We are just about to post the photos of the setup process of the improved Box of Slugs 1.0, which will be known as BoS 1.1. It will have two broodstock tanks, a hatchery and a sump/Bryopsis storage tank. In the meantime, here are a few more photos from the January trip to Bonaire. Nothing to do with slugs, but the photos are at least of marine organisms. I hope you enjoy them. As always, click on the images to enlarge.
The Elysia literature is rich, varied, and growing constantly. From time to time, I will highlight a recent paper that strikes my fancy.
The paper of the moment is a recent review by de Vries and his colleagues that, from my point of view, demonstrates how the field of kleptoplasty in sacoglossans is maturing as a growing number of researchers apply diverse methods and approaches. Although I will summarize some of the highlights below, it is worth reading this short, nicely written paper for yourself (de Vries et al., 2014, Acta Soc Bot Pol 83: 415-421)
This paper illustrates a larger point that I have had to learn repeatedly during my career: as much as possible, one needs to look at the data and the biology, unfiltered by the way you think it should work. Almost invariably, one generates a mental model to organize one’s observations about a biological system. This model forms the basis for additional experiments, which can potentially support the model, but which almost always show you how naïve and simple your initial model was. The model is essential to focus one’s thinking, but any biological system is more complex (and therefore more interesting) than your limited human mind can imagine. So, there comes a time to listen to what the biology is telling you, and to think very hard about the next generation of models.
The paper by deVries and colleagues highlights that the field is at a point where researchers can, and must, think more deeply about the mechanisms and functions of kleptoplasty. They lay out a series of fundamental unanswered questions regarding the biology of solar sacoglossans.
The paper ends with a quote from Ed Yong that echoes my introduction to this blog post, but is much more eloquent: “Science is about resisting the easy pull of conclusions. It’s about testing stories that seem like they should be right to see if they actually are right.”
As the study of kleptoplasty evolves from “gosh, wow, a photosynthetic slug” to a more complex and interesting view of the animals’ biology, the questions become more focused and more sophisticated. It will be fun to watch, and with a little time and effort, participate in the process.
In the previous post, I mentioned that there are at least five species of slugs in Box of Slugs 2.0. Four Elysia species were described in that post, but the one in the picture below is odd enough to warrant its own spotlight. This tiny beauty was clearly different from the Elysia, having branched rhinophores, and being adorned with cerata, outgrowths from the dorsal surface. It is more reclusive than the others, rarely making its presence known except for the occasional appearance in the early morning before lights-on. It is also very quick to become scarce, diving behind or into a clump of algae, if the lights come on when it is out and about. These behaviors, combined with my limitations as a photographer, explains the quality of the images so far.
When I first saw this creature, with its frilly cerata, I believed it to be a nudibranch. In general, marine aquarists are not pleased about seeing nudibranchs in their tanks. They tend to be specialist predators, focusing on one or a few species of prey items. Prey can be sponges, bryozoans, corals, or even other molluscs. As a result, nudibranchs can either be pests, destroying prized species, or, more likely, will starve to death in the absence of preferred foods
I had removed the slug to a small container, and was debating its fate, when Joanna made it clear that killing the slug was the less preferred option. Realizing that I did not yet know what I was dealing with, I took a few macro shots and started to go through field guides. Looking more closely, I realized that it was not a nudibranch, but a sacoglossan like Elysia. The lack of external gills on the posterior end (see photo below) mean that this animal is not a nudibranch, and the combination of cerata and branched rhinophores suggest that it is a species of Cyerce. Cyerce antillensis comes from the right part of the world to have ridden in on some macroalgae, and it lives on Penicillus, so it is a promising candidate. It is a variable species, and is the closest match I have found so far, but I have not yet seen a photo of Cyerce antillensis that is convincingly similar to our slug. In any case, the slug was returned to the Box of Slugs as a harmless curiosity.
The photos above and below show a body filled with green stuff, presumably chloroplasts removed from its food plant(s). According to the literature, however, Cyerce is not kleptoplastic.
With time and luck, opportunities for better photographs will present themselves. Who knows, there may even be enough of them to start seeing young ones.
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.
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. crispata may be the the glamour slug of the tank. The bluish hue and large white spots make these slugs very eye-catching.
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).
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.
The different species wander around the tank constantly, but tend to focus on their food plants.
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.
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.