The vibrant MacFarland’s nudibranch chomps on sponges from…

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Nudibranch Flabellina trilineata and look-alike amphipod crustacean Podocerus cristatus. Photos and description by Jeff Goddard, UCSB

“Whoa—what is THAT?!”

“Look at this thing! What is it?! It’s a shrimp of some kind… An amphipod, maybe? Come on, keep the camera steady… Wow that thing is tiny! That’s so crazy… And what’s up with the orange puffball pattern?! Duuuuuude…”

Such was my highly eloquent inner dialogue when I saw my first Podocerus amphipod. The hilarity continued back on the dive boat—on a vessel filled to the brim with experienced research divers and species identification books, none of us could figure out what this thing was on about, other than a sponge.

The only thing we could all agree on was that the critter looked rather slug-ish, its orange/white color combo very reminiscent of Triopha catalinae—the clown nudibranch.

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That one, there.

Maybe this conniving crabby was a colorful copy-cat? On that hunch, I sent the video along to UC Santa Barbara’s Dr. Jeff Goddard—a king of local nudibranchs and expert slug photographer—to see what he thought of the confounding crustacean cutie.

Imitation is the sincerest form of flattery

“That amphipod is slugging nearly 100%” — Dr. Goddard (paraphrasing)

Our mystery shrimp had a name! Podocerus spongicolus, a close cousin to Podocerus cristatus, a notorious imposter for Flabellina sea slugs!

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Amphipod crustacean mimic Podocerus cristatus, Diablo Canyon, San Luis Obispo County, central California. Photo: Mike Behrens, SeaSlugForum.com

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Flabellina idionea — the Spanish shawl nudibranch, Santa Barbara County, California. Photo by Todd Huspeni, SeaSlugForum.com

While Triopha is not a likely muse for mimicry by these mini-Machiavellis, many smaller Flabellina sea slugs have similarly bright, contrasting colors. And for much the same reason: these nudibranchs are trying to look disgusting.

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Gross.

Many sea slugs dine on noxious prey, and through some creative digestion, become very nasty themselves. Some Flabellina slugs can even use their prey’s stingers for their own self-defense! Their bright colors are meant to give predators a big ol’ case of the NOPEs, thereby granting the slugs—and anything that looks a lot like them—diplomatic immunity to forage at large and unbothered by weary predators.

Knowing your audience: a key to local success.

And Podocerus amphipods aren’t a one-trick sea-pony. Dr. Goddard recently published a note describing how Podocerus amphipods appear to be locally adapted to mimic the area’s dominant slug detractor, down to the tiniest details of antenna color and body stripes.

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A fascinating pattern of local adaptation by Podocerus amphipods to the surrounding slug-fest noted by Dr. Goddard. Photos and graphic by Jeff Goddard, Gary McDonald, Allison Vitsky, and Ali Hermosillo; map via GoogleMaps.

But we don’t know whether each local mimic is a separate species that specializes on a particular form of nudibranch, or if each color morph is an iteration of one highly variable species found up and down the coast. Dr. Goddard says that this relationship “begs further study”—perhaps some youngster should dive into this system! Any takers?

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