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.

In 2001, Hatfield Marine Science Center oceanographers Alex de Robertis and Richard Brodeur used a remote-controlled robot to study and collect krill from a continental shelf break off the coast of Astoria. They found a mass mortality of E. pacifica on the sea floor at depths ranging from 722 to 1,804 feet (220 to 550 meters).

"We found the parasite on several cruises but never thought they could cause mass mortality," said Gómez-Gutiérrez. "Alex and Rick recorded it with the robot's video camera and even collected a few specimens."

The collected carcasses from the mass mortality exhibited the same infection signs as infected krill the researchers had studied in their ship-based laboratory. Like the other infected krill, the parasite eluded precise identification.

The researchers determined the krill killer was of the genus Collinia but different than the parasite Collinia beringensis, which has been found to infect the krill Thysanoessa inermis in the Bering Sea. Gómez-Gutiérrez and colleagues are in the process of naming the species.

Reid noted that infection of this type has not been seen anywhere else. "Such parasitism has not been recorded in all species of krill. It has not been seen in Antarctic krill [Euphausia superba] or in the Northern krill [Meganyctiphanes norvegica] in the North Atlantic."


Gómez-Gutiérrez said that he does not know how the parasites get inside the krill, but that once inside they eat all the krill's organs. The krill, which are usually transparent with small red spots, turn orange and their shells swell.

The parasites then multiply rapidly, forming cells that are ready for transmission to a new host. These transmission cells burst out of the krill body, leaving a ruptured carcass on the ocean floor.

The researchers say that infected krill die within 24 to 72 hours. Once the parasite has killed the host and spread transmission cells into the water it has approximately two to three days to find a new victim in order to continue its lifecycle.

Gómez-Gutiérrez and colleagues suggest the parasite is most successful in krill swarms, given its need to find a new host quickly. Krill form swarms to improve their ability to capture prey, find mates, and avoid predators such as whales, salmon, and other fish. However, the dense swarms also make it easier for parasites to spread, the researchers say.

Reid agrees that the swarming nature of krill likely increases their susceptibility to the parasites. "However, there are a suite of positive aspects of forming swarms that one would presume outweigh the potential negative consequences of doing so. If this were not the case, we would not find krill living in swarms," he said.

In future studies, Gómez-Gutiérrez and colleagues hope to discern how this parasite impacts the food chain.

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