Fun with Science Club®


			Are there more female eels in the north? Carrie Straight 05.18.01 American eels (Anguilla rostrata) are one species that live in fresh water, and move into salt water to breed. This movement from fresh to salt water is called catadromous. The odd thing about this eel is that when they are young and found in the saltwater, the baby eels are neither male nor female. Later in an eels life when they move into the fresh water, the reproductive organs develop in the young eels making them either male or female. Scientists have long hypothesized about what makes the little eels male or female. Some believe that the latitude (distance from the equator) of area where the young eels are determine their sex. Researchers in Maine, USA wanted to find out if there was a relationship between numbers of male and female eels and the latitude of the streams where the eels became either males or females. The scientists collected eels from 5 different rivers and determined whether they were males or females. One river had almost all male eels and two other rivers had an equal number of males and females. Because all of the rivers are on the same latitude, the differences in the sex-ratio (the number of male compared to number of female eels) from one stream to another showed that the latitude did not determine whether an eel would become a male or a female.


			Oliveira, K., J. D. McCleave, and G. S. Wippehauser. 2001. Regional variation and the effect of lake: river area on sex distribution of American eels. Journal of Fish Biology 58: 943-952.