Sea urchins are small, spiny marine animals found on ocean floors worldwide. They belong to the class Echinoidea and have a hard shell called a test, covered in movable spines. Sea urchins play a vital role in their ecosystems, helping to maintain the balance of marine life by grazing on algae. These creatures primarily feed on algae, but some species also consume small invertebrates. Sea urchins reproduce by releasing eggs and sperm into the water, where fertilization occurs. They are also known for their unique ability to regenerate lost spines and other body parts, making them resilient in their environments.