Bahia II: Elysia diomedea Tastes Bad

As described in the previous post, I had very modest goals for this summer in Bahia.  Because I am getting more interested in the role of kleptoplasty in chemical defense, I thought it would be worth assessing the palatability of Elysia diomedea.  Some Elysia species are known to taste bad because of chemicals assimilated from their food plants (see, e.g., Rasher et al., described in this post).  E. diomedea is known to produce interesting derivatives of plant compounds (e.g., Ireland et al., 1978, J. Am. Chem. Soc. 100:1002), but, as far as I can tell, there is no evidence regarding the slugs’ palatability.

Fortunately for me, there is a relatively easy way to get a quick sense of their palatability.  When snorkeling at the field station, one is generally followed by a small parade of large bullseye puffers (Sphoeroides annulatus) waiting for tasty morsels to be stirred up.  What would happen if I dropped a slug in the water column and allowed the fish to eat it?  One might expect a puffer to eat anything.

After a day in the field, I had time for a snorkel, so it was a perfect opportunity.  After a short survey along the subtidal, I found a few Elysia in a small bunch of Codium (surprise!).  I pulled out this little beauty, apologized to her and carried her to the surface.

Elysia diomedea in the shallows in front of the Bahia de los Angeles field station. 5/23/17

If you click the link below for the short video (note: large-ish file), it is pretty clear that the puffer does not find the little Elysia to its liking.

Puffer spitting out Elysia diomedea. Bahia de los Angeles, 5/23/17

Not only one, but three puffers rejected the Elysia.  After the first spat out the slug, a second tried it, then a third.  In no case did one of the puffers as much as chew, they rejected it as soon as it was in their mouths.  Very good for the slug, and suggests that it may be something secreted in the mucus that repels the fish.  One might also conclude that puffers don’t learn from their friends, since each had to try it.

Based on one slug (but three puffers), we can tentatively conclude that E. diomedea tastes bad.  Are the bad tasting compounds derived from products made by the kleptoplasts?

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