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Animals, Plants and Habitat

Sea Star Identification on King County Beaches

Equal-arm Sea Star
Mediaster aequalis

photo

Even though this bright-red sea star can be as much as eight inches in diameter, it has relatively short arms (R/r = 2.4 to 3.0)*. It can be found on almost any substrate but it seems to prefer gravel and sand. It eats encrusting sponges, sea pens, loose algae on sand and flesh from dead animals. This sea star seemed to "bloom" near Point Robinson on Vashon Island in 1996.

* 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