II
Fishy Sex-Uncovering the
The public has had an unusual number of sex
stories to mull over of late. Scarcely a week goes by without a report of
another politician’s scandalous sexual foray or a gossip column piece on some
stars in vitro fertilization.
But for science buffs with an appetite for bizarre
sex yarns, new findings on the antics of fish may prove more satisfying.
For starters, that
grouper you dined on at the beach this summer had probably undergone a complete
sex change. What’s more, studies suggest that
a few species of fish living In and around coral reefs
reverse gender as many as 10 times. Some species have two kinds of males—the
family man and the run-around. Yet others, such as the common bluehead wrasse, reverse sex , and
have two very different types of males.
Why this prurient
interest in piscine sex? Scientists hope that investigating what goes on in the
brains and bodies of these mutable creatures may help uncover some of the
mysteries of human sexuality.
Sex-changing fish are
known as successive hermaphrodites—each fish can make either testes or ovaries
and produce either eggs or sperm. The simultaneous hermaphrodite, however,
comes equipped with both testes and ovaries. These fish would have to check
both the male and female boxes on a personal questionnaire.
One species of sea bass,
Serrunus torrugarum, releases
eggs during about half of Its 14 or so daily spawns and contributes sperm for
the other half. It can switch from providing sperm to releasing eggs within 30
seconds.
S. torrugarum develops more female gonadal tissue as it grows bigger, causing it to release
more eggs and fewer sperm. The fish’s spawning behavior remains the same,
however. lts increased egg production may boost reproductive
success, since almost all eggs get fertilized but not all sperm find eggs
Other simultaneous
hermaphrodites develop more male tissue as they grow. These fish eventually
change sex completely, however, becoming successive hermaphrodites. Scientists
have studied more than 100 species of successive hermaphrodites but suspect
that many more exist. Most of the
species found so far reside in shallow ocean waters.
In the most common type
of sex reversal, females mature, reproduce, and then become males. Their ovaries
disappear entirely (or almost entirely, they grow testes, and their hormonal
systems switch. Many fish also change color. They act like males, which in some
species includes protecting their nests, behaving aggressively, and courting
females. Less commonly, some species change from male to female.
Either way, sex reversal
is a very dramatic change in almost all aspects of the life of the fish.
Most fish undertake the
change only once. For many species. It occurs in response to an altered
social environment, as when a dominant male dies
or leaves his social group and one of the females becomes male and takes his
place. Changing sex can take as little as 4 days. What keeps fish from doing it
more often remains unclear. Some researchers suggest that the dominant male’s
aggression inhibits sex change in subordinates. Others argue that it’s much
more complicated than that.
Some females, for
example, won’t become male if their group includes fewer than two to five
other females. The benefits of changing sex may not outweigh the costs when so
few females exist for males to mate with.
If, as ecologists think,
a fish’s primary goal is to reproduce as often as possible, then becoming a
male makes a lot of sense. Females often outnumber males, so males get a good
rate of return on their mating efforts.
But becoming a female
has benefits too. Anemone fish live in small groups. Only the largest female
and male reproduce and the female outweighs the male.
If this female dies or leaves, her mate becomes a female the largest juvenile
fish in the group becomes male and mates with the new female Douglas Shapiro at
EMU explains in The Differences Between
the Sexes (1994, Cambridge U Press.)
Despite its advantages, acquiring a new sexual
identity takes a toll on fish. During the process. they cant reproduce, yet they probably expend a lot of
energy and don't defend themselves well..
Some fish dont stop with one sex change. Scientists have identified
at least three species of reefdwelling fish that
change sex several times: Paragobiodon echinocepfiahss, Cirrhauchzhss auit-us. and Tnnrnru okincuuxse.
No doubt other fish perform the same trick.
However, scientists know of no other vertebrate that undergoes multiple sex
changes. Only one other vertebrate, an African frog, can reverse its gender.
Earlier
laboratory studies revealed that the female T
okinwae, a tiny Japanese goby, becomes male in
response to the departure of a dominant male. But such males revert back to
female when a larger male comes on the scene. The frequency of gender changes
depends on the stability of the social system.
The researchers wondered
whether all these adjustments left the fish behaving and looking different from
female males or females. In a new study it was found that fish going for round
two as a male or a female look just like fish that have never entered the ring.
Two scientists, Grober
and Sunobe induced females to become males, males to
become females, and females to become male and then female again—all by
swapping their companions. The fish completed a single gender change its as few as 4 days.
Unlike many other sex-changing species, T. okinauiae always carry with them a remnant of the opposite sex’s gonad.
“The {gonad} cells are already there (and) can
probably generate rapid changes in hormones.” Grober
speculates. Scientists don’t know whether the other two fish that reverse
their sex more than once retain such cells.
In just those few days, the fish’s gonads, genital
papilia (a flap of skin sensitive to steroid
hormones), brain cells, and other
anatomical parts changed. The fish’s sex hormones adjusted, the authors assume,
but they are only now examining them.
The scientists knew that the number and size of
cells that produce peptldes important to reproduction
differ in male and female bluehead wrasse, one-time
sex reversals. So they examined cells in T oknawae that produce arginine vasotocin (AVT) neuropeptides,
which regulate reproductive behavior in many vertebrates, to see whether they
changed.
The cells enlarged when males became females and
shrank when females became males. When these males went back to being females,
the cells enlarged again, Grober and Sunobe find. The total number of cells didn’t vary
Behavior also served as a good indicator of the fishs
current sexual status, Grober says. Males, even those
new to the job, behaved aggressively, defended their nests and performed a
traditional courtship dance.
A trained eye couldn’t
miss the differences between the males and the females, he contends. it’s like walking into a singles bar. Is there any issue as
to who is a male and who is a female? Absolutely not.”
Sometimes, there a even a clear distinction between the males. Take the saddleback wrasse, Thaksssoma duperrey. Like
salmon and many other species, it has
two types of males. They vary in size and take very different approaches to life,
including mating.
Grober and his
colleagues are finding clear differences in their brains is
well. In one section of the brain, the larger male has many more cells that
produce AVT than the smaller males or females do.
Among vertebrates, almost no other animals except
certain fish species have two distinct classes of males, and no vertebrates
are known to have two types of females, says Andrew H. Bass of
Over the years, Bass has compiled a detailed
picture of the two versions of male midshipman fish living off the West Coast
of the
The type I male midshipman takes longer to mature,
which gives him time to grow bigger and develop a stronger vocal system for
courting. Type II males put their energy into becoming sexually mature and
developing their gonads, which make up 9 percent of their body weight. Type I
males’ gonads account for a mere 1 percent of their weight. The two males also
have different types of testosterone, Bass explains.
Type I fish build and guard their nests,
activities type II males never bother with. The nest builders also hum to
attract females and grunt in defense of their home. Type II fellows avoid the
courtship, trying instead to invade their neighbors
nests to mate with the females.
Midshipman males may
resemble comical stereotypes of some human males, but marine scientists say
that fish and people actually have enough in common to make the study of fish
worthwhile.
Research on fish may shed light on how environment
and stress inhibit human reproduction and on the role of the brain in human
sexual orientation, scientists say. Fish also emphasize the inflexible nature of most other vertebrates’ sexuality, Grober says.
“The speed of (sex changes in fish}the diversity
of body systems involved, and their reversibility are remarkable, relative to
the rigid nature of these characteristics in most vertebrates,” Grober and Sunobe contend.
All of this sexual variation and gender gymnastics
helps fish to reproduce more successfully, scientists suspect. But maybe
another benefit exists. Maybe. having
walked in each othe~s shoes. male and female fish get along
better.
Would the battle of the sexes never have begun if humans had more fishlike qualities?
1. How often do some reef fish reverse gender?
2. What are sex-changing fish known as?
3. Describe how sea bass (S. tortugarum) spawn?
4. What is the most common type of sex reversal?
5. Some fish only change once. What causes this to occur?
6. Name 3 species of reef fish that change sex
several times?
7. Do fish change appearance when they go back
to the one sex each time?
8. Which fish has two types of males? How are they different? What about type I & II?
9. How can one determine the sex of a
fish...like the males?
10. How is this sex reversing a benefit?