Echinoidea – Sea Urchins
The sea urchins, also referred to as sea hedgehogs, are a class in the Phylum of the Echinodermata. Echinoidea include about 950 known species. The scientific name derives from the ancient Greek “echinos”, which is called “hedgehog”.
In the anatomy of the sea urchins one differentiates between regular (Regularia) and irregular sea urchins (Irregularia).
The regularia, ie typical sea urchins, have a radially symmetric skeleton of calcium carbonate. This corresponds, depending on the type, to a hemisphere. This can be somewhat flattened or conical.
Irregularia consist of heart urchins (Spatangoida) and sand dollars. The skeleton is flattened and even disc-shaped in the sand dollars. Contrary to the Regularia, there is a front, rear, right and left at the Irregularia. The skeleton is thus no longer radial-symmetrical.
The spines also differ. While regular Echinoidea have well-defined spines, these are more like a kind of hair in the case of heart urchins, and strongly re-formed or not present in sand dollars. The functions of the spines are manifold. They are, of course, primarily defense and are poisonous in some species. In addition, some species can use them as drilling, catch or movement tools.
Sea urchins have pedicellaria, tonglike tools with which they can clean their surface and, by poison, also attack larger predators.
They also have tube feet, at the end of which are small suction cups. This allows them, for example, to hold mussels, stones or algae in order to protect themselves or to camouflage themselves.
For food, they have a special rasping device, the Aristotle’s lantern. This consists of 40 skeletal plates and five teeth, which are connected to one another in a mobile manner.
Habitat and Distribution
Sea urchins live benthic to almost 7000 meters of depth. They live exclusively in seas, from the poles to tropical waters. They occur on both rocky and sandy substrates. Some live in coral reefs or even buried in the sand.
In general, echinoids have separated sexes. There are occasional hermaphrodites, but this is not common. In order to increase the probability of successful fertilization, several individuals usually gather. Partially so close that they touch each other. Egg and sperm cells are then released simultaneously into the open water, where fertilization takes place.
Eat and be eaten
Sea urchins mainly feed on algae. In addition, their diet consists of sea cucumbers, mussels, polychaete worms, sponges, brittle stars, sea lilies and feather stars. In rarer cases, cannibalism may also occur.
Among their predators, in the Canary Islands, are the Grey and the Ocean Triggerfish, the Spotfin Burrfish and several starfish. In addition, in some Mediterranean countries, as well as in Chile and Korea, the gonads (sex glands) of Echinoidea are a popular starter. In Japan they eat the eggs.