Since about mid-July, whenever we walk the local beaches, we've been seeing blue — this has been an extraordinary year for By-the-wind Sailors (Velella velella). Then on 30 August, we encountered Blue Buoy Barnacles (Dosima fascicularis). And now, on 21 September, we found these.
The extreme close-up above might be a little puzzling at first. I'll zoom out a little more, and then I'll show some pictures of the entire animal.
Okay, here we go, the next two photos will give it away.
This is Fiona pinnata, a pelagic nudibranch. I first wrote about Fiona on 19 October 2012, but in that case those individuals were brown. Fiona's color depends on its food. When eating barnacles, they're brown, but when eating By-the-wind Sailors, they're intensely blue! This is the first time we've seen the blue form of Fiona pinnata.
Eric's sharp eyes first spotted Fiona's egg capsules on Velella sails washed up on the beach. Then I found one Velella with both eggs and a nudibranch! We started looking for others, and counted at least 20 Velella sails with Fiona egg cases and at least 12 blue nudibranchs!
Below are two examples of what they looked like on the beach (the off-white blobs are the egg masses):
We weren't sure what kind of condition they'd be in after a journey through the surf zone, but to document their occurrence in Bodega Bay this year, we brought a few back to the lab for photographs. The next image was taken in a small aquarium:
The whorled egg masses are packed with developing embryos:
When the nudibranchs turned on their sides, you could see what looked like developing eggs inside:
If you're wondering about the blue color...well, so am I. I watched Fiona actively feeding on Velella. Fiona's digestive tract extends into its cerata (the structures on its back). So the reason you're seeing blue is because you're seeing fragments of ingested Velella. Benjamin Kropp wrote about this after an experiment in 1931. When he starved or fed blue Fiona different food, they lost their blue coloration within 2-3 days. When he fed them Velella again, they regained their blue coloration within 2-4 hours! [See Kropp, B. 1931. The pigment of Velella spirans and Fiona marina. Biol. Bull. 60: 120-123.]
It's officially fall now (when I'm writing this), but I can't help wonder if another blue animal will be discovered before the water turns cold again?