Sea cucumberHolothuroidea

A sea cucumber eats sand and gives it back clean.

It is an echinoderm, a relative of the sea star, built on the same five-part plan but stretched along its axis and lying on one side, so the five rows of tube feet run down the length of it instead of radiating. The skeleton is still there, reduced to microscopic plates scattered through the leathery skin like grit, and the shapes of those plates are how the species are told apart.

What it does all day is process sediment. A ring of branched tentacles around the mouth shovels sand in, the gut strips off the film of bacteria, diatoms and organic debris coating each grain, and the cleaned sand comes out the other end in neat coils you can find all over any lagoon floor. On a busy reef flat the entire top layer of sand passes through sea cucumbers repeatedly, which keeps it aerated and stops it from souring.

It also breathes through its anus. Two branched respiratory trees fill much of the body cavity and open into the hindgut, and the animal pumps water in and out through the anus to work them. Sit and watch a resting sea cucumber and you will see the opening dilate and close on a slow rhythm. The same opening does defensive duty. Threatened hard, some species fire a tangle of sticky white threads out of it, the Cuvierian tubules, which stretch and turn adhesive on contact with water and glue a crab's legs together. Others simply expel their internal organs and grow a replacement set over the following weeks.

None of that helps against a diver with a mesh bag. Dried and sold as beche-de-mer, sea cucumbers are the most heavily fished invertebrate in this region: they are slow, unarmed, shallow and easy to see, and a lagoon can be stripped in a season. 🇪🇬 Egypt closed its own fishery after the northern stocks collapsed. Recovery is slow, because they spawn into open water and a thinned-out population no longer has the density for the eggs to meet anything.