Sea urchin facts for kids

Quick facts for kidsSea urchin
Scientific classification
Kingdom:	Animalia
Phylum:	Echinodermata
Class:	Echinoidea (sand dollars) Order Holectypoida; ; ; Perischoechinoidea Order Cidaroida (pencil urchins); ; -->

Sea urchins are the Class Echinoidea of the Phylum Echinodermata. Like the rest of the Echinoderms, literally "spike-skinned", they are entirely marine. They are usually globe-shaped, and protected by calcareous plates and spines. Urchin is an old word for hedgehog, and in many foreign languages these animals are called "sea hedgehogs".

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.

Sea urchins mostly feed on algae and small animals. They have a special chewing apparatus called Aristotle's lantern, after the Greek philosopher Aristotle who was fascinated by sea urchins. With this apparatus they can scrape organisms stuck to the surface over which the urchin is moving.

Spines

The spines are long and sharp in some species, and protect the urchin from predators. The spines inflict a painful wound when they penetrate human skin, but are usually not dangerous. Echinoids also have pincer-like pedicellaria all over their body between the spines. The job of these is to deal with anything (like larvae) settling on the test (outer shell). It is not known if the spines are venomous (unlike the pedicellariae between the spines, which are venomous).

Typical sea urchins have spines that are 1 to 3 centimetres (0.39 to 1.18 in) in length, 1 to 2 millimetres (0.039 to 0.079 in) thick, and not terribly sharp. Diadema antillarum, familiar in the Caribbean, has thin, potentially dangerous spines that can reach 10 to 30 centimetres (3.9 to 11.8 in) long.

Teeth

Sea urchins have five jaws with one tooth each. The teeth grow throughout the sea urchin's life. The teeth of the pink sea urchin chip against each other to stay sharp.

Ecology

Echinothrix calamaris, a sea urchin with huge banded spines. The sphere, top, middle, is its anus.

Sea urchins feed mainly on algae, but can also feed on sea cucumbers, and a wide range of invertebrates such as mussels, polychaetes, sponges, brittle stars and crinoids. Sea urchins are one of the favorite foods of sea otters and wolf eels. Without predators to eat them, urchins breed in large numbers and eat almost all the nearby kelp, creating an urchin barren, devoid of macroalgae and associated fauna. Sea otters have re-entered British Columbia, dramatically improving coastal ecosystem health by eating sea urchins.

Fossil record and evolution

The first echinoid fossils are from the Lower Ordovician period. The earliest forms had flexible tests, with plates that could slide over each other. Echinoids were a relatively unimportant part of the biota in the Palaeozoic. Only one group survived the P/Tr extinction event to form the basis of all later echinoids. The group which survived into the Triassic, the Cidaroids, radiated into all other modern groups, which are known as the Euechinoids.

The break with perfect symmetry, which happened in the Jurassic, gave them definite front and back ends. This opened up new habitats, in particular, the burrowing habitat. Sand dollars and heart urchins have been highly successful. The living echinoids are now important members of the biota, especially in shallow and in-shore waters.

Sea urchins as food

Some humans eat the reproductive organs of sea urchins (which they call roe). This is popular in Korea. In Japan, this kind of sushi is called uni.

Tube feet in action: how a sea urchin moves

As tools

In the Pacific, bunches of heavy spines from slate-pencil urchins (Heterocentrotus species) were used as abrasion tools for shipbuilding.

Aquaria

A fossil sea urchin found on a Middle Saxon site in Lincolnshire, thought to have been used as an amulet

Some species of sea urchins, such as the slate pencil urchin (Eucidaris tribuloides), are commonly sold in aquarium stores. Some species are effective at controlling filamentous algae, and they make good additions to an invertebrate tank.

Folklore

A folk tradition in Denmark and southern England imagined sea urchin fossils to be thunderbolts, able to ward off harm by lightning or by witchcraft, as an apotropaic symbol. Another version supposed they were petrified eggs of snakes, able to protect against heart and liver disease, poisons, and injury in battle, and accordingly they were carried as amulets. These were, according to the legend, created by magic from foam made by the snakes at midsummer.

Images for kids

There is a wide diversity of shapes in sea urchins. This "slate-pencil sea urchin" (Heterocentrotus mamillatus), despite its big, wide spines, is a regular sea urchin and not a cidaroid: its spines are not covered with algae.
Sea urchin anatomy based on Arbacia sp.
Podia of a purple sea urchin.
Oral surface of Strongylocentrotus purpuratus showing teeth of Aristotle's lantern, spines and tube feet
The flower urchin is a dangerous, potentially lethally venomous species.
The five white dots on this Diadema setosum are believed to be visual captors.
The development of a regular sea urchin.
Sea urchin larva, called "Pluteus".
Wolf eel is a highly specialized predator of sea urchins.
Antarctic sea urchin (Sterechinus neumayeri) inhabits frozen seas.
Echinolampas ovalis, Middle Eocene, Civrac-en-Médoc, France.
Pseudocidaris club-shaped spine; Matmor Formation (Middle Jurassic) of Makhtesh Gadol, Israel.
Fossil of an Archaeocidaris brownwoodensis (Pennsylvanian).
Sea urchins belonging to the Irregularia like this Clypeaster have lost their spherical shape and their radial symmetry.
Japanese uni-ikura don, sea urchin egg and salmon egg donburi
Open sea urchins in Sicily, Italy
Paracentrotus lividus, a regular sea urchin (Euechinoidea, infraclass Carinacea).
A sand dollar, an irregular sea urchin (Irregularia). They live in the sediment, and feed on it.
Phyllacanthus imperialis, a cidaroid sea urchin (Cidaroidea).
Test of an Echinus esculentus, a regular sea urchin.
Test of an Echinodiscus tenuissimus, an irregular sea urchin ("sand dollar").
Test of a Phyllacanthus imperialis, a cidaroid sea urchin. These are characterised by their big tubercles, bearing large radiola.
Close-up on the test of a regular sea urchin. One can see an ambulacrum (yellow) with its two rows of pore-pairs, between two interambulacra (green). The tubercles are non perforated.
Close-up on a cidaroid sea urchin apical disc : the 5 holes are the gonopores, and the central one is the anus ("periproct"). The biggest genital plate is the madreporite.
A sea otter feeding on a purple sea urchin.
A crab (Carpilius convexus) attacking a slate pencil sea urchin (Heterocentrotus mamillatus).
A wrasse finishing the remains of a damaged Tripneustes gratilla.
Purple sea urchins at low tide in California. They dig a cavity in the rock to hide from predators during the day.
Dermechinus horridus, an abyssal species, at thousands of meters deep.
Aspidodiadema hawaiiense, an Aspidodiadematoida
Diadema antillarum, a Diadematoida
Caenopedina hawaiiensis, a Pedinoida
Salenocidaris hastigera (dried), a Salenioida
Arbacia lixula, an Arbacioida
Strongylocentrotus franciscanus, a Camarodonta
Stomopneustes variolaris, a Stomopneustoida
Asthenosoma marisrubri, an Echinothurioida
Pourtalesia wandeli (dried), a Holasteroida
Maretia planulata, a Spatangoida
Echinoneus cyclostomus, an Echinoneoida
Peronella lesueuri, a Clypeasteroida
Echinolampas depressa, an Echinolampadoida
Eucidaris metularia, a Cidaroida