Short is good for scaling waterfalls, but tall is better for evading predators along the way
BOSTON — Evolution has stuck Hawaiian fish called stream gobies between a rock and a scary place.
A Sicyopterus stimpsoni goby shorter than a thumb has to climb the rocks of waterfalls, sometimes several hundred feet high, on a juvenile migration upstream. But to reach those falls, the fish has to pass through a kill zone of lowland waterways full of predatory fish, explains Richard Blob of Clemson University in South Carolina.
Darting away from predators favors a taller body shape with bigger fins instead of the squat, rock-hugging form ideal for waterfalls, according to new work from Blob and his colleagues. Trade-offs between squat and tall in different island geographies could explain some of the variety in shape within this species, Blob said January 4 at the annual meeting of the Society for Integrative and Comparative Biology.
“What’s novel is the comparative study” of gobies on different islands, says Thomas Kunz of Boston University
The theme of conflicting evolutionary pressures on the same structure runs deep in nature. Kunz studies bats, but he says he’s starting to think about how the many uses of wings affect their structure.
To test for predator effect on body shape, Blob and coworkers put S. stimpsoni gobies from Hawaii's Big Island in tanks containing a major goby-eater: a fish called the sleeper, or Eleotris sandwicensis. When sleepers had caught half the gobies, a job that took about three days, the researchers measured the body dimensions of the survivors.
Gobies that evaded predators tended to have a greater relative body depth than usual, Blob reported. This shape makes it easier to generate thrust while swimming.
The higher profile, alas, also means more drag in the struggle up a waterfall, Blob says, based on earlier research with Heiko Schoenfuss of St. Cloud State University in Minnesota, who also worked on the current study. With torrents pounding down, waterfall climbers inch up rock faces by gripping the surfaces with their mouths and a specialized pinhead-sized sucker on their undersides. “In human terms, it’s like a marathon,” Blob says.
S. stimpsoni gobies
get caught in the opposing forces for body shapes since the fish live an
out-to-sea-and-back lifestyle a little like mainland salmon. These gobies hatch