Echinoderms are members of the phylum Echinodermata, from the Greek words for “spiny skin.” This group of marine animals lives only in salt water and includes sea lilies, brittle stars, sea cucumbers, sand dollars, sea urchins, and starfish. Though not all echinoderms actually have spiny skin, in most cases it does have a very rough texture, and some species also wield toxic spines as a defense mechanism. These complex invertebrates possess many distinctive features, but here are five features that the group share:

Echinoderms all have radial symmetry

An adult echinoderm has a body made up of equal sections, usually five or a multiple of five, that surround a central point. In starfish and brittle stars, each of these sections is in the form of an arm-like appendage that points away from the center like a wheel spoke. Often each body section will house a duplicate set of internal organs.

An Echinoderm can regenerate its body parts

Echinoderms possess the remarkable ability to regrow lost limbs or other body parts, even internal organs. When attacked, many will gradually regenerate a damaged or severed limb after the wound has closed up. Also, some species can use regeneration to reproduce by deliberately breaking themselves apart, after which each piece will grow into a whole new organism.

Echinoderms don’t have blood

Without blood or a heart, an echinoderm instead utilizes a water vascular system to carry oxygen to its vital organs. After drawing seawater into its tubular feet, the tubes then squeeze oxygenated water through the rest of its body. This system is also used for movement, by which expansion and contraction of their tube feet allows echinoderms to grasp surfaces and prey items.

Echinoderms Have No Eyes

They don’t have a brain either, only a rudimentary nerve network. However, many starfish do possess light-sensitive organs on their arms. Called “eyespots,” these simple organs provide no detailed images but can sense varying degrees of light, allowing the starfish to seemingly have some idea of where it’s going.

Some Echinoderms Feed By Ejecting Their Stomachs

Starfish in particular will digest prey by extruding their stomachs through their mouths. Many species favor shellfish especially, using their tube feet to grip and pry open the shell. The starfish then ejects its stomach into the shell, which secretes digestive juices to dissolve the contents, before sucking it back up into its body along with its liquefied meal.

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.