Life History and Behavior
The life cycle of krill is relatively well understood, despite minor variations in detail from species to species. After krill hatch, they experience several larval stages—nauplius, pseudometanauplius, metanauplius, calyptopsis, and furcilia, each of which divides into sub-stages. The pseudometanauplius stage is exclusive to species that lay their eggs within an ovigerous sac: so-called "sac-spawners". The larvae grow and moult repeatedly as they develop, replacing their rigid exoskeleton when it becomes too small. Smaller animals moult more frequently than larger ones. Yolk reserves within their body nourish the larvae through metanauplius stage. By the calyptopsis stages differentiation has progressed far enough for them to develop a mouth and a digestive tract, and they begin to eat phytoplankton. By that time their yolk reserves are exhausted and the larvae must have reached the photic zone, the upper layers of the ocean where algae flourish. During the furcilia stages, segments with pairs of swimmerets are added, beginning at the frontmost segments. Each new pair becomes functional only at the next moult. The number of segments added during any one of the furcilia stages may vary even within one species depending on environmental conditions. After the final furcilia stage, an immature juvenile emerges in a shape similar to an adult, and subsequently develops gonads and matures sexually.
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