At least a little.

As often happens, I kept too many babies from a recent brood, and they rapidly consumed the large growth of Bryopsis in their tank., Algae production was a little slow in the system, so I chose to reduce their ration.  I did not exactly starve them, but there was not always Bryopsis in the tank.  There was , however, quite a bit of the pest algae Valonia,  known as “bubble algae,” because it grows as clusters of large unicellular vesicles.  Although I periodically have tried to remove it, it was thriving in the tank with the young slugs.

Elysia clarki, about 3 cm long, with empty Valonia bubbles. A large, empty bubble can be seen by the slug’s tail. 2/26/19

The slugs appeared to be feeding on the algae, and some of the bubbles, which are normally intense green, became clear.  Although it would require DNA sequence analysis of the slugs’ kleptoplasts to be certain, the circumstantial evidence indicates that they are sucking sap from the algae.  It makes sense, because Valonia bubbles are large single cells, which would allow a slug to have a big meal with just one puncture.

A recent paper by Mike Middlebrooks and collaborators (Biol. Bull 236:88, 2019) demonstrates that Elysia crispata (with which E. clarki is most probably synonymous) eats a wide variety of algae in the wild.  In the aquarium, it looks like we can add one more species.

The good news is that the slugs seem to be wiling to make use of Valonia when necessary.  Unfortunately for anyone with an outbreak, the slugs consume them so slowly that there is little chance they would eradicate an infestation.