A number that represents the relative importance of viscous forces and inertial forces in a fluid. As Re increases, inertial forces become more important. In sea water, Re increaseswith increasing water velocity and with the size of the object in the water.

Water flow can be described as laminar or turbulent. Laminar flow is characterized by smooth motion: neighboring particles advected by such a flow will follow similar paths. Turbulent flow is dominated by re-circulation, whorls, eddies and apparent randomness. In such a flow particles which are neighbors at one moment can find themselves widely separated later.

Reynolds number is the ratio of inertial forces to viscous forces. As the size of an organism and the strength of the current increases, inertial forces will eventually dominate, and the flow becomes turbulent (large Re). As the size and strength decrease, viscous forces eventually dominate and the flow becomes laminar (small Re).

Biologically there is an important distinction between plankton and nekton. Plankton are the aggregate of relatively passive organisms which float or drift with the currents, such as tiny algae and bacteria, small eggs and larvae of marine organisms, and protozoa and other minute predators. Nekton are the aggregate of actively swimming organisms which are able to move independently of water currents, such as shrimps, forage fish and sharks.

As a rule of thumb, plankton are small and, if they swim at all, do so at biologically low Reynolds numbers (0.001 to 10), where the viscous behaviour of water dominates and reversible flows are the rule. Nekton, on the other hand, are larger and swim at biologically high Reynolds numbers (103 to 109), where inertial flows are the rule and eddies (vortices) are easily shed. Many organisms, such as jellyfish and most fish, start life as larva and other tiny members of the plankton community, swimming at low Reynolds numbers, but become nekton as they grow large enough to swim at high Reynolds numbers.