We've only found a few of them, but all of them were this exact shape. The pictures I'm showing tonight are of the most recent one found on 10 April 2015. We estimate it was about 20-22 mm across.
Below is another angle, showing the points at one end — they almost look like small "horns." If you look very closely, you'll also see another important characteristic — four raised knobs in an arc at the rounded end (it's easier to see them in the first picture).
We hadn't seen this animal before, so at first we just started calling them "horseshoes." We'd be walking along the beach, and when the sun glinted off one and we were close enough to see the shape, one of us would say, "Oh, there's another horseshoe!" They were very different than a nondescript blob on the beach.
Well, you know me...it wasn't enough to call something so distinctive a "horseshoe" and not know what it was. My best guess was that it was part of a siphonophore, so I started looking up possibilities. I was struggling, so I gave in and requested help from Phil Pugh in the U.K. Phil was very generous and responded with an identification of this siphonophore:
What a great name! Here's an illustration of the nectophore or swimming bell of Hippopodius, along with one of my pictures to match the same orientation:
From A Synopsis of the Siphonophora by A.K. Totton (1965)
I mentioned that this is one nectophore, i.e., it's just one part of the whole colonial siphonophore. Hippopodius has many nectophores that fit together tightly around a long central stem (see below). The nectophores are used for swimming while the stem supports structures used for feeding and defense (e.g., stinging cells) and reproduction.
Modified from Jacobs, W. 1937. Beobachtungen über das Schweben der Siphonophoren. Z. vergl. Physiol. 24: 583-601; and Totton (1965), see above.
There are a couple of really fun facts that I can't help sharing:
- The name Hippopodius means "horseshoe"! It's very appropriate.
- Hippopodius is a warm-water species. It's rare to find them in northern California.
- Hippopodius is known for being bioluminescent — I think even the lone nectophores would light up in response to stimulation. If we find another one, we'll have to look for it.
P.S. For introductory information about siphonophores, review the posts from 23 January 2012 and 3 April 2013.
P.P.S. Since Hippopodius is somewhat long, we started calling it "Hippo" for short, leading to the title of this post.