Role of the Setae in an Ectoparasitic Seal Louse in Reducing Surface Drag: Numerical Modeling Approach

Echinophthirius horridus, an ectoparasitic seal louse adapted for living on diving wildlife in the marine environment, exhibits unique cuticular morphology with dense body coverage of characteristically-shaped setae. This study investigates their potential role in reducing drag during the host's diving activities. Scanning electron microscopy (SEM) and confocal laser scanning microscopy (CLSM) examines E. horridus setae morphology, revealing stair-like elevations and gradual sclerotization increase from base to tip. Numerical simulations using movable cellular automata (MCA) demonstrate that optimal inclination of surface protrusions leads to vortex formation, potentially reducing friction and energy losses. Vertical protrusions cause stronger flow perturbations and higher energy dissipation compared to natural inclination. Over time, as flow self-organizes, total power losses decrease, suggesting natural selection optimized surface structure inclination and spacing to minimize friction and energy losses. Comparisons with shark scales reveal morphological similarities but different drag reduction mechanisms, with seal louse setae utilizing a “ball-bearing” effect and shark scales relying on a “riblet effect.” This study provides insights into surface topography's influence on fluid dynamics at small scales, with potential applications in understanding biological surfaces and designing reduced surface drag artificial surfaces.