Hydrodynamic interaction between two copepods: a numerical study

Abstract

Numerical simulations were carried out to compute the flow field around two tethered, stationary or swimming model-copepods with varied separation distances between them and for different relative body positions and orientations. Based on each simulated flow field, the power expended by each copepod in generating the flow field and volumetric flux through the capture area of each copepod were calculated. The geometry of the flow field around each copepod was visualized by tracking fluid particles to construct stream tubes. The hydrodynamic force on each copepod was calculated. Also, velocity magnitudes and deformation rates were calculated along a line just above the antennules of each copepod. All the results were compared to the counterpart results for a solitary copepod (stationary or swimming) to evaluate the hydrodynamic interaction between the two copepods. The calculations of the power and volumetric flux show that no energetic benefits are available for two copepods in close proximity. The results of the stream tube and force calculations show that when two copepods are in close proximity, the hydrodynamic interaction between them distorts the geometry of the flow field around each copepod and changes the hydrodynamic force on each copepod. Two beneficial roles of the hydrodynamic interactions are suggested for copepod swarms: (1) to maintain the integrity of the swarms and (2) to separate the swarming members with large nearest neighbour distances (usually more than five body lengths). To prevent strong hydrodynamic interactions, copepods in swarms have to avoid positions of strong interactions, such as those directly above or below their neighbours. The results of the velocity magnitudes and deformation rates demonstrate that the hydrodynamic interaction between two copepods generates the hydrodynamic signals detectable by the setae on each copepod's antennules. Based on the threshold of Yen et al. (1992), the results show that the detection distance between two copepods of comparable size is about two to five body lengths. Copepods may employ a simple form of pattern recognition to detect the distance, speed and direction of an approaching copepod of comparable size.