How about another one?
And a couple more?
We think these wonderful isopods are Dynamenella dilatata. Eric spotted them in a Bull Kelp holdfast. They ranged in size between 5-10 mm long.
There are a couple of important characteristics to pay attention to. Believe it or not, the dynamite color pattern isn't one of them!
According to the identification key in The Light and Smith Manual (2007), Dynamenella dilatata has (1) a distinctive "forehead" — that is, the frontal margin projects forward as a quadrangular process; and (2) inflated antennules — that is, the first two articles of the antennules are dilated.
Here's a closer look at both of those characteristics. Note how there's a flat "shelf" in front of the eyes, and that the bases of the antennules are quite large. The inflated antennule bases give this isopod a "moustached" appearance.
In this diagram of Dynamenella dilatata (see below), you can see the projecting frontal margin on the head, and the inflated antennules. In the lower drawing, note also the three parallel ridges in the center of the pleotelson (basically the tail).
From Richardson, H.R. 1899. Key to the Isopods of the Pacific Coast of North America, with Descriptions of Twenty-two New Species. Proceedings U.S. National Museum 21: 815-869.
These three raised ridges were easy to see on the larger isopods. Look for them in the following image.
If you want to look at one more thing...or perhaps you've already noticed this? The smaller individuals (first two images) have a shallow notch at the tip of their pleotelson. The larger individuals (images three and four) have a very deep notch, like a keyhole (and see image directly above).
We don't know if the depth of the notch is related to the size of the isopod, and if all individuals will develop deeper notches as they get larger? Or if the depth of the notch might indicate a difference between males and females? Does someone out there know the answer? The deeper notch was a little confusing to us at first because it isn't mentioned or illustrated in any of the identification keys.
We had trouble finding information about Dynamenella dilatata. But Jim Carlton was kind enough to answer a few questions for us. It appears as though their range extends from British Columbia to southern California. [Jim told us that he knows of records between Washington and southern California, and there's an unidentified picture of what appears to be Dynamenella dilatata in Marine Life of the Pacific Northwest by Lamb and Hanby (2005).]
I don't know why Dynamenella dilatata is so colorful, or so variable, but it's fun to wonder about!