It might be a little hard to tell from a photograph, but one of the arms (the lowest one) is much wider than all of the others. Aside from the width of the "double-wide" arm, we couldn't see anything unusual, until we looked at the other side:
From above (photo #1), this sea star has 5 arms (the normal number), but from below (photo #2) it has 6 arms, although two of those arms are fused, creating the extra width seen from above.
Here's an even closer look at that unusual "arm":
Note that the arms are completely fused throughout their length. There are two parallel ambulacral grooves — the grooves from which the tube feet arise (they're the yellowish grooves in the image above).
I was curious about the partial gap near the tip of the arm. By partial I mean that from above this area is completely fused, but from below there is a slight separation. Here's a close-up of that area:
When we looked at the very tip of the arm, we were intrigued. Here's the tip, curling up slightly. Remember that a sea star has a small red eyespot at the tip of each arm. Can you see the eyespot?
Okay, that was a bit of a trick question. There are two eyespots! Below is an even closer view of the arm tip, showing both eyespots. They're partially hidden by the spines (white bumps), but I think you'll be able to see both of them:
Well, we've looked at thousands and thousands of Ochre Sea Stars, and we've never seen one like this. Have you?
We did some research, and there are a few records of sea stars with "double ambulacral grooves," but it's very rare. Hotchkiss (2000) listed 11 individual examples, most of which were Asterias spp., and only one was Pisaster ochraceus. That specimen was collected over 70 years ago in southern California.
Hotchkiss proposed that this abnormality was due to an injury and abnormal regeneration of the arm. (The other possibility is a developmental irregularity during metamorphosis.)
The sea star we encountered had no obvious signs of an injury or regenerated arms (e.g., all of the arms were similar in length, ~5 cm long). If the unusual arm arose from an injury, perhaps the injury happened quite a while ago when the sea star was much smaller.
Interestingly, the earlier Pisaster record is slightly different. The arms are fused almost to the tip, but then they split:
Image from Fisher, W.K. 1945. Unusual abnormalities in sea-stars. Journal of the Washington Academy of Sciences 35: 296-298.
Who knows if we'll encounter another sea star with a "double-wide" arm. If we do, I'm sure it'll raise our eyebrows again while we continue to wonder about this unusual phenomenon.
P.S. The reference mentioned above is: Hotchkiss, F.H.C. 2000. Inferring the developmental basis of the sea star abnormality "double ambulacral groove" (Echinodermata: Asteroidea). Revista Chilena de Historia Natural 73: 579-583.