Skip to main content
King County logo



The purple or blue stripe down each arm distinguish this ten armed sea star.  It can be very pale yellow to completely blue on it's smooth surface.  The arms are moderately long (R/r = 2.3 to 4.4)*.  This animal is usually found on cobbled beaches but can be  found on sand.  It is a carnivore which feeds primarily on sea cucumbers. The animal on the  right is not in distress but takes on this shape so that it can tumble across the bottom in the along-shore currents.  The author assumes that it moves from one feeding area to another by this method.

* 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