When two species turn into five

An interesting exercise in how parsimony and preconceptions can mislead.

As the most recent posts suggest, slugs have been added, and have appeared, in Box of Slugs 2.0.  Here is a streamlined sequence of events from my point of view.

The tank was set up, and slugs (E. clarki) & macroalgae (Penicillus, Udotea and Avrainvillea) were purchased from a collector. A few days after the new organisms settled in, we left for a little over a week.  After we got back (as described here), not only were there some E. clarki eggs masses (one of which I watched being deposited), but also some tiny sluglets.  Logical conclusion: the small slugs were baby E. clarki.  The timing seemed a bit off, because the youngsters appeared a little too soon based on the incubation times observed for E. clarki in Box of Slugs 1.0 (about 16-17 days), but [mutter about temperature or previously deposited eggs or something].

As the little guys grew, they looked like they might be more than one species (see this post), and the one that looked most closely like E. clarki stilll did not look quite right.  Maybe the big, ruffly parapodia develop as they mature?  Are they really laying eggs at such a small size? Although I did not know what baby E. clarki looked like, suspicions were becoming aroused.

It all came into focus over the past week, as a sizable cohort of baby slugs appeared and started to grow quickly on a diet of Bryopsis.  What do baby E. clarki look like?  It turns out that they look like little teeny versions of their parents, complete with broad, ruffly parapodia and loosely rolled rhinophores.

Very small E. clarki in Box of Slugs 2.0.  2/24/15

Very small E. clarki in Box of Slugs 2.0. 2/24/15

Baby E. clarki, dorsal view

Baby E. clarki, dorsal view

Baby Elysia clarki, ventral view. 2/27/15

Baby Elysia clarki, ventral view. 2/27/15

Baby E. clarki, dorsal view.

Baby E. clarki on Derbesia, dorsal view.

No doubt, they look like tiny versions of the adults lumbering around the tank.  They may have hatched from eggs laid by one of the residents, or from a clutch of almost fully developed eggs that I added a week or so back.

It turns out that, rather than the two species of slugs that were purchased from KP Aquatics, we have five.  The adult clarki and crispata that I purchased are the real deal.  Both species have the same general shape, with ruffly parapodia and loosely rolled rhinophores.  Both are very fond of Bryopsis, at least in captivity.

E. clarki are uniformly green with white spots, and seem to grow and thrive better than the other species in their captive algal world.

E. clarki in Box of Slugs 1.0

E. clarki in Box of Slugs 1.0

E. crispata may be the the glamour slug of the tank.  The bluish hue and large white spots make these slugs very eye-catching.


Elysia crispata in Box of Slugs 2.0.

After looking at a lot of photos, especially at the Sea Slug Forum, I have identified the other species with some confidence.  What I had originally identified as E. clarki is most likely E. papillosa.  Looking at it side by side with E. clarki in the photo below, it is clearly not the same species.  The parapodia of E. papillosa are much smaller and simpler, the rhinophores are pinkish and more tightly rolled, and the spots are smaller and more sparse.  Unlike clarki and crispata, papillosa does not appear particularly fond of Bryopsis, preferring to hang out and feed on Penicillus most of the time. Another interesting difference is that E. papillosa uses its parapodia to swim from time to time.  Despite hundreds of hours of observations of E. clarki, including the slugs floating in the water column, I have never seen them use their parapodia for propulsion.  Maybe the less elaborate parapodia are more useful for swimming (think using a square dancing skirt vs a wedding dress).


Large adult E. clarki (left) with presumed E. papillosa (right).

The final Elysia species, which has very small parapodia, is presumed to be E. tuca.  The bright green coloration, the white rhinophores and the white areas on the head, along with the small parapodia, are anatomical features of E. tuca.  Combined with the species tendency to spend its time on Halimeda and its common occurrence in the Florida Keys, where I assume the algae were collected by KP Aquatics, and it’s a pretty good bet that these are E. tuca, and that they rode in with the first shipment of macroalgae.  They show no interest in Bryopsis, spending most of their time associated with Halimeda, and making the occasional trip to Penicillus or Avrainvillea.

Elysia tuca (left) and E. papillosa (right) on Penicillus that has been overgrazed and overgrown.

Elysia tuca (left) and E. papillosa (right) on Penicillus that has been overgrazed and overgrown.

The different species wander around the tank constantly, but tend to focus on their food plants.

A trio of Elysia: papillosa (left), crispata(top),  and tuca (bottom right)

A trio of Elysia: papillosa (left), crispata(top), and tuca (bottom right)

As far as I can tell, they do not interact socially.  In the photo below, E. tuca crawls over E. crispata as it would any other obstacle.


E. papillosa, crispata and tuca, making a dogpile.

So, rather than the two species of Elysia I purchased, there are four species.  On top of that, at least a few of them are reproducing successfully.  Not bad.

But didn’t I mention five species?  There is one more sacoglossan species in the tank that has not been discussed. Although the species above are interesting enough in their own right, the final species warrants a post of its own.

Stay tuned.


  1. Reply
    Hans Ruppel March 1, 2015

    That is most interesting and clever detective work. I am, however, puzzled by the connection to parsimony and preconceptions. I can pretend I understand the latter but not the former. Perhaps, might you elaborate? It sounds like there won’t be serious competition for the same resources; is that true?

  2. Reply
    Joanna Ruppel March 1, 2015

    I too could use a bit more explanation in connecting the dots. But I am waiting in anticipation for the blog on #5. Box of Slugs 2.0 is truly magical!

    • Reply
      Dave March 2, 2015

      The idea goes something like this. If I were to put one species of slug into an aquarium and then see babies appear, the most parsimonious explanation would be that they are offspring of the adult slugs that were added. This led to the preconception that the youngsters were baby E. clarki, which colored my thinking. Despite the fact that neither species really looked like E. clarki, I assumed at least one of them was and viewed them through that filter. So, even though E. papillosa bears only a loose family resemblance to E. clarki, I did not recognize the difference.

      Just a minor embarrassment, but a reminder that one needs to look at the data that one observes, and not bias it too strongly with expectations.

      As far as resources, Hans, I am hoping I can maintain enough of all algae species to keep everyone happy. They seem to have their favorites, but each will feed on just about any species in the salad bar. The challenge will be to balance the slug population with algal growth.

  3. Reply
    Andrea March 3, 2015

    How wonderful to have additional species! It’s like finding another present or two after you’ve unwrapped everything you thought you had. Or finding more chocolate in a second layer in the box when you thought there was only one layer.

    I enjoyed the framing of your story with the alliterative parsimony and preconceptions. I wonder, though, if the title could be construed as misleading, as one species does not turn into another one, certainly not on this timescale. Maybe a subtitle “The mis-identification of Elysia”? Or perhaps that’s a topic for several more posts?

    Thanks for the descriptions, it’s a great break for me.

    • Reply
      Dave March 4, 2015

      Interesting issue. I had not thought about the other possible interpretation when I wrote the post. Might even get a few more clicks if people think it’s about rapid evolution, rather than mis-identification. I suppose it is naughty, but it may encourage a few more people to read a little, and possibly learn a little, on order the find out the truth.

      Glad you’re enjoying it. On this end, it’s like having a public record of my learning process. Not always pretty.

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