Starfish
A male and female starfish commonly reproduce by releasing sperm and egg into the water which will, hopefully, be fertilized. To increase the chance of fertilization, starfish gather in groups when they are ready to spawn. They may use environmental signals to coordinate timing or sometimes chemical signals to indicate readiness. Some species of starfish brood their young. The gametes released from the male fertilize the eggs a female carries on the surface of her body or in the pyloric stomach of the starfish. Male and female starfish are not distinguishable from the outside. You need to see the gonads of the starfish located on each arm.