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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