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The Purple Sea star is a medium size animal found on rocky shores where it feeds primarily on barnacles. Its arms are relatively short (R/r = 2.7 to 4.1)* Although its common name implies that it is most often purple, it is as likely to be some shade of orange or brown but never pink. The arrangement of spines on the topside forms a netlike pattern. Some years, numbers of this species seems to "bloom" to the extent that large boulders show large white patches where the they have consumed the barnacles leaving just the cement that held their shells to the rock. These animals also eat mussels.

* Two measurements need to be made in order to determine the relative length of the arms of a sea star. Simply stated, a sea star is made up of arms and a central area to which they are attached called the disk. Since a sea star has radial symmetry, their dimensions are referred to radii. "

R" is the length of the arm as measured from the center of the animal to the tip of the arm.

"r" is the radius of the disk and is measured from the center to a notch between the arms.

R divided by r (R/r) is the relative length of the arm.

For instance: Distance from center of the disk to the tip of the are is 10" and the distance from the center to the notch between the arms is 2 1/2", then: R/r = 10/2.5 = 4