Unformatted text preview: vertebrates, fish dominate. With some thirty thousand species ranging in size from about one-third of an inch long (the Philippine Island goby) to about fifty feet long and weighing several tons
(the whale shark), no other vertebrate species compares to fish for variety and sheer numbers. Among those of us with backbones, there are
more fish than any other creature. And when it comes to sex, fish have
evolved some of the most varied and interesting approaches among all
More than (perhaps a lot more than) one hundred species and
twenty families of fish are hermaphroditic, and here we begin to stretch
the limits of what we mean by hermaphroditism. Hermaphroditic
fish come in two common forms—simultaneous hermaphrodites and
sequential hermaphrodites. Simultaneous hermaphrodites have the
reproductive organs of both sexes at the same time. Sequential hermaphrodites have ovaries for parts of their lives and testes during other
parts of their lives. Hamlet fish are simultaneous hermaphrodites and
have both female and male sexual organs as adults. The same is true
for some types of salmon. Hamlet fish are small, gold and yellow fish
found mostly in the Caribbean and the Gulf of Mexico. And though
they have both male and female organs, these fish do not mate with
themselves. Perhaps this is simply a consequence of their shape and
size, or maybe it’s an evolutionary adaptation that helps maintain or
generate diversity among hamlet fish. Regardless, when hamlet fish do
mate with one another they take full advantage of their hermaphroditism. During mating, hamlet fish take turns being the male and the
female partner. Hamlet fish trysts involve multiple matings that last for
up to three nights. So each fish has several opportunities to try out the Where Our Sexes Come From 111 role of each sex. For such small fish, their lust, not to mention their
creativity, is great.
Some hermaphroditic sea bass, on the other hand, do in effect mate
with themselves. These bass may spawn as many as twenty times in a
single day. And as they spawn they alternate between being egg-laying
females and sperm-spouting males. It takes a sea bass only about thirty
seconds to go from male to female, from laying eggs to fertilizing those
same eggs with a dose of bass sperm. Because sea bass have both ovaries and testes, these animals are by definition simultaneous hermaphrodites. But they don’t function as males and females simultaneously;
rather, they switch from one to the other with surprising speed and
seemingly at will. And as these fish age, things get even more unusual.
As sea bass pass into their golden years, their ovaries enlarge, and
they begin to produce mostly eggs and little to no sperm. That seems
to increase their reproductive rate, since most eggs do get fertilized (by
other fish) while most sperm do not find eggs. Thus, in effect, these sea
bass end their lives as females.
Other species of fish do just the opposite and end their lives as males.
Because they are not true sequential hermaphrodites, fish like these sea
bass are sometimes called successive hermaphrodites, meaning there is
one point in their lives when they do not have organs of both sexes and
one point at which they do. Wrasses (brightly colored fish often seen
in saltwater aquariums), parrotfish (nearly iridescent fish found widely
distributed among oceanic coral reefs), and some gobies (small, cigarshaped fish that make up the largest group of marine fishes) are all also
These fish find themselves beyond the sexes for one phase of their
lives—not exactly female or male—but end their lives as more or less
males or females. They are fish with a flair for change, fish with no concept of sex as we think we know it. Whole schools of fish fool with our
heads and confound any simplistic idea we might like to have about sex.
But the story gets even better.
For decades, maybe centuries, people have known that some fish
change sexes during their lifetimes. But it wasn’t until 1972 that marine
biologists began to figure out what motivated these fish to up and aban- 112 Between XX and XY don their lives as males or females and sprout the genitalia of the opposite sex.22 Not surprisingly, it turns out that the whole motivation thing
is complicated.23 Every fish seems to have its own set of rules and reasons for swapping sexes.24
Beyond successive hermaphroditism, some wrasse (Labroides dimidiatus) are sequential hermaphrodites—at times female, at other times
male. Most of these small coral-reef fish begin life as female. And as
they grow, a complex social structure develops so that by the time these
fish achieve adulthood they live in harems of female fish controlled by
a single dominant male wrasse. This alpha male, through his physical domination and perhaps his chemical presence, forces the females
to remain female. Among the females of the group a pecking order is
quickly established, and the alpha female runs the show. But her most
important job is yet to c...
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This document was uploaded on 02/04/2014.
- Spring '14