There is so much going on in the world of solar slugs! I came across a couple of nice videos, and thought I’d share.
A broad collection of European slug scientists has launched a project to sequence the genome of Elysia timida. That will be a wonderful resource for all of us who are interested in slug science. The video below provides an explanation of the motivation, along with excellent general information.
The video below does a great job of describing the biology of E. viridis, with some beautiful footage. Can you see the minor error? (Hint: what is it eating?)
I recently had an epiphany about how to grow Bryopsis more efficiently. In retrospect, it was pretty obvious, and I wonder why it took 2 years to get to this point.
It seemed as though culturing adequate Bryopsis was under control. However, there’s nothing like 6+ weeks of travel to turn things upside-down. I had hired a service to come in and keep things going while I was away, but their primary task was to prevent biological meltdown, or worse, a big salty disaster that would have me forever on the naughty list of the facilities people. I am happy to say, there were no smelly or wet disasters.
Unfortunately, I was not there to give the algae cultures the kind of attention they need, and by the time I was back in the office, the system was overrun with Derbesia, and the slugs had devoured the Bryopsis that I had left for them. The 20-gallon slug tank and both algae culture tanks were full of felty, green hair algae. In retrospect, I should have taken photos, but I was more focused on cleaning up the mess and getting ready to teach a summer class.
Over a week or so, I pulled out the algae tanks off the system, cleaned out at least two pounds of green glop, salvaged the remaining Bryopsis, and set the tanks back up. Somewhere along the line, I came across a few posts about “algae reactors,” cylindrical chambers with water flowing through and some sort of light source. My first thought was “maybe I should buy one of those things.” Then I remembered that I had two media reactors sitting idly in the basement. They are clear cylinders, designed to have water flow through them, which is exactly what I wanted.
The amount of Bryopsis remaining was so small that it didn’t seem worthwhile to have two algae tanks. Instead, I shut down the 10-gallon tank and stuck some of the remaining algae into a reactor. The reactor was hooked up to one of the valves, and connected to the drain, and sat under the grow light where the 10-gallon tank had been.
Within a week or so, it became clear that the experiment was working, so I added the second reactor. I had enough Bryopsis to harvest some for the slugs, and the culture in the first reactor had seeded a sponge that I could use to start the second reactor.
It was working. Time to make things a little less clumsy. I built a rack from 3/4″ PVC, setting the reactors at an angle to optimize the connections to the input valves and drains.
Things went so well, I begged a few unused reactors from local aquarists. This one is from Alan (of unidentified algae fame), and I have one more waiting in the wings.
At this point, it was clear that my former method of rearing Bryopsis in aquaria was not very efficient. Raising Bryopsis in reactors allows me to play with growth parameters, like flow and nutrients, much more easily. Further, keeping multiple separate cultures will make it much easier to eradicate unwanted algae. At some point, it should be straightforward to maintain cultures free of unwanted algae and invertebrate pests (I am all for biodiversity, except when it eats slug larvae) by UV sterilizing the water going into the chambers.
As far as I can tell, this was a successful experiment, so I converted the 15 gallon algae tank into the second slug tank, shutting down the 10-gallon slug tank.
When I walked in today, the system had two slug tanks (top and middle left), and one remaining algae tank (top right)
Five hours of cleaning and rearranging later, there were two slug tanks and three algae reactors. In the process, the system now has two fewer circulation pumps and one fewer light fixture.
In the future, you can expect a few more reactors. Now it’s time to play with parameters to maximize algae growth. Maybe we’ll finally see some consistent egg production from the slugs.
To many, this post is akin to “how to give fleas to your dog,” or “how to grow ragweed.” Bryopsis is a benthic (substrate-associated) genus of algae that most aquarists dread seeing in their tanks. Bryopsis can thrive in reef aquaria, to the point that it can smother and kill corals. Many methods of eradication have been explored, including near-toxic levels of magnesium, and the anti-fungal medication fluconazole.
On the other hand, I really want the stuff. As far as I can tell, it is the favorite food of Elysia clarki. E. clarki will eat other macroalgae, such as Penicillus, Avrainvillea or Halimeda, but, given the choice, they will go straight for Bryopsis and stay there. Further, they have consistently produced eggs only at times when they have had unlimited availability of Bryopsis. One of my primary goals is to maintain a self-sustaining colony of slugs, so I very much want a constant, adequate supply of algae.
I’d be perfectly happy if I could count on the outbreaks of local aquarists to supply my needs. Unfortunately, sources have been spotty. My primary source, Justin, who had a beautiful outbreak in a 500-gallon tank, unfortunately managed to get his infestation under control. He swears the controlling factor for him is the level of phosphate; if PO4 levels creep up, he gets an outbreak. Others have provided quality material as well, but not as consistently. Yet others have offered species that turn out not to be Bryopsis. The take home lesson is that people who are trying to eradicate something are not the most dependable source.
How hard can the stuff be to grow? It is a pest, after all. Like any aquatic plant, it needs light, nutrients, and circulation. It gets a little more difficult when you try to provide for the plants’ requirements on a consistent basis. As far as I can tell (and PLEASE email or comment if I am wrong), nobody has published specific requirements of Bryopsis for PAR (photosynthetically-available radiation), macronutrients (e.g., carbon, nitrogen, phosphorous), or micronutrients (like iron, copper and cobalt). When marine aquarists have outbreaks, much of the nutrient load is derived from fish food (well, really fish poop), which is not measured. In fact, most reef aquaria don’t have Bryopsis outbreaks, which suggests that the unlucky few have managed to combine all the right ingredients. For a constant supply of Bryopsis, I need to provide for all of its needs.
If I wanted to grow phytoplankton, the single-cell, free-floating relatives of Bryopsis, I would be in luck. Because of the important role of phytoplankton in our oceans, and because there is a growing industry using them to generate hydrocarbons from sunlight, growing conditions have been explored and published at length. The most popular formula is Guillard’s F/2, so named because it consists of his formula F diluted to 50% (Guillard & Ryther, 1962, Can. J. Microbiol. 8:229), is readily available through suppliers such as Florida Aqua Farms, or even Ebay. The formula provides a balanced blend of nitrogen and phosphorous (with an option to add silicon if you want to grow diatoms), along with a mix of trace elements and vitamins. Mix it up, add a starter of the phytoplankton of your choice, bubble in some CO2 as a carbon source, and you’ll soon have a thriving culture of green water.
I started dosing the Florida Aqua Farms version (“Plant Fuel Too”) almost a year and a half ago, using the complete dry mix (minus silicates). The Bryopsis started to look darker and happier pretty quickly. Perhaps unsurprisingly, there was also a phyoplankton bloom that turned the water green and generated a layer of green scum on the surface of the water. Also, the added nutrients seemed to increase levels of nuisances like cyanobacteria (a.k.a., red slime algae). That would have all been tolerable, but the biggest sticking point was that Bryopsis would go through cycles of boom and bust, which is not conducive to long-term management of a slug colony. I needed to get the right balance of nutrients that would get the Bryopsis to grow maximally and out-compete the species I don’t want.
With all of the possible factors (e.g., light, circulation, temperature, nutrients) to be within the right ranges, there were a lot of variables to play with:
In order to separate the variables a bit better, I started using Florida Aqua Farms original “Plant Fuel,” which contains all of the trace elements and vitamins, but no nitrogen or phosphate. That would allow me to play with levels of trace elements, N, and P separately. I also added a source of carbon. Because white vinegar is cheap, always available, and is readily broken down into CO2 by bacteria, it seemed like a good choice.
By adjusting the relative input of all four ingredients (C, N, P and trace elements), combined with very high flow in the algae culture tank, I was able to get significantly better growth. The final improvement was something that I had not expected to have such an impact: rebuilding the system. Within about a week of the big move, the algae was thriving on the racks of plastic eggcrate I was using as a substrate. I suspect that the 50% water change and improvements to circulation in the new system played a large part in the improved growth.
By growing on plastic racks, Bryopsis cultures can be moved to slug tanks to be consumed, and then removed for regrowth.
So we’re making forward progress. We are controlling the growth of food algae to the point where it seems to be sustainable. As a side benefit, the other macroalgae are also thriving. For example, the three deep green Penicillus in the photo below are new sprouts from the ratty old plant in the center.
At the moment (and likely to change) the system receives major elements at the (molar) ratio of 150:30:1 C:N:P. This is roughly in the range reported for other macroalgae, and does not support much growth of cyanobacteria or phytoplankton. In other words, it looks like I am providing nutrients at about the rate at which the are being consumed by the desired algae.
There is still plenty of room for improvement. The priority for the moment is eliminating or reducing dinoflagellates in the algae culture tanks. They do not seem to cause too much trouble in the slug tanks, but they form a slimy film on the Bryopsis in the culture tanks, presumably reducing growth, competing for nutrients, and producing toxins.
At higher magnification they are kind of pretty. Based on the shape, size and general appearance, it looks a lot like Ostreopsis ovata (Faust and Gulledge, 2002, Contr. U.S. Natl. Herb. 42: 1-144). Like many dinoflagellates, O. ovata produces toxins, some of which are toxic to mice (Nakajima et al., 1981, Okinawa Bull. Jpn. Soc, Sci. Fish. 47: 1029). The effects on slugs or other invertebrates is not known.
At the moment, I have reduced the photoperiod (the time the lights are on), and am rinsing the Bryopsis racks in clean artificial seawater periodically to reduce the population. At some point, it might be necessary to use harsher methods, such as antibiotics (metronidazole seems to affect dinoflagellates somewhat specifically) or starting a clean culture of Bryopsis and preventing entry of other species using UV sterilization.
For now, there is plentiful food for the slugs, which is a step forward.
The trip to Bonaire ended too soon. Six days of dives, with a bonus of kayaking and snorkeling in the mangroves on our no-dive day. The best slug-watching happened in the first few days.
The site with the highest slug count this year was Jeff Davis Memorial (named for the physician, not the Confederate leader). It is usually a favorite of ours, but this year seemed to have more dead coral than usual. On the positive side, we easily found at least a dozen E. crispata among the rubble.
The blue of this one was so intense, it jumped out from many feet away. The camera did not really capture the intensity.
The slug below was more acrobatic than most. The average E. crispata just sits there, or moves along gracefully, but this gal seemed to have somewhere to go. Eventually she fell off, and found herself in a new spot.
We also got a chance to explore the mangroves on the east side of the island. Although no actual Elysia were found this year, the conditions look good. For example, the clump of Halimeda and Caulerpa below would be a great place to find E. tuca, among other species. Maybe next year?
The sponges, tunicates and hydroids, like those below, would also be excellent places to hunt for nudibranchs.
We’re taking a bit of a break from winter and work, and have escaped to Bonaire for about a week. As always, the diving is amazing, with hordes of beautiful fishes, along with healthy corals and amazing sponges. But you are not an avid fan of this site because of fluff like that.
Although our search for other species of Elysia has not been successful as yet, there have been plenty of E. crispata. One does not even need to venture very far to find them. After a dive from the Bonaire Dive and Adventure dock to check some recently repaired equipment, who did we find right next to the exit in a few feet of water? This little beauty was waiting as if to wish us a good evening.
One usually finds these guys on surfaces that appear devoid of food plants. However, looking closely, a short, sparse turf of green algae can be seen. With a vivid imagination, you might even be able to make out some little branches on the filaments, suggesting the presence of Bryopsis. Or maybe not.
Time for more adventures with untamed slugs. More soon.
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.
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.
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.
The Washington Post reported that a species of photosynthetic nudibranch has made the SUNY Environmental Science and Forestry list of the Top 10 New Species of 2015. The field was large, about 18,000 species in all, but Phyllodesmium acanthorhinum made the list based on what the animals tell us about the evolution of the symbiosis between the slugs and the photosynthetic algae they host.
Like Elysia, species of Phyllodesmium steal the ability to perform photosynthesis from their food organisms and maintain the required components in sacs extending from the gut called digestive diverticula. There are some important differences, though. Unlike Elysia, Phyllodesmium is a true nudibranch, and it feeds on corals rather than macroalgae. Another important difference arises from the different biology of the algae that Elysia eat and the corals upon which Plyllodesmium feeds. Photosynthetic corals, such as Xenia, contain symbiotic algae (dinoflagellates, actually) called zooxanthellae, which provide the corals with most of their nutritional needs. When Phyllodesmium feeds on Xenia (or other coral species, depending on the species of Phyllodesmium), it steals the zooxanthellae and stores them in the diverticula. In this way, Phyllodesmium has it a bit easier, the stolen algae are autonomous cells, and the slugs do not need to worry about maintaining isolated chloroplasts.
So how did this species end up in the top 10? A recent paper describing Phyllodesmium acanthorhinum and analyzing the interrelationships of species within the genus (E. Moore and T.Gosliner, 2014, The Veliger 51:237) provides some new insight into how the ability to maintain zooxanthellae evolved within the group. Earlier work had suggested that the branching of the diverticula, and their extension into the cerata (the frills on the back of the nudibranch) increases with the increased ability to sequester and maintain zooxanthellae. In other words, species that simply digest the zooxanthellae have minimal branching, while those that maintain large collections of active zooxanthellae have more elaborate diverticula that branch deeply into the cerata. Based on the descriptions of P. acanthorhinum and another species, P. undulatum, both of which are relatively less specialized for maintaining zooxanthellae, Moore and Gosliner provide additional support for this hypothesis. Further, they suggest that the larger body sizes achieved by more derived species, i.e., those that are better able to maintain populations of zooxanthellae, result from the additional nutrients produced by the symbionts.
Once again, slugs find a way of hijacking photosynthesis from their food. Because Elysia and Phyllodesmium are only distantly related, and their biology and that of their food are so different, the two forms of theft-based photosynthesis must have evolved independently. The similarities are striking, though. It does make one wonder if there is some aspect of the biology of sea slugs that predisposes them to separate chloroplasts or entire zooxanthellae from their food and maintain them in digestive diverticula.
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.