Predation as an explanation for a latitudinal gradient in arm number among featherstars

Abstract

Aim The role of biotic interactions in generating broad patterns in organismal phenotypes is a central question in macroecology. We investigate global patterns in feeding morphology among featherstars, a globally widespread group of suspension‐feeding echinoderms whose evolutionary history has been demonstrably shaped by predators. Location World's oceans. Taxon Crinoidea. Methods We tested for global patterns in the featherstar suspension‐feeding apparatus, a filter made up of 5–200 arms which is the main interface with predators. We investigate a geospatial dataset of 23,950 occurrences in 442 species using statistical analyses including quantile regression and a new permutation‐based phylogenetic comparative approach appropriate for testing for a broad range of patterns in data with strange distributions. Results We find that featherstars exhibit a latitudinal gradient in arm number: arm number is both greater on average and more variable between species at lower latitudes. This pattern holds across depths and hemispheres and is not a spurious result of either the latitudinal diversity gradient or phylogenetic autocorrelation. Tropical featherstars that conceal themselves have fewer arms, and also appear to experience less intense predation. Main conclusions Temperature, primary productivity and substrate type do not adequately explain the latitudinal gradient in arm number. We attribute it instead to a corresponding gradient in predation intensity: many armed featherstars can withstand more intense arm loss to predators. Concealment and other alternate solutions to the problem of predation, along with reproductive costs associated with having many arms, explain why the trend is wedge‐shaped rather than linear. Our findings constitute a latitudinal gradient in functional diversity, paralleling recent findings in other taxa. The gradient may be a consequence of shallow tropical reefs; inasmuch as reefs as centres of biotic interactions promote functional richness, changes in the distribution of reefs through deep time probably entailed shifts in the global deployment of ecological diversity.