ably act as camouflage, mimicking wave-dappled 

 sunlight in the water or perhaps a school of small 

 fish. If so, an important function of the markings 

 may be to conceal juvenile sharks from predators. 



Individual sharks have unique markings. Recent- 

 ly a team led by Bradley M. Norman, a marine bi- 

 ologist with the marine conservation group Ecocean 

 in Perth, Western Australia, adapted a computer al- 

 gorithm, originally devised for mapping stars, to 

 identify individual whale sharks from photographs 

 of their spots. The group is building a database of 

 identifiable whale shark photographs (available at 

 www.whaleshark.org), which should help biologists 

 track the animals and learn more about their travels 

 and behavior. 



Ningaloo Reef lies along a lonely 160-mile 

 stretch of outback coast and is Western Aus- 

 tralia's answer to the Great Barrier Reef. Although 

 smaller and less well known than its east-coast coun- 

 terpart, Ningaloo is famed for the large marine an- 

 imals — humpback whales, manta rays, whale sharks, 

 and others — that gather seasonaDy in its waters. I first 

 visited the reef in 1997 and spent nearly every day 

 of my two-week stay snorkeling with whale sharks 

 and photographing them. By the time I left, I'd be- 

 come curious to know much more 

 about these gentle giants, and sur- 



prised by how little science could tell me. Nine 

 months later, I enrolled in a Ph.D. program at the 

 University of Western Australia in Perth to study the 

 species at Ningaloo. I wanted to know why the whale 

 sharks gather on the reef each year, and why more 

 sharks come in some years than they do in others. 



Whale sharks, like many other shark species, seg- 

 regate themselves by sex. At Ningaloo Reef most 

 whale sharks are immature males, suggesting that 

 they come to feed, not to mate. The region's 

 oceanography may explain why they — and perhaps 

 some of the other large creatures — visit the reef. 

 Flowing southward along the continental shelf, the 

 Leeuwin Current dominates the area. But a smaller 

 countercurrent called the Ningaloo Current flows 

 northward between the Leeuwin Current and the 

 reef. Each fall, the two currents join to form a gyre, 

 which may keep nutrients and prey in the area rather 

 than flush them out. It hardly seems coincidental that 

 that's when the whale sharks arrive on the reef. 



Still, whale shark abundance varies widely from 

 year to year. Some years, as many as a few hundred 

 sharks come to Ningaloo Reef; in other years, the 

 numbers are much lower. To determine why, I be- 

 gan by looking at patterns in whale shark abundance 

 derived from records of shark interactions that com- 

 mercial tour-boat operators must keep as part of 

 their permit requirements. Many years of such 



Gill slits 



Whale shark is a "suction filter-feeder" on dense congregations of minute prey such as krill. It strains 

 mouthfuls of food-filled seawater (brownish-blue arrows) through porous gill plates and consumes the 

 prey that remains in its mouth. Channels behind its gill plates direct the filtered water over its gill fila- 

 ments, which extract oxygen for respiration. The filtered water (blue arrows) is then released through 

 the whale shark's gill slits. The animal can generate suction to draw in its meals, perhaps by expanding 

 its oral cavity and depressing its basihyal, a tonguelike structure on the floor of its mouth. 



Vestigial teeth 



April 2006 NATURAL HISTORY 



45 



