Colorful discs

Last night I mentioned we've been finding some kelp washed up on the beach.  Recently, one Bull Kelp specimen still had a relatively large holdfast at the bottom of the stipe, shown at the right in the photo below.  (A smaller kelp, Laminaria setchellii, was also attached to the Bull Kelp holdfast.)

 

Eric and I have been doing surveys of holdfasts for several years, so we took a closer look at this one.

Almost immediately, Eric noticed some very nice Daisy Brittle Stars (Ophiopholus kennerlyi) on and inside the holdfast.

Here are two close-up examples:

 

 

We kept looking, and it turns out there were more than 35 individual brittle stars on this holdfast (and we probably missed some).  We were especially fascinated by the diversity of color patterns on their central discs. 

So we took pictures of many of the individuals, and then Eric did something pretty special, just for you!

You can click on the photo below to see a slightly larger version (and to truly appreciate these spectacular brittle stars!).

 

In some ways, it's hard to believe this variety is seen in a single species.  Daisy Brittle Stars are known for amazing variation in the colors and patterns of their central discs (and their arms).

Moment (1962) seems to have been one of the first scientists to document this phenomenon in a related North Atlantic species, Ophiopholus aculeata:

"...out of hundreds of individuals collected at low tide in about an inch of water in a single cove on the coast of Maine it is not possible to find two exactly alike."

Here are examples of the patterns in Moment's brittle stars:

 

From Moment, G.B.  1962.  Reflexive selection: A possible answer to an old puzzle.  Science 136: 262-263.

Moment (1962) proposed a possible reason for the variation.  He called it "reflexive selection."  The basic idea is that variable color patterns make it harder for a visual predator (e.g., a fish) to develop a search image for one particular color variant...so predators impose selection for variation per se.  This process can generate something known as "massive color polymorphism" where just about every individual is unique.  According to Owen and Whiteley (1986), "The massive diversity thwarts the learning processes of the predators..."

Eric's fascinating collage is a wonderful demonstration of this individual variation.  Of the twenty shown, do you have a favorite color pattern...or, do you just like the diversity?