for National Geographic News
Samuel "Doc" Gruber, a marine biologist at the University of Miami, has studied juvenile lemon sharks in the Bahamas for three decades. His research, conducted largely in a Bimini Islands lagoon nursery, has produced one of the world's most comprehensive shark studies.
It has also spotlighted how little we know about species.
"We've been looking at [lemon sharks] for maybe 30 years, and as a result we know a tremendous amount about their early lifebut nothing whatsoever about the adult stages," the scientist said. "That's why I'm so excited about [my current] work."
Gruber's current work involves the dramatic aggregations of several hundred adult lemon sharks off the coast of Jupiter, Florida, each year. The mass gatherings, which offer scientists a unique opportunity to meet adults face-to-face, are believed to be part of the animal's reproductive cycle.
Surprisingly, scientists have never observed lemon sharks mating.
Like salmon and certain other marine species, lemon sharks (Negaprion brevirostris) return to their natal grounds to give birth.
"If you were mating with your sister or your mother, you might have some inbreeding problems," Gruber said. "If a whole population was less than 100 or 200 animals, you might expect to see sharks with twelve fins." Yet lemon sharks appear normal. "This perplexed us."
To avoid inbreeding problems within their relatively small populations, the sharks appear to have developed a mating strategy as yet unobserved in other shark species: Though female lemon sharks return to their natal grounds each year, males remain nomadic.
The strategy ensures genetic diversity among different lemon shark populations. It may also have spawned the phenomenon of large lemon shark gatherings, like the ones found near Jupiter, Florida. Such gatherings guarantee that the two sexes get together.
"We believe that a bunch of females go to a certain place where there's a current and put out pheromones to attract males," Gruber said. "When they have attracted a great enough numbera critical massthey begin to mate."
"I suspect that quite a few [shark] species could do this," he added. "But nobody knows."
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