Effects of Gulls Predation on Different Color Morphs of Pisaster ochraceus and Evasterias troschelii along a wave exposure gradient in Washington state
Abstract
The sea star Pisaster ochraceus displays color polymorphism and can be brown, purple or orange. A distinct color distribution pattern has been observed across Washington state. 95% of P. ochraceus are purple in the Puget Sound region with very few if any orange morphs whereas on the open coast up to 28% of the sea stars may be orange (Harley et. al. 2006). Many hypotheses have been proposed to explain this pattern in color distribution, but the cause is unknown (Raimondi et. al. 2007). One hypothesis is that gulls, Larus spp., may feed preferentially on the orange color morph in Puget Sound, but feed less on sea stars in more open coast locations. Using models of P. ochraceus and live E. troschelii in predation trials along intertidal zone transects our study found that the median number of attacks on orange models was significantly different from the median number of attacks on gray models (H=29.99, df=3, p<0.0001). Orange models had the highest difference in reflectance from rock substrate. Orange E. troschelii were also attacked most frequently during trials. They were attacked significantly more times than either the brown morph (F= 4.553, df= 2, 38, p= 0.039) or the gray morph (p= 0.006). E. troschelii also had the highest difference in reflectance from rock substrate. Overall, predation rate differed greatly between the open coast and the Puget Sound (F= 3.089, df= 3, 30, p= 0.012). This information suggests that gulls may be responsible for the observed color distribution pattern of P. ochraceus across Washington state.