Skip to main content
King County logo

How to use this key

  1. Start at 1a.
  2. Count the arms on the sea star you wish to identify and then click on the appropriate statement. You will be moved to the next appropriate pair of statements.
  3. Click on the appropriate statement in that pair.
  4. Repeat this process until your animal is identified.

Start Here- Click either statement 1a or 1b below.

1a. Animal has five arms
1b. Animal has more than five arms
      Go to 2
2a. Animal has six arms
2b. Animal has more than six arms
3a. Animal has 9 or 10 arms, smooth "skin"
3b. Animal has more than 10 arms; spiny "skin"
4a. Animal has short arms (R/r less than 3)
* See notes
4b. Animal larger or shaped otherwise
5a. Prominent spines along the margin of arms
Hippasteria spinosa
5b. Smooth along margins of the arms
6a. Animal bright orange or red
6b. Animal grayish; soft; leathery 
Leather Star  Dermasterias imbricata
7a. Arms smooth; red on top, yellow beneath
7b. Arms spiny
8a. Long arms (R/r =5 to 8)
See notes  
8b. Short arms (R/r=3 to 4.5)
 See notes
9a. Color generally pink; animal can be up to two feet across
9b. Color purple, brown or orange


* 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.