Understanding Color Polymorphism in the Sea Star, Pisaster ochraceus, Through Activity Levels and Dietary Preference

Abstract

Color polymorphism is found in a wide array of organisms ranging from copepods to black bears. Pisaster ochraceus, an intertidal sea star on the North Pacific West Coast, shows a striking color polymorphism including a range of orange, brown, maroon, and purple. Pisaster shows extensive geographic color variation, with >95% frequency of bright purple Pisaster in the Vancouver region; while the open pacific coast has a frequency distribution of 6-28% orange, 68-90% reddish-brown to dull purple, and a small percentage of bright purple. Maintenance of color polymorphism remains unknown, but one hypothesis suggests an underlying genetic component with regional-scale variation controlled by an ecological factor. Two ecological factors suggested are diet and salinity. This study analyzes the differences in dietary preference and activity levels between the color morphs in Bamfield, British Columbia. Pisaster was binned into two colors, orange and purple, for statistical analysis. Feeding laboratory trials showed no preference between the color morphs for Mytilus edilus or Mytilus californianus. Self-righting trials were performed in a low (20 psu) and controlled (35 psu) salinity and no significant difference was found between the color morphs. Interestingly, the activity levels of orange color morphs were not significantly different between low and control salinity. The results suggest there are ecological differences between the color morphs that still need to be evaluated.