I've been keeping aquarium fish for years, and I have long known that fish gender is nowhere near as starkly defined and inflexible as it is in higher animals. It has long been known that many species of fish can and do switch gender, sometimes routinely. The process is not well understood, though.

Earlier Glenn Reynolds reminded me of this when he linked a piece with the provocative (some might even say "transphobic") title of Something in the Water Is Feminizing Male Fish. Are We Next?

It's one thing to worry about pollutants in our freshwater supply. It's another to find out that all across the country, male fish swimming in some of that water are becoming "intersex," their male sex organs producing immature female eggs. Although the condition occurs naturally in some species, it shouldn't happen to black bass. But a new study shows that it is, and in numbers far greater than ever suspected. The phenomenon raises serious concerns about the pollution levels in our rivers and could threaten several species.

The basics are discussed in "Hermaphroditic Fish Can be Two Sexes at One Time -- Fish Can Change Gender to Increase Their Reproductive Potential:

Not all fish can change sex, but many can. In fact, the majority of reef fish will change gender at some point in their lives. These fish are considered hermaphroditic and such fish have a few options. Some fish are simultaneous hermaphrodites meaning they are both genders at the same time and could potentially mate with any other individual in their species. Other fish are sequential hermaphrodites and these fish change sex at some point in their lives. Protandry is when a male becomes a female and protogyny is when a female becomes a male. Some fish can change their gender more than once, going back and forth between genders.

Why Do Fish Change Sex?

Fish will change their gender in order to maximize their fertility. Maximizing reproductive success is influenced by the amount of resources around, the current male to female ratio in the fish population, and the size of the current males and females. Some fish species will start off all female and a large aggressive male will form a harem, using aggression to keep the smaller fish female. If he dies off the largest female fish can change sex and use her size to maintain dominance over the harem. In other species the juveniles are all males waiting for their chance to mate with the large female. This is useful to fertility because larger females can produce more eggs. When the female dies the largest male fish will change gender.

How Do Fish Change Sex?

The endocrine system is almost certainly involved in the gender switch. The endocrine system is in charge of releasing hormones into the body and includes the gonads. However, while progress is being made the exact mechanisms are still not known. Scientists are still researching which hormones, genes, and chromosomes are involved.

Gender changes in cichilds have been known since at least the 1970s. One researcher's explanation is that the sex of the fish varies according to social signals related to size advantages. Another researcher states "In fishes, almost anything goes when it comes to ways to reproduce and raise young."

This is complicated stuff, and I am no expert, but I think it's worth reminding people that the phenomenon is anything but new.

From a 1984 New York Times piece titled SEX CHANGE IN FISH FOUND COMMON:

Conversions from female to male are now known to occur in species belonging to at least 14 families, while conversions from male to female are known in eight families.

Nevertheless, the process by which fish change sex remains a mystery to scientists. ''No one has ever come up with an answer'' as to how they do it, Dr. Klaus D. Kallman of the New York Aquarium, a fish geneticist, said recently.

Gender-change is just a phase in life for the black sea bass. They are born in the wild as females, and when they reach about 2-5 years of age, they change to a male. Black sea bass in captivity can change their gender much faster than those in the wild. This is causing a problem for the fish breeder in commercial fishing industry.

The concern over intersex fish is not new, except that in 2003 the culprit was said to be cattle hormones used in feedlots. Now it's pee from women using birth control pills.

I'm not saying that hormones in the water aren't messing with the fish, for how could I hope to prove such a thing? But shouldn't the people who are pointing to humans as the culprit have to prove something beyond the assertion of a correlation?

I hate to sound like a parent in denial, but what if the fish are just going through a phase?

MORE: From the abstract of "Widespread occurrence of intersex in black basses (Micropterus spp.) from U.S. rivers, 1995-2004":

Intersex was not found in largemouth bass older than five years and was most common in 1-3-year-old male largemouth bass. The cause(s) of intersex in these species is also unknown, and it remains to be determined whether the intersex we observed in largemouth and smallmouth bass developed during sex differentiation in early life stages, during exposure to environmental factors during adult life stages, or both.