Hierarchical Design of Basking Shark Gill Rakers and their potential for High-volume Suspension Filters

Abstract

Basking sharks, like baleen whales, are planktivores, filtering thousands of litres per hour, indicating possession of a highly efficient filter. The combination of splayed pharyngeal arches and attached comb-like gill raker arrays surely provide this filter efficiency, but the actual mechanisms are unknown due to logistical challenges of studying these immense fish, and decisive data on raker anatomy are still lacking. We characterized basking shark filter anatomy at multiple size scales, from clinical CTs of shark heads down to high-resolution micro-CTs of rakers, combining diverse material techniques (e.g. SEM, spectroscopy, nanoindentation), linking raker morphology and materials with filtering biomechanics. Although the evenly spaced, thread-like rakers in the pharynx appear similar to keratinous baleen, we show they are modified tooth-like denticles of apatite-reinforced collagen, with a hypermineralized outer enameloid acting jointly with a softer, dentin-like inner layer to allow simultaneous flexibility and wear resistance. Stability and performance of the raker array are supported by several features of individual raker threads — e.g. drag-reducing hydrofoil cross sections, elongated sigmoid morphologies and subtle surface microstructures — which promote canting of adjacent rakers relative to flow. Surprisingly, digital models of raker morphology from our tomographic data indicate raker surface texturing can generate local suspension vortices that may enable indirect filtration. We will verify this with physical models informed by in vivo field observations of head, raker and water motion to understand specific links between anatomy, kinematics and filtering ecology, contributing to conservation of this threatened species and providing bioinspiration for dynamic, high-volume suspension filters.