Sea urchins or echinoids (Echinoidea) are a class belonging to the phylum of Echinoderms, which currently has approximately 950 species and are characterized by a compact, semi-spherical body with pentaradial symmetry, and closed in an external skeleton formed by calcareous plates joined together. Covering the external skeleton, they have spines, which through internal ligaments and muscles are able to move at will.
Sea urchins move thanks to their ambulatory feet (arranged in 5 radial regions), as well as by the movement of the spines. Between the spines, we find structures called pedicellaria, which consist of small tweezers formed by 3 jaws that are located at the end of long stems, and whose main function is to keep the body of the animal clean. Certain species can have poison in their pedicellaria and adopt along with the spines defensive functions. The mouth, located at the end in contact with the sea floor, has a robust, complex and quite developed structure, which has 5 teeth and is known as Aristotle's lantern. This is used by sea urchins to scrape Algae from the substrate and crush other food, since they are omnivorous species. They can also use the Aristotle's lantern to gnaw on the rocks they are in, thus generating cavities in which to shelter and in which their sharp thorns prevent predators from accessing them.
The distribution of sea urchins is very wide in all seas, from intertidal regions to the ocean depths. These are benthic species, which we generally find associated with rocky bottoms, although there are species that live partially or totally buried in sandy/muddy bottoms.
Sea urchins have separate sexes, releasing eggs to the water column, where they are fertilized. After fertilization, a larva is formed that passes a planton stage of several months, to later perform the metamorphosis and settle on the sea floor.