NICHE BREADTH AS A LINK BETWEEN FEEDING MORPHOLOGY AND EXTINCTION RISK: AN EXPLORATION WITH EXTANT CRINOIDS

Niche breadth has frequently been invoked as a predictor of extinction risk: species that occupy a narrow range of habitats – stenotopes – are thought to be less resilient to environmental change and therefore tend to persist for shorter spans of geological time. Fossil crinoids in particular have yielded important insights into the link between stenotopy and extinction risk: geological duration has been shown to be lower in species that occupy fewer lithological facies and whose morphology is more atypical for a given taxonomic group. Furthermore, the “density” or fineness of the crinoid filtration fan delimits the range of currents in which they can feed efficiently, and has been implicated in extinction risk. Here we test the relationship between feeding morphology and ecological specialization for the most widespread and species-rich group of crinoids, the featherstars. Using a database of 11,045 modern occurrence records, we explore the relationships among geographic range, bathymetric range, and maximum inhabited depth. We test for relationships between these response variables and feeding morphology. Arm number, which varies from five to nearly 200, is used here as a proxy for fan density. Geographic range is weakly but positively correlated with bathymetric range. Featherstars occur from 0-6000m, but many-armed (>45 arms) forms are restricted to shelf depths (<200m). Many-armed featherstars have relatively low geographic and bathymetric ranges. We discuss and attempt to sidestep statistical pitfalls relating to differences in sample size and phylogenetic autocorrelation. Our results support arm number as an important control on crinoid niche breadth, and experimental and computational approaches suggest that it does so by affecting the range of hydrodynamic regimes in which crinoids can efficiently feed. Stenotopy explains the putative link between crinoid feeding morphology and geological duration in the fossil record, although the strength of this link remains unclear.