Sea urchin
Sea urchins or urchins () are typically spiny, globular animals, echinoderms in the class Echinoidea. About 950 species live on the seabed, inhabiting all oceans and depth zones from the intertidal to . Their tests (hard shells) are round and spiny, typically from across. Sea urchins move slowly, crawling with their tube feet, and sometimes pushing themselves with their spines. They feed primarily on algae but also eat slow-moving or sessile animals. Their predators include sea otters, starfish, wolf eels, and triggerfish.
Strongylocentrotus droebachiensis
Sea urchins are dioecious, meaning they either contain male or female reproductive organs. They contain five gonads tucked under the test. These are located close to the anus and are protected by genital plates. One of these plates is perforated, and also acts as the madreporite. Sea urchins all release their eggs or sperm directly into the water column at the same time to ensure fertilization. It is not understood what causes "S. droebachiensis" to release their sperm or eggs, but it may have to do with temperature, because they usually reproduce in early spring.