On the mention of sea cucumbers, perhaps the first thing that comes to mind would be the slippery and gelatinous seafood, often braised in a broth of shiitake mushrooms to make a highly coveted Chinese delicacy. Given its common name, one may also be inclined to think that sea cucumbers are plants. However, did you know that sea cucumbers are in fact animals, and some are even native to Singapore?
Sea cucumbers belong to the phylum Echinodermata, along with other animals such as sea stars and sea urchins. “Echinodermata” is derived from two Ancient Greek words — echinos (ἐχῖνος), meaning “hedgehog”, and derma (δέρμα), meaning “skin”. The direct translation of this name applies literally, as all echinoderms, including sea cucumbers, are characterised by hard, spiny body covering or skin. All echinoderms are also marine and have pentaradial body symmetry.
Ranging from almost perfectly round to elongated, from 3 mm to 3 metres long, and from vibrantly coloured to transparent, sea cucumbers come in a plethora of shapes, sizes and colours. Unlike their sea star counterparts, sea cucumbers lack spiny body coverings as well as arms, and do not lie on their mouths. Instead, they lie on their sides, and have soft skin made of tissue and muscle which can harden up to protect themselves from danger. To make up for their soft bodies and slow movement, sea cucumbers have a wide range of defense mechanisms, from having toxins to regurgitating their intestines and organs, or even ejecting sticky strings from their anuses. An example can be seen in the picture on the bottom-right corner of this collage. Sea cucumbers are found in both shallow and deep waters alike, present in several continents all around the world. On the shores of Singapore, there is also a great variety of sea cucumber species!
The sea cucumber we encountered during the Pulau Semakau Intertidal Walk was a Garlic Bread sea cucumber (Holothuria scabra), which lives in seagrass meadows of the Northern and Southern shores of Singapore. Named after their appearance, Garlic Bread sea cucumbers have a loaf-like shape. It is usually between 15-20 cm in length, but can be up to 40 cm long and 1.5kg in weight! The upper side is dark brown in colour and has folds or black lines, while the distinct underside is flat and pale brown or white. Like most sea cucumbers, the Garlic Bread sea cucumber have appendages known as tube feet distributed all along their bodies. Most tube feet enable the sea cucumber to cling to surfaces, while there are around 20 tentacles around its mouth which have been modified into branched feeding tentacles. These tentacles are often covered with mucus, creating a large surface area to volume ratio for feeding, helping to increase the efficiency of gathering detritus on the seafloor. In fact, the diet of sea cucumbers also helps to clean the sea floor of detritus and other waste, thus regulating the balance of the ecosystem. Garlic Bread sea cucumbers also ventilate using a system of internal branching tubes, known as a respiratory tree, allowing water to enter and exit the animal via the anus. Sometimes, the comfortable environment created may also cause pea crabs to call the opening of these respiratory trees home.
Unfortunately, the harmless nature of the Garlic Bread sea cucumber causes it to be widely exploited by humans, for purposes such as collection and dining, and the coastal development of Singapore may also have caused the population to dwindle further. As of now, the Garlic Bread sea cucumber is listed as ‘Vulnerable’ on the Red List of threatened animals of Singapore. This is a worrying phenomenon as sea cucumbers can make up as much as 90% of the marine ecosystem, and play a pivotal role in enhancing the survival of other organisms such as seagrasses.
Currently, efforts have been made to culture sea cucumbers for dining, although these are still in the infant stage. However, with concerted effort and time, it is hopeful that these fascinating creatures can be conserved for the future, at the same time maintaining a constant supply of our popular delicacy!
References:
- Connor, S. (2011, October 27). Ready to acquire a taste for the ‘vacuum cleaners of the sea’? Retrieved from https://www.independent.co.uk/news/science/ready-to-acquire-a-taste-for-the-vacuum-cleaners-of-the-sea-2202601.html
- Garlic bread sea cucumbers (Holothuria scabra) on the Shores of Singapore. (n.d.). Retrieved from http://www.wildsingapore.com/wildfacts/echinodermata/holothuroidea/scabra.htm
- Sea cucumbers (Holothuroidea) on the Shores of Singapore. (n.d.). Retrieved from http://www.wildsingapore.com/wildfacts/echinodermata/holothuroidea/holothuroidea.htm
- Sea cucumbers have a secret superpower. (2017, April 20). Retrieved from https://www.popsci.com/secret-superpower-sea-cucumbers
- SEAA. (2015, March 31). 10 things you didn’t know about sea cucumbers. Retrieved from http://seaa.rwsentosablog.com/sea-cucumber/
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