rubble, using its tentacles to pick up deposited detritus. There are numerous papillae on its body surface, and it readily ejects cuverian tubules when disturbed. It can grow up to almost a metre.

Holothuria fuscocinerea
Holothuria fuscocinerea is another species found under rocks or coral rubble and readily ejects cuverian tubules. It is usually light brown with darker patches, reaching lengths of about 50cm.

Holothuria notabilis
Holothuria notabilis is found in sandy areas and usually burrows into the substrate. It is usually brown with numerous yellow or white spots, growing to about 30cm long.

Holothuria ocellata
Holothuria ocellata is another species found in sandy areas, burrowing deeply into the sand. On its body is numerous ring-like patterns, each with a spiky papila inside. It can grow to about 30cm long.

Holothuria cf. impatiens
Several Holothuria species remains unidentified. The above is possibly a Holothuria impatiens, and is usually found under rocks or coral rubble too. It can reach length of almost 50cm.

Holothuria sp.
This is another unidentified species, possibly a Holothuria sp., that is seldom seen.

Family Stichopodidae

Sea cucumbers from the family Stichopodidae mostly have a somewhat squarish or rectangular cross-section, and many have tube feet in rows on the underside. Many have a warty or bumpy appearance. They do not eject cuverian tubules when threatened, but many can eject their internal organs if attacked by predators. Some species can shed off their skin to distract predators.

dragonfish sea cucumber (Stichopus horrens)
The Dragonfish Sea Cucumber (Stichopus horrens) is commonly found in seagrass meadows and adjacent reef flats. It has numerous spiny bumps on its yellowish brown body, and the cross-section is squarish. While the entire animal is seldom collected for food, natives in the pacific are known to cut it open and its intestines can be eaten raw, sometimes with a bit of lemon. The animal is then returned to the sea, where it will regenerate its lost intestines. The body fluid of this sea cucumbers is also harvested to make air gamat, a traditional Malay tonic used to aid healing, especially for women after delivery.

dragonfish sea cucumber (Stichopus horrens)
Interestingly, this sea cucumber can detach part of its skin when attacked by predators or when stressed. Like other species of the genus Stichopus, they are known to become very limp, as if they are melting, and eventually disintegrate if they are taken out of water for long period of time. They are able to reverse the process if returned to water in time. The Dragonfish can grow to about 30cm long, but those in the intertidal area seldom reach 20cm.

ocellated sea cucumber (Stichopus ocellatus)
The Ocellated Sea Cucumber (Stichopus ocellatus) is closely related to the dragonfish sea cucumber. "Ocellated" means "having little eyes", refering to the eye-like bumps on the yellowish body surface. These bumps are believed to have some find of sensory functions and also help the sea cucumber to hold on to hard surfaces. It can grow to a huge size, about 50cm long and 15cm wide.

curryfish sea cucumber (Stichopus herrmanni)
The Curryfish Sea Cucumber (Stichopus herrmanni) is also found in Singapore. This species is also collected for the sea cucumber trade. The colour is quite variable, ranging from cream coloured, to brownish to dark greenish brown. The body is covered with orange or whitish papillae, and with short tube feet. It can grwo to near 50cm long.

Stichopus vastus
Stichopus vastus is more commonly seen by divers in deeper waters, but occasionally they can be seen in the intertidal areas too. It can grow to almost 50cm long, and the body is covered with big lumps marked with numerous dark stripes.

Family Cucumariidae

Sea cucumbers of this family are usually small, and have ten branching tentacles of which the lowest two are much reduced and easily missed. They are filter feeders, spreading their branching tentacles to filter plankton from the water. Most species are found on sandy substrates or among seagrasses, but seldom found in coral reefs.

thorny sea cucumber (Colochirus quadrangularis)
The Thorny Sea Cucumber (Colochirus quadrangularis) appears to be seasonally abundant, and sometimes hundreds of them are found among seagrasses, rocks and other structures in the intertidal area. It is a suspension feeder which gathers tiny food particles from the water with its colourful tentacles. Only 8 of the tentacles are easily seen above, and if you look closely, you will see the lowest 2 being very much reduced and are touching the mouth. This sea cucumber is pink with lots of spiky protrusions. It is quite small, with the biggest ones seen about 10cm long.

warty sea cucumber (Cercodemas anceps)
The Warty Sea Cucumber (Cercodemas anceps) is less abundant than the Thorny Sea Cucumber, though good numbers can also be seen when they are in season. It can be differentiated from the latter by the yellow patches on the pink body, and having wart-like structures instead of spiky structures. Both the above sea cucumbers are brightly coloured, possibly to advertise their toxicity. It is about 10cm long.

sea apple (Pseudocolochirus axiologus)
The most colourful and prettiest local sea cucumber should be the Sea Apple (Pseudocolochirus axiologus). It is usually bright red with some parts in purple and yellow, and appears like a ball, reaching length of about 15cm.

Many local Cucumariidae species remained unidentified, such as the one in the photo above.

Family Synaptidae

synaptid sea cucumber
The sea cucumbers of this family are commonly called Synaptid Sea Cucumbers. They are another group of sea cucumbers commonly seen on our shores, but many of them are not yet identified. They appear worm-like without tube feet. Instead, they have tiny hook-like structures on their skin, which feel somewhat sticky to the touch and allow the animal to hold on to rocks or seaweed and seagrass. When are usually seen with their oral tentacles extending and lashing around, picking up organic matter from their surrounding.

synaptid sea cucumber (Opheodesoma sp.)
Many synaptid sea cucumbers can grow to great length - Opheodesoma sp. can grow to over 1.5m long, and is usually found in seagrass meadows. It has rows of bumps running down the sides of its body, which is usually orange or brown in colour, sometimes with darker blotches.

Synaptula sp.
This Synaptula species can stretch to almost 1 m long, and is found in seagrass meadows too. It is pinkish red in colour, with a smoother body surface.

Lampert's sea cucumber (Synaptula lamperti)
These are probably Lampert's sea cucumber (Synaptula lamperti), from the same genus as the previous species, but much smaller about 5-10cm long. They are usually found congregating on sponges.

Family Caudinidae

Members of this family have a thin body wall and no tube feet. They usually live in U-shaped burrows in sandy or muddy substrates, extending their tentacles out of the burrows to catch plankton. Often, their back end also extend above the surface of the substrate, believed to aid gas exchanges.

sea cucumber (Paracaudina australis)
This Translucent Sea Cucumber (Paracaudina australis) is a burrower on our sandy shores. You can even see the double stripes of muscles along the sides of its body. The body is usually smooth, and even slimy.

Paracaudina sp.
This smooth sea cucumber is also a Paracaudina sp., if not a colour variation of the above species.

Family Phyllophoridae

The phyllophorids are usually determined to the species through their calcareous ossicles, which form a complex ring making a mosaic pattern, arranged in a tube. The burrowing species have U-shaped bodies while in the sand, but shape-shift into a ball when they are dug out. Others may have spindle-shaped bodies.

ball sea cucumbers (Phyllophorus sp.)
The Ball Sea Cucumbers (Phyllophorus spp.) are found in sandy areas, and are usually round in shape when out of the sand. The body is surrounded by tube feet, and the colour ranges from orange to brown to grey.

ball sea cucumbers (Phyllophorus sp.)
Most of the time when they are burrowing, only the feeding tentacles can be seen. These sea cucumbers are collected for their body fluids, which are used to make air gamat.

Family Sclerodactylidae

Members of this have a complex ring of ossicles near the front end, but unlike those of the phyllophorids which form long tubes, those of the sclerodactylids form either short tubes or may not even take the form of tubes at all. Many species of this family are able to reproduce by fission.

Afrocucumis africana
The Afrocucumis africana is a species which is usually found hiding under rocks near the upper shore. This small sea cucumber is dark purple or black in colour, and is known to reproduce asexually by fission. Hence, you can usually find more than one under the same rock.

Unknown Sea Cucumbers

sea cucumbers
These little brownish sea cucumbers are usually found under rocks, and they are yet to be identified.

The small reddish sea cucumbers have a somewhat scaly appearance. They are occasionally found under rocks or by the sides of rocks.

These small pinkish sea cucumbers are often seen in huge numbers among seagrasses.

Sea cucumber
This brownish sea cucumber is sometimes seen in seagrass meadows as well.

Sea cucumber
This whitish sea cucumber grows to about 20cm long, and is sometimes seen among rocks on rocky shores.

Like most other marine organisms, the sea cucumbers in Singapore are threatened by loss of habitats and collection.

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