On the advantage of size

I went out collecting for snails again last week because the weather was lovely and I needed them for an experiment...and also because I really enjoy getting up at 4:30 to go to the beach. Who wouldn't?

Anyway, processing the samples inspired me to share a fun fact of reproduction in Crepidula, the genus of snails that I study:

All of the species in this family of snails (Calyptraeidae) change sex as they age. Individuals start out as male and become female as they get older. This is called protandry, a type of sequential hermaphroditism. Evolutionary theory predicts that it's a good strategy to maximize lifetime reproductive success. The basic idea is that sperm are cheap, but eggs are expensive -- so it's better to make sperm when you are small and wait to make eggs when you are a bigger individual. This is called the size-advantage hypothesis.

A graphical depiction of the size-advantage hypothesis, from Munday et al. (2006).

When the blue line is above the red line in the graph above, it is advantageous to be female. When the red line is on top, it is advantageous to be male. Crepidula would be modeled by the graph in panel (b): better to be female at a larger body size. The point where the lines cross is the size at which you expect sex change to occur.

Working with sex change questions in Crepidula means spending lots of time investigating snail genitalia. Perhaps more on that at some point soon -- both work I have done in the past and some stuff I have going on in the lab now.