Sea urchins are spiny, globular echinoderms in the class Echinoidea. About 950 species of sea urchin are distributed on the seabeds of every ocean and inhabit every depth zone from the intertidal seashore down to 5,000 meters. The spherical, hard shells (tests) of sea urchins are round and covered in spines. Most urchin spines range in length from 3 to 10 cm, with outliers such as the black sea urchin possessing spines as long as 30 cm (12 in). Sea urchins move slowly, crawling with tube feet, and also propel themselves with their spines. Although algae are the primary diet, sea urchins also eat slow-moving (sessile) animals. Predators that eat sea urchins include a wide variety of fish, starfish, crabs, marine mammals, and humans.
Sand dollars are species of flat, burrowing sea urchins belonging to the order Clypeasteroida. Some species within the order, not quite as flat, are known as sea biscuits. Sand dollars can also be called "sand cakes" or "cake urchins".
Meoma ventricosa, known by the common names cake urchin and red heart urchin, is a large species of sea urchin which lives in shallow waters in the Caribbean. It may reach a diameter of twenty centimeters and is covered in reddish-brown spines. It has both pentagonal radial symmetry and bilateral symmetry, giving it a sand-dollar appearance; however, two of its five sections are merged more closely than the others.
The common starfish, common sea star or sugar starfish is the most common and familiar starfish in the north-east Atlantic. Belonging to the family Asteriidae, it has five arms and usually grows to between 10–30 cm across, although larger specimens are known. The common starfish is usually orange or brownish in color, and sometimes violet; specimens found in deeper waters are pale. The common starfish is found on rocky and gravelly substrates where it feeds on mollusks and other benthic invertebrates.
Dendraster excentricus, also known as the eccentric sand dollar, sea-cake, biscuit-urchin, western sand dollar, or Pacific sand dollar, is a species of sand dollar in the family Dendrasteridae. It is a flattened, burrowing sea urchin found in the north-eastern Pacific Ocean from Alaska to Baja California.
Oreaster reticulatus, commonly known as the red cushion sea star or the West Indian sea star, is a species of marine invertebrate, a starfish in the family Oreasteridae. It is found in shallow water in the western Atlantic Ocean and the Caribbean Sea.
Ophiura ophiura or the serpent star is a species of brittle star in the order Ophiurida. It is typically found on coastal seabeds around northwestern Europe.
Clypeaster, common name "cake urchins" or "sea biscuits", is a genus of echinoderms belonging to the family Clypeasteridae.
Heterocentrotus is a genus of slate pencil urchins, part of the familia Echinometridae. They are mainly found in the Indo-Pacific basin, especially in Reunion or Hawaii. This genus appeared in the Miocene and spread throughout the warm Indo-Pacific.
Phormosoma placenta is a species of sea urchin in the order Echinothurioida. It is a deepwater species, seldom being found at depths less than 500 m (1,600 ft), and occurs on either side of the Atlantic Ocean on the continental slope.
Fragum fragum is a species of cockle, a marine bivalve mollusc in the family Cardiidae. It is commonly known as the white strawberry cockle and is found in the western Indo-Pacific Ocean. It is the type species of the genus Fragum.
Glycymeris nummaria is a species of saltwater clam, a marine bivalve mollusc in the family Glycymerididae, the bittersweet clams.
Irregularia is an extant infraclass of sea urchins that first appeared in the Lower Jurassic.
Eunice norvegica is an aquatic polychaete worm found in deep water on the seabed of the northern Atlantic Ocean as well as in the Pacific and Indian Oceans. It is a tubeworm and is often associated with deep water corals.
Pseudocentrotus depressus, commonly known as the pink sea urchin, is a species of sea urchin, one of only two species in the genus Pseudocentrotus. It was first described in 1864 by the American marine zoologist Alexander Agassiz as Toxocidaris depressus, having been collected during the North Pacific Exploring and Surveying Expedition undertaken by Captain Cadwalader Ringgold and later Captain John Rodgers.
Clypeaster rosaceus, the fat sea biscuit, is a species of sea urchin in the family Clypeasteridae. It occurs in shallow water in the western Atlantic Ocean and was first scientifically described in 1758 by Carl Linnaeus.
Clypeaster humilis is a species of sea urchin in the family Clypeasteridae. This species was first scientifically described in 1778 by the German biologist Nathanael Gottfried Leske. It occurs in the tropical Indo-Pacific region.
Clypeaster japonicus, the Japanese sea biscuit, is a species of sea urchin in the family Clypeasteridae. This species was first scientifically described in 1885 by the German zoologist Ludwig Heinrich Philipp Döderlein.
Leodia sexiesperforata, commonly known as the six-holed keyhole urchin, is a species of sand dollar, in the echinoderm order Clypeasteroida. It is native to tropical and sub-tropical parts of the western Atlantic Ocean where it buries itself in soft sediment in shallow seas.
Goniocidaris umbraculum is a species of cidaroid sea urchin that inhabits the continental shelf off the southern coasts of New Zealand. It is plentiful on a seabed composed of seashell and bryozoan rubble at a depth of 95 m (310 ft) off Otago.
