Fish reproduction an intriguing process

In 1958 my mother went into labor during a Labor Day picnic. The pandemonium that ensued, ending with the delivery of a healthy baby boy, made such an impression on my 10-year-old mind that since then I’ve linked Labor Day with the labor of giving birth.

Today, given my interest in marine life, I’ve extended that link to include the birth of fish.

Fish don’t deliver live babies in the way of us mammals (sharks and seahorses being exceptions), but in a massive ocean teeming with predators, fish have methods of producing offspring that are a category of labor all its own.

The most common kind of fish reproduction is called broadcast spawning, a system in which males and females release sperm and eggs into the water. And that’s the end of it for the potential parents. Spew and split. Job done.

The chances of free-floating eggs and swimming sperm connecting on a coral reef is small, and offshore even smaller. But fish have ways of upping the odds. One is to produce an enormous number of eggs and sperm. This is where labor comes in. When it’s time, as dictated by moon phase, day length, water temperature or something we don’t know, female reef fish produce tens of thousands of eggs and males manufacture millions of sperm.

Open ocean species churn out even more.

The queen of egg production is the ocean sunfish (scientific name Mola mola, hence the common name, mola). In one 4.5 foot-long female, researchers found an estimated 300 million eggs in her one ovary, several orders of magnitude greater than the average broadcast spawner. Since molas grow to about 6 feet long, it’s likely that a full grown female would have even more eggs.

Broadcast spawners’ tiny floating eggs and sperm become part of the drifting plant and animal soup known as plankton, the beginning of the food web. Plankton eaters gobble up most of those sex cells, but some fulfill their destiny of continuing of the species.

Researchers measure a fish’s reproductive effort by comparing the size of its gonads to the size of its body. Charles Darwin proposed the idea that because fish ovaries are considerably larger (30 to 70 percent of the female’s weight) than testes (5 to 12 percent of body weight), female fish invest more energy in reproduction than males.

Today, however, males get more credit. Because random spilling of sperm is a waste, males expend considerable energy to attracting females, establishing territories and running off trespassers.

Some male fish send out pheromones that cue a female to begin egg maturation. Others signal their fitness by turning bright colors or performing lively dances. And when the going gets tough, the tough change sex. Wrasses, parrotfish and anemonefish change sex as circumstances demand.

Studies show that larger female fish of all species produce more and bigger eggs, resulting in higher offspring survival.

That’s why it’s crucial in this era of depleted fish stocks to leave some of the biggest fish in the ocean.

Most people don’t associate Labor Day with reproduction like I do. But in nature, it’s the only labor that counts.