RAIMONDI, PETER; SAGARIN, RAPHAEL*; AMBROSE, RICHARD; GEORGE, MAYA; LEE, STEVEN; LOHSE, DAVID; MINER, C. MELISSA; MURRAY, STEVEN; UNIVERSITY OF CALIFORNIA, SANTA CRUZ; UNIVERSITY OF CALIFORNIA, LOS ANGELES; UNIVERSITY OF CALIFORNIA, LOS ANGELES; UNIVERSITY OF CALIFORNIA, SANTA CRUZ; UNIVERSITY OF CALIFORNIA, LOS ANGELES; UNIVERSITY OF CALIFORNIA, SANTA CRUZ; UNIVERSITY OF CALIFORNIA, SANTA CRUZ; CALIFORNIA STATE UNIVERSITY SANTA CRUZ: Color change and consistency in the sea star Pisaster ochraceus

The sea star Pisaster ochraceus is among the best known systems for the study of intertidal ecology. It is also one of the most conspicuous members of north eastern Pacific rocky shores due to its dramatic color variation, ranging from bright yellowish orange to brown to deep purple. Despite a large body of ecological study concerning Pisaster ochraceus, few studies have rigorously examined color patterns or their causes across its geographic range. We take advantage of thousands of observations of sea star color and size taken as part of multi-investigator intertidal monitoring efforts that stretch from southern California to northern Oregon to determine size-related ratios of purple (including brown forms) to orange sea stars. We find that ratios in large (> 80 mm radius) sea stars are consistently between 3 and 4 to 1 across a wide geographical range. We consider several ecologically-based alternative hypotheses for this consistent color pattern but conclude that color is most likely determined by a fixed, polymorphic, selectively neutral trait, making this one of the first observations of color polymorphism without obvious ecological significance. Additionally, at several sites, ratios of purple to orange sea stars were much greater than 4 to 1 among small (< 60 mm radius) sea stars and thus we propose that color changes ontogenetically in P. ochraceus. These novel findings point to the need for renewed study of the basic biology of this key ecological species.