In some finned species, like the Atlantic silverside — as well as in many reptiles — sex is determined not by genetics but by temperature: the undifferentiated embryo develops testes or ovaries on the basis of whichever option conveys evolutionary advantages for that particular environment. Now, in a study published in the July 30 edition of the scientific journal Public Library of Science, Natalia Ospina-Alvarez and Francesc Piferrer have gone a little further in explaining how that mechanism works. In laboratory tests, they have demonstrated that higher water temperatures result in more male fish.
"We found that in fish that do have temperature-dependent sex determination [TSD], a rise in water temperature of just 1.5 degrees Celsius can change the male-to-female ratio from 1:1 to 3:1," says Piferrer, the study's co-author. In especially sensitive fish, a greater increase can throw the balance even more out of whack. Ospina-Alvarez and Piferrer have found that in the South American pejerrey, for example, an increase of 4 degrees Celsius can result in a population that is 98% male.
What makes these findings especially troubling, of course, is that the International Panel on Climate Change (IPCC) predicts that ocean-water temperatures are likely to rise by 1.5 degrees over the course of this century — and they may even go up a few degrees more. "If climate change really does result in a rise of 4 degrees, which is the maximum the IPCC predicts, and if species can't adapt in time or migrate, then in the most sensitive cases of TSD, we're looking at extinction," says Piferrer.
Most research into fish sex determination has been done in the lab (for obvious reasons), but the pejerrey is one of the few species that scientists have been able to study in the field. And those studies have revealed that already, its proportion of males to females is skewed. "It could be because of chemical pollution or it could be because of climate change. We don't know," cautions Piferrer. "But the field data matches our predictions."
At this stage, it is hard to tell what these results bode for already declining fish populations around the world. Of the estimated 33,000 piscatorial species, only 5,000 have had their sex-determination mechanism affirmed. But the study by the two CSIC scientists also suggests that the percentage of TSD fish is lower than previously believed. In tests of 59 species believed to be reproductively sensitive to temperature, only 40 proved to be true TSDs.
That would be good news in this grim era of climate change if it weren't for one factor: even genotypic sex determination can be affected by anomalous conditions, including anomalous temperature. "Basically, if you freeze it or cook it enough," says Piferrer, "you can get whatever sex you want."