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

Notes

* 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.