Meeting Abstract

124.3  Monday, Jan. 7  Ontogenetic color variation in the sea star Pisaster ochraceus as an adaptation to avoid predation by gulls KRAUSZER, M.; LEIKEN, A.; ELLIOTT, J.K.*; Univ. of Puget Sound, Tacoma jkelliott@ups.edu

Early life history stages of many species are often camouflaged to reduce detection by visual predators because they are more vulnerable than older/larger individuals. We have studied a variety of ontogenetic stages of the sea star Pisaster ochraceus in Puget Sound, WA. Juveniles are grey/ brown, and at a size of approximate 5 cm arm length they change to their characteristic adult color of purple, brown, or orange. Small sea stars (< 7 cm arm length) of P. ochraceus are most abundant in habitats with high structural complexity (e.g. cobble), and are found under rocks or in crevices at low tide. In contrast, large sea stars are often observed out in the open during low tide. We observed gulls foraging under rocks at low tide and feeding on small sea stars, and we hypothesized that the grey/brown coloration of juveniles was an adaptation to reduce detection by foraging gulls. To test this hypothesis we placed different colored clay models (grey, brown, purple, orange) and live sea stars (grey, brown, orange) in the intertidal to determine whether gulls would preferentially prey on certain colors. We also used reflectance spectrometry to compare the brightness of each color morph in relation to their background as a measure of conspicuousness. Orange sea stars were most conspicuous, and they experienced the highest predation rates. Grey and brown sea stars were the least conspicuous and had the lowest predation rates. Selective predation by gulls on small brightly colored orange sea stars may be a factor causing the purple color morph to be predominant in Puget Sound, whereas low predation rates by gulls in more exposed coastal locations may allow orange color morphs to occur at higher frequencies.