Vital ocean prey play active role in environment

Conceptual model of krill’s life history sheds light on forces that drove its evolution

Although Antarctic krill (Euphausia superba) are one of the best-studied organisms of the open sea, key aspects of their life cycle have remained murky. Understanding krill is important because they are vital prey for fish, birds, and marine mammals, yet they are vulnerable to fishing pressure and environmental change. In the February 2006 issue of BioScience, the monthly journal of the American Institute of Biological Sciences, Stephen Nicol of the Australian Antarctic Division presents a new conceptual model of the species’ life history that emphasizes krill’s active role in exploiting a highly seasonal environment.

Nicol’s model separates the species into different life stages: larvae, juveniles, gravid females, and other adults. These groups are believed to remain geographically separate. In summer, adults remain close to the edge of the continental shelf, which allows them access to open-water food supplies. Females carrying eggs move offshore to deep water to spawn, where the embryos sink. The embryos hatch into free-swimming larvae at depth, then swim upward over the course of several weeks. The larvae are at this stage dependent on microorganisms found on the underside of sea ice for food, and as they grow they drift with pack ice. After they metamorphose in late winter and early spring, the juveniles move with the currents back to inshore waters. In summer, the juveniles are found inshore of the adults. The life cycle thus keeps krill in younger stages separate from adults, which reduces competition for food and lessens the chance that adults will prey on younger stages. Further studies of krill can lead to an improved picture of the Southern Ocean ecosystem, Nicol writes.