With enlarged parapodia, they swim to the surface of the water where both sexes release gametes, and then die.
Colonies of the same species release gametes together, over one, two or three nights around a full moon.
In the shallow waters, where Hydractinia are found, colonies release gametes on a light cue.
These tiny, new medusae (which are either male or female) mature and spawn, releasing gametes freely into the sea in most cases.
This is one reason they are believed to live in large groups and tend to release gametes at the same time into the water column.
As with other sea urchins, breeding involves releasing gametes into the water column.
Corals must rely on environmental cues, varying from species to species, to determine the proper time to release gametes into the water.
These have tiny caps which open to release gametes into the sea.
Each arm contains two gonads, which release gametes through openings called gonoducts, located on the central disc between the arms.