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Animals, Plants and Habitat

Sea Star Identification on King County Beaches

Mottled Sea Star
Evasterias troschelli

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The mottling on this sea star is fairly distinctive. It might be confused with the purple sea star (Pisaster ochraceus) but the mottled has a greater arm to disk ratio (R/r = 5 to 7.5)*. That ratio for the purple sea star ranges from 2.7 to 4. Usually mottled green, red, brown or orange with white spines. Usually on rocks and cobble but is also found on sand. It feeds on clams, jingle shells, barnacles, chitons, sea squirts and snails. Usually has a commensal scale worm.

* 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