Current and Benthic Organisms: Chronic Instability of the Surface Horizon of the Substrate
Any organism living in flowing water—a rheophile—has mechanisms adapted to resist the current. These mechanisms are morphological and behavioural. The tendency to face the current and move against it is quite common in rheophilous animals, as is their dynamic morphology. The most common morphological characteristics are a hydrodynamic form and dorsoventral flattening. For animals living in open water, the form presenting the least resistance to the current is that of a hydrodynamic body in which the largest transversal section is located at a little more than one third of the total length. This is what is observed in many types of fish. Dorsoventral flattening appears more as an adaptation to avoid the current in benthic invertebrates. They are small, adhere to the substrate, and are protected in the boundary layer. They thus escape the pull of the current. Examples are Turbellaria, Ancylidae Mollusca, or even larvae of Ephemeroptera such as Heptageniidae (Fig. 13, Table 2).