Divernet

It was brown, and lay perfectly still. I was baffled by what appeared to be a small log on the seabed at 12m. I had noticed it simply because it was so dull among the bright colours of the Bunaken reef, and according to my basic laws of physics, werent logs supposed to float
Suddenly, having decided that I was getting a little too close for comfort, the startled log pointed its anus and fired a projectile of long, sticky white tubes at my face. Equally startled I moved away, wondering what my curiosity had made this huge sea-slug do. Would it now die like a bee that has released its single fatal sting No, it would just regenerate its lost parts, I was informed by our knowledgeable dive guide.
That was my first encounter with the marrow-shaped sea cucumber, or Holothurian, and the beginning of a study that taught me never to judge a book by its cover.
When provoked, certain species of sea cucumber shoot a network of the culvian tubes that line its anus at the intruder. Computed as foe, I had encountered this bizarre self-defence strategy.
If small enough, the attacker becomes entangled in the sea cucumbers sticky web.
Crabs and small crayfish can die a slow death this way. Fortunately for the sea cucumber its attackers are few and far between, and its internal organs are a highly poisonous and effective deterrent to most predators.
Even stranger is how this creature deals with a life-threatening situation. Its sides split open and it voluntarily disembowels itself, tossing most of its internal organs over-board.
Whether this bizarre phenomenon - called evisceration - is a method of defence is debatable. One explanation is that it occurs at a time of low water oxygen levels or high temperature, when its living conditions are threatened or simply unsuitable.
It makes sense to shed any unnecessary baggage as an energy-saving device - the sea cucumbers metabolism is far lower without its internal organs, and it can manage without them. After evisceration, as with loss of the culvian tubes, when conditions improve the lost parts simply regenerate.
Adaptation is the key to survival, and if sheer numbers are anything to go by, the sea cucumber in its strange way is a master of the game.

There are more than 1000 species of Holothuroids, and they are almost as varied as they are numerous. Ranging from 2cm to 2m long, with soft, cylindrical bodies and warty, leathery, cucumber skin, they are also among the few animals whose habitat is not restricted by ocean depth.
So numerous are the deep-sea species, Elasipods, that they account for 50 per cent of life-forms at 4000m and 90 per cent at 8000m! Steven Spielberg could have a field-day with these creatures of the deep, which have adapted to their environment by developing sail-like fins, except that such docile characters would never pass the screen test.
Although the dried and smoked skin of the sea cucumber is regarded as an aphrodisiac and delicacy in Asia, trepang or beche-de-mer, as it is called, is farmed on a sustainable, cottage-industry basis only. Holothuroids are not an endangered species.
Normally dull in colour, sea cucumbers are found either burrowing or slowly grazing sandy seabeds. Burrowing species swallow the silt and sand, extracting animal and plant remains as the sediment passes through the intestine.
Feeding is a full-time occupation, and even at a burrowing speed of 2-3cm an hour, the sea cucumber still manages to digest, filter and empty some 45kg of seabed a year. Large populations therefore play an important role in the changing shape and composition of the ocean floor, as they sluggishly spring-clean the seabed.
Other, non-burrowing species, feed using the 10 to 30 tentacles around their mouths. Species such as Holothuria and Cucumaria use these either to sweep the ocean floor for microbes, extend them out in the water to catch passing small organisms and plankton, or sweep the ocean floor for any unfortunate algae or diatoms, which are then drawn into the mouth opening.
Not only can a sea cucumber shed parts of its anatomy, but being a back-to-front sort of creature it breathes through its anus as well as discharging waste from it.
Water is drawn in through a series of muscle contractions, and oxygen is absorbed into the walls of one or two respiratory trees lining the anus before being expelled. The deep-sea varieties are the only exceptions to this remarkable adaptation, obtaining oxygen through their entire body surfaces.
One creature that takes advantage of the movement of water to the respiratory trees in the sea cucumber is the pearlfish, or cucumber fish (Carapidae).
Having developed a rather indelicate and guilt-ridden fixation for peering into the bottom of every sea cucumber - in the name of research, and on a strictly look-but-dont-touch basis - I have been fortunate enough to see the pearlfish in residence on a few occasions.
About 15cm long, this fish, like its passive host, could easily be overlooked for its lack of striking colours. Its long, thin shape, however, suits it for living in the gut of its host. Perfectly hidden during the day, it emerges at night to forage.
Most such symbiotic relationships hold some kind of benefit for both animals, but the sea cucumber appears to gain little from the pearlfish.
Beside the risk of accidental injury caused by the fishs anal entry, and coupled with having to work that bit harder to get water into the respiratory trees, the sea cucumber also has to contend with a tenant that is not averse to nibbling the walls of its home! Once again the resilient sea cucumber uses its powers to repair any damage.

The pearlfish is not the only creature to take advantage of the regenerative powers of its hardy landlord. Growing up to 1.3 metres in length, one particular snail, Parenteroxenos doglieli, is another parasite that lives in the body cavity of the sea cucumber, whilst scaleworm, harlequin and pinnotherid crabs reside externally, having an occasional nibble too.
Reproduction is the most normal thing about sea cucumbers, and as with most fish involves external fertilisation. The male and female face each other, rise up cobra-like and release sperm and eggs into the water, creating microscopic embryos.
Within a few days these are free-swimming, until they settle on the sea-bottom and continue to grow to adulthood.
There are exceptions to the rule. Apart from certain asexual species, some brood their young in small pockets in the skin surface, while in the case of Thyone rubra and Leptosynapta the young develop in the colon, with the sides of the mother splitting open to release her developed offspring into the ocean.
Years after my first underwater encounter, I am still fascinated by the sea cucumber and can only marvel at the bizarre and wonderful capabilities camouflaged within this unassuming plodder. Well, as they say: Its the quiet ones you have to watch.