The hydrodynamics of contact of a marine larva, Bugula neritina, with a cylinder

Marine larvae are often considered as drifters that collide with larval collectors as passive particles. The trajectories of Bugula neritina larvae and of polystyrene beads were recorded in the velocity field of a vertical cylinder. The experiments illustrated that the trajectories of larvae and of beads may differ markedly. By considering a larva as a self-propelled mechanical microswimmer, a mathematical model of its motion in the two-dimensional velocity field of a long cylinder was formulated. Simulated larval trajectories were compared with experimental observations. We calculated the ratio η of the probability of contact of a microswimmer with a cylinder to the probability of contact of a passive particle with the cylinder. We found that depending on the ratio S of the swimming velocity of the microswimmer to the velocity of the ambient current, the probability of contact of a microswimmer with a collector may be orders of magnitude larger than the probability of contact of a passive particle with the cylinder: for S≈0.01, η≈1; for S≈0.1, η≈10; and for S≈1, η≈100.