hundred whale shark sightings had been reported 

 worldwide; in the past two decades, human interac- 

 tion with them has grown substantially. 



Yet despite the increasingly frequent contact 

 between people and whale sharks, and despite their 

 presence throughout the world's tropical and tem- 

 perate seas, including the waters of some 1 25 nations, 

 surprisingly little is known about them. Marine bi- 

 ologists don't know much, for instance, about how 

 whale sharks reproduce: no one has ever observed 

 their courtship, mating, or birth. How they interact 



hark feeds passively on small prey by swimming with 

 th open. A snorkeler watches from above. 



socially is anyone's guess. What they do on the pro- 

 longed, deep dives they make is yet another mystery. 

 No one knows how many there are, or whether their 

 populations are rising, stable, or declining. Given their 

 largely unregulated harvest and vulnerability to cap- 

 ture, however, decline seems most likely. 



To a small troop of biologists — myself included — 

 those gaps in knowledge present a challenge. Our re- 

 cent research into the whale shark's feeding habits, 

 diving behavior, and migrations is slowly giving us a 

 better understanding of its role in the marine envi- 

 ronment. Our hope is that we will be able to use this 



knowledge to help ensure the species' survival, before 

 it becomes another casualty of the changing world. 



At least some facts about whale sharks are clear. 

 First, the name "whale shark" is somewhat 

 misleading: the animals are indeed sharks, but they 

 are "whales" only by virtue of their size. They grow 

 more than forty feet long (the length of a luxury 

 motor home), and there are unsubstantiated reports 

 of a sixty-five-footer that weighed thirty-seven tons. 

 Unlike most sharks, though, whale sharks are filter 

 feeders. They share that behavior, fittingly, with the 

 world's biggest animal, the blue whale. Whale sharks 

 suck dense concentrations of minute prey, such as 

 krill and other zooplankton, fish spawn, and small 

 fishes, into their enormous mouths. To collect the 

 prey, they filter out the accompanying water 

 through sievelike gill plates, and then expel it 

 through their gill slits. 



Whale sharks often feed passively by swimming 

 slowly with their mouths agape. They can also as- 

 sume a head-up, tail-down feeding posture, some- 

 times bobbing up and down near the surface to 

 pump prey-filled water over their gills [see illustra- 

 tion on opposite page]. Oddly, they are not closely re- 

 lated to the other two filter-feeding sharks, the bask- 

 ing shark and the megamouth shark. Instead, their 

 closest relative is the nurse shark, a bottom-dwelling 

 predator. In spite of their filter-feeding ways, whale 

 sharks possess some 27,000 minute teeth, similar to 

 teeth in the fossil record that date to about 55 mil- 

 lion years ago. Little else is known of their evolu- 

 tionary history. 



Scientific knowledge of whale shark reproduc- 

 tion is based on a single female, harpooned off Tai- 

 wan in 1995, that carried 301 embryos in various 

 stages of development. Biologists know from that 

 catch that the pups are born alive when they are 

 about two feet long. (The eggs hatch inside the 

 mother.) Studies of growth rings in vertebrae sug- 

 gest that whale sharks reach sexual maturity when 

 they are between twenty and thirty years old, and 

 may live for several decades more. Young whale 

 sharks less than ten feet long are rarely seen, lead- 

 ing some investigators to speculate that they occu- 

 py deep, offshore habitats during that most vulner- 

 able stage in their lives. Newborns have been re- 

 covered from the stomachs of a blue shark and a blue 

 marlin. The adults likely have few natural predators, 

 except perhaps great white sharks and killer whales. 



The whale shark's most prominent feature — 

 other than its sheer magnitude — is its distinctive 

 markings. Pale spots speckle a grid of bars and 

 stripes atop the shark's blue, gray, or brownish back 

 and flanks (its belly is white). The markings prob- 



AL HISTORY April 2006 



