Here's a picture of a juvenile marine invertebrate that is more easily recognized as an adult:
Can you tell which species of sea star this is?
The number of rays, or arms, is a clue. As is its speed. I had trouble taking pictures of this individual because it kept crawling very quickly out of view!
Have you counted the number of rays? There are 8 total, of 3 different lengths. Five long, one medium, and two short.
This number of rays rules out most of the local sea stars. I'll also tell you that this individual was found in the rocky intertidal zone, that it was flexible (not stiff), and spiny (not smooth).
Have you guessed? I'll reveal the answer below the next photo. This one shows the size, at about 14 mm (~1/2 inch).
This is a juvenile Sunflower Star (Pycnopodia helianthoides)! The adults are much larger (up to 4 feet across) and have many more rays (sometimes over 20).
Interestingly, when they first undergo metamorphosis from the planktonic larval stage, Sunflower Stars only have five rays. Then they add one more to make an even six. After that they start adding rays in pairs. So that's what you're seeing in this tiny juvenile.
This process was first discovered and illustrated by William Ritter and Guilelma Crocker in 1900. Below is a drawing from their paper. It shows the first five rays (I-V), the sixth ray (A), and the two newest rays (small buds to either side of A). Future rays will be added in a bilateral arrangement adjacent to the newest rays. [Compare this drawing to the pictures above!]
Illustration modified from Ritter, W.E. and G.R. Crocker. 1900. Multiplication of rays and bilateral symmetry in the twenty-rayed starfish, Pycnopodia helianthoides (Stimpson). Proc. Wash. Acad. Sci. 2: 248-274.
Before Eric found this tiny Sunflower Star, I had no idea that this is how they added their rays...and that William Ritter, the first marine biologist at the University of California and the founder of the Scripps Institution of Oceanography, was involved in the discovery!
One more Pycnopodia picture for you — an adult Sunflower Star, so you can visualize what this juvenile will become!