Smallest echinoderm

The smallest species of echinoderm is Psammothuria ganapatii, a minute species of sea cucumber or holothuriid, which measures no more than 4 mm in total length as an adult. Native to India, it is so small that it inhabits the interstitial spaces between individual grains of beach sand on India's Waltair coast, and remained undescribed by science until May 1968, when it was shown to be so different from all other sea cucumber species that it was housed within its very own genus. Although almost 50 years have passed by since then, no smaller echinoderm species has ever been discovered.

This tiny species is a synaptid sea cucumber, i.e., a member of the taxonomic family Synaptidae, whose generally elongate-bodied members lack many anatomical features that are typically possessed by a wide range of other sea cucumbers, and even other non-holothurian echinoderms too. These include tube feet, respiratory trees or cuvierian organs, tentacle ampullae and the radial canals of the water-vascular system.