Sea urchins or urchins () are the class Echinoidea within the echinoderms. Approximately 950 species live on the seabed, inhabiting all oceans and depth zones from the intertidal zone which actually is not a fixed number but varies greatly, usually defined by the range between the highest and lowest tide, to deep seas of . They typically have a globular body covered by spiny protective tests (hard shells), typically from across. Sea urchins move slowly, crawling with their tube feet, and sometimes pushing themselves with their spines. They feed primarily on algae but also eat slow-moving or sessile animals such as crinoids and sponges. Their predators include sharks, sea otters, starfish, wolf eels, triggerfish, and humans. When unchecked by predators, urchins can create urchin barrens, damaged environments devoid of large algae and the animals associated with them.
Like all echinoderms, adult sea urchins have pentagonal symmetry with their pluteus larvae featuring bilateral (mirror) symmetry; The latter indicates that they belong to the Bilateria, along with chordates, arthropods, annelids and molluscs. Sea urchins are found in every ocean and in every climate, from the tropics to the polar regions, and inhabit marine benthic (sea bed) habitats, from rocky shores to hadal zone depths. The fossil record of the echinoids dates from the Ordovician period, some 450 million years ago. The closest echinoderm relatives of the sea urchin are the sea cucumbers (Holothuroidea), which like them are deuterostomes, a clade that includes the chordates.
The animals have been studied since the 19th century as model organisms in developmental biology, as their embryos were easy to observe. That has continued with studies of their genomes because of their unusual fivefold symmetry and relationship to chordates. Species such as the slate pencil urchin are popular in aquaria, where they are useful for controlling algae. Fossil urchins have been used as protective amulets.
Diversity
Sea urchins are members of the phylum Echinodermata, which also includes starfish, sea cucumbers, sand dollars, brittle stars, and crinoids. Like other echinoderms, they have five-fold symmetry (called pentamerism) and move by means of hundreds of tiny, transparent, adhesive "tube feet". The symmetry is not obvious in the living animal, but is easily visible in the dried test.
Together with sea cucumbers (Holothuroidea), they make up the subphylum Echinozoa, which is characterized by a globoid shape without arms or projecting rays. Sea cucumbers and the irregular echinoids have secondarily evolved diverse shapes. Although many sea cucumbers have branched tentacles surrounding their oral openings, these have originated from modified tube feet and are not homologous to the arms of the crinoids, sea stars, and brittle stars. They have a rigid, usually spherical body bearing moveable spines, which give the class the name Echinoidea (from the Greek 'spine'). The name urchin is an old word for hedgehog, which sea urchins resemble; they have archaically been called sea hedgehogs. The name is derived from the Old French , from Latin ('hedgehog').
Like other echinoderms, sea urchin early larvae have bilateral symmetry, but they develop five-fold symmetry as they mature. This is most apparent in the "regular" sea urchins, which have roughly spherical bodies with five equally sized parts radiating out from their central axes. The mouth is at the base of the animal and the anus at the top; the lower surface is described as "oral" and the upper surface as "aboral".