Sea urchins eat both plant and animal matter, but they feed mostly on algae. Sea urchins also eat sea cucumbers and a number of invertebrates, such as mussels, sponges and brittle stars.
The mouth of a sea urchin is located in the middle of the urchins' underside and is surrounded by lips made of soft tissue. Inside the mouth are multiple tiny, bony teeth.
Sea urchins can be found in almost all climates and marine ecosystems. Most are located on tropical coasts and in shallow water, although some species are found several thousand meters deep. As of 2014, there are close to 200 different species of sea urchins, typically ranging in size from 6 to 12 centimeters.