National Geographic : 2018 Oct
Elizabeth Kolbert’s book The Sixth Extinction won a Pulitzer Prize; she wrote about race and genetics in the April issue. David Liittschwager, the Richard Avedon of obscure but beautiful creatures, has shot 13 features for the magazine. up in the Black Sea. A native of the western Atlantic, it presumably had been transported in a ship’s ballast water and then been discharged. In the Black Sea it reproduced so prolifically that by 1989 it had reached densities of up to 11 per cubic foot of water. Fish couldn’t com- pete with the jellies for food—sea walnuts eat as much as 10 times their body weight a day— and many fish became food for the jellies. Local fisheries collapsed. In other parts of the world, swarms of jellyfish have menaced swimmers and clogged fishing nets. In 2006, beaches in Italy and Spain were closed because of a bloom of jellyfish known as mauve stingers. In 2013 a Swedish nuclear plant temporarily shut down because moon jellies were blocking its intake pipes. Situations like these led to a spate of reports that jellyfish were taking over the seas. One web- site warned of the “attack of the blob.” Another predicted “goomageddon.” But scientists say the situation is more com- plicated than such headlines suggest. Jellyfish populations fluctuate naturally, and people tend to notice only the boom part of the cycle. “A big jellyfish bloom makes the headlines, while a lack of a jellyfish bloom isn’t even worth reporting,” says Lucas Brotz, a marine zoologist at the University of British Columbia. While some jellyfish species seem to thrive on human disturbance—off the coast of Namibia, for example, overfishing may have tipped the ecosystem into a new state dominated by com- pass and crystal jellyfish—other more finicky species appear to be declining. Researchers in a couple parts of the world have reported a drop in the number of jellyfish species they are encountering. Meanwhile, if people are having more unpleas- ant encounters with jellyfish, is it because they’re taking over the seas or because we are? “Anytime we have an adverse encounter with jellyfish, it’s because humans have invaded the oceans,” Haddock says. “ We’re the ones who are encroaching into their habitat.” Jellyfish are only doing what they’ve been doing generation after generation for hundreds of millions of years— just pulsing along, silently, brainlessly, and, seen in the right light, gorgeously. j SOREN WALLJASPER, NGM STAFF; LAWSON PARKER SOURCE: LUCAS BROTZ AND OTHERS, HYDROBIOLOGIA, 2012 PACIFIC OCEAN INDIAN OCEAN ATLANTIC OCEAN INDIAN OCEAN NORTH AMERICA SOUTH AMERICA AFRICA ASIA EUROPE AUSTRALIA ANTARCTICA Observed Jellyfish Populations Increase Stable Decrease High confidence Low confidence TROPIC OF CANCER EQUATOR CIRCL E ARCTIC CAPRICORN TROPIC OF ANTARCTIC CIRCLE Mediterranean Sea Black Sea SPAIN ITALY JAPAN SWEDEN NAMIBIA JELLYFISH INVASION? Some scientists are concerned that climate change and overfishing are changing the oceans in favor of jellyfish. But it’s unclear whether observed increases in jellies reflect actual ones or just better reporting. Limited knowledge Scientists often must rely on sporadic observations during blooms—a natural part of many jellies’ life cycles—making them less confident in their findings.