Form and Function in Mobulids: A Comparative Analysis of Filter Morphology With Bioinspiration Applications.

Mobulas (manta and devil rays) are large-scale ram filter feeders that separate planktonic food particles from large volumes of water with minimal clogging. This contrasts with most human-made filters that can suffer from problematic clogging requiring additional mechanisms for clearing blocked surfaces and maintaining performance. Prior studies have shown that mobulas employ a unique mechanism referred to as ricochet separation to filter feed, whereby captive vortices in filter pores cause particles to bounce off the filter surfaces and away from the filter pores. This mechanism enables the filtration of particles smaller than the pore size and reduced clogging. However, few studies have examined how the morphology of the filtering structure varies across the diversity of mobulid species, and little is known about how this variation may impact filtration efficiency or prey selectivity. This study conducts a systematic investigation of the gross morphology of the filtering structure in seven mobilid species using a combination of computed tomography and macro photography. Examination of filter anatomy suggests that some features are highly variable while others are well-conserved across species. In particular, a reconstruction of the phylogenetically corrected morphospaces indicated that the primary pore dimensions of the filter lobes are a major driver of morphological variation across species. Additionally, inspection of the gross anatomy revealed a pronounced asymmetry in the anterior and posterior filter plates of each gill arch. This asymmetry suggests that water may impinge on the filtering structures at different angles than has previously been speculated. Here, the functional ramifications of the observed morphological variations were interpreted using recent modeling studies. Most mobulid fishes have a filter morphology that should be capable of high filtration efficiency and low hydrodynamic resistance, but may also be sensitive to flow conditions. A deeper understanding of the mechanics of filter-feeding in mobulid fishes would generate needed insights into the ecology of these species and could provide a firmer framework for the development of bioinspired filtration systems. These findings highlight the value of integrating detailed anatomical studies into bioinspired design efforts and pave the way for the development of bioinspired filter systems with improved performance.