Over the last two years, one of the most popular invertebrates along the Gibraltar coastline have been the Holothurians. This was unexpected, especially with some of the other candidates: the octopus, starfish, and Triton snails. It goes to show the importance of keeping our environmental assumptions at bay, especially when teaching children.
When looking at a Holothurian, or sea cucumber, you would be forgiven for thinking that the animal was a worm of some description. Although a member of the Echinoderms (spiny skin), these animals initially seem to lack the pentagonal symmetry which is externally visible in their cousins: the starfish, the brittle stars, and the sea urchins. It is in fact present, but one needs to look at the internal structures in order to see it. As an example, they have five feeding tentacles which they extend outside the opening on one side that we would associate with a mouth.
Gibraltar is home to five species of sea cucumber with the ones most commonly found being Holothuria tubulosa, the common sea cucumber, and Holothuria forskali, the cotton spinner. Both species can be easily found off all of our local beaches with the highest abundances on the western side. Obviously, these two species live in a shallower depth range which is why they are so easily spotted. The remaining three species tend to live either on sandy beds or in much deeper water, making spotting them a rarer occurrence.
Typically up to 30cm long, Holothurians are detritivores and hoover up the organic matter that accumulates within the sediments. They play an important role in recycling and are a good indication that the ecosystem in question is in balance. All sea cucumbers have tubular feet; a series of sucker-type structures which they use to stick to objects, similar to the suckers on an octopus except they extend out. The have an endo-skeleton made up of calcareous ossicles within the skin. This is further supported by water pressure which maintains the shape of animal, much like a water balloon in the sea.
The cotton spinner has a series of tricks up its backside to warn off potential predators. In the short term they have a bundle of Cuvierian tubules which produce sticky glue-like strands called saponins. In some species the saponins are actually their internal organs. This substance has such a strong adhesive property that most potential predators leave the cucumber well alone due to the level of discomfort it can cause if entangled within it. A lesser-known defence is that these cucumbers store parasitic copepods, Asterocheres boecki, within their tubules or body walls. This combined with the saponins can be ejected from their behinds as a way of warding off any predator which might be considering a nibble.
As unbelievable as it might seem, Holothurians also have a type of brain. It is not as centralised and as developed as our brains, but it is a ring of neural tissue which was the precursor for our brains. This simple brain is associated with feeding, but studies have demonstrated that it is superfluous for the animal’s survival. Furthermore, the animals have no defined senses associated with their nervous system. However, there is some evidence to demonstrate that they are sensitive to both touch and light.
Sea cucumbers are dioecious, distinct male and female forms, and subsequently eggs must be fertilised before the young begin to develop. The eggs are fertilised inside the body cavity and young Holothurians develop with it until they grow large enough to escape from a small rupture in the body wall.
Another incredibly interesting fact is that of all the phyla of invertebrates it is the Echinoderms, and subsequently the Holothurians, which share a common ancestor with the chordates. The Chordates are the precursor to the vertebrates and consequently Holothurians are one of our closest living invertebrate relatives – a fact that is very hard to believe when looking at them.
Human cultures across the world have commonly harvested Holothurians for food and even medicine. Within the Mediterranean, however, it is only the edible cucumber Parastichopus regalis that was considered to be a culinary delicacy in the Victorian era. Today it’s pretty rare to see it locally on a food menu, although globally still consumed in China and Japan. Heston Blumenthal famously experimented with recipes for these animals on his cooking programme and was quoted as saying: “If I am going to use this strange sea creature in my feast, I have got to find a way of drawing out its flavour without freaking my guests out by the way it looks!”.
Despite its ugly duckling appearance, Gibraltar’s Holothurians have proved a hit in the local school educational outings, with many projects confirming this bias in favour of them. Gibraltar might not be serving up sea cucumber stew anytime soon, but environmentally these animals have secured a future by winning local hearts. If you have not had the pleasure of an encounter yet, then please feel encouraged to visit them at beach near you. But be warned, if you decide to disturb them they might have something to say about it – and they are stickier than you might imagine!