  We study the dynamics and the statistics of dilute suspensions of gyrotactic
swimmers, a model for many aquatic motile microorganisms. By means of extensive
numerical simulations of the Navier-Stokes equations at different Reynolds
numbers, we investigate preferential sampling and small scale clustering as a
function of the swimming (stability and speed) and shape parameters,
considering in particular the limits of spherical and rod-like particles. While
spherical swimmers preferentially sample local downwelling flow, for elongated
swimmers we observe a transition from downwelling to upwelling regions at
sufficiently high swimming speed. The spatial distribution of both spherical
and elongated swimmers is found to be fractal at small scales in a wide range
of swimming parameters. The direct comparison between the different shapes
shows that spherical swimmers are more clusterized at small stability and speed
numbers, while for large values of the parameters elongated cells concentrate
more. The relevance of our results for phytoplankton swimming in the ocean is
briefly discussed.
