The immensity of the oceans can create challenges for small animals trying to find a mate. But being surrounded by water also offers advantages. Eggs can stay suspended for long periods without drying out, making it possible for them to be fertilized outside of their parents' bodies by swimming sperm. External fertilization allows many species to reproduce without ever having to seek out a mate, or without moving around at all.

	GEOGRAPHY CONE SNAIL Conus geographus

Some invertebrates release large numbers of eggs and sperm in a strategy known as broadcast spawning. Synchronized spawning, in which enormous numbers of individuals release their eggs and sperm at precisely the same time, greatly increases the odds of fertilization. The brief abundance also helps some escape being eaten. Many species time their breeding using cues in the environment. Seven days after the 12th or 13th full moon of the year, palolo worms release the back ends of their bodies, filling the water with wriggling polychaete tails, each releasing its store of eggs or sperm. Many other plants and animals, including corals and fishes, also engage in synchronous spawning.

	COMMON BARNACLE Balanus improvisus

Other Ways of Reproducing
Most snails and leeches use traumatic insemination. Both have male and female organs; they mate when one stabs a detachable sperm-filled harpoon into the other's body.

Barnacles are rooted in place. To reproduce, they extend a long, flexible appendage to inseminate other barnacles that happen to be anchored nearby.

Mussels and clams do not synchronize their broadcast spawning. They dump millions of sperm and eggs into the water and rely on them to mix and fertilize by chance.