Hydrodynamic effects of spines: A different spin

Many small planktonic organisms bear spines, some of whose potential functions have been explored, for example, in increasing drag during gravitational settling or in defense against predators. Using an immersed boundary framework, we performed computational fluid dynamic simulations that examine the rotational dynamics of model diatoms in shear flows with varying spine number, length, and angle. We found that the motion of spined cells could be accurately predicted from simple theory for motion of spheroids by applying that theory to the smallest spheroid that could inscribe the cell inclusive of its spines. The poorest fits were for small numbers or extreme angles of spines that left large volumes of the inscribing spheroid unoccupied by any spines. Although the present work provides a simple means of predicting motions of rigid, spined cells in shear flows, the effects of spines on nutrient exchange remain to be explored.