Sea urchins range in size from as small as a couple of inches in diameter to over a foot in diameter. They have a moutn located on thei upper part of their body (also known as the oral surface) although some sea urchins have am outh located towards one end (if their body shape is irregular).
Sea urchins have tube feet and move using a water vascular system. Their endoskeleton consists of calcium carbonate spicules or ossicles. In sea urchins, these ossicles are fused into plates that form a shell-like structure called a test. The test encloses the internal organs and provides support and protection.
Some species of sea urchins have long, sharp spines. These spines serve as protection from predators and can be painful if they puncture the skin. It has not been determined in all species whether the spines are venomous or not. Most sea urchins have spines that are about an inch long (give or take a bit). The spines are often rather blnt at the end although a few species have longer, sharper spines.
Sea urchins have separate sexes (both male and female). It is difficult to distinguish between the sexes but males usually select different micro habitats. They are usually found in more exposed or higher locations than females, enabling them to dispers their spermatic fluid into the water and distribute it better. Females, in contrast, select more protected locations to forage and rest. Sea urchins have five gonads located on the underside of the test (although some species only have four gonads). They release gametes into the water and fertilization takes place in open water. Fertilized eggs develop into free-swimming embryos. A larva develops from the embryo. The larva dvelops test plates and descends to the sea floor where it completes its transformation into an adult form. Once in its adult form, the sea urchin continues to grow for several years until it reaches its mature size.
Sea urchins can sense touch, chemicals in the water, and light. They do not have eyes but their entire body seems to detect light in some manner.
Sea urchins have a mouth that consists of five jaw-like parts (similar to the structure of brittle stars). But in sea urchins, the chewing structure is known as the Aristotle's latern (so named for the description in Aristotle's History of Animals). The teeth of sea urchins sharpen themselves as the grind food. The Aristotle's lantern encloses the mouth and the pharynx and empties into the esophagus which in turn connects to the small intestine and caecum.
Sea urchins feed on algae for the most part although some species also feed occasionally on other invertebrates such as sponges, brittle stars, sea cucumers and mussels. Although they appear to be sessile (attached to the sea floor or substrate) they are capable of moving. They move over surfaces by way of their tube feet and spines. Sea urchins provide a food source for sea otters as well as wolf eels.
Fossil sea urchins date back about 450 million years ago to the Ordovician period. Their closest living relatives are the sea cucumbers. Sand dollars evolved much more recently than sea urchins, during the Tertiary, about 1.8 million years ago. Sand dollars have a flattened disk test, instead of the globe-shaped test sea urchins have.