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Animals, Plants and Habitat

Sea Star Identification on King County Beaches

Identification Key for Common Intertidal Sea Stars of Puget Sound

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 Go to 4
1b. Animal has more than five arms Go to 2
2a. Animal has six arms Six arm Sea Star, Lepasterias hexactis
2b. Animal has more than six arms Go to 3
3a. Animal has 9 or 10 arms, smooth "skin" Sun Star, Solaster stimpsoni
3b. Animal has more than 10 arms; spiny "skin" Sunflower Star, Pycnopodia helianthoides
4a. Animal has short arms (R/r less than 3)
* See notes
Go to 5
4b. Animal larger or shaped otherwise Go to 7
5a. Prominent spines along the margin of arms Hippasteria spinosa
5b. Smooth along margins of the arms Go to 6
6a. Animal bright orange or red Equal arm Sea Star Mediaster aequalis
6b. Animal grayish; soft; leathery  Leather Star  Dermasterias imbricata
7a. Arms smooth; red on top, yellow beneath Blood Star, Henricia leviuscula
7b. Arms spiny Go to 8
8a. Long arms (R/r =5 to 8)
See notes  
Mottled Sea Star, Evasterias troschelli
8b. Short arms (R/r=3 to 4.5)
 See notes
Go to 9
9a. Color generally pink; animal can be up to two feet across Short Spined Sea Star, Pisaster brevispinus
9b. Color purple, brown or orange Purple Sea Star, Pisaster ochraceus


* 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.