To trade, or not to trade; that is the question

The black hamlet is a simultaneous hermaphrodite fish that does not fertilize its own eggs and whose reproductive success is strongly related to the number of eggs of other fish that it fertilizes. Its own eggs are an inducement to other fish to let the fish fertilize their eggs. Consequently, the hamlet engages in egg trading, a mating process in which one fish lets another fertilize its eggs in exchange for allowing it to fertilize the eggs of the other fish.