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We often intuitively classify animals as male or female. But members of many species can be both sexes at the same time or switch between them. Maryna Lesoway, a postdoctoral scientist at the University of Illinois, first became interested in hermaphrodites when she heard about a fierce mating ritual in the flatworm.

As simultaneous hermaphrodites -- meaning both male and female sex organs are functional at the same time -- two mating flatworms must decide who plays what reproductive role. What is the deciding factor? The worms stab at each other in a duel called “penis fencing,” battling to be the first one to inseminate the other and be the male.

Now, Lesoway is at the Marine Biological Laboratory as a Whitman Center Early Career Investigator. She is studying sex change in the snail Crepidula, commonly known as “slipper limpets” or “slipper shells.” According to Lesoway, Crepidula make a great model system for studying sex change because they grow well in the lab and much is known about their early development.

As sequential hermaphrodites, Crepidula are born male but at a point in their life cycle they become female. “I’m trying to learn more about how Crepidula change sex, and part of that is looking at where the gonads actually come from and how they make male and female parts,” Lesoway says. What makes Crepidula interesting is although they make both sperm and egg cells in the same individual, they function as either sex and not both simultaneously.

Work led by Lesoway’s postdoctoral advisor, Jonathan Henry, has shown that  give rise to the germ line in Crepidula. Using a technique called lineage tracing, Lesoway will determine how these cells give rise to sperm and egg cells. By using dyes to follow their movement, cells from one stage of embryonic development can be traced to different locations at later stages. Lesoway will also remove each of the primordial germ cells with laser ablation to see if male or female gametes arise from one or both cells. She also plans to identify genes known to be associated with the germ line and with turning on sperm or egg cell development.

So why change sex? For Crepidula (and other species), there are reproductive advantages that correlate with size. In Crepidula males, reproductive success does not change with size but as females get larger, their reproductive success increases. Thus, as males grow larger, becoming female may offer an evolutionary advantage. But things are not so simple. Sex change in Crepidula also seems be influenced by their surroundings. When in direct contact with female snails, males delay sex change.

While the Crepidula reproduction system may seem so vastly distant from that of many animals, studying its development can enhance our understanding of reproductive systems of different species.