Aquarium Invertebrates: Sea Slugs - Part 1
I have just returned from MACNA XVI in Boston, and as usual, it was a great opportunity to catch up with many old friends, and meet some new ones. It was also a great to be able to finally attach faces to many of the Email names I have corresponded with over the years.
On my way back to Hawaii after the meeting, I stopped off to visit my family in Edmonton, Alberta. I was quite surprised to see how much the reef specialty shops in town had expanded since I had visited last, and was also surprised to see the variety of animals that were now available for sale there. Not only had the extent and sophistication of the hobby in Edmonton expanded, but there was even a petshop selling marine fish & invertebrates in my hometown of Devon, which has now grown to a massive population of roughly 5,000 people!
This was both encouraging and discouraging to me. I remembered it being difficult to find a wide selection of reef animals and supplies from many years ago. That had certainly changed! Unfortunately, many of the animals that I saw for sale in some shops have extremely low chances of survival in an aquarium, and I wondered if people were being informed of that when shopping for new additions to their tank. Many of the conversations I had at MACNA involved the sustainability of our hobby, education to prevent trade in animals that have low survival success, supporting captive rearing of fish and invertebrates, and trying to better educate newbies to avoid many of the problems that we had inevitably encountered over the many years of reefkeeping.
As I was cruising the petshops in Edmonton, one tank in particular that caught my eye contained about 50 sea slugs. As I crouched in front of the tank, I couldn't help but notice that the primary substrate for the animals was a large mass of plastic plants that resembled Caulerpa algae. I was pleased to see that the two species of sea slugs in the aquarium happened to be 2 of the more likely sea slugs (sea hares and lettuce sea slugs) to survive in captivity, and there were none of the more colorful and infinitely more difficult nudibranchs that are so often imported. As I was writing this article, I was going to discuss sea hares, lettuce slugs and nudibranchs all in the same article, but it ended up running so long that I have split the discussion into two parts. I'll save the discussion of nudibranchs for next time.
Getting back to my story, I was very encouraged that the only slugs in the shop were the ones that had a reasonable chance of survival. I figured that meant that the staff must be pretty well informed, and were making a conscious decision to only stock animals with a reasonable chance of survival in tanks. Almost as soon as the thought entered my head, I overheard a salesperson telling a shopper that "these lettuce nudibranchs are reef-safe and would be a great addition to your tank."
Now, as I said, I do have to give the shop credit for stocking animals that had a non-zero chance of survival in captivity (the sea hare Aplysia and the lettuce slug Elysia ), but this proclamation bothered me for a couple of reasons. First, neither of these slugs are actually a nudibranch, and second, both species feed on specific algal prey that were absent from any tank in the entire shop. The fact that the salesperson did not ask the perspective customer about their setup, the inhabitants, or tell them anything about the biology or feeding requirements of the sea slugs was a little disturbing to me. Couple that with the observation that there was nothing in the shop that could have been provided to these animals as food, and I had a sinking feeling; either the effort of the person doing the ordering was being undermined by a lack of knowledge on the sales floor, or there wasn't quite the conscientious effort for responsible stocking that I had given them credit for in the first place…
What Is In A Name?
So what do I mean that these animals are not really nudibranchs, and who really cares? First, let's back up a step and define the terms 'sea slug' and 'nudibranch'. Many people use the terms interchangeably to cover the more than 3000 species of opisthobranch sea slugs, which is simply incorrect. Nudibranch is actually a proper taxonomic description - the naked-gilled sea slugs. Sea slug, on the other hand, is a descriptive term that can be applied to any of the shell-less gastropods that live in the sea.
Together with their shelled cousins, Opisthobranchs are members of the Molluscan class Gastropoda. In the simplest terms, the subclass Opisthobranchia is simply a group of snails that do not produce a shell - unlike their more familiar cousins, the snails. The subclass Opisthobranchia contains several different groups of sea slugs, but the most diverse and spectacular members are the Nudibranchia. Members of the Nudibranchia also happen to be the most commonly sea slugs offered for sale in the pet trade, so it is easy to see why some people might make the mistake of considering 'nudibranch' and 'sea slug' as synonyms. In addition to the order Nudibranchia, the other common orders of sea slugs are the Sacoglossa (also known as the ascoglossans), the Anaspidea, the Notaspidea, and the Cephalaspidea (see Brusca and Brucsa 1990 or Ruppert and Barnes 1994 for more detail). Because each of these groups have the same taxonomic status (they are all orders), it is technically incorrect to refer to a sacoglossan or cephalaspidean as a 'nudibranch' and the generic term 'sea slug' is preferable if you do not want to use the proper name of opithobranch.
I make this distinction for several reasons. First of all, I think it is important to use the right names for the animals in our tanks to avoid confusion. Second, there are actually some researchers who do not think that all of these sea slug orders are really all that closely related to one another, and if those people are right (i.e., different snail ancestors have converged on a slug-like body and they are not all closely related to one another), then there may be at least as much divergence between the different groups of 'sea slugs' as there is between fish and mammals! People would never lump all fish and mammals under a single common name, and it is one of my pet- peeves that it is done with sea slugs. However, it is more than simply a pet- peeve of mine: the biology, life-style and aquarium care of groups that are this distantly related is almost guaranteed to be very different from one another. If no other reason, the similarities among closely-related, and the differences among distantly-related, groups should be sufficient reason for you to care about the proper classification of these animals.
Finally, and most importantly, there are literally thousands of species within the Nudibranchia, but every one of them that have been studied to date are exclusively predatory (i.e., they feed on only animal matter, not algae). Because all known members of the Order Nudibranchia that have been studied to date prey entirely on animal matter, and many of them are so specific in their diets that they are able to recognize only a single species or two of prey. If that prey (which is usually a sessile invertebrate such as a cnidarian, bryozoan, tunicate or sponge) is not available within the aquarium, then the animal will simply wander endlessly around the aquarium searching for prey, and slowly digesting itself until it can no longer sustain its metabolic needs and finally starves to death (this is usually a matter of one to several months for a well-fed and healthy animal that is introduced to an aquarium). Given these specific food requirements, none of the nudibranchs can be considered either easy to keep or particularly reef friendly. I will discuss these animals in more detail in Part 2 of this article.
General Advice To Avoid Buying Sea Slugs
First of all, I have to say that unless you are able to determine the species of a sea slug and its exact requirements in captivity from a reliable source, I would always recommend against buying any of these animals. In their book Reef Invertebrates, Anthony Calfo & Bob Fenner (2003) have an entire section devoted to opisthobranch molluscs and discuss the selection, care and feeding of many of the species offered for sale in the hobby. They discuss the members of these groups in more detail than is possible in a short article here. Julian Sprung (2001) also has some very nice photographs and descriptions of some of the more common sea slugs to be sold in the hobby in his book, Invertebrates. If you don't have one of these books, and are unsure of the identification of the animals offered for sale in your local shop, ask your dealer to show you their copy of the books to compare. Although there are some spectacular pictures in these books, there are only a few of the more than 3000 known species mentioned, and your best resource for up-to- date information about the taxonomy and biology of an unidentified sea slug will probably be Bill Rudman's Sea Slug Forum or Michael Miller's Slug Site. Despite the fact that there are many beautiful sea slugs out there, unless you know exactly what you are buying and their feeding requirements, the rule of thumb is to always avoid ever purchasing any sea slug. Sadly, the vast majority of sea slugs are beautiful, but virtually impossible to care for animals in the aquarium.
OK, so having said that, I also wanted to give you a little more encouraging news. If you are really interested in maintaining a sea slug in your reef tank, there is a decent possibility that an herbivorous species from one of the other Orders (such as Sacoglossa or Anaspidea) could actually be kept successfully in an appropriate aquarium. Whenever I am asked about sea slugs in general or nudibranchs in particular, I almost always give a blanket recommendation that they should be avoided. I stick by that still unless you know a reasonable amount of detailed information about the biology of animal you intend to purchase, and what are the specific food(s) that the animal requires. I am not going to change that advice here - I still believe that no one should ever consider buying one of these animals unless they know exactly what the animals require to thrive in captivity.
I know that I am repeating myself, but I am belaboring this point because almost all sea slugs have extremely specific food requirements, and they will inevitably starve to death while searching if their preferred prey is not available. Frequently the only nudibranch sea slugs that tend to thrive in reef aquaria are those that are considered pests, such as the soft coral specialistpredators of the genus Phyllodesmium , whose members feed on corals such as Sarcophyton, Briareum and Xenia. I will discuss this in more detail in Part 2 of this article.
Given that the tank full of slugs that got me started on this article were both non-nudibranch sea slugs, I figured that I would start out with a discussion of those two groups. The Anaspidean sea slugs, often called "sea hares" or "sea bunnies" in the trade, get their common name from the two long ear-like extensions that apparently made them look like a rabbit to someone in the distant past. I can't honestly say that I see that resemblance, but maybe you can. Unlike nudibranch sea slugs, that lack a shell, the anaspideans have a reduced shell plate. This shell plate is not always present, and even when it is, the shell is internal, making it seem like they have no shell to most people. In general they also have wing-like extensions that fold back over the reduced shell, and which can be used for short bursts of awkward swimming if the slug is in a hurry. Some of these animals can get very large: in his book Pacific Coast Nudibranchs, Dave Behrens reports that the largest sea hare, Aplysia vaccaria, can grow to about 1 metre long and can weigh nearly 14kg!
Most sea hares tend to protect themselves by the exudation of a bright purplish ink when they are alarmed. There remains some debate about the exact function of the ink, but research suggests that the purplish ink is a concentrated secretion of defensive compounds isolated from their algal food (primarily fleshy red or green algae or cyanobacteria, depending on the species). The best data support the hypothesis that the ink functions for defensive purposes. Observed behavioral changes from potential predators included: (1) bristle erection by fireworms; (2) increased mucus production by other opisthobranchs; (3) reduced feeding behavior, increased grooming behavior, and temporary pauses in heart and scaphognathite beating by crabs; (4) reduced or increased activity by cryptic and exposed sea urchin species, respectively; and (5) rapid swimming by fish (Carefoot et al. 1999). When threatened, these animals apparently eject their defensive ink to confuse and irritate any potential predators while they make a hasty retreat. This ink is not particularly toxic (although it is certainly a stressful irritant to most species), but it is a potential hazard for pollution and stress if not removed from the aquarium. Fortunately, the ink is easily removed with the use of activated carbon, or by water changes.
Like the sacoglossans I will discuss later, the anaspideans are exclusively herbivorous, but the degree of diet specialization depends on the species in question. Some species (e.g., Bursatella ) are specialists on microalgae growing on the surface of sand grains, and actually ingest mouthfuls of sand to digest away the cyanobacteria and diatoms growing there. In this sense, they function very much like the popular sand-mopping sea cucumbers known as Tiger-tails (e.g., Holothuria hilla ). Obviously an animal such as this will be better suited to a tank with an established deep sandbed than one with a bare bottom. Also, Bursatella would be a much better choice for inclusion in an aquarium than any of the specialist predators that are frequently offered for sale in the hobby, but unfortunately I have never seen these animals in any petshop.
Chemical Defenses Of Sea Hares
These sea hares are of interest to aquarists, because they are among the few species that tend to feed on these highly distasteful nuisance algae that are avoided by the majority of the animals in our aquaria. It was previously thought that sea hares fed specifically on these distasteful prey species to collect and sequester the defensive compounds that prevent other animals from feeding on these algae. However, more recent research suggests that sea hares have generic mechanisms for sequestering algal metabolites rather than mechanisms that are tightly linked to particular compounds, that these mechanisms do not differ dramatically between species, and that sequestered secondary metabolites are not located optimally for defense (Pennings & Paul 1993). Furthermore, bioassays found that the distasteful compounds located in the skin of sea hares was produced by the animals themselves and not derived from their algal diet (Pennings et al. 1999). Researchers found that algal- derived secondary metabolites play a role in chemical defense of sea hares via the ink, but are not responsible for the unpalatability to potential predators on the reef of the skin or eggs (Pennings et al. 1999). They concluded that accumulation of dietary-derived metabolites that function as defensive compounds in the digestive gland may occur primarily to detoxify a chemically rich diet, rather than to deter predators (Pennings et al. 1999).
Captive Breeding Of Sea Hares
The animals that were in the shop in Edmonton were a species of Aplysia. As I said previously, I was encouraged to see these animals for sale because this slug happens to be one of the few that is well-suited to aquarium life. Their diet is known, and most sea hares are generalist grazers on filamentous and large, fleshy red or green algae. The techniques to raise them are well-known; in fact Aplysia was one of the first opisthobranchs to be reared from eggs to reproductive adults in captivity (Kriegstein et al. 1974). That knowledge was then scaled up to large-scale mariculture and Aplysia mass production began in 1983. The NIH/University of Miami National Resource for Aplysia Facility now ships over 25,000 animals a year from their facility. Despite the fact that all of these factors suggest that these animals should be ideally-suited to the aquarium trade, my enthusiasm was quickly dampened when I realized that there were no algae in the entire shop that could be fed to these magnificent animals. Furthermore, very few of the sea slugs for sale in the aquarium hobby are captively-bred at this time, because all the Alpysia raised at the University of Miami are sold to researchers rather than hobbyists.
Furthermore, depending on the species, feeding these animals in captivity can be more difficult than it sounds. For some sea hares, the dietary requirements depend to a large extent on the age, because like many marine invertebrates, they can show marked size-related shifts in their feeding preferences (Pennings 1990). Thus, even knowing the species of the slug, and the fact that it is herbivorous may not be sufficient to provide a suitable environment for the animal if you were unaware of the shifting requirements of the animal as it grows. The biology and feeding preferences of these animals have been the focus of a great deal of research, and interested readers can find more information from the compiled list of research papers on sea hare feeding posted on the Sea Slug Forum here.
As I mentioned above, the mating systems of these slugs is well-suited to culture in captivity. Aplysia are simultaneous hermaphrodites that mate repeatedly in the field acting as both males and females (Angeloni et al. 2003). They lay gelatinous egg masses from which free-swimming larvae hatch out and spend several weeks feeding in the plankton before they become competent to settle and metamorphose into the adult body form. For this reason, they are not likely to reproduce in the aquarium. However, it is obviously possible to supply the hobby demand for these animals if the University of Miami is able to sell tens of thousands of slugs they raise each year. For the home aquarist, culture of larvae is more difficult, because they produce long-term feeding larvae although the techniques to raise planktonic marine larvae are well-established (e.g., see Toonen 1996 for more info). One of the major hurdles to face when culturing animals such as these is knowing what cues they respond to when they settle. Fortunately Aplysia tend to metamorphose in response to a wide range of macroalgae (particularly reds, see Pawlik 1988) and compared to their nudibranch cousins, they are relatively easy to breed and grow in captivity. Likewise, their preferred food item, the red alga Gracilaria, is also relatively easy to culture and is readily available in the trade.
Hopefully captive-bred sea hares will be more widely available to aquarists in the near future.
Lettuce Sea Slugs
The slugs identified in the shop as the "Lettuce Nudibranch" were most likely the sacoglossan sea slug Elysia crispata, which was known as Tridachia crispata until it was recently renamed (Gosliner 1995). Even after Gosliner's recent re-examination of the genus, E. crispata remains one of the most variable sea slugs in the world, and there remains some debate whether this is a consequence of extreme individual variability or poorly understood taxonomy. Some experts (such as Bill Rudman and Kathe Jensen) appear to believe that these slugs are probably members of a morphologically-similar species complex for which the taxonomy (and specific biology) is not yet understood.
In support of the hypothesis that E. crispata may be a species complex rather than an extremely variable single species, there is a considerable amount of variation among how these animal appear to behave in both the aquarium and the wild. There are reports of differences in feeding preference and reproductive mode that may accompany the variation in color seen among different varieties of this slug. If true, then there are some major differences in the aquarium requirements and proper care of the various color morphs that have been reported. That would also make it more likely that there are at least a couple of different species within this complex that are waiting for a proper name.
From the hobbyist perspective, however, even more important than knowing the real species name for these animals, we need to understand what exactly the natural history of each group is, what they feed on, and what are their specific biological requirements within the aquarium? Only after we understand these outstanding questions and the specific site of origin for the animals in the hobby can we reliably predict what they need to survive in the aquarium.
Biology of Lettuce Slugs
As I mentioned above, these animals are not nudibranchs, either. They are a Sacoglossan sea slug that makes a living by sucking the contents from the living cells of their prey. Fortunately for the adventurous aquarists among us who really want to keep a sea slug, some of the members of these non- nudibranch groups (especially sacoglossan, cephalaspidean & anaspidean) sea slugs may actually be reasonably well-suited for certain aquaria.
In general, sacoglossans are among the more colorful of these slugs, but none of the groups are typically as colorful or striking as their nudibranch cousins, however, there are some spectacular exceptions that could be perfectly suitable for a reef aquarium. For example, some species, such as Lobiger are herbivores that could actually provide a useful service to aquarists. Although I say these animals are herbivores, there is really a little more to it. They are actually specialist feeders that have a modified radula (the hardened ribbon of teeth used by most molluscs to feed) that can "surgically" remove the contents of individual algal cells and allow them to digest the cellular contents while saving the choloroplasts (the photosynthetic organelles plants use to produce energy from sunlight) to incorporate them into their own body. By stealing these photosynthetic organelles to use for a time, the slugs are able to take advantage of their function for a short time much as they did in the plant from which they were removed. This amazing ability actually allows these animals to generate some of their own food by making themselves "solar-powered sea slugs!"
In this case, the slugs that I saw in the petshop, Elysia (formerly Tridachia ) crispata, happen to fall into the "solar-powered sea slug" category. That was one of the reasons why I was happy to see these animals being offered for sale in the local petshop. Unlike their carnivorous cousins the nudibranchs, the sacoglossans (of which E. crispata is a member) are largely herbivorous, and you could think of them as sort of a marine mosquito for algae. I know that sounds a little odd, but how else do you describe an animal that makes a living sticking a fine tube into their host and carefully sucking out their fill of the contents before moving on?
Most of the lettuce slugs imported for the hobby appear to be the form that specializes on the nuisance 'hair algae' Bryopsis. However, as I mentioned above, there are reliable reports of variation in feeding preferences and reproductive modes reported among the various color morphs, and some lettuce slugs will completely ignore Bryopsis in the aquarium. For example, Elysia collected from the Great Barrier Reef appear to specialize on the alga Chlorodesmis rather than Bryopsis (Hay et al. 1989). Furthermore, some slugs within this species appear to be direct-developers that reproduce well in aquaria, whereas others produce planktonic larvae that do not generally survive in the aquarium. Given this degree of variability, and the fact that researcher cannot currently tell these things apart in a reliable manner, it is obviously going to be very difficult for you as a hobbyist to make sure that you know what you're getting when you buy one of these animals in the local pet store.
As I mentioned earlier, the Lettuce Sea Slug ( E. crispata ) is among the most variably colored sea slugs in the world, ranging from solid pale olive to individuals with brilliant yellow, red & blue highlights to solid electric blue animals, and it seems likely that additional study will reveal that some of this variation is a result of there being multiple species in this complex. However, other species in the genus, such as Elysia viridis, are quite cryptic, resembling their food (primarily the algae Codium, Bryopsis and Halimeda ) so much that most people would miss them even after they were told that there were sea slugs in the aquarium!
Using Lettuce Slugs For Algae Control In The Aquarium
Although these sea slugs are often sold as a means to eradicate nuisance algae from reef tanks, the simple truth is that they will rarely perform that task. Very few sea slugs have been shown to control population density of their prey in nature, and efforts at biological control often have unexpected results (reviewed by Secord 2003). In fact, some experiments at using sea slugs for biological control have had exactly the opposite effect that was expected. For example, in an effort to control the spread of the invasive alga Caulerpa taxifolia researchers experimented with the specialist predator sea slug Lobiger serradifalci. Instead of destroying the algal fronds as the researchers hoped, the slugs actually fragmented them into tiny living portions that were each capable of regenerating (Zulijevic et al 2001). Thus, the introduction of L. serradifalci may actually facilitate the spread of C. taxifolia rather than controlling it (Zulijevic et al 2001). That is pretty much the exact opposite effect that we would want for a biological control agent!
Despite our hopes for using sea slugs to eradicate nuisance species in the aquarium, I doubt that most will be any more successful at accomplishing that role in the aquarium than they are in nature. Lettuce sea slugs will definitely feed on Bryopsis, but they will rarely provide a means to completely eradicate this nuisance alga from an aquarium. Unless they are introduced in very high density, they have little effect beyond drastically slowing the growth of the algae. In fact, if you manage to introduce enough slugs to actually eradicate the algae from the tank, they will promptly starve to death once the algae on which they feed has been eliminated. However, the presence of a specialist sea slug (such as E. crispata feeding on Bryopsis, E. viridis feeding on Halimeda, or L. souverbiei feeding on Caulerpa ) will often slow the growth rate of the alga to the point that they are less likely to become problematic in a reef aquarium and are more easily controlled by a reasonable effort from the aquarist. Almost any of these slugs would be infinitely more appropriate for the average reef tank than any of the true nudibranch species I will discuss next time.
Chemical Defenses Of Lettuce Slugs
Another interesting note about Elysia is that the slug appears to be well defended via distasteful chemicals (Hay et al. 1989). The source of the antifeedon chemistry in these slugs is not well-studied, but they appear to sequester a distasteful compound called chlorodesmin from this alga that provides some level of defense to the slugs. However, that cannot be the complete story, because food that contained extractions of the compounds found in Elysia (including the chlorodesmin) was readily eaten by wrasses of the genus Thalassoma, whereas the slugs themselves are not (Hay et al. 1989). On the other hand, another common sacoglossan sea slug from the region, Cyerce nigricans sequestered metabolites from Chlorodesmis algae in relatively small amounts, was much more distasteful to fishes: Cyerce were never attacked by fishes and incorporation of the crude organic extract of this slug into food strongly deterred feeding by wrasses in laboratory assays (Hay et al. 1989).
Fortunately, these chemical defenses tend to be effective only at preventing the slugs from being eaten, and not as a toxin that may affect tankmates. Provided that your pets do not try to eat your Lettuce Slug, there is no risk of keeping one in your tank.
Conclusion & Summary
Well, that is a bit of a diatribe that was started from my recent trip home. I hope that I have conveyed that anyone but experienced hobbyists should studiously avoid ever purchasing a sea slug for their tank. Remember that unless you know the specific identification and requirements of the animal that you are looking to purchase, it is a good idea to leave them be, because if you don't know the species, then just by chance, you're most likely to get an animal for which it will be impossible to provide appropriate care. I also hope that I have given the adventurous and experienced aquarists who read this a better idea of what to expect if you decide to ignore my advice and get one of these animals for your tank anyway. So, for those of you that have a good supplier who provides you with detailed information about the animals you plan to purchase, then you may be able locate one of the species of non-nudibranch sea slugs that has a decent chance of surviving (and maybe even thriving) in captivity…
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