Scientists Discover Mystery Krill Killer

John Roach
for National Geographic News
July 17, 2003

Scientists have discovered a tiny, one-celled parasite that causes a grisly and fatal infection in krill. Masses of the parasite grow inside the krill, eat its organs, divide, and then burst out of their host's dead body in search of new victims.

The discovery sheds more light on a key player in ocean's food chain. Scientists previously thought most animals like krill were either eaten by larger predators or simply starved to death. The find shows that parasites also play an important role.

The scientists, led by marine biologist Jaime Gómez-Gutiérrez, a researcher at Mexico's Centro Interdisciplinario de Ciencias Marinas in La Paz and currently a Ph.D. graduate student at Oregon State University in Corvallis, found infected krill—called euphausiids—at more than a dozen sites along the Oregon and Washington coasts.

At a site off the coast of Astoria, Oregon, researchers found a massive die-off in a krill swarm, with carcasses littering a 0.9-mile (1.5-kilometer) stretch of the sea floor.

"Because they cause mass mortality, they literally compete with other predators of euphausiids," said Gómez-Gutiérrez. "If the mass mortality occurs frequently, they can have a significant impact on euphausiid and predator production."

Keith Reid, a krill expert with the British Antarctic Survey in Cambridge, England, said the research highlights "a potentially important cause of mortality that should be considered along with traditional contributors to mortality."

Prior to this research, which is published in the July 18 issue of the journal Science, mass mortality of krill caused by a parasite had not been recorded.

New Parasite?

The exploding krill were first discovered during a research cruise off Newport, Oregon, in July 2000 by William Peterson, a fisheries oceanographer at the National Oceanic and Atmospheric Administration's Hatfield Marine Science Center in Newport.

Gómez-Gutiérrez, Peterson's graduate student, and colleagues carried out detailed studies of the phenomenon. On several research cruises in 2002 they found infected krill at seven percent of the 313 sites they sampled.

The parasite infects three out of the 14 species of krill that live in this region: Euphausia pacifica, Thysanoessa spinifera, and Thysanoessa gregaria. The infected creatures were mostly found along continental shelf breaks—areas where the ocean floor steeply drops and where krill are most abundant.

"Most of the euphausiids have a very big concentration there, and the parasites prefer those locations for infection," said Gómez-Gutiérrez.

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