One of the members of the Washington DC Area Marine Aquarist Society (WAMAS) recently posted on the club forum that he had an outbreak of small slugs.  When I looked at the photos, I was pleased and amazed to see that his slugs looked like small Elysia.  Amazed because his was a traditional coral reef tank, which, given the hazardous pumps and lack of appropriate food, are not conducive to survival, much less propagation of Elysia.  Nonetheless, there they were.

Ryan, the slugs’ accidental owner was happy to meet up and hand off a few.  It took a few weeks to find the right time, but he gave me a baggie of six little slugs yesterday.  Meantime, most of the little guys had done what Elysia usually do in reef tanks, and had climbed or been blown into the filtration system to their doom.

Even a quick, unmagnified look at the slugs suggested that they were not the usual suspects (clarki, crispata, papillosa…).  Once under the microscope, they were clearly unlike the other species that I have either purchased or been lucky enough to obtain as hitchhikers on live plants.  They are quite small – about 7 mm – have reduced parapodia, stubby, tubular rhinophores, little white bumps, and squarish hearts.

Two unidentified Elysia. Total magnification 70X. 6/28/15

Unidentified Elysia. Ruler shows how tiny the slug is. 6/28/15.

Small unknown Elysia. Each mark represents 1 mm. 6/28/15

They are now in the growout tank of the hatchery system, where I hope they will find a species of algae to their liking.  Of the species that fit the rough description on the Sea Slug Forum, Elysia serca, and Elysia flava, two western Atlantic species, or E. obtusa, from the Pacific seem, to be the closest fits.  However, none of them seems perfect.  The mystery species lacks the characteristic trio of white spots on the heart and parapodia of E. serca, as shown in the photo below, but the small body size, large head, and small rhinophores look like a fairly close match.  E. flava and E. obtusa appear much more translucent in photographs, but the pattern of white specs strongly resembles E. obtusa.  If they settle in and produce progeny, there should be some opportunities for proper analysis.

Elysia serca from the Sea Slug Forum

Elysia obtusa from the Sea Slug Forum

E. serca, feeds on seagrasses, such as Thalassia (turtle grass) and Halophila (tape grass), which are true vascular plants rather than the macroalgae that serve as food for most of the genus.  The food plants of the other two candidate species do not appear to be known.  In their new home, there are at least a half dozen species of macroalgae, plus some shoal grass plants (Halodule) that rode in with them, so there is a decent chance they will find something to eat.

It would all be made a lot easier if we knew where they came from.  As far as Ryan knew, no plants or macroalgae were placed into his system, so there is no obvious way for the slugs to have ridden into the tank. At this point, we don’t even know which ocean they came from.

Here’s a final look, in their new home. Will she thrive or fade?  We’ll see.

Mystery slug in growout tank. 6/29/15.