What is a Sea Urchin?

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.

A sea urchin is a small, spiny, globular animal that belongs to the class Echinoidea within the phylum Echinodermata. They inhabit the ocean floor worldwide and are known for their hard shells, which are covered in spines. Sea urchins play a significant role in marine ecosystems, often grazing on algae and affecting the distribution of marine plants.

Sea urchins move using a unique water vascular system that powers their tube feet. These tube feet extend from among their spines and operate through hydraulic pressure, allowing the urchin to adhere to surfaces, move slowly, and manipulate objects. This system is not only for locomotion but also aids in feeding and respiration.

Sea urchins are primarily herbivores, feeding on algae and kelp. However, some species are omnivorous and may consume a variety of invertebrates as well. Their feeding habits are facilitated by a specialized mouthpart known as Aristotle's lantern, which consists of five tooth-like plates that can scrape food from rocks and other surfaces.

While sea urchins are not aggressive, they can be dangerous if stepped on or handled carelessly due to their sharp spines. Some species have venomous spines that can cause pain and swelling. It's advisable to wear protective footwear when walking in areas where sea urchins are present and to handle them with caution if necessary.

Sea urchins typically reproduce through a process called spawning, where males and females release their sperm and eggs into the water column simultaneously. Fertilization occurs externally, and the resulting larvae are planktonic, drifting with ocean currents until they settle and metamorphose into their adult form.

Sea urchins play a crucial role in maintaining the balance of marine ecosystems. They help control algae growth, which can otherwise overwhelm coral reefs and other marine habitats. However, when sea urchin populations become too dense, often due to overfishing of their predators, they can cause significant damage to kelp forests and coral reefs through overgrazing.