Euphausiids (krill) are often the dominant pelagic crustacean in a given water column and are of vital importance to the productivity of the world's oceans. It has been assumed that the two most important factors in the regulation of euphausiid populations are predation and starvation; the impact of parasitization has been largely overlooked. In a Brevia article in the July 18 Science, Jaime Gómez-Gutiérrez and colleagues at the College of Oceanic and Atmospheric Sciences show that infection by a ciliate parasite can substantially reduce krill numbers in an individual population (Science, 301:339, July 18, 2003).

Over the course of six summer oceanographic cruises, Gómez-Gutiérrez et al. collected water samples off the coast of Oregon. They collected 71 euphausiids of the species Euphausia pacifica, Thysanoessa spinifera, and T. greagaria that had been parasitized by an apostomatid ciliate of the genus Collinia. Analysis in the shipboard...

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