Some species produce bioluminescence, which is light produced as a result of a chemical reaction within specialized structures in the dinoflagellate cell. Agitation of seawater containing dinoflagellates will stimulate light flashes. The phenomenon was first noted in the genus Noctiluca in the 1800's, but the ability to produce bioluminescence is now known to occur in several marine species of dinoflagellates.
Bioluminescence acts as a burglar alarm.
The fish is more liable to find and eat the larger copepod than the miniscule dinoflagellate cell. In this way the predators of the dinoflagellate are removed. When the copepod sees the luminescent flash it gives a jump, because staying put means it is vulnerable to predation.