Sheaves of gills?

  

From the "Little wonder" post from ~1 week ago, I think you can tell that we've been looking closely at a couple of nudibranchs that live on the patches of bryozoan found on Giant Kelp (Macrocystis pyrifera).  Above, Eric is scanning the bryozoan colonies for signs of the nudibranchs.

We've found both Corambe pacifica and Corambe steinbergae, but these nudibranchs are small (often under 1 cm long), so it can be difficult to tell which species you're looking at. 

To make it a little easier for other folks to know what to look for when identifying these two species, Eric put together a series of video clips highlighting the differences between them (see the video below!).

In the video, first you'll see the bryozoans that the nudibranchs feed on.  The zooids emerge and extend their tentacles to feed.  When the nudibranchs appear, watch for these features:

Corambe steinbergae:
- the edge of the nudibranch is smooth around the entire perimeter
- the rhinophores are simple (no extra "decorations")
- there is a gap between the two rows of gill plumes on either side of the midline (and the gill plumes look a little more linear)

Corambe pacifica:
- the edge of nudibranch has a deep notch at the posterior end
- the rhinophores are sculptured or fluted
- instead of a gap between the two rows of gill plumes, in the center there are several gill plumes that are pinnate = with short branches aligned on either side of the main stem (they look a bit like feathers)

For reference, it also might help to see how the gills plumes of these two species were illustrated in the original descriptions:

 

Above, Corambe pacifica — the entire animal with posterior notch; a close-up of a fluted rhinophore; a close-up of the gill plumes from below (drawings by Olive Hornbrook MacFarland).

Above, Corambe steinbergae — the entire animal without a posterior notch; showing the simple rhinophores; and illustrating the gill plumes with a gap between the two rows.

One thing I didn't mention yet is that the gill position of these two species is unusual.  Many nudibranchs have gills (or other respiratory structures) on the upper surface.  In this case, the gill plumes are positioned on the underside!  But luckily Eric was able to film the nudibranchs from above and below, so this is an excellent view of the gills and the differences between the two species.

Check it out!  [And if you can't see the video player in an e-mail, just click on the title of the post to watch it on the NHBH website.]

Nicely done, Eric!  The best views of the differences between Corambe steinbergae and Corambe pacifica captured on film!

P.S.  Corambe pacifica's pinnate gill plumes have an interesting shape.  When viewing them I kept thinking about a Henri Matisse cut-out called The Sheaf (La Gerbe):