The Nature of Flow and the Reaction of Benthic Cnidaria to It

Abstract

Fluid flow is an environmental factor acting upon organisms in its path. All organisms live in a fluid, liquid or gas. A fluid moving relative to a body imposes mechanical forces on that body. These forces, which tend to carry the body downstream, are known as drag forces. Organisms depending on moving fluid to transport them from place to place have various morphological features that maximize drag, whereas organisms that locomote through a fluid tend to have structures that minimize drag. Sessile organisms such as benthic cnidarians risk being dislodged or broken by drag forces, yet they depend on the fluid moving over them to bring them food and essential substances, to carry away their wastes, and to disperse their gametes or young. Various compromises between maximizing and minimizing the effects of flow can be recognized among the benthic cnidarians.