What Does “A School of Krill” Mean?

“School” is a general term borrowed from fish terminology and occasionally applied to krill, though it’s a less precise fit than “swarm,” the collective noun most commonly and accurately used for these tiny crustaceans. Krill aren’t fish, but their tendency to move in tightly coordinated, densely packed groups makes the borrowed fish terminology understandable, even if “swarm” better captures the sheer scale and density of a real krill aggregation.

Whichever word is used, the underlying behavior is genuinely dramatic. Krill gather by the billions in some of the densest animal aggregations on the planet, performing coordinated vertical migrations toward the surface at night to feed and back into deeper water by day. That level of large-scale coordination is exactly what both “school” and “swarm” are reaching for, even if one term has become the more standard choice.

Example: “A school of krill drifted past the research vessel, dense enough to show up clearly on sonar.”

Related: Collective Nouns for Crustaceans · What Is a Group of Krill Called? · A Swarm of Krill