Sitting comfortably on a riverbed might seem to be an easygoing lifestyle, but you try doing it in a fast running stream. Yet tiny North American darters appear to be perfectly at home holding still in fast flowing water, apparently without gripping on. Rose Carlson from Fordham University, USA, explains that she is fascinated by how animals' body shapes evolve. She says, ‘I was interested in whether darters' body and fin shape might contribute to their ability to stay on the bottom.’ She explains that environmental factors often drive an animal's evolution, so she and George Lauder from Harvard University decided to find out more about the turbulent fluid flows the fish frequent (p. 1181).
‘This project started out being very exploratory. We just wanted to describe the patterns of flow over different substrates,’ says Carlson. Placing simulated riverbeds in a flow tank and shining a plane of laser light in the water as it flowed at speeds ranging from 0 to 31 cm s–1, Carlson and Lauder were able to visualise the turbulent flows as the water rushed over the surfaces. Comparing the flows over the gravelly river beds with the flow over a smooth Plexiglas surface, Carlson and Lauder were surprised to see that water close to the gravel flowed much slower than water in the main body of the ‘river’. This region of relatively tranquil water above the Plexiglas increased from a few millimetres to almost 2 cm as the water tumbled over the gravelly riverbeds. And when the duo introduced a large rock into the flow, the water behind the obstruction even began flowing backward slightly. Friction between the water and the coarse surface was slowing the flow near the riverbed significantly to produce a tranquil boundary layer, but how could this help the darters?
Carlson realised that the fish were almost the same height as the newly discovered region of reduced flow. This layer of slower flowing water was deep enough to shelter the fish and help them sit tight, but the fish must be modifying the fluid flows in some other way to help them stay put. Working with Lauder, Carlson began visualising the fluid flows around the darters' pectoral fins as the fish held them out wide while they sat still on a simulated riverbed and a Perspex surface.
According to Carlson, some fish, such as sharks, produce downward directed forces with their pectoral fins; however, when she and Lauder analysed the flow patterns trailing from the darters' widespread fins, they were surprised to see that they were not producing enough force to pin the fish to the bottom, even in the slow flowing water. ‘They are probably using some other mechanism to generate a lot of friction between themselves and the substrate, and those frictional forces are probably the important force helping them to stay in place,’ she says.
Having found that darters take advantage of slow flowing water in the boundary layer to remain in place, Carlson is curious to find out more about how this could have shaped the explosion of darter species in North America. She explains that there are over 240 species and says, ‘There is an important phenomenon that often precedes adaptive radiation [when new and ecologically diverse species appear], which is the availability of unoccupied ecological space.’ Carlson would like to find out whether the darters' reduction in size and loss of a swim bladder allowed them to occupy the tranquil – and under-used – boundary layer, giving them the ‘ecological opportunity’ to diversify into the wide range of species we see today.