Hydrodynamic effects of spines on zooplankton: how horns and tails alter flow and forces on barnacle nauplii

Many planktonic animals have body extensions (spines) that could affect the hydrodynamic performance of ecologically important functions. Because crustacean naupliar larvae have a wide range of spine morphologies, they provide useful model systems to study spine hydrodynamics. Here, we focus on nauplii with long anterior (horns) and posterior (tail) spines: the feeding nauplii of common flotsam-dwelling barnacles, Lepas anserifera, which have a long pelagic larval duration. We investigated the hydrodynamic consequences of anterior and posterior spines using dynamically similar physical models of these nauplii with horns and tails that could be removed. Models were towed through a viscous fluid at speeds to match the Reynolds numbers of locomoting and feeding L. anserifera nauplii. Hydrodynamic forces and torques on the models were measured, and particle image velocimetry (PIV) was used to quantify flow velocity fields around them. We found that both horns and tail spines increase drag, which resists swimming. However, they improve stability by counteracting the moments generated by beating appendages. Additionally, spines prolong the residence time of food-carrying water parcels near the particle-capture region of a nauplius, which is likely to improve feeding performance. These findings highlight the trade-offs between the performance of different ecological functions, such as swimming versus feeding, that can exert selective pressures on morphology.