Morphological Variation of Pisaster Orchraceus in Response to Wave Exposure
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Pisaster ochraceus (the Ochre Sea Star) is a keystone predator in the NE Pacific that regulates intertidal diversity through consumption of space-competing organisms. Individuals inhabit a broad range of habitats ranging from sheltered coves to exposed cliffs and experience large temporal and spatial variability in water flow throughout their lifetime. However, it is largely unknown how sea star body shape changes between wave-exposed and wave-sheltered environments throughout an organism's lifetime. Wave exposure was measured at sites near Charleston, OR using dissimilar metal dissolution and intertidal zonation of sessile organisms. At these same sites, I measured shapes and sizes of Pisaster juveniles and adults and analyzed how morphology changed as a function of wave exposure. Average zinc anode mass loss differed significantly between sites during seasonal trials (p < 0.001 ). Mean upper intertidal zone limits were significantly higher at the Middle Cove and 01MB Boathouse sites for Ba/anus g/andula (p < 0.01 ), Mytilus spp. (p < 0.01), Neorlwdomela oregona (p < 0.001), and Sacclwrina sessilis (p < 0.05). Adult sea star populations the Middle Cove site had longer, narrower arms (p < 0.00 l) and smaller central discs (p < 0.001) than individuals from the 01MB Boathouse or Bastendorff Jetty sites for a given weight. Juveniles appeared to exhibit similar morphological trends to respective adult populations but results were inconclusive. The lack of a significant relationship between wave exposure and Pisaster morphology is likely due to errors in measurements of exposure and demonstrates that exposure alone does not determine body shape.