Thus in effect these sea bass end their lives as

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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 the animals. 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 successive hermaphrodites. 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.

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