Sea urchins are small, spiny echinoderms belonging to the class Echinoidea. The approximately 200 extant species occur in all oceans from pole to pole and play an important role in many marine ecosystems as both predators and prey. Sea urchins have a round body shape and usually measure between 3 and 10 cm across. They are covered with strong, sharp spines which protect them from predators. The name “sea urchin” is derived from the Old English word for hedgehog, urc (pronounced “erk”).
Sea urchins are found on the seabed in a wide range of habitats, including rocky shores, coral reefs, kelp forests, deep sea beds and estuaries. They are most commonly found in shallow water close to the shoreline but some species can live at depths of up to 6 km.
The diet of sea urchins consists mainly of algae but they will also eat other plants and animals if available. Their mouth is located on the underside of their body and they use their ever-growing teeth (made of calcium carbonate) to scrape food off rocks or other surfaces. Sea urchins are important grazers in many marine ecosystems and help to keep algal growth under control. In turn, they provide an important food source for a variety of animals such as fish, turtles, seals and birds.
Sea urchins have a unique reproductive system which involves releasing eggs and sperm into the water column where fertilisation takes place. The resulting larvae develop into juvenile sea urchins which settle onto the seabed after around two weeks. It can take up to two years for a sea urchin to reach maturity depending on the species involved. Some sea urchin species are capable of regenerating lost body parts including arms, legs or even their entire body! This ability makes them one of the longest-lived invertebrate animals on Earth with some individuals living for over 100 years!