World's Biggest Fish Is a Delicate Feeder
Monday, September 3, 2007
ISLA HOLBOX, Mexico -- The whale shark -- a massive, polka-dotted creature stretching about 23 feet long -- slowly swam up alongside the scientists' boat. As it neared, they tossed a handful of pre-soaked rice in front of its enormous mouth. The rice rushed in along with the seawater, and the whale shark made a shuddering motion.
"There's the cough!" exclaimed Philip Motta, a University of South Florida biology professor, using the catchphrase he and his colleagues coined recently to describe the shark's abrupt way of swallowing.
Cruising the waters off the Yucatan for hours in the broiling sun, Motta and doctoral student Kyle Mara, along with several other researchers, repeated the ritual again and again. Hoping to figure out in detail the complex feeding mechanism of the biggest fish in the sea, the scientists trailed whale sharks of different ages, sizes and sexes, using a laser beam to measure each gigantic mouth and videotaping as the rice flowed in.
Whale sharks are not mammals but got their name because of their immense size and whalelike tendency to filter-feed on the ocean surface. Most sharks are known for their intimidating teeth, but whale sharks fascinate scientists for the opposite reason: They have a unique, three-part filtering system that is more complex than those of whales and other filter-feeding sharks. The setup is so effective that modern bottling plants employ it.
Until recently, it was difficult to study their filter feeding in detail, since whale sharks can be elusive. Then researchers discovered that hundreds of the animals come to Isla Holbox each summer to feast on plankton carried to the surface by an upwelling of water between the Gulf of Mexico and the Caribbean. Scientists began making annual pilgrimages to this former pirates' lair about 90 miles northwest of Cancun, where the fish are called domino sharks because of their spots.
Then, just this year, the deaths of two whale sharks at the Georgia Aquarium gave scientists the opportunity to study the anatomy of the sharks' feeding systems.
Motta's work, part of a research program directed by the Sarasota, Fla.-based Mote Marine Laboratory and the Mexico-based conservation group Project Domino, is coming closer to answering a question that has eluded scientists for years: How did a fish weighing thousands of pounds and stretching the length of a school bus develop its unusual feeding apparatus?
"How do you go from an animal that sits on the bottom and eats fish and crustaceans to one that swims on the surface and consumes a large volume of water to eat some of the smallest creatures of the sea?" asked Robert Hueter, who directs Mote's Center for Shark Research. "That's a huge evolutionary step."
The still-unexplained deaths at the aquarium have also contributed to the research. The two males -- imported from Taiwan and dubbed Ralph and Norton by aquarium staff -- died within the past year, one as recently as June. Motta and Hueter helped perform head dissections on the dead sharks, allowing them to examine the sharks' filtering system up close.
The aquarium, which has given scientists $250,000 to study whale sharks off the tiny island of Holbox, still has two male and two female whale sharks in its 6.3 million-gallon Ocean Voyager tank.
Motta and his colleagues have observed them from the top of the tank and by entering the water with them during routine veterinary exams.
Aquarium officials are also planning to test radio tagging equipment on the captive whale sharks, to ensure that the technology scientists want to use in the wild will work properly. Ray Davis, the aquarium's senior vice president of zoological operations, said officials are talking with wildlife tagging manufacturers to develop devices that will monitor whale sharks' physiological changes as they dive and will transmit the data back.
"We're able to start ground-truthing these things," Davis said. "We're able to test out some of these techniques."
The marriage of research on the captive animals and real-time observations of the hundreds of whale sharks that gather in the wild each summer near Holbox allowed the scientists to discover that whale sharks apparently have what Motta calls a unique "cross-flow filtration system" that lets them consume massive amounts of plankton each day without clogging their gills.
The scientists found that the sharks ingest about nine pounds of plankton an hour while swimming through a dense plankton bloom. The tiny sea creatures and massive amounts of water flow into a whale shark's throat, where 20 filtering pads with tiny holes lie. The pads take up more than 6.5 square feet, but the pads' pores are tiny, just 0.07 inches across. The water hits the pads at an angle, which ensures that as it flows through the pads and the shark's gill apparatus, the food particles are swept into the back of the throat, near the opening to the stomach.
As a result, the plankton does not clog the filtering pads as the water moves through and returns to the ocean. When the whale shark accumulates enough plankton "slurry," as Motta describes it, the whale shark swallows in the "cough" motion.
It is only coincidence that beverage companies use a similar approach to reducing clogging in their commercial operations, Motta said, but the fact that engineers rely on cross-flow filtration testifies to its effectiveness.
Other researchers are exploring whether some bony fishes, which share a distant common ancestor with whale sharks, have developed the same sort of filtration system. If they have, Motta said, it would demonstrate "convergent evolution," whereby two radically different species of fishes that are only distantly related have independently developed a similar solution to filter feeding. Baleen whales separately evolved their own form of filter feeding.
Some animal rights activists have challenged the use of the aquarium-held whale sharks for research.
"Whatever you learn by keeping these animals in captivity, whether it's a marine mammal or a whale shark, is of very limited value," said Naomi Rose, a marine mammal scientist at the Humane Society of the United States. The fact that two whale sharks died at the aquarium, Rose added, is "a pretty high price to pay for what they've learned."
But David Santucci, a spokesman for the two-year-old Georgia facility, said the Holbox project demonstrates that the aquarium is contributing to science. The aquarium's nutritionist, Mike Maslanka, journeyed to Holbox in August to help identify the types of plankton in the water so that he and other researchers could get a better sense of what the whale sharks eat.
"A year ago people were asking, 'What's the Georgia Aquarium going to do for science?' " Santucci said. "It was a difficult question to answer. Science takes a lot of time. We're just starting to reach that point where we're starting to do science."
Staff researcher Eddy Palanzo contributed to this report.