Its basically first one back gets the bets position. In areas where there are no crevices to hide black sea urchins group together to help protect them selves from their predators. The black sea urchin s main predators are Port Jackson sharks, the Blue Groper and of course man. Currently there are restrictions on the number of Black Sea Urchins a person can take from the water. Refer to the fisheries homepage for more info see references section at the bottom.
Divers also kill a great number is sea urchins every year to feed other creatures.
These days more and more divers are being educated not to kill urchins as they too play an important role in the underwater ecosystem. Sea urchins are also fished commercially due to demand for their gonads in countries overseas. As expected they don t get too close to one another. The male and the female sea urchins shoot their eggs and sperm directly into the ocean and hope for the best. Scientist are not so sure about the factors that set off this spawning, but it is believed it has something to do with the length of the days and the sea conditions.