I want to get a nudibranch for my tank. I just saw one at the pet shop, but I need to know some background on these things before I get one. I know that they are supposed to be difficult to keep, and I have read advice against adding them to a reef tank in Salt Solutions and elsewhere, but I still want to try. The local shop has a nudibranch right now (sorry I don’t know which one, but it’s the one on the fasTesT test kit box), and the owner tells me that it just cruises along the glass all day eating algae. I wanted to check exactly what these sea slugs eat and anything else you can tell me about their care and requirements before I buy it. I have seen many beautiful nudibranchs at various shops and think they look so cool – I *really* want one. Are they really algae eaters and are they suitable for a small (20g) reef tank? Anything you can tell me about them will be greatly appreciated. Thank you for your time and have a good day!
Let me start off by saying that I am very glad to hear that you are researching your purchase before buying it, no matter how badly you want the animal in your tank. That is great to see, and I encourage everyone to follow that example! My guess on the identity of the slug you saw in your local shop is Hypselodoris bullocki and it is indeed a spectacular animal – unfortunately with very few exceptions nudibranchs are a poor choice for a home aquarium, and the vast majority of them are doomed to slowly waste away as they slowly starve to death over a period of about 3-6 months. I'll come back to this animal and it's care in a while, but I want to clear some things up first.
First, let’s try to define exactly what you and I are each talking about here. Many people use the common name "nudibranch" to cover all the 1500-2000 species of opisthobranch sea slugs, which is simply incorrect. There are 5 primary orders of opisthobranch snails (Cephalaspidea, Anaspidea, Sacoglossa, Notaspidea & Nudibranchia), of which the nudibranchs are the most diverse and spectacular members, and therefore more commonly imported for the pet trade, but there are many other types of sea slugs in addition to the nudibranchs. Although many people use the common name “nudibranch” as synonymous with sea slug, it is technically inappropriate for members of the other orders of the opisthobranch sea slugs to be called nudibranchs. The reason I want to make this distinction is that there are actually a reasonable number of people who do not think that all these sea slug orders are that closely related to one another, and if these people are right that many different snail ancestors have converged on a slug-like body, then there may be as much difference between the different groups of “sea slugs” as there is between fish, birds and mammals! People would never lump all fish, birds and mammals under a single common name, and it is one of my pet-peeves that it is done with sea slugs. In case you didn’t notice, I always bristle a little at people lumping all of this amazing diversity under the name “nudibranch.” :)
In any case, because of this confusion, it is difficult for me to answer your question, and I am actually unsure what you are asking for information on here – sea slugs in general, lumped under the common name “nudibranch” or true nudibranchs? If you are asking are there any sea slugs that can survive in an average reef aquarium, then the answer is yes. For example, there are many sacoglossan slugs that specialize on various algae and do quite well in a well-established aquarium, and there are some cephalaspideans and anaspideans (sea hares) that will provide general grazing functions and should do very well in an established reef as well. There are also some true nudibranchs that can do well in aquaria, and one of the most popular species Berghia verrucicornis, are being actively cultured for addition to aquaria plagued by the proliferation of those pesky glass anemones, Aiptasia. In fact, there is even a filter-feeding dendronotid nudibranch, Melibe leonina, that can be maintained on feedings of enriched brine shrimp in aquaria! Some of these animals are quite capable of breeding in an aquarium, and some species of captively-raised sea slugs are now available from vendors such as Inland Aquatics (www.inlandaquatics.com
In general these other groups of slugs that are more suitable for a reef aquarium (the sacoglossan, cephalaspidean and anaspidean sea slugs) are not as colorful or striking as their nudibranch cousins, however there are exceptions. Some species, such as Elysia (formerly Tridachia) crispata specialize on the nuisance “hair algae” Bryopsis, and are among the most variably colored sea slugs ranging from solid pale olive to individuals with brilliant yellow, red & blue highlights to solid electric blue animals. Other species, 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! Some species, such as Lobiger souverbiei and E. crispata (mentioned above), could provide a useful service to aquarists in keeping their algae under control because 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 use for a time, making themselves “solar-powered sea slugs!” Although these slugs will rarely provide a means to eradicate nuisance algae from an aquarium, they can drastically slow the growth of various species (such as E. crispata feeding on Bryopsis or L. souverbiei feeding on the common macroalga Caulerpa) such that they are less likely to become problematic in a reef aquarium. Any of these slugs would be infinitely more appropriate for the average reef tank than a true nudibranch species.
If you really do mean a true 'nudibranch' rather than 'sea slug' in the more general sense in your question, however, then with few exceptions (such as the aeolid Aiptaisa predator, Berghia verrucicornis, I mentioned above) the answer is almost certainly no. There are 4 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 which they can rapidly consume. In general these slugs are not something that you want to discover, let alone intentionally add to your tank! Aeolids are the second largest group of slugs, are most easily identified by not belonging to one of the other 2 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) which most species use to 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 cnidarians, they must actively feed upon them. 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 group, 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 (which is easily obvious in the picture of the slug on that fasTesT box you mentioned). There are 3 major groups of dorid nudibranchs (based on their morphology) and 2 of these groups are primarily sponge predators, while the third group feeds primarily on bryozoans and tunicates.
OK, having cleared that up, lets come back to your specific question about the dorid in your local pet shop. To be blunt, your LFS is either gravely misinformed or trying to sell you a line, if they claimed that any dorid is herbivorous. 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 your local shop, or (unfortunately this is not all that uncommon) a ploy to get you to buy the animal before it starves to death in the shop.
As far as I know, there are no specific feeding studies on the species you saw, but all the members of the genus to which it used to belong (Chromodoris) and to which it has recently been reclassified (Hypselodoris) are sponge specialists, many accepting only a single sponge species as suitable prey. There are reports of H. bullocki feeding on some sponges of the genus Halichondria, but no one knows how accurate these reports are or whether the slug will accept other sponge species. Given that sponges are often considered more difficult to keep than are sea slugs, the chances that you have an abundant supply of Halichondria on hand to feed these guys is pretty slim. I’m afraid that I have to recommend that you pass on the purchase of this animal. Unless you know specifically which slug you are buying and what their preferred prey are (e.g., the Aiptaisa predator I keep mentioning, 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!). You might also want to keep in mind that not all shops provide buyers with accurate and/or honest information, and if this is the quality of information your current shop consistently provides, you might just want 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 and if followed would guarantee that I fail with my new animal...