Monday, April 29, 1996
Fish's sex lives may offer insight
to human mating
WHEN I was in sixth grade, my friends and I became
keenly interested in biology. One of our favorite pastimes was searching
for sex words in an older sibling's biology textbook.
We whispered over gonads, giggled about gametes and
blushed at the mention of genitalia.
One day, my friend came to school with a new word,
hermaphrodite, from her big brother's book. We looked it up: an organism
having both male and female reproductive organs. We stared at each other.
You were either a girl or a boy, weren't you?
The concept was so confusing, we chalked it up as
another of the many secrets of sex.
Now my interests have turned to the marine world, and I
still find hermaphrodism a mystifying subject. On the reef, being male or
female is anything but clear-cut.
Take nudibranchs. These snails-without-shells, also
called sea slugs, are true hermaphrodites, having male sex organs on one
end, female organs on the other. When these creatures have sex, they line
up end to end, sometimes several individuals at a time, creating a kind of
Researchers also have discovered more than 100 species
of hermaphrodite fish, and suspect that many more exist. Fish, however,
vary in hermaphroditic types.
FISH equipped with both testes and ovaries are called
One such type of sea bass spawns about 14 times a day.
About half the time, the individual releases eggs; the other half sperm.
This fish can switch from one to the other in 30 seconds.
As these sea bass grow older, their female gonads grow
larger, causing the fish to release more and more eggs and less and less
sperm. This probably increases the species' reproductive success rate
since most eggs get fertilized, but most sperm don't link with eggs.
Other simultaneous hermaphrodite fish grow more male
tissue as they age. Usually, these fish keep changing, eventually becoming
all male. Fish that change sex completely like this are called successive
hermaphrodites. Wrasses, parrotfish, some gobies and several other reef
fish are in this group.
Successive hermaphrodites perform sex-change acts in a
variety of ways, depending upon the species and circumstances.
In clownfish (those cute orange fish that live with
anemones), only the largest female and male of a group reproduce. If this
large female dies, her mate becomes a female. Then the largest juvenile in
the family moves up, becoming the new male.
Other fish make more than one sex change. In Japanese
reef gobies, a female in a group will become male if the dominant male
leaves. If a larger male joins the group, the changed fish reverts to her
former female self. This sex change takes only four days.
OTHER fish, such as Hawaii's saddleback wrasses (hinalea
lau-wili), have two different types of males in one species. Each varies
in size and mating approach.
Such a two-male species, the midshipman of Northern
California, has been studied extensively. Type I males take longer to
mature, but grow bigger and develop strong vocal systems for courting.
These males, whose gonads account for only 1 percent of
their weight, hum to attract females to their carefully built nests.
Type II males, however, mature early. Their gonads
account for a whopping 9 percent of their body weight. (This would be 16
pounds of testicles on a 180 pound man). And humming to attract mates?
Forget it. These males use invasion to get mates, stealing both nests, and
the females in them, from type I males.
This sure sounds familiar. In fact, many marine
scientists believe fish and people have enough in common to make studies
of fish sex worthwhile. Perhaps learning what goes on in the brains and
bodies of fish will unlock some of the mysteries of human sexuality.