Holothuriida

Holothuriida is an order of sea cucumbers. Members of this order were once placed in the order Aspidochirotida, but that grouping was found to be polyphyletic in 2017, leading to some species being moved into the clades Synallactida and Persiculida.

These sea cucumbers are set apart by their flattened, often leaf-like tentacles, which lack the large appendages seen in the related order Elasipodida. They have no introvert or retractor muscles. Their tube feet often form a distinct sole on the underside.

Around the mouth are 15 to 30 shield-shaped tentacles that can be retracted. The body wall is thick and leathery, containing ossicles, including some shaped like tables. Gas exchange occurs through respiratory trees.

The mesentery of the posterior gut loop attaches to the right ventral interradius, and the longitudinal muscles are arranged in five double bands. When threatened, they can release sticky white threads called cuvierian tubules from their cloaca to entangle or distract predators. They typically inhabit exposed shallow-water environments.

Their popularity as a luxury seafood in some Asian countries poses a serious threat to the order.

The order contains two families: Holothuriidae (with 5 genera and over 200 species) and Mesothuriidae (with 2 genera and 32 species).