Hydatina physis is a common tropical cephalaspidean sea slug that is commonly called a “paper bubble” because of their lightly calcified shells. This slug is a highly specialized predator, feeding only on cirratulid polychaete worms.

Last time, I explained what sea slugs are, and why the common name “nudibranch” should not be applied blindly to the over 3000 species of opisthobranch molluscs (for more detail, consult a good invertebrate textbook such as Brusca and Brucsa 1990; Ruppert and Barnes 1994). I also discussed a couple of the non-nudibranch opisthobranch sea slugs that can be reasonably kept in an aquarium. For the most part, I have always advised against the purchase of any sea slugs because it is not common for many hobbyists or retailers to differentiate among the groups that may be suitable for some reef aquaria (some anaspideans, cephalaspideans or sacoglossans) and those that are almost certainly not (virtually all nudibranchs). In the first part of this article, I discussed some basic biology about the anaspidean and sacoglossan sea slugs. In this article I will focus specifically on the nudibranch sea slugs.

Nudibranch Sea Slugs

With only a couple of exceptions (see below), nudibranchs are completely inappropriate animals for home aquaria. Most large public aquariums have difficulty in keeping these animals alive for any length of time, and their success rate in home aquaria is abysmal. There are a variety of reasons for this low success rate, but their delicacy and highly specific feeding needs are two of the primary hurdles to keeping them in aquaria. Combine those with a lack of information about the basic needs of the vast majority of species (both in the scientific and hobby community), and you have a recipe for disaster.

Hypselodoris bullocki is often mislabelled as a “reef safe herbivore” ideal for aquarium clean-up crews. Quite to the contrary, this nudibranch is, like every true nudibranch ever discovered, a highly specific predator. There are several color morphs that each appear to feed on only one or two species of sponges.

OK, so which opisthobranch sea slugs are actually nudibranchs? Well, there are four suborders of Nudibranchs, although only 3 would ever be likely to appear in a pet shop: the Dendronotacea, Aeolidacea and Doridacea. The dendronotids are the smallest of these groups and are characterized by a cup-like sheath surrounding their rhinophores (the “antennae”). With few exceptions, dendronotids feed exclusively on cnidarians, and are most commonly found by aquarists as unwanted hitchhikers on Xenia or Sarcophyton. These animals are one of the exceptions to the rule that most nudibranchs starve to death in captivity. Although these animals often do reasonably well in the aquarium when accidentally introduced with the corals on which they feed, they can rapidly consume their coral prey in the aquarium (e.g., see Figure 8 in Ron Shimek’s article here). Thus, these slugs are not generally something that you want to discover in your tank, let alone intentionally add! Aeolids are the second largest group of slugs, and are most easily identified by not belonging to one of the other two groups I am describing… Aeolids typically have well developed cerrata (finger or feather-like extensions along the back of the slug which contain a branch of the digestive system) in which many species store the unfired nematocysts (stinging cells of cnidarians) from their prey to use in their own defense when attacked. Obviously if they collect and store the stinging cells of their cnidarian prey, the nudibranchs must actively feed upon these animals. I will come back to this below. The aeolids are all predatory, and the vast majority of them feed on hydroids (although some also prey on other opisthobranchs or their eggs, corals, gorgonians, sea anemones, bryozoans or tunicates). The final group, the dorids, are the largest, with more species than all the other nudibranch groups combined. They are most easily distinguished by the presence of a ring of gills around the anus at the back of the slug. These are some of the most spectacularly colored and commonly photographed of all opisthobranch molluscs. There are three major groups of dorid nudibranchs (based on their morphology) and once again, all are specialist predators on other invertebrates. Two of the dorid groups are primarily sponge predators, while the third feeds primarily on bryozoans and tunicates.

General Advice To Avoid Buying Sea Slugs

I need to reiterate a point I made in the last article just to make sure that the message gets through – even if you didn’t read Part 1. 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 in the aquarium.

Micromelo undata is another spectacular cephalaspidean sea slug. The lightly calcified shell is obvious in this photo, but little is known about their feeding habits. These animals typically live in sandy habitats in which they burrow periodically for protection which keeps their shell clean from fouling.

However, as I mentioned last time, there are some sea slugs that are reasonably well-suited to life in the aquarium. In fact, there are now some species of captively-raised sea slugs now available from vendors such as Inland Aquatics, IndoPacific Sea Farms, or Stockly’s Aquarium. You should also keep in mind that all opisthobranchs are relatively short-lived, and you may be paying a lot of money for an animal that will only live several months under the best of conditions! If you’re determined to add a sea slug to your aquarium, however, I’d recommend that you look into one of the captively-raised species – not only will you get an animal well-adapted to life in an aquarium, the supplier will be able to tell you exactly what conditions are required to keep the animals alive! And, because their requirements are known and can be met, you may find that one of these species actually thrives in your aquarium and are able to reproduce.

Reef-Safe Herbivores??

If you do a search for “reef safe nudibranch” on the web, there are literally hundreds of hits, but the vast majority of these are either gravely misinformed or trying to sell you a line. For example, I found several sites that listed “algae eating purple and yellow nudibranchs” for sale that claimed these slugs were “incredible algae eaters that will not harm any invertebrates or corals!!!” Well, that information quite simply wrong. First of all, no dorid nudibranch is herbivorous, and there is a page of the Sea Slug Forum devoted to dispelling some of the common aquarium trade misinformation such as this. Quite simply, although dorids are the most diverse group of opisthobranchs, they are also among the most specific feeders of all molluscs. The animals studied thus far are almost without exception specialist predators that require one or at most a few species of specific prey to survive, and in most cases the animals will not even recognize perfectly suitable alternatives as food! In any case, there are no true nudibranchs yet discovered that are herbivores, so this is either a big misunderstanding on the part of these suppliers, or (and I hope this is not the case!) a deliberate ploy to get consumers to buy a beautiful but completely inappropriate animal before it starves to death.

These lovely purple and yellow nudibranchs are Hypselodoris bullocki. This species used to belong to the genus Chromodoris, but has recently been reclassified as Hypselodoris. Contrary to the reports from many suppliers, these beautiful little slugs are sponge specialists, and many accept only a single sponge species as suitable prey. In fact, it turns out that these nudibranchs don’t even recognize other sponges as food, let alone algae, and the claim that buying these nudibranchs will add “beautiful color and take care of algae at the same time” is absolutely false and misleading. Aside from wasting your money as a consumer, it is terrible to sentence these animals to a slow death via starvation because even well-intentioned aquarist have been mislead into providing inappropriate food for their animals. Unfortunately, in this case, scientists are not even sure about the exact species of sponge on which the various color forms of this nudibranch feed. There are reports of H. bullocki feeding on some sponges of the genus Halichondria, while others report that they feed exclusively on Dysidea or Aplysilla. Sadly, no one knows whether the reports of these slugs being found on a given sponge represent actual feeding, or whether the slug will accept any other sponge species even if the feeding reports are all accurate. Given that most reef sponges are considered more difficult to keep than even these delicate sea slugs, the chances that you have an abundant supply of suitable food on hand to feed these guys is pretty slim. However, biology is full of exceptions to the rule: Bill Rudman reports that the endemic species of Rostanga in New Zealand, by some freak of biochemistry, was found to prefer an introduced sponge from Europe over all the native New Zealand sponges that it normally fed on.

Phyllodesmium is a common nudibranch predator on a variety of soft corals, and often mimic their preferred prey. The cerrata on the back of this nudibranch have an amazing likeness for the polyps of the coral Xenia, on which this slug feeds exclusively.

Regardless of whether the ultimate cause of the misinformation at the retail level is a lack of education or a deliberate ruse, the ultimate message is that you as a hobbyist should never consider purchasing one of these animals. Unless you know specifically which slug you are buying and what their preferred prey are (e.g., the Aiptaisa predator slug, Berghia verrucicornis ), you should avoid any true nudibranch sea slugs (no matter how badly you want one), because their feeding requirements are so specific that very few people can keep them alive in aquaria (even large public aquaria with huge feeding budgets generally avoid them!). Even well-intentioned suppliers and advanced hobbyists can be easily misinformed about the animals they are trying to keep, and our knowledge of sea slug taxonomy and biology is still sadly incomplete. Add to that fact that there are more species of nudibranchs than there are of all of the remaining sea slug orders combined and that many are undescribed or their feeding biology is unknown, and it becomes very likely that a random sea slug in your local shop is going to starve to death in your tank if you were to buy it.

Some Sea Slugs That Can Thrive In Captivity (Whether You Want Them Or Not!)

OK, I have to come clean and admit that there are some true nudibranchs that can do well in aquaria. Most of these exceptions are actually considered pests in the aquarium. Almost all dendronotid sea slugs feed exclusively on cnidarians, and these animals are most commonly found by aquarists as unwanted hitchhikers on Xenia or Sarcophyton in the reef aquarium. The most common nudibranchs to prosper in the aquarium tend to be members of groups such as Phyllodesmium and Doto which prey exclusively on aquarium corals such as Briareum, Xenia, Carijoa and Sarcophyton, to name a few. It is becoming more-and-more common that small aeolid nudibranchs have been found consuming prized colonies of SPS corals in reef tanks. Many of these slugs are tiny (less than 1 centimeter in length) and are very easy to overlook. Unfortunately for hobbyists, these animals appear to some of the few sea slugs that reproduce well in the aquarium, because they lay tiny egg masses that house the babies through the entire larval stage until they hatch as crawl- away juveniles. This is a particular worry because once they are introduced to dealer’s tanks, they can be easy spread into the home aquarium by hitchhiking either as eggs or well-camouflaged adults. In general these slugs are not something that you want to discover, let alone intentionally add to your tank!

There are some cases where we try to take advantage of the specific prey habits of these slugs for biocontrol in the aquarium. There are a variety of species that feed specifically on aquarium pests and are therefore sold as a remedy to remove these pests. One of the most popular such species, Berghia verrucicornis, is being actively cultured for addition to aquaria plagued by the proliferation of those pesky glass anemones, Aiptasia (e.g., Carroll and Kempf 1990). Unfortunately for both the animals and the aquarist who has spent a considerable amount of money on them, most slugs are quite sensitive and frequently die shortly after being introduced to a new aquarium. Another concern is that they often fall short of the lofty goal of eradication because of their extreme diet specificity. These animals often reduce the population of the pest species in the tank to the point that they starve. Sadly, they usually can’t find those last few pests in the tank before they succumb to starvation and therefore pack it in just short of complete eradication. After the nudibranchs starve to death, the pests are again able to grow unchecked and the plague continues.

But there is at least one cool exception to that rule. There is a filter- feeding dendronotid nudibranch, Melibe leonine, that can be maintained on feedings of enriched brine shrimp in aquaria (Monterey Bay Aquarium has a magnificent display of these animals)! Unfortunately for the average aquarist at home, this species is temperate, and can only be maintained in a cool water aquarium representative of their natural range from Alaska through the Gulf of California.

Dorid Nudibranchs – The Spanish Dancer

The dorids are the largest group of nudibranch sea slugs, with more species than all the other nudibranch groups combined. They are most easily distinguished by the presence of a ring of gills around the anus at the back of the slug. There are three major groups of dorid nudibranchs (based on their morphology) and two of these groups are primarily sponge predators, while the third group feeds primarily on bryozoans and tunicates (Brusca and Brucsa 1990; Ruppert and Barnes 1994). Although dorids are the most diverse group of opisthobranchs, they are also among the most specific feeders of all molluscs. The animals studied thus far are almost without exception specialist predators that require one or at most a few species of specific prey to survive, and in most cases the animals will not even recognize perfectly suitable alternatives as food! Many species are specifically adapted to deal with the defensive chemistry of their prey and often take
advantage of those chemical defenses for their own defense as well.

For example, the Spanish Dancer ( Hexabranchus sanguinensis ) is a nudibranch that is often offered for sale in petshops. Like most of its brightly colored relatives, the Spanish Dancer defends itself with distasteful chemical compounds (antifeedon chemical defenses) that it derives from its prey. Like the vast majority of dorids, which prey specifically on one or a few species of invertebrate prey, H. sanguinensis is a specialist predator that consumes only sponges. Like many other nudibranchs, these animals concentrate and/or modify distasteful chemicals from their prey and incorporate them into their skin to repel attacks by their own predators. Many (if not most) interactions between feeding by nudibranch species and their prey are not particularly well studied, but H. sanguinensis happens to be one of those that is well-known (which is why I picked it as an example). The defensive compounds (trisoxazole macrolides, if you are into such things), are derived from the black sponge ( Halichondria sp.) on which they primarily prey. However, it turns out that H. sanguinensis is one of the lesser specialized species of dorid in this respect, and although they will only eat sponges, the list of sponges that they will consume includes Ancorina, Callyspongia, Cliona, Haliclona, Adocia, Mycale, Zygomycale, Pachastrella, Paraesperella, Petrosia, Stelletta, Xestospongia, Halichondria, and Leucetta (Francis 1980; Pawlik et al. 1988).

Halichondria sponges have a variety of nasty defensive chemicals (e.g., (Yamagata et al. 1990), that are highly effective in protecting the sponge from being eaten on the reef (Dunlap and Pawlik 1996). However, the specific defensive chemicals isolated from the flesh of the nudibranch are not actually found in the sponge. Instead, the defensive compounds used by this slug are modified directly from sponge products during the digestion process after the sponge is eaten (Pawlik et al. 1988). The Spanish Dancer then incorporates the toxic compounds derived from the sponges on which it feeds into its egg masses as well as its own brightly-colored flesh. In this case, the defensive chemicals also happen to have an antimicrobial function, and are thought to protect the eggs from both predation and infection simultaneously (Pawlik et al. 1988). Like the other nudibranchs, these animals are simultaneous hermaphrodites, and the eggs which they produce hatch into feeding veliger larvae which feed on phytoplankton for 4-6 weeks before metamorphosing into tiny adult slugs. The biggest problem is that the chemical cue to which these larvae respond and on which they metamorphose is unknown. People have hypothesized that the cue is associated with the sponge species on which they prey, but no one has yet managed to isolate the metamorphic inducers for this (or most other) opisthobranch species (Pawlik 1992). So even if it were possible to breed them and raise the larvae, there would be no way to induce them to settle, so they would never turn into adults.

Having said that, obviously any animal you see in the pet shop is wild-caught, and despite the impressive list of diverse sponges on which this species will feed, you are unlikely to be able to find a reliable supply of any of them in your local shop. Thus, adding one of these beautiful animals to your aquarium guarantees that it will be doomed to slow starvation, no matter how well established or diverse. Sadly, by buying one of these animals, you also encourage the pet shop to bring in more to sell. That is bad because even the most experienced aquarist will be unable to feed it properly (even professional aquariums such as Monterey Bay and Waikiki have a difficult time keeping nudibranchs well-fed and healthy), and they will inevitably die in home aquaria.

Aeolid Nudibranchs

Aeolids are the second largest group of nudibranch sea slugs after the dorids, and are most easily identified by not belonging to one of the other 2 groups I have described above (for a more detailed explanation, see Brusca and Brucsa 1990 or Ruppert and Barnes 1994). Aeolids typically have well developed cerrata (finger or feather-like extensions along the back of the slug which contain a tubular branch of the digestive system). These cerrata function jointly as gills, and for the digestion and assimilation of their prey. The wonderfully unique feature of these gills that that most species use their cerrata to store the unfired nematocysts (stinging cells of cnidarians) from their prey to use in their own defense when attacked. These nudibranchs rip off mouthfuls of their cnidarian prey, but have figured out how to prevent the nematocysts from their food from discharging and stinging them. Instead, the nematocysts are collected and stored in the tip (often very obvious and bright white) of the cerrata of the nudibranch for their own defense. The slugs are so good at providing suitable storage for these stolen nematocysts (called kleptocnidae) that even immature isolated nematocysts are capable of maturing and being ready to fire in defense of the slug when attacked. Nematocysts don’t last forever, though, and as the cells age and become incapable of firing their defensive threads, they are expelled from the tips of the cerrata and digested.

Once again, the aeolids are all predatory, and there are no herbivorous members that could be considered tank cleaners. The vast majority of aeolid species feed on hydroids, corals, gorgonians, and sea anemones (although some also prey on other opisthobranchs or their eggs, bryozoans or tunicates).

Summing Up

All true nudibranchs should be considered “very difficult to keep” and even experienced and advanced aquarists should avoid purchasing them. Unless you know the identity of the slug to determine the specific requirements of the animal and its prey species, and are able and willing to provide for both, no one should ever consider buying a nudibranch for their reef tank. In my last column I discussed the general biology of some of the sea slugs that are more likely to be suited to life in the aquarium than the nudibranch sea slugs discussed here. Granted, there are some exceptions, such as Berghia verrucicornis which I mentioned above. However, there are only a handful of such species from the more than 3000 described opisthobranch sea slugs out there. The simple fact is that the vast majority of nudibranchs should be avoided in the aquarium trade.

Even in the case of well-known nudibranch species where we know the specific food requirements of the animal (such as the Spanish Dancer discussed above), the problem remains that most preferred prey species are rarely available in the hobby. Obviously, we need both the animal and its food to care for it in captivity, and the sale of one without the other is deceitful and irresponsible. Sadly, even this example in which the sale of the Spanish Dancer is frequently done without proper consideration of the prey availability is actually better than most nudibranchs. At least someone who is really determined to keep a Spanish Dancer knows in advance what is needed to attempt this feat. That is not true for the vast majority of these amazing molluscs; not only is the prey unavailable, but in the vast majority of cases, it is completely unknown.

Finally, before I sign off for this article I have to get on my soap-box and say that not all shops have staff that is qualified to provide buyers with accurate information about the animals they sell. Keep in mind that the shops are not just hurting your chances of keeping an animal if they provide blatantly false information about their care (such as the “reef safe herbivores” section above or the examples I mentioned last time that got me started on this series). If that kind of misinformation is indicative of the general quality of information a given supplier provides you, then you probably ought to look around for a shop that provides you with more reliable information on your future purchases. Personally, I’d much rather hear “I don’t know” from the shop employees than something that is totally inaccurate! At least “I don’t know” can be followed up with research that gives the animal a chance of survival, but blatantly misleading information is essentially a guarantee that
you will fail with your new animal. Only by letting the dealers take the hit with animals dying in their tanks and never being sold can we ‘cast our vote’ to stop the spread of misinformation and the importation of inappropriate animals to the pet trade…

References:

1. A link to all my aquarium articles
2. Brusca, R. C., and G. J. Brucsa. 1990. Invertebrates. Sinauer Associates, Inc, Sunderland, Mass.
3. Calfo, A. & R. Fenner, 2003. Reef Invertebrates: An Essential Guide to Selection, Care and Compatability. Reading Trees, Monroeville, PA.
4. Carroll, D. J., and S. C. Kempf. 1990. Laboratory culture of the Aeolid nudibranch Berghia verrucicornis (Mollusca, Opisthobranchia): Some aspects of its development and life history. Biological Bulletin 179:243-253.
5. Dunlap, M., and J. R. Pawlik. 1996. Video-monitored predation by Caribbean reef fishes on an array of mangrove and reef sponges. Marine Biology 126:117-123.
6. Francis, M. P. 1980. Habitat, food and reproductive activity of the nudibranch Hexabranchus sanguineus on Tongatapu Island. The Veliger 22:252-258.
7. Gosliner, T. M. 1995. The genus Thuridilla (Opisthobranchia: Elysiidae) from the tropical Indo-Pacific, with a revision of the phylogeny and systematics of the Elysiidae. Proceedings of the California Academy of Science 49:1-54.
8. Hay, M. E., J. R. Pawlik, J. E. Duffy, and W. Fenical. 1989. Seaweed- herbivore-predator interactions: Host-plant specialization reduces predation on small herbivores. Oecologia 81:418-427.
9. Paul, V. J. 1992. Explorations in Chemical Ecology: Ecological Roles of Marine Natural Products. Cornell University Press, Ithaca, N.Y.
10. Pawlik, J. R. 1992. Chemical ecology of the settlement of benthic marine invertebrates. Oceanographic and Marine Biology Annual Reviews 30:273-335.
11. Pawlik, J. R., M. R. Kernan, T. F. Molinski, M. K. Harper, and D. J. Faulkner. 1988. Defensive chemicals and the Spanish dancer nudibranch Hexabranchus sanguineus and its egg ribbons: Macrolides derived from a sponge diet. Journal of Experimental Marine Biology and Ecology 119:99-110.
12. Rudman, W. B. 1999 (May 6). Hypselodoris bullocki (Collingwood, 1881)
13. Page 1. Sea Slug Forum:http://www.seaslugforum.net/hypsbull.htm.
14. Ruppert, E. E., and R. D. Barnes. 1994. Invertebrate Zoology. Saunders College Publishing, Harcourt Brace Jovanovich Publishers, Orlando, FL.
15. Shimek, R.L. 2002. Naked…gills on snails. A Spineless Column. Reefkeeping 09/2002.
16. Sprung, J. 2001. Invertebrates: A Quick Reference Guide. Sea Challengers, Danville, CA.
17. Yamagata, K., Y. Yamagiwa, and T. Kamikawa. 1990. Synthesis of chiral long-chain α-hydroxy acids from L-ascorbic acid: Useful components for the synthesis of cerebrosides. Journal of the Chemical Society Perkin Transactions I:3355-3357.