Life in turbulent water – whether it is the deep sea or a flowing river – is unimaginably tough. The restless, tumbling currents prove a great challenge for organisms living within. The powerful thrusts may push creatures some distance away, and even harm their bodies. Evolutionary processes have allowed the development of a wide range of strategies adopted by different organisms for settling in these harsh environments.
Water flows at different speeds under various conditions. Friction between water molecules and solid substrates (such as rocks and sand) will slow and even stop water flows. Hence a boundary layer may form right above a sea bed or river bed—and the rougher the surface, the thicker is the boundary layer. Research has shown that boundary layers up to 1 to 2 centimetres thick can be formed above rocky surfaces. If an organism “hides” within this layer, it can avoid the turbulent currents. Many benthic insects and fishes have developed suckers or powerful appendages to hold onto the substrate; or in other cases, evolved to have a flat, small body that allows them to shelter within the boundary layer.
While small organisms hide in the boundary layer, what about the larger ones? With a wider cross-section facing the water flow, the resistance gets higher. Therefore, many organisms adapt by having a streamlined body to reduce friction. Better still, such as in the case of large seaweeds, a soft and relatively long structure allows them to “pulse” along with the sea waves, and avoid being broken by the force of the current.
For other organisms, changes in behaviour can help reduce the impacts of water currents. For instance, many stream fishes swim in a direction that minimises their cross-sectional area facing the water flow. Scientists have also noticed that gathering together is an effective way to reduce water friction for fish in the middle of the shoal, where swimming becomes much easier.