| The Magazine of the CALIFORNIA ACADEMY OF SCIENCES |
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Horizons White Shark Any runner knows that warm muscles cooperate better than cold ones. Good thing that we are endotherms--able to make our own heat to warm our bodies before trying to outsprint a speeding locomotive. Some hard-earned research data suggest the same may be true of one of the ocean's top predators, the great white shark. If so, it would be the first case of true endothermy in anything but a bird or mammal. Ken Goldman took the stomach temperatures of great white sharks (Carcharodon carcharias) at the Farallon Islands after they attacked and ate young elephant seals, their main meal. He found that the fish stays warm to the core; in other words, this may be a mammal-like fish that hunts fish-like mammals. In Fall 1991, Goldman, a biology graduate student at San Francisco State University and a part-time aquatic biologist at Steinhart Aquarium, made his first trip to the Farallons, 27 miles west of San Francisco's Golden Gate, where he joined ongoing efforts by the Point Reyes Bird Observatory to observe shark feeding behavior. Refusing to chum the water to artificially attract sharks, the researchers wait at Southeast Farallon Island's lighthouse for a shark to make the first move. After spotting a shark attack, Goldman and colleague Scot Anderson take their boat to the site. They throw in a piece of blubber--collected from dead elephant seals on the island--that has been stuffed with a transmitter to record the shark's depth and stomach temperature. The blubber lure is attached to a square, blue float that sometimes draws the shark's attention instead. "Getting the sharks to eat these transmitters is not an easy task," says Goldman. "Every now and then you get lucky." During two seasons of effort, Goldman got lucky three times. In 1991, he followed a 14-footer named Boots for a total of five days. In 1993, Goldman fed transmitters to 16-foot-long Ambergris and twelve-foot Phoebus (formerly Phoebe until Goldman learned that she was a he), and he tracked them for a total of eleven and seven days, respectively. Each shark maintained a mean stomach temperature of 24.6-27.3 °C, regardless of water temperature. Their body cores consistently stayed ten to 14 degrees warmer than the water. (Goldman did not monitor water temperature at all depths, but he says there's an average of only one degree difference between water temperature at the surface and at 25 meters deep.) In 1985, Academy aquatic biologist John McCosker, working in Australia, took the first ever stomach temperature of a white shark. His results were virtually identical to Goldman's, even though the water temperature there was three degrees higher than the warmest water at the Farallons. When he began his research, Goldman thought that white sharks might share a similar physiology with tuna. Swift-swimming tunas expend a vast amount of energy and eat as much food as they can when they find it. After eating, tunas raise their stomach temperature by twelve to 15 °C for up to 20 hours to help them digest. Body temperature then falls back to five or six degrees above the ambient water. Like tunas, sharks swallow large helpings of rich food. "What are they digesting? Fat," says Goldman. "Fat is very difficult to digest but it's very nutrient-rich." But Goldman's data show that the white shark's stomach temperature stays high all the time, dropping slightly only during the initial gulps when sea water inadvertently gets swallowed. Since white sharks retain more heat longer than tunas, they apparently heat up for more than a temporary digestive aid. Goldman points out that most predatory sharks inhabit warm waters, but whites, the largest species, must chase swift, agile, and warm-blooded prey through cold water. Goldman suspects that by keeping constantly warm, white sharks can be more successful cold-water hunters. But how do they do it? Out of 370 shark species, only seven, including the great white, possess a rete mirabile or "wonderful net." Also found in tuna, retes are sites of heat exchange among arteries and veins that are sandwiched together and allow a shark to keep internally generated heat rather than lose it through the gills as all other fish do. White sharks have three retes, located above the eyes and brain, in front of the liver, and in the muscles above the viscera. Through the work of Francis Carey at Woods Hole Oceanographic Institution, Goldman knows that blood can be shunted around a shark's liver rete; the retes may thus provide a mechanism for white sharks to control their body temperature. That remains a mystery. While Goldman is convinced that the sharks thermoregulate, he needs more data before concluding whether they are true endotherms. But, he says, "They're certainly putting the best effort into being endothermic of any fish you'll ever see."
Dead Whales Tell Tales Conservationists now have a powerful tool to uncover evidence of illegal whaling. Two molecular biologists are using DNA analysis to determine which whale species show up on sale as meat in Japanese markets. Their results raise hope, as well as concern, for the future of cetaceans. Stephen Palumbi of the University of Hawaii's Kewalo Marine Laboratory and Scott Baker of the University of Auckland have studied whale genes for five years as a way to reconstruct the marine mammals' evolutionary history. Two years ago, the Hawaii-based environmental organization Earthtrust asked them whether their method could reliably identify whale species from pieces of fresh or frozen meat. Earthtrust suspected that protected whales were being killed illegally and sold to markets, where packaged meat bears only the generic label kujira or "whale." The International Whaling Commission (IWC), the organization that sets quotas for whaling, legally permits the Japanese to hunt 300 minke whales per year from the Southern Hemisphere for research and domestic meat sales. Fin whales, however, have not been hunted since 1989, and humpbacks have been protected worldwide since 1966. "If you can't tell the difference between a minke whale and, say, a fin whale when it's wrapped in cellophane, how can you police that? That's where the genetic technique is really powerful," says Palumbi. Earthtrust arranged for meat samples to be purchased in several markets. Then Baker took a portable laboratory to Japan in order to multiply the DNA within the meat using polymerase chain reaction (PCR), a technique for quickly making millions of copies of a gene. (He had to travel to Japan because wildlife trafficking laws restrict the export of pure whale DNA but not copies of it.) Baker got DNA out of fresh meat, sold sliced for sashimi, as well as dried, marinated meat. He was able to multiply genes from about half of the 41 samples and obtained good data from 16 of those. The copies then journeyed to Hawaii where the scientists sequenced particular mitochondrial genes and compared them with DNA reference collections from several species. Some sequences exist for nearly all baleen whales, but DNA has been obtained from only a fifth of the toothed whales and dolphins. From this analysis, fourteen samples could be positively identified. Eight turned out to be from minke whales, a small baleen whale, four were from fin whales, the second largest species, and two others came from toothed whales or dolphins. One sample proved more puzzling. Its sequences matched both minke and humpback whales. Since the sample consisted of marinated meat strips, Palumbi considers it likely that two species got processed together. Still, the ambiguity of this and another result drew criticism of the method from the Japanese Fisheries Agency (JFA) and a Washington, D.C.-based lobbyist for the whaling industry. The JFA claimed that the minke and fin whale meat had been stored frozen for years and had been processed before international protection took effect. Yet it seems odd that four-year-old frozen meat could be passed off as sashimi selling for nearly $200 per pound (not to mention that the humpback sample would have had to remain in storage for 27 years to predate that species' protection). Baker says that the lobbyist's response was "wide of the mark from a scientific point of view." The lobbyist contended that since genetic sequences from the market samples don't exactly match the reference genes, the samples could have come from dolphins or other unprotected whales. But Baker says that one fin whale sequence is identical to reference sequences from Iceland and the Mediterranean. The minor differences in others, say the scientists, reflect normal genetic variation. Despite its criticism, the JFA announced last September that it will create a genetic database of coastal cetaceans and will start testing whale meat in markets using the same technique. The announcement came two weeks after Baker and Palumbi published their results in the journal Science and recommended that whale meat carry labels identifying the species, where they were caught, and when they were processed. Whether the Japanese use the test to expose or cover up an illegal whaling problem remains to be seen. "It's not fair to blame the Japanese entirely. This is an international problem," says Baker. "Japan just happens to be one of the markets." Baker and Palumbi's results hint at the international scope of illegal whaling. Because some whale species show distinct genetic differences between oceans, the scientists could pinpoint several of their samples to a population in a particular ocean. While most minke whale sequences suggested a Southern Hemisphere source, one was more similar to North Atlantic sequences. A fin whale sequence matched others in the database from the North Atlantic, and the humpback whale sequence probably came from a North Pacific animal. The detection of meat from supposedly protected species calls into question the IWCUs statistics for setting sustainable whaling quotas. Because about half the baleen whales sampled had been caught outside the Southern Hemisphere, says Palumbi, more whales are being killed than the IWC realizes. Says Palumbi, "It's assumed that if a species is protected it isn't being harvested....We need to know the impact of illegal harvesting on some of these animals." Fortunately, the IWC Scientific Committee has shown interest in Baker and Palumbi's research, which helped prompt a recent resolution for member nations to monitor illegal whaling. Meanwhile, Palumbi, who co-authored The Simple Fool's Guide to PCR, and Baker will teach their identity test to others and make it faster and easier to use. Blake Edgar is Associate Editor of California Wild. |
Winter 1995
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