An echinoderm is any deuterostomal animal of the phylum Echinodermata, which includes starfish, brittle stars, sea urchins, sand dollars and sea cucumbers, as well as the sessile sea lilies or "stone lilies". While bilaterally symmetrical as larvae, as adults echinoderms are recognisable by their usually five-pointed radial symmetry, and are found on the sea bed at every ocean depth from the intertidal zone to the abyssal zone. The phylum contains about 7,600 living species, making it the second-largest group of deuterostomes after the chordates, as well as the largest marine-only phylum. The first definitive echinoderms appeared near the start of the Cambrian.
Sea urchins or urchins are typically spiny, globular animals, echinoderms in the class Echinoidea. About 950 species live on the seabed, inhabiting all oceans and depth zones from the intertidal to 5,000 metres. Their tests are round and spiny, typically from 3 to 10 cm 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. Their predators include sea otters, starfish, wolf eels, and triggerfish.
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".
Keyhole sand dollar refers to five living species of sand dollars in the genus Mellita, plus the extinct †Mellita aclinensis. They are found on the Atlantic coasts of the Americas, ranging across the Caribbean Islands, from the southern United States at the north, to the southeastern coast of Brazil at the south. Their range includes the Pacific coast of equatorial countries, such Central American countries and near, in the north sporadically across the Pacific coast of Mexico.
Astropyga radiata, the red urchin, fire urchin, false fire urchin or blue-spotted urchin, is a species of sea urchin in the family Diadematidae. It is a large species with long spines and is found in the tropical Indo-Pacific region. It was first described in 1778 by the German naturalist Nathaniel Gottfried Leske.
Lytechinus variegatus, commonly called the green sea urchin or the variegated sea urchin, is a species of sea urchin that can be found in the warm waters of the western Atlantic Ocean and Caribbean Sea.
Heterocentrotus mamillatus, commonly known as the slate pencil urchin, red slate pencil urchin, or red pencil urchin, is a species of tropical sea urchin from the Indo-Pacific region.
Tripneustes ventricosus, commonly called the West Indian sea egg or white sea urchin, is a species of sea urchin. It is common in the Caribbean Sea, the Bahamas and Florida and may be found at depths of less than 10 metres (33 ft).
Toxopneustes pileolus, commonly known as the flower urchin, is a widespread and commonly encountered species of sea urchin from the Indo-West Pacific. It is considered highly dangerous, as it is capable of delivering extremely painful and medically significant stings when touched. It inhabits coral reefs, seagrass beds, and rocky or sandy environments at depths of up to 90 m (295 ft). It feeds on algae, bryozoans, and organic detritus.
Echinometra mathaei, the burrowing urchin, is a species of sea urchin in the family Echinometridae. It occurs in shallow waters in the Indo-Pacific region. The type locality is Mauritius.
Lanice conchilega, commonly known as the sand mason worm, is a species of burrowing marine polychaete worm. It builds a characteristic tube which projects from the seabed, consisting of cemented sand grains and shell fragments with a fringe at the top.
Echinocardium cordatum, also known as the common heart urchin or the sea potato, is a sea urchin in the family Loveniidae. It is found in sub-tidal regions in temperate seas throughout the world. It lives buried in the sandy sea floor.
Paracentrotus lividus is a species of sea urchin in the family Parechinidae commonly known as the purple sea urchin. It is the type species of the genus and occurs in the Mediterranean Sea and eastern Atlantic Ocean.
Spatangus purpureus, commonly known as the purple heart urchin, is a species of sea urchin in the family Spatangidae. It is found in the eastern Atlantic Ocean and the Mediterranean Sea, where it lives immersed in the sediment.
Eucidaris metularia, the ten-lined urchin, is a species of sea urchins in the family Cidaridae. It is found in shallow parts of the Indo-Pacific Ocean and is characterised by its sparse covering of banded, flat-tipped spines.
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.
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.
Echinocyamus pusillus, commonly known as the pea urchin or green urchin, is a species of sand dollar, a sea urchin in the family Fibulariidae, native to the northeastern Atlantic Ocean and the Mediterranean Sea. It buries itself in gravel or coarse sand at depths down to about 1,250 m (4,000 ft).
Montacuta substriata is a species of small marine bivalve mollusc in the family Lasaeidae. It is found on the eastern side of the Atlantic Ocean where it is often associated with a sea urchin, such as Spatangus purpureus. This species was first described in 1808 by the English naturalist George Montagu who gave it the name Ligula substriata. It was later transferred to the genus Montacuta, making it Montacuta substriata.
