13 April 2005

Cast from the past: Thomas Say’s male and female pulmonates

While everyone is busy reading what got published today, I will turn my gaze back to the early 19th century. The pioneering American naturalist Thomas Say was a member of an expedition from Philadelphia to Florida in 1817. One result of that expedition was a paper Say published about a year later in the then fledgling Journal of the Academy of Natural Sciences of Philadelphia1.

In that paper, Say established the genus Polygyra and described three Polygyra species. What is most interesting about Say’s descriptions is that for two of the species, P. auriculata and P. septemvolva, he gave separate dimensions for the shells of what he claimed were female and male snails. Obviously, in 1818 Say didn’t know that those particular species he was describing were, in modern terminology, simultaneous hermaphrodites, that is, they can produce both sperm cells and ova at the same time and have both female and male genitalia.

But how did Say distinguish between "female" and "male" Polygyra? Say didn’t say, but a clue emerges from the dimensions he gave. For both species, his females are larger than his males. We also know that the dimensions of the shells of Polygyra species tend to be quite variable. So it appears that Say noticed and attributed this variability to sexual dimorphism and decided that "female" snails were larger than "males".

When and how did Say wise up? That will be the subject of another post.


1. Say, T. 1818. Account of two new genera, and several new species, of fresh water and land shells. Journal of the Academy of Natural Sciences of Philadelphia, 1:276-284. Full text

No comments: