[1] Acartia tonsa is a species of calanoid copepod that can be found in a large portion of the world's estuaries and areas of upwelling where food concentrations are high.

[2] The wide distribution of Acartia tonsa may be a result of these copepods being transported as ballast in ships.

In cooler climates including the North Atlantic, it is frequently the dominant zooplankton in the spring and summer.

[8] Several studies indicate they aggregate near the ocean floor during the day and rise closer to the surface at night.

This behavior likely helps A. tonsa avoid predators who rely on vision to locate and capture prey.