Follow the Leader

ASC team dives in Catalina

ASC on Catalina Island 2012

When the leader of a group is no longer in the picture, a new leader steps forward to take the reins. But with certain species of fishes, leading a group or assuming control over a territory involves a switch beyond changing from follower to leader.

For the bluebanded goby, an aspiring leader may have to make a more extreme change—altering its sex.

“Sex change is not uncommon in fishes, about five percent of fish are sex changers,” said Lock Rogers, assistant professor of biology at Agnes Scott College. “There are no mammals who are naturally sex changers, so as mammals, it does feel like a strange thing to strategically alter one’s sex, yet it’s not. It still brings a smile to my face to think about how simultaneously weird and completely sensible it is for these organisms.”

When they reach a certain size, bluebanded gobies can switch their sex from female to male in situations where being male is an advantage. For instance, one male typically leads and mates with a group of many females, called a harem. If the male dies or leaves and there is no male to take its place, the largest female will then become male and assume control of the harem.

To find out more about this interesting sex and power dynamic, Rogers and Megan Williams ’13, a biology student at Agnes Scott, spent a few weeks in 2012 studying bluebanded gobies at the USC Wrigley Institute for Environmental Studies on Catalina Island, 20 miles off the coast of Los Angeles.

Rogers and Williams dove off the coast of the island, where the gobies live, to collect harems and temporarily observe their behavior and reproductive rates in aquaria at the institute before releasing them back into the ocean.

“When the male dies and the largest female becomes male, there are sudden pulses of growth among the other females and everyone grows pretty quickly and moves up the ranks. We’re interested in how these fish allocate resources for growth versus reproduction, depending on the dynamics of the harem,” Rogers said.

Rogers and Williams’ experiment involved setting up harems to include a male plus two females—one large and one medium-sized—in half of the aquaria, and one male plus a medium and a small female in the other half. With this group dynamic, half the medium-sized fish were next in line to take over the harem and as a result had better incentive to use their energy to grow larger and eventually become male instead of producing eggs. The other half were not in a position to take over the harem, so they had lower incentive to forgo reproduction and grow larger.

Though they’re still working to analyze their results, Rogers and Williams expect that the medium-sized females paired with a smaller female will exhibit relatively low reproductive rates and relatively high growth rates.

They’re particularly interested in whether these findings will be consistent with the behavior of other sex-changing fish Rogers has studied. Rogers, Williams and Kelly Smith ’12 conducted a related research project in St. Croix, U.S. Virgin Islands, last summer studying the bluehead wrasse.

“While the field work is always fun and beautiful, seeing the data and trying to work out whether they reject or confirm our hypothesis back at Agnes Scott is very exciting,” Rogers said.