Sea urchins are spherical spine-covered marine animals from the echinoderm family. On average, they are 2 to 4 inches (about 5 to 10 cm) in diameter and come in a wide variety of colors, including black, red, and white. The sea urchin moves around the ocean floor using tube feet with suckers on the end, and many varieties also have movable spines that they can use to aid in locomotion. These animals generally require salt water to survive, and they are widely distributed across the world’s oceans. The word urchin actually means "like a hedgehog" in Greek, and they were given this name because of the similar coating of defensive spikes.
Most varieties of sea urchin are nocturnal creatures. They often dig themselves into crevasses and spend most of the daylight hours hidden away. At night, they come out and feed, usually eating kelp and other marine plants. These animals are thought to serve an important ecological purpose by helping to maintain balance in kelp forests, but if there aren't enough predators around, urchins can exert a lot of pressure on marine plant populations. When this occurs, they can sometimes leave behind long stretches of desolation on the ocean floor.
Sea urchins reproduce by releasing sperm and eggs into the water. These particles are then carried by the current and intermingle, eventually resulting in fertilization of the egg. A sea urchin can go through several larval stages as it matures, but the exact sequence for development varies quite a bit between different kinds of urchins. Spawning generally happens on a yearly basis in the spring.
When attacked, sea urchins rely on their spiky surface as a defensive weapon. They can be one of the most common marine animals to cause human injury, and it sometimes can take surgery to remove the needle-like spikes from an injured body part. Some urchin species have venomous spines and the toxin can cause a wide variety of symptoms, including pain, inflammation, shortness of breath, and vomiting.
Despite their defensive mechanism, sea urchins have numerous natural enemies that have found ways to bypass the protections. Human beings are probably the biggest sea urchin predators, as urchins are a common ingredient in many foods, especially in Japanese cuisine. Sea otters and starfish often prey on urchins, and other animals, such as seagulls, have been known to dine on them when the opportunity presents itself.