An echinoderm is any member of the phylum Echinodermata. The adults are recognisable by their radial symmetry, and include starfish, brittle stars, sea urchins, sand dollars, and sea cucumbers, as well as the sea lilies or "stone lilies". Adult echinoderms are found on the sea bed at every ocean depth, from the intertidal zone to the abyssal zone. The phylum contains about 7,000 living species, making it the second-largest grouping of deuterostomes, after the chordates. Echinoderms are the largest entirely marine phylum. The first definitive echinoderms appeared near the start of the Cambrian.
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".
The water vascular system is a hydraulic system used by echinoderms, such as sea stars and sea urchins, for locomotion, food and waste transportation, and respiration. The system is composed of canals connecting numerous tube feet. Echinoderms move by alternately contracting muscles that force water into the tube feet, causing them to extend and push against the ground, then relaxing to allow the feet to retract.
A pedicellaria is a small wrench- or claw-shaped appendage with movable jaws, called valves, commonly found on echinoderms, particularly in sea stars and sea urchins. Each pedicellaria is an effector organ with its own set of muscles, neuropils, and sensory receptors and is therefore capable of reflex responses to the environment. Pedicellariae are poorly understood but in some taxa, they are thought to keep the body surface clear of algae, encrusting organisms, and other debris in conjunction with the ciliated epidermis present in all echinoderms.
Kelp forests are underwater areas with a high density of kelp, which covers a large part of the world's coastlines. Smaller areas of anchored kelp are called kelp beds. They are recognized as one of the most productive and dynamic ecosystems on Earth. Although algal kelp forest combined with coral reefs only cover 0.1% of Earth's total surface, they account for 0.9% of global primary productivity. Kelp forests occur worldwide throughout temperate and polar coastal oceans. In 2007, kelp forests were also discovered in tropical waters near Ecuador.
In biology, a test is the hard shell of some spherical marine animals and protists, notably sea urchins and microorganisms such as testate foraminiferans, radiolarians, and testate amoebae. The term is also applied to the covering of scale insects. The related Latin term testa is used for the hard seed coat of plant seeds.
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.
Strongylocentrotus droebachiensis is commonly known as the green sea urchin because of its characteristic green color. It is commonly found in northern waters all around the world including both the Pacific and Atlantic Oceans to a northerly latitude of 81 degrees and as far south as Maine and England. The average adult size is around 50 mm (2 in), but it has been recorded at a diameter of 87 mm (3.4 in). The green sea urchin prefers to eat seaweeds but will eat other organisms. They are eaten by a variety of predators, including sea stars, crabs, large fish, mammals, birds, and humans. The species name "droebachiensis" is derived from the name of the town Drøbak in Norway.
The heart urchins or Spatangoida are an order of sea urchins.
The Atelostomata are a type of sea urchins. They are distinguished from other sea urchins by their irregular shape and the absence of a feeding lantern. The group includes the well known heart urchins, as well as some less familiar and extinct forms.
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.
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.
Lytechinus williamsi, the jewel urchin, is a sea urchin in the family Toxopneustidae. It occurs on shallow reefs off the coasts of Panama, Belize, the Florida Keys and Jamaica.
Ossicles are small calcareous elements embedded in the dermis of the body wall of echinoderms. They form part of the endoskeleton and provide rigidity and protection. They are found in different forms and arrangements in sea urchins, starfish, brittle stars, sea cucumbers, and crinoids. The ossicles and spines are the only parts of the animal likely to be fossilized after an echinoderm dies.
Luidia maculata is a species of starfish in the family Luidiidae in the order Paxillosida. It is native to the Indo-Pacific region. It is commonly known as the eight-armed sea star because, although the number of arms varies from five to nine, eight arms seems to be the most common.
Irregularia is an extant infraclass of sea urchins that first appeared in the Lower Jurassic.
Clypeaster reticulatus, the reticulated sea biscuit, is a species of sea urchin in the Family Clypeasteridae. This species was first scientifically described in 1758 by Carl Linnaeus. It lives on the sandy seabed of shallow seas, semi-immersed in the sediment.
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).
Luidia savignyi is a species of starfish belonging to the family Luidiidae. The species is found in the tropical and subtropical Indo-Pacific region. It is a large starfish and preys on other echinoderms.
