Sea cucumber releasing spermSea cucumbers are a very common sight in the Mediterranean Sea, moving imperceptibly slowly along the sea bed on their longitudinal rows of tube feet. They don't appear to be very interesting but they have a secret sex life that is revealed if you dive in the right place at the right time.

Resembling big brown gherkins, some species can reach up to 40 centimetres in length. Although they don’t look very appetising, the species Stichopus regalis (Royal cucumber), and Holothuria tubulosa (Cotton-spinner) are fished and eaten in the Mediterranean. Thus the species that we encounter more often when diving are Holothuria stellatti (Brown sea-cucumber), Holothuria forskali (Black sea cucumber - although it is reddish-brown in colour), and Holothuria polii (White-spotted cucumber), which is almost black with white spots at the end of each of the small protuberances that cover its body.

Sea cucumbers live in a variety of habitats from rocky reefs and algae beds to seagrass meadows, feeding on sea bed detritus. At the slightly narrower front end there are stumpy tentacles that surround the mouth, which is underneath. These shovel in sand to be stripped of any nutrition within the long gut and deposited from the anus as signature globules.

Sea cucumbers seem oblivious to life around them and only become interesting subjects for divers when under serious threat or in mating mode. Few fish could manage to swallow a sea cucumber, but if a crab or an octopus were unwise enough to attack a mass of strong and sticky white threads would be ejected from its anus to entangle and immobilise the predator. The ability to eject these Cuvierian tubules earns the sea cucumber the common name of Cotton Spinner. Despite thousands of Mediterranean research dives over the past fifteen years, I have never witnessed this “cotton-spinning” behaviour, nor seen any other creature take an interest in a sea cucumber.

 However, I have seen and recorded sea cucumbers’ mating behaviour, which is quite surreal to watch. In a demonstration of “survival of the fittest”, males adopt an upright pose, often leaning against a rock or some seagrass for support, in order to reach as high as possible before releasing white sperm into the water column. This results in a patch of “milky sea” surrounded by clear visibility, usually in July.

There is no visual clue to differentiate males and females, but I have been lucky enough to come across a group of several males, spaced out over a few meters, periodically ejecting sperm, and one female adopting the same stance and every few minutes releasing a cloud of tiny pink eggs. In calm conditions, the eggs slowly rise to be fertilised by the sperm of the strongest and most persistent male.