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Animals, Plants and Habitat

Sea Star Identification on King County Beaches

Short Spined Pisaster
Pisaster brevispinus

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This animal prefers relatively shallow water and a sand/mud mud bottom where it feeds on clams. The Short Spined Pisaster can grow to be more than 2 feet across with arms more than 3 inches thick. It is similar in appearance to the Purple Sea star but it has somewhat longer arms (R/r = 2.8 to 5.0)*. and the Purple Sea star is never pink. The Short Spined Pisaster has been sighted at Burton Acres on Vashon Island during low tide events.

* 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