Ocean Watch

Monday, September 1, 1997

Labor’s reward found in
offspring, one or many

My longtime readers may remember my story about a Labor Day many years ago when my mother gave birth to my baby brother. The event was such a sensation in my 11-year-old mind that, since then, Labor Day has had a double meaning to me.

Besides commemorating workers, the day also honors those who perform the supreme labor: that of perpetuating life on Earth.

In the world of fish, this labor comes in many forms, ranging from the release of eggs and sperm into open water and leaving the rest to nature, to the provision of nutrients to embryos developing inside a female fish's body.

Tunas are typical of the first hit-and-miss type of reproduction. These schooling fish spawn in the warm upper layer of the open ocean. Each female tuna releases about 50,000 floating eggs per pound of body weight, and each male releases millions of sperm in the vicinity. The eggs that get fertilized hatch in about 30 hours.

It's a lonely, perilous world these youngsters face. When the tiny tuna, only 0.1 inch long, begins its life in the marine world of fish-eat-fish, its parents are long gone. The mortality rate is staggering.

Although not many tuna hatchlings make it to maturity, not many have to. Of the millions, only two need to reach adulthood to keep the tuna population stable.

This type of reproduction may seem easy on the parents, but the cost is high.

Producing millions of eggs and sperm at each spawning requires tremendous amounts of energy.

At the opposite extreme are sharks, which produce fewer eggs and less sperm but use considerable energy giving their offspring a head start.

All sharks have internal fertilization, meaning the males deliver sperm directly inside the female through extensions of their pelvic fins. In many species, the female retains her eggs inside her body until they hatch, then gives birth.

Some kinds of sharks, such as sand tigers, threshers, makos and maybe great white sharks, have a unique way of nourishing their unborn pups.

One embryo remains in the mother's body, eating its later-arriving siblings. The young of these sharks have the advantage of entering the world already fairly large.

Most sharks found in Hawaii have a more familiar way of feeding their unborn babies. A tube, called a pseudo-umbilicus, connects each embryo to the mother's tissue. When the embryos are large enough to survive, the little sharks are born.

Between these extreme reproductive labors lie variations as vast as the ocean itself:

Flying fish lay eggs bearing sticky threads that attach to floating seaweed. This natural cover likely gives hatchlings more protection than they have simply floating free in the open ocean.

Pipefish and seahorses are a human female fantasy. In these fish, it is the males who become pregnant. The female deposits her eggs into the male's stomach pouch, then takes off. The distended male incubates the eggs for 10 to 50 days, after which his tiny babies pop out.

In seahorses, the youngsters immediately head to the surface for a gulp of air, which helps them swim upright.

Cardinalfish are also a female dream-come-true as far as the work goes. During the spawning season, females lay masses of eggs. Males fertilize them, then collect them in their mouths, holding them until they hatch. Sometimes the males' mouths are so full of eggs, they can't close their jaws completely.

Then there are the female wrasses and parrotfish that get tired of all that egg-laying and simply turn into males. Many wrasse species spawn in groups, releasing eggs and sperm in a rapid upward rush. I've watched this happening and it looks like pure fish ecstasy. Procreative labor does have its rewards.