About two years ago, I shared a post about the Scarlet Sea Cucumber, Lissothuria nutriens (see post on 22 January 2015). At that time, I noted this species was rare in Bodega Bay, but we also wondered if this southern sea cucumber might increase in abundance if sea water temperatures became warmer. It appears that has happened!
The Scarlet Sea Cucumber reaches its northern range limit in Bodega Bay. There are some records from Duxbury Reef in Bolinas, but so far we haven't been able to find any records from further north. (This would be a good time to look for them in northern California!)
Remember that this is a small sea cucumber. Here's a picture from the rocky intertidal zone with some fingertips for scale. It shows several Scarlet Sea Cucumbers on the rock — look for three reddish blobs in the center of the picture:
When the cucumbers are out of water, their tentacles are retracted. And note that they cover their bodies with bits of debris. (Recognizing this "covering" behavior is helpful when searching for these sea cucumbers — it differentiates them from red sponges and sea squirts.)
Here's another example, this time a close-up of a single Scarlet Sea Cucumber:
Because this species has been rare in our area, we wanted to be certain about the identification. So Eric double-checked the ossicles (the calcareous plates inside the body wall, tube feet, and tentacles that help identify different species of sea cucumbers).
Below is a diagram illustrating the different types of ossicles found in Scarlet Sea Cucumbers:
Ossicles of Lissothuria nutriens. (A) Curved supporting plate from tube foot, (B) end plate from tube foot, (C) smaller, hourglass-shaped bodies from dorsal body wall. Modified from Deichmann, E. (1941). The Holothuroidea collected by the Velero III during the years 1932 to 1938. Part I. Dendrochirota. Allan Hancock Pacif. Exped. 8: 61-194.
For comparison, here are a few of the ossicles from one of the local specimens. You can see that the shapes match the drawings above, confirming the identification:
If you get lucky and spot a Scarlet Sea Cucumber in a tidepool, along with the red oblong body you might see the tentacles extended. Interestingly, the tentacles vary in color from fiery orange to scarlet red:
These sea cucumbers feed by catching organic material on their tentacles and then moving it to their mouth in the center of the tentacles. Eric filmed some of this behavior under a microscope in the lab, and he made a wonderful video clip so that you can see the tentacles in action. (Also watch for the shiny ossicles embedded in the tentacles and tube feet.)
We have counted quite a few Scarlet Sea Cucumbers in the low intertidal zone while conducting surveys this past week. We're also wondering if the warm ocean temperatures during the last couple of years might have allowed this species to move even further north, beyond Bodega Bay. We'd be very interested in any Scarlet Sea Cucumber observations in northern California, so let us know if you spot one!