The Armor of a Keystone Predator: Intraspecific Aboral Spine Variation in the Ochre Sea Star, <i>Pisaster ochraceus</i> (Brandt)

AbstractThe abrasive, high-impact environment of rocky intertidal zones produces important abiotic stressors that many calcifying organisms are armored against. The ochre sea star (<i>Pisaster ochraceus</i>), a slow-moving but ecologically important predator in the wave-swept rocky intertidal of the eastern North Pacific, is covered aborally with calcium carbonate ossicles that are modified into spines. These spines may act as armor, providing lightweight protection for <i>Pisaster</i> against impacts and abrasion from abiotic particles in the intertidal. We used digital image analysis and scanning electron microscopy to characterize variation in the aboral armor of <i>Pisaster</i> across a range of sizes and intertidal habitats that vary in the joint impacts of water motion and abrasion. We found that <i>Pisaster</i> from different habitats exhibited distinct relationships between size and investment in aboral armor (spine density and areal spine coverage). Aboral armor was significantly higher in <i>Pisaster</i> from wave-exposed shores on which joint impacts of water motion and abrasion were highest, especially in smaller-sized individuals. We also identified specific aboral spine morphotypes, the proportions of which varied according to habitat. Individuals from wave-swept rock benches and boulder fields had short, blunt, and convex spines, whereas individuals from protected embayments had a higher proportion of upright, tapered, or columnar spines. Our results suggest that increased density and areal coverage of wider, convex spines in <i>Pisaster</i> are adaptive for more vulnerable life stages and in more abrasive and high-impact habitats. Future work should determine whether habitat-associated variation in the armor of <i>Pisaster</i> reflects a functional trade-off with feeding ability and whether it is primarily genetic or phenotypically plastic.