Known to science as Holothuria leucospilota, the black sea cucumber is the most common sea cucumber in Hong Kong, but is considered endangered in mainland China because of overexploitation. Dive or snorkel anywhere on the eastern half of Hong Kong and you are certain to find a few!
While relaxed it can be up to 40 cm (16 in) long, but it can stretch to about a metre (yard). It is sausage-shaped, tapering towards the back end. At the head end, there are twenty oral tentacles with branched tips. These surround the mouth which is on the under side of the body and help shovel food into its mouth. The animal is soft and covered with small fleshy protrusions called papillae. The usual colour is charcoal grey or reddish-black with pale grey tube feet on the underside.
Holothuria leucospilota is normally found in shallow water from east Africa to the western Indo-Pacific region including the South China Sea and Hong Kong waters. It is also a common species on the north-east coast of Australia where it is found on reefs and rocky coasts, often partly concealed under a boulder. It is generally more common near boulders, corals and seaweed clumps than on the open seabed at depths from 0-15 m. A study from Singapore found that the Black Sea cucumber is relatively tolerant of changes in the salt content and temperature of seawater. During identical salinity and temperature changes, the Japanese sea cucumber (Apostichopus japonicus) shrank in size, eviscerated (spewed out its own guts!), and died within three days. The black sea cucumber, however was fine.
Holothuria leucospilota is a scavenger and when feeding it usually has its back end anchored underneath a rock or in a crevice so that it can contract back out of sight if disturbed. It feeds by using its tentacles to shovel organic debris lying on the seabed into its mouth. In the process it swallows a lot of sand, which passes through the gut. In doing this it can remove and feed on the thin layer of bacteria and micro-algae that coats the grains of sand – this layer is normally called ‘biofilm‘.
Here is a nice clip of a black sea cucumber feeding on sand:
And here is a lovely clip of one feeding on detritus floating its way in the water. You can also see its tiny tube feet on the glass:
A study by HKU found that the sea cucumber can detect the sand patches with the most biofilm, because the sand it eats has about twice the amount of organic matter than the average sand patch. It churns through just under half a kilo of sand a year and feeds 3-4 times faster in summer than in winter.
Black sea cucumbers are commercially harvested in Malaysia, China and Vietnam, processed by eviscerating and freeze drying. Although the black sea cucumber is not highly prized: wholesale prices are around USD 5 per kg compared with USD 42 per kg for some other species.
The 1st Horror
As a boy I learned that when threatened or handled (especially out of the water) the black sea cucumber can release a sticky white substance from its bottom called Cuvierian tubules. These can entangle potential predators and lets the sea cucumber escape. As the name implies the substance is actually made up of very fine tubes and these form inside the sea cucumber.
Warning! It gets a bit gross now: when stressed, the sea cucumber faces away from the attacker and contracts its body muscles sharply, causing the wall of the cloaca to tear, the anus to gape open and the free ends of some of the tubes to be ejected. I warned you…
The Cuverian tubules are presumably what give the black sea cucumber its other common name ‘white thread fish’ which is pretty silly as it isn’t a fish by any stretch of the imagination. Sea cucumbers , or Holothurians to scientists, are related to starfish and sea urchins and are invertebrates….but I digress.
The Cuverian tubules lengthen when they come into contact with seawater and become amazingly sticky when they meet objects or human hands! It takes fifteen to eighteen days to regenerate these tubules. Now that you know what it takes to project the sticky tubules (painful rectal tear) and that it takes 15-18 days to regenerate leaving the sea cucumber vulnerable, spare a thought for the sea cucumbers and don’t deliberately stress them just to see the effect. Especially not because the sticky white stuff actually has a toxin called holothurin (sea cucumbers are known in science as Holothurians from the Greek word meaning water polyps).
Instead you can watch someone in this YouTube clip “stressing” a black sea cucumber:
My personal experience with the Black Sea cucumber is that skin contact with the toxin does not cause any harm other than general unpleasant stickiness, but the general advice is to avoid contact with it completely and wash affected areas properly to avoid any contact between contaminated skin and the eyes. This is because people have reported holothurin to cause conjunctivitis and blindness! So whatever you do, do not let the tubules come into contact with your eyes (likely via your hands)!
The 2nd Horror
The black sea cucumber sometimes plays host to the worm pearlfish (Encheliophis vermicularis), a fish that lives as a parasite inside sea cucumber guts where it feeds on the sex organs (gonads). It can grow to 15 cm…nearly half the body length of its host sea cucumber! Oh yeah – and they like to live in male-female pairs inside the sea cucumber. They enter and exit the sea cucumber through its anus and seem to be attracted chemically to the Cuverian tubules – the sticky white substance mentioned before. They also somehow manage to not get entangled in the tubules because black sea cucumber (its favorite host) does not eject the tubules in response to the worm pearlfish’s “comings and goings”.
Here is a clip showing an example of different species of sea cucumber and pearlfish “interacting”…
Sex or cloning? Cloning, please
The black sea cucumber has two methods of reproducing: sexual and asexual. In sexual reproduction the sea cucumbers spawn releasing sperm and eggs into the water in the hope that some eggs will get fertilized. These then develop into larvae that float and feed in the plankton until they are big enough to settle and transform into juveniles. Asexual reproduction takes place by “transverse fission” – basically the sea cucumber splits itself in half down the middle an each half grows into a full sea cucumber. Cool, huh?
In Taiwan this species reproduces between June and September. Unfortunately, I couldn’t find any information the reproduction of the black sea cucumber in Hong Kong waters.
And finally, here’s is a short list of some of the interesting research being done on the black sea cucumber:
– evaluation of its polysaccharides in the treatment of tumours
– evaluation of its chemical compounds as antimicrobial or anti-fungal treatments
– using them to absorb the organic detritus falling to the seabed under fish farms while producing a secondary commercial crop of sea cucumbers