Fish Are Surprisingly Gender Fluid

The following is an excerpt from the new book What a Fish Knows by Jonathan Balcombe (FSG, 2016): 

"Fishes . . . are characterized by a level of sexual plasticity and flexibility that is unrivalled among other vertebrates." —Thavamani J. Pandian, Sexuality in Fishes

True to their magnificent diversity of forms, fishes exhibit a Full Monty of breeding systems—thirty-two in all. They have as many different kinds of reproductive behaviors and strategies as exist in all the other vertebrates combined. There are promiscuous fishes, polygamous fishes, and monogamous ones, including fishes that mate for life.

The great majority of fishes exhibit a familiar pattern with an obscure name, gonochorism, in which individuals are either male or female throughout life. But you can guess what that implies: there are scores of fishes who cross gender lines. For some reason, reef living in particular has had a diversifying effect on sexual expression. More than one-quarter of all fishes on a reef can transition from male to female, or vice versa—with no need for expensive surgery. Other fishes opt for a unisex approach, assuming both male and female identities simultaneously, or sequentially.

Of the species that produce both sperm and eggs at the same time (simultaneous hermaphrodites for jargon lovers), most are found in the vast darkness of the deep sea. Being able to fertilize yourself is a very useful adaptation where the daily prospects of finding another of your kind are almost as dim as your surroundings. The sex-changing fishes (sequential hermaphrodites) are not so restricted, and they benefit by being different sexes at different ages and sizes. For example, in a mating system in which one male may monopolize many females, it pays to start out as a female, then become a male when you are large and more physically capable of rebuffing the challenges of competitors. Often, all younger members of a species are females, with a harem male occupying the top-dog position. In other cases, the chain of command is reversed, with a string of lesser males awaiting the future prospect of becoming a breeding female.

The popular clownfishes of Finding Nemo fame rely on size, hierarchy, and sex change to maintain social order. They live in groups of two large and several smaller individuals. The big ones are the breeding pair, the larger of whom is the breeding female. The subordinates, all males, are ranked hierarchically according to size. Although these lower-ranking fishes may be as old as the spawning pair, the behavioral dominance of the sexually mature individuals keeps the subordinates from growing or developing. Hans and Simone Fricke, who studied this strict mating system, described the low-ranking males as being, in essence, psychophysiologically castrated. Each retains his place in the queue until there is a job opening in the executive office. If the breeding female dies, the chief male changes sex to female and the next largest fish in the subordinate group bumps up to chief male. So there is always hope for a suppressed male in a clownfish family. (All of this reveals a slight inaccuracy in the course of events in Finding Nemo. The fact is, upon Nemo’s losing his mother, his dad, Marlin, should have become his new mother.)

Sex-changing fishes behave appropriately, performing male-typical or female-typical sexual behaviors according to their current gender assignment. Sexual behavioral plasticity can also be observed in fishes that do not normally change sex, but who are subjected to hormonal manipulation. Although how this happens is not clear in a fish, results of field and laboratory observations suggest that some bony fishes (as distinct from the cartilaginous sharks and rays) have a sexually bipotential brain that can manage two types of behaviors, unlike most other vertebrates, which have a discrete sex differentiation of their brain and can only perform gender-typical sexual behavior.

The ability of individual fishes to change sex shows just how fluid gender divisions can be in nature. If you are at all tuned in to societal trends, you’ll know that gender lines are becoming more blurred in humans, too. The book Becoming Nicole, for example, explores the social challenges faced by a human family whose son, an identical twin, sought gender reassignment at an early age. As medical advances expand our options to assert our true gender identities, we unwittingly become more like a fish.

Excerpted from WHAT A FISH KNOWS by Jonathan Balcombe, published by Scientific American/Farrar, Straus and Giroux, an imprint of Farrar, Straus and Giroux. Copyright © 2016 by Jonathan Balcombe. All rights reserved. 

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