We've gone through a few different possible identifications with this cucumber now. And I should be clear that part of the problem stems from that fact that we've looked at several individual small white sea cucumbers all found in Bull Kelp holdfasts. We now think there might be at least two different species involved.
Tonight I'm going to clarify the identity of this particular cucumber. At first I proposed it might be a Stiff-footed Sea Cucumber (Eupentacta quinquesemita). And then I corrected myself and proposed that it might be Pentamera trachyplaca.
On 24 February, I said we were going to check with experts about that identification. However, before writing to a sea cucumber expert, we decided to do more homework by consulting the online specimen listings in the California Academy of Sciences' Invertebrate Zoology Collection. We discovered that another species of Pentamera had been documented in this area (Pentamera rigida), but it wasn't in the books we had been consulting. We found a research paper that described Pentamera rigida and realized we needed to look at more ossicles to see if our cucumber might be this species.
Eric checked the ossicles at the base of the tubefeet and was a little surprised to discover some ossicles that didn't look anything like the ossicles described for Pentamera (neither Pentamera trachyplaca or Pentamera rigida).
They were very three-dimensional. Eric thought they looked somewhat like clusters of grapes:
Well, we were stumped. We searched the sea cucumber book for a match for these ossicles, but couldn't find anything that looked like this in the ossicle pictures. Then I started reading all of the written descriptions for small white sea cucumbers. And that's when I encountered this text:
"Skin ossicles: vary from thick, knobbed, four-holed buttons to larger knobbed plates, some are elongated and quite thick with a spiny handle at one end, often referred to as pine-cone ossicles; three-armed ossicles from tube feet."
Pine-cone ossicles! That seemed like another way to describe the shape of the ossicles from this cucumber. Here's another view:
This description is for a sea cucumber called Pseudocnus lubricus. We checked the other characteristics of this species, and most seem to match, so we're feeling pretty good about it. We'll still check with an expert, but I feel confident enough to share this identification correction with you...and these fascinating pine-cone ossicles!
I don't know how he did it, but Eric even got pictures of these ossicles in a tubefoot! Below, look for the pine-cone ossicles at the base of the tubefoot (on the left) and the three-armed ossicles throughout the rest of the tubefoot. (I've included a picture of the three-armed ossicles below the tubefoot image as a reminder so you know what to look for.)
And I can't help but show one more tubefoot picture. It's so much fun to see both types of ossicles in the tubefoot!