Sea urchins (phylum Echinodermata) are small marine species that have a worldwide distribution. All are free-living and solitary in nature; some 800 species have been identified to date. The body is characterized by its rounded or oval shape and, in most species, by the presence of large numbers of sharp spines of varying lengths. The underside is usually flattened in contrast to the convex upper surface. The term Echinodermata is taken from the Greek words echinos (spiny) and derma (skin) and is used to describe a wide range of animals, including starfish (Asteroidea), brittle stars (Ophiuroidea), sea lilies (Crinoidea), sea cucumbers (Holothuroidea), and the closely related sand dollars in the same taxonomic class, Echinoidea. In appearance, sea urchins may be black, brown, green, white, red, purple, or a combination of these colors. Most species measure from 2.4-4.7 in (6-12 cm), but some tropical species may reach a diameter of 13.8 in (35 cm). The entire body is contained within a toughened skeleton, or test. This consists of a number of closely fitting plates arranged in rows. The spines are usually circular and taper to a fine point; some may bear poisonous tips. The spines are attached to muscles in the body wall and, through a special ball and socket type arrangement, can be moved in any direction. The entire test, spines, and other external appendages are covered in a thin layer of tissue.
Adult sea urchins are radially symmetrical with unsegmented bodies. The body is made up of five equal and similar parts. They possess a spacious body cavity, which houses the digestive and reproductive organs as well as the large feeding parts and other organs. All echinoderms have a unique organ called a water vascular system which serves as a filtering mechanism and fluid circulating system.
Sea urchins are highly mobile and move by means of hundreds of tiny tube feet, called podia, which arise from pores in the test. When moving, these are extended in one direction and then shortened, pulling the body along in the process. The spines may also assist with movement. Most often sea urchins are found on rocky shorelines, rock pools, and sheltered depressions of coral reefs. Many remain attached to seaweed fronds. Some species that live in exposed habitats—for example, where wave action is strong—can burrow into soft rocks by continuously rubbing the spines against the rock substrate. In this way species such as Paracentrotus lividus and Strongylocentrotus purpuratus are able to obtain shelter. The tube feet, which may also function as tiny suction cups, enable sea urchins to climb wet rocks and steep cliffs with ease.
Sea urchins feed on a wide range of species, with an apparent preference for algae and sessile animals such as corals. Some species are carnivorous, while many deep sea species are thought to be detritus feeders. All sea urchins have an elaborate feeding mechanism known as Aristotle's lantern, after the Greek philosopher who first described this apparatus. This is made up of five large calcareous plates, each of which is sharply edged and forward pointing. Supported by a framework of rods and bars, the plates are capable of moving in all directions and provide the urchin with an effective rasping and chewing tool.
In between the spines are large numbers of tiny organs known as pedicellariae. These are small pincer-like structures that are used to remove debris from the surface of the body, but are also used to capture prey and pass food particles towards the mouth, which is located on the underside of the body.
All sea urchins are dioecious-either male or female. When mature, the gonads release large quantities of sperm and eggs into the sea. Fertilization is external in most sea urchins, although a few cold water species may retain their eggs near the mouth opening where they are protected by spines. The resulting larvae, known as an echinopluteus, are free-swimming and join the myriad of other tiny organisms that make up the plankton of the sea. As the echinopluteus matures, it begins to develop a hard outer covering. When this happens, it settles on the sea bed and undergoes a complex process of metamorphosis, the resulting organism being a minute (usually measuring less than 0.04 in or 1 mm) replica of the adult.
Despite their apparently formidable suit of armor, sea urchins are frequently eaten by seabirds, many of which drop the urchins from a height to break the hard outer test. Sea urchins are also preyed upon by crabs and a wide range of fish, such as parrot fishes, which are specialized at chewing hard materials such as corals. One specialist feeder on sea urchins is the sea otter. When the otter dives to find sea urchins, it also retrieves a small rock from the sea bed; when it surfaces, it lies on its back, places the stone on its abdomen, and smashes the urchins against the stone, breaking through the test and reaching the flesh. Some of the larger tropical species such as Tripneustes ventricocus are also collected as a source of protein by island dwellers in the West Indies. Many other sea urchins are also collected and dried for sale to tourists. Overharvesting of certain species has led to laws limiting their collection in some areas.