Eric was looking at a bryozoan, and he noticed this small, delicate cluster next to it:
He recognized it as an octocoral (or soft coral), but also realized it looked different than the species we've encountered most often in this area. (For scale, the large polyp above is ~5 mm across, measuring from one tentacle tip across to the tip of the opposing tentacle.)
Here's a view from the side with multiple polyps extended:
Soft corals may be found under rocky ledges in the low intertidal zone. In that position, the polyps hang upside down with tentacles extending downward:
The photo above is useful as it shows an important feature of this species. Can you see the small, shiny pieces in the column — next to the tentacles and at the base of the polyp? I'll zoom in so you can see them better:
The photo above shows the "necklace" of shiny sclerites encircling the polyp at the base of the tentacles.
In the next photo, look for the larger, rough-looking sclerites at the base of the polyp where it is attached to the substrate (the tentacles are retracted). There are so many sclerites embedded in the base of the polyp that it can look as if there are sand grains gathered around the colony (see second photo of this post).
Sclerites are small, calcareous skeletal elements. They can be important for structural support and may also deter predators. (They are also helpful for species identification.)
This is the first time we've encountered Thrombophyton trachydermum. Jeff Goddard documented it on Bodega Head years ago, but we haven't found it ourselves until now.
What a treat!
P.S. For views of two other soft corals found in northern California, see Cryptophyton goddardi on 25 January 2012 and Discophyton rudyi on 10 May 2012.
P.P.S. Thrombophyton means "lumpy animal" and trachydermum means "rough skin" (probably in reference to the numerous sclerites). So the scientific name of this coral basically means "a rough-skinned lump." It seems this wonderful coral is worthy of a prettier name!