Hey, Pisces, what's your latitude?

Hey, Pisces, what's your latitude?

Male-female relationships can be confusing, even whenyou're in the swim of things. But for those species where the offspring become male or female based on environmental-- rather than genetic--factors, finding a member of the opposite sex is no breach party. Especially if you're a male silverside fish cruising the waters off South Carolina in June.

The silverside is a species whose ratio of males to femalescan be influenced by temperature of the environment during larval development, an example of the "adaptive sex ratio theory.' Yet others in the species apparently have sex ratios determined only by genetic factors, with sex determined at conception. Which case of sex selection applies seems to be a matter of latitude, according to a report in the April 2 NATURE.

David O. Conover and Stephen W. Heins, of the MarineSciences Reseaerch Center at State University of New York in Stony Brook, transferred fish embryos from various ocean sites to the laboratory to study the strange case of the silverside. Their previous field studies had suggested that, for silverside fish along the mid-Atlantic coast of the United States, most offspring become female if produced early in the breeding season, when water temperatures are lower. By the end of the season in June, when the water is warmer, the sex ratio shifts in favor of males. Allowing the females a longer growing period appears to increase their reproductive capability. In contrast, fish living in waters off Nova Scotia, for example, have shorter breeding and growing seasons, and maintain equal numbers of males and females despite fluctuations in water temperature.

Several thousand larvae were distributed among tanks ofdifferent temperatures and allowed to grow until sex could be determined. The researchers found a direct correlation between the amount of sex-ratio fluctuation and the water temperature, with the greatest effect seen among fish collected from areas with longer growing seasons. The smallest variation was seen among fish from Nova Scotia, an observation the authors attribute to the short season--during which there would be little advantage for such adaptability.