Sex in the Sea

Marine creatures rely on some surprising strategies to reproduce in their watery world

  • Daphne Gail Fautin
  • Feb 01, 1991
During two decades as a marine biologist, never had I seen anything like it: puddles of white and orange, each the size of a quarter, dotting the rocks in a California tide pool. Looking closer, I saw that the orange spots were eggs, thousands of them, and the white were perhaps millions of sperm. Both had just been released by sea anemones attached to depressions in the rock. When the tide came in, eggs and sperm would mix, giving way to a new generation.

To a casual observer, the sight might have scarcely raised an eyebrow. Rut I was fascinated: Those puddles contained the seeds of life itself. I wondered, what if sperm and eggs matured randomly, so that a male sea anemone released sperm one day and a female released eggs on another? The two would never find each other in the wide Pacific Ocean. Besides, eggs and sperm (collectively called gametes) live for only a few hours: most would die before they could join forces.

Clearly, successful reproduction at sea is no casual affair. Rather, it can demand precision planning and split-second timing. As I have learned, even some of the most primitive-looking of the ocean's inhabitants have developed extraordinarily sophisticated strategies for ensuring that at least some of their young will survive.

Most marine creatures, like the sea anemone, spawn freely into the open water, "broadcasting- their eggs and sperm by the millions, if not billions. Fertilized by sperm, a sea anemone egg develops into a furry, pear-shaped larva that bears scant resemblance to its tentacled parents. Most of the young are lost to predators. The few survivors drift in the open sea, sometimes for weeks, then suddenly metamorphose into smaller versions of their parents and attach themselves to rocks.

Considering the tortuous path that sea anemone larvae and the offspring of other species take to adulthood, it's remarkable that any of them make it. However, some wondrous adaptations improve the chances eggs and sperm will meet and the young will survive. For one thing, gametes of most species ripen only during part of the year. Males of most sea creatures release sperm only in response to nearby females spawning, a simple mechanism that ensures sperm will not go into the water lacking eggs. Fish generally discharge eggs or sperm only in the presence of the other sex, and often only after some kind of courtship behavior. In some cases, gamete release is tied to environmental cues, such as tides, water temperature, day length, even phases of the moon.

When it comes to good timing, few creatures can match the clocklike punctuality of the Palolo worm, which lives in the tropical Pacific. Precisely seven days after the full moon following the September equinox, the worms travel from the sea floor and into the open, where their rear ends snap off. The pieces, which contain either sperm or eggs, swim to the surface and break open all at once, allowing their contents to mingle.

On Australia's Great Barrier Reef, scientists have discovered another example of exquisite sexual timing. For about four hours after dusk every evening during the week following October's full moon, nearly 150 species of coral send out streams of sperm and eggs, which accumulate on the surface in huge, pink slicks.

Only a few groups of marine creatures have developed the specialized anatomy and behavior necessary for copulation, the most efficient method for delivering sperm to an egg. The technique for sea animals is similar to that used on land — with some interesting differences. A type of shell-less snail called the saccoglossan, for instance, can insert its penis and inject sperm into any part of its partner's body.

Cephalopods (which include octopuses and squids) have perhaps the most unusual sperm-delivery system of all. Using a specialized arm, the male grasps a packet of sperm from inside its body and inserts it into the female, near the oviduct. The tip of the male's arm then breaks off, leaving the sperm packet lodged near the tube through which eggs are emitted.

Nudibranchs, octopuses and many other sea animals lay eggs by the thousands in sacs, which the females attach to rocks. Left on their own, eggs hatch into larvae, which float about and often are eaten by other animals. In a number of species, including some clams, jellyfish, sea anemones and tunicates, the fertilized eggs develop inside the female.

Brood care may seem superior to broadcasting vulnerable larvae into the open sea, but there are definite tradeoffs. One price a female pays for keeping all her eggs in one basket, so to speak. is fewer offspring. And because brooded eggs must contain enough yolk to support the young, they are generally larger than broadcast eggs, whose occupants feed in the sea.

Whatever the reproductive strategy, the goal is the same: to produce enough youngsters so that as many as possible will become sexually mature adults. For most sea creatures, the odds of failure are great. The rewards of success, however, are nothing less than survival of the species.

Daphne Gail Fautin, a specialist in reproductive biology of marine creatures, teaches at the University of Kansas. Photographer Fred Bavendam lives in New Hampshire.

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