Sea urchins, often called sea hedgehogs or simply referred to as urchin, are echinoderms that belong to the class Echinoidea. They are small in size and have a globular body that is covered in numerous spines. Their appearance has given them the name which comes from an old English term “urchin”, used to describe hedgehogs. There are around 950 described species of sea hedgehogs, and they are dispersed through all the oceans of the world, from 5000 meter deep to intertidal areas.
The test of the animal, also called the shell, is round in shape, typically measuring between 3 and 10 centimeters. The test is covered in numerous spines of various forms, sizes and colors. Urchins are diversely colored in red, orange, black, olive, blue and green. Because of their body shape, presence of spines and absence of proper locomotion mechanisms, they move slowly on the sea floor, eating mostly algae. Their body has a bilateral symmetry in the developing larval stages, only to become radial fivefold at maturity. However, there are species of echiniods that are oval in shape and have distinguishable front and rear ends, which makes them bilateral even at maturity.
Most sea hedgehogs feed on algae, although there are numerous species that feed on various invertebrates such as sea cucumbers, sponges and mussels. The distribution of population largely depends on the type of habitat, and more abundance in species is usually found in barren areas as opposed kelp forests. In barren areas, diversity and densities are usually higher in shallower waters, although abundance in urchins may also be found in deeper water in case there are waves moving the water.
Lower densities can be observed in winter, especially during storms, when most sea hedgehogs hide in shelters such as crevices or beneath underwater structures to protect them. However, and interesting case is that of the shingle urchin, Colobocentrotus atratus which has adapted to living exposed on shorelines, being resistant to the action of waves.
Sea hedgehogs are the preferred food source for a variety of animals such as the California sheephead, sea otters and they also represent the main food source for wolf eels. When these and other species fail to control sea urchin populations, the latter may be destructive towards their environment. Urchins may devastate areas and cause what scientists call urchin barren – empty of any marcroalgae and fauna that depends upon it.