Echinodermata

As an amateur marine life enthusiast, I am continually frustrated by the lack of readily available information on the animals I see. When my 'popular' books fail me, I do not have access to experts, or to a specialist library where I can hunt through monographs and journals. It can be years before I come across the perfectly well-known answer to a simple question. The result is that I often have to work out the possible or probable answer for myself from observation and guesswork, and hope that eventually a reference or chance acquaintance will confirm it.

The Cotton-Spinner, a big sea cucumber properly called Holothuria forskali, provoked several such questions a while ago. One has now been answered; the answers to the others remain speculative.

I see Cotton-Spinners regularly when diving over rocky grounds in the south-west; they are said to be found occasionally at extreme low tide level. The adults are noticeable animals which resemble huge black warty sausages 20 cm (eight inches) or more long, slightly flattened underneath, with the back covered in small conical projections. The upper surface is black or dark brown, the underside dull orange or yellow; sometimes the yellow colour extends onto the back between the projections.

Tube Feet

Although they look very different, sea cucumbers are related to sea urchins and starfish, and like them have numerous extensile tube-feet which can be modified for various purposes. In Holothuria, those on the underside have suckers, and the animal walks and clings on with these; it can climb vertical rock, but is most often seen on ledges and near-horizontal surfaces where sediment collects. The conical papillae on the back seem to be modified tube-feet with a sensory function (my books are not entirely clear on this point). And around the mouth, highly specialised tube-feet form oral tentacles which are used for feeding.

Feeding

Holothuria is an unselective deposit feeder, eating large quantities of silty sediment, digesting what is edible, and ejecting the rest as droppings which look like chains of tiny silt-coloured sausages. The mouth is at one end and the anus at the other; the oral tubercles are retracted into the mouth when the animal is not feeding, so the two ends usually look identical. The first time I noticed that many Cotton-Spinners had one end slightly flattened and widened underneath, as if it was pressed against the rock, I thought they must be feeding.

However when I looked it up, my books definitely stated that feeding was at night, and were agreed that the mouth was on the end of the body, not underneath. Furthermore, the short stubby oral tentacles are always illustrated projected forwards – which seemed a curiously inefficient orientation for scooping sediment off the rock into the mouth.

What were the sea cucumbers doing if they were not feeding? I picked up several of those with flattened ends and turned them over. The centre of the splayed area was always contracting rapidly to a tiny puckered hole which could certainly have been the mouth, and although no tentacles were visible, they might have retracted in the second or two it took to detach the animal. There was certainly no trace of a mouth on the end of these animals.

After some months of puzzlement (I do not often get the chance to dive in the south-west) I watched an individual in which the tips of blunt dull-orange projections kept appearing and disappearing under the edge of the flattened 'head'. On the same dive I picked up another specimen which was gripping the rock very loosely with its tube-feet; I turned it over fast enough to see orange projections disappearing into the contracting mouth. They could only be the oral tentacles; despite what the books said, the animals were feeding.

Close Observations

I have since watched several specimens feeding actively, both while diving and in a public aquarium. When at rest, the mouth is at the end of the body, tightly closed, with the oral tentacles retracted and invisible. To feed, the front end of the body is curved downwards towards the substrate (usually rock with a thin layer of silt) and the mouth opens wide; the ring of papillae surrounding it just touch the rock and may be sensory, assessing the nutrient content of the sediment. The oral tentacles are short, branched and flattened, either yellow\orange or black, and move regularly in and out; they often remain within the ring of papillae, but can extend well beyond. Their undersides appear to be adhesive, for instead of shovelling sediment into the mouth, as books describe, they are lowered onto the substrate, then lifted again with some adhering sediment and retracted into the mouth.

Misleading Illustrations

It took a little longer to discover why my books were so misleading, but the answer appears to be very simple; most book illustrators and at least some authors have never seen a live Holothuria behaving normally. They are not very often kept in aquaria, and until recently it was usual for illustrators to draw preserved specimens. For many animals this works reasonably well, but a narcotised sea cucumber dies in a semi-relaxed position it does not normally adopt while alive.

I knew already that the crevice-living, suspension-feeding sea cucumbers are totally unidentifiable from drawings of 'pickled' specimens, which show short thick tentacles at the end of a cylindrical body, while live animals have the body almost entirely concealed and the tentacles fully extended in a delicately-branched bushy mass. You only see the tentacles as they are illustrated, three-quarters retracted, if you disturb the animal, and then only for a second before they vanish into the mouth for safety. A narcotised Holothuria also relaxes into an unlifelike posture with the mouth at the end of the body but the tentacles partially protruded outwards, and is carefully and misleadingly illustrated and described like that.

Bionomics

Why books insist they always feed at night is more speculative, but is probably based on aquarium observations made before scuba diving became common. Possibly observers, 'knowing' from preserved specimens that the mouth and oral tentacles are at the end of the body, failed to recognise the unobtrusive feeding behaviour, and assumed that feeding must be at night because copious droppings appeared although the animals were not seen to feed. A more likely explanation is that small individuals really do hide by day and feed at night to reduce the risk of being eaten by a hungry carnivore. Since aquarists usually prefer smaller specimens because they are easier to obtain and take up less tank space, they would observe that small individuals fed at night and assume, wrongly, that this applied to all Cotton-Spinners.

Juveniles

Small Cotton-Spinners are very rarely seen by divers; those I see commonly measure 8 - 10 inches. Small specimens are unusual enough to attract attention, and I have not seen one less than 60 mm (4 inches long). Since Holothuria has a planktonic larval stage, it must be minute when it settles onto the seabed and metamorphoses into a juvenile sea cucumber; but they are conspicuous creatures, and youngsters 25 mm (1 inch) or more long could not easily be overlooked if they occupied the same habitat as the adults. So where do they live?

A rockpooler told me the few Cotton-Spinners he has found were invariably small and extremely well hidden. I have no other evidence that young Holothuria, both on the shore and sublittorally, hide in crevices and under rocks by day and only venture out at night, but it would make good sense. The young of some, perhaps many, species do this to reduce the risk of being eaten by a hungry predator while they are small and vulnerable.

Small crabs, baby Edible Sea Urchins and young dogfish all seem to live concealed while the adults are active, or at least visible, by day. It seems the only sensible explanation for the apparent absence of little Cotton-Spinners.

Defences

The third puzzle is the Cotton-Spinner's defensive behaviour.When attacked, it is described as turning its rear end towards the threat and expelling a stream of long sticky white threads forcibly from the anus. These swell and elongate, confusing or entangling the would-be predator. 'If disturbed will readily throw out sticky white threads'¹; 'The British species Holothuria forskali will perform in this way when handled, and the mess one individual can make has to be seen to be believed'² are typical remarks.
 

Human Interaction

It seems probable that most Cotton-Spinners found near low water by non-divers are small individuals; in many species, juveniles and adults have similar depth ranges except that the youngsters extend into shallower water, moving offshore as they grow. And if young Cotton-Spinners are those that feel more vulnerable to predation, they may use the only defensive measure available to them - ejecting threads - much more readily than the larger and less vulnerable adults that divers meet. This is still more likely if they are discovered hidden under rocks; a concealed nocturnal animal which suddenly finds itself exposed in full daylight would probably feel seriously threatened and take defensive action. And any sea animal that is lifted from the water into air, whether by a rockpooler or in a dredge, will be extremely alarmed and distressed, so probably almost all Holothuria will eject threads in this situation. If I am right, until diving became common almost every Cotton-Spinner people saw had either been exposed by turning rocks on the shore or brought up in a dredge, and ejected its sticky threads in panic; so this was accepted as normal behaviour at any disturbance.

I have corresponded with one or two people about the Cotton-Spinner, but I would welcome any readers' observations which shed further light on these animals, whether confirming my speculations or not. Letters can be sent to The Editor at the address on page two, who will pass them on to me.

12 March 2008 This Cotton-Spinner, a sea cucumber properly called Holothuria forskali, was washed up dead on White Strand beach, Cahirciveen, Co. Kerry. I'd seen live ones in lobster pots but this was the first 'stranded' one I've seen.   Report by Rosemary Hill

¹Erwin, D. & B. Picton, 1987, Guide to Inshore Marine Life
²Nichols, D., 1966, Echinoderms