Effects of gravity on copepods: an experimental investigation of spatial distribution and swimming behavior

Zooplankton performing vertical migrations need to overcome gravity to move upwards. In this work, we carried out a series of laboratory experiments to investigate how gravity influences the spatial distribution and swimming behavior of calanoid copepod Pseudodiaptomus annandalei. The experiments were conducted in a quasi-two dimensional vessel with large number densities (400-10000 inds. L^-1), corresponding to the cases of zooplankton aggregations in nature. By lying the vessel vertically or horizontally, the effects of gravity on copepods were disentangled. It is found that the spatial distribution of copepods without gravity effect follows a random Poisson process, which becomes uneven in the vertical-lying cases. Kinetic and directional statistics reveal that while copepods increase sinking and jumping behaviors in the vertical direction when their motion is subjected to gravity, their mobility becomes ∽20% smaller. The mean square displacement also shows a lower diffusive coefficient in the vertical-lying cases. This could be an effective swimming strategy for calanoid copepods to respond to gravity.