Monday, September 1, 1997
Laborís 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
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
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
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
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.