A Spineless Column by Ronald L. Shimek, Ph.D.

Naked ...Gills on Snails

Well, it got your attention, didn't it? Nudibranch means "naked gills" and this month I thought I would briefly review these gorgeous and wonderful animals; animals that, almost without exception, have no place in aquaria, but which find their way there anyway.

Now, I will freely admit, right up front, I don't have many pictures of beautiful tropical nudibranchs, and so I won't put any pictures of them in this column. However, I will refer you to a site that has a lot of good images; tropical eye candy, if you will. Nudibranchs are exceptionally interesting creatures, and I will concentrate my efforts in this column discussing how to recognize them. In doing so, I hope to give you enough information that you can recognize a nudibranch when you see one, which is not a difficult proposition, and decipher a bit of its biology simply from its appearance, which is a bit more difficult.

For much information and a lot of good images, and references, go to The Sea Slug Forum.

Nudibranchs are mollusks. Following from last month's column on molluscan diversity, they therefore must have all those typical molluscan characters such as a radula, a foot, and specific types of gills, guts, and nervous system. This, they do, and possibly more importantly for the topic at hand, they reproduce in a standard molluscan manner. All nudibranchs are hermaphrodites. This means they have the sexual organs and plumbing of both genders, and they are frequently simultaneously active. During copulation, they both give and receive sperm. They cannot, however, fertilize themselves.

After mating, they lay eggs, often by the millions, all encased in some sort of jellylike mass. All of these develop into embryos, and these embryos develop into small larvae, which hatch from the egg mass. These larvae are called veligers, and each of them has the beginnings of small snail shell. The shell is transparent, because at this stage of the animal's life it can't secrete the calcareous shell of an adult snail; nevertheless, it is a perfectly good little snail shell. Nudibranch larvae look like any other good planktonic snail larvae, and live much the same way. They swim in the plankton, and feed on phytoplankton.

If they get enough food, and if it is the correct food, they will grow, and develop further into recognizable snails, which do have a calcareous shell. They are normally still planktonic when this occurs. During this period they are getting larger, and their internal structure is developing as they get more complex. Eventually, they become able to live on the bottom in the benthic environment. Biologists, who study larvae, refer to them at this stage as being "competent" to settle.

Competent larvae don't just rain out of the plankton down to the ocean floor and change into small snails. Instead, they swim down until they encounter the substrate and then they chemically analyze it. Or, to throw in a highly technical, jargon term from biology, they "taste" it. These larvae will not settle from the plankton until they find a suitable substrate, and if they don't find one, they perish. What constitutes an appropriate substrate varies with each nudibranch species. Often, it is something that is correlated with their adult prey. So, for example, a coral-eating nudibranch may regard a species of coralline algae typically found near the corals on which the adults feed as the appropriate substrate, as it provides a cue for future food.

If they find the substrate, they settle or touch down on the substrate. Following this they metamorphose, or change their body, into a small juvenile nudibranch. In this process, they discard the shell; it is simply shed into the water. They also generally reabsorb the larval feeding/swimming organ called a velum. Typically, the gut reorganizes; the cells capable of digesting phytoplankton die or change, and from this point forward the animal can only digest animal prey. All nudibranchs are carnivores. This process takes from a few hours to a few days, and during this time, the little nudibranch lives on fat reserves it accumulated during its larval planktonic period; it can't begin to feed until the gut has completed its changes. Additionally, during this time in the life of most nudibranchs, gills develop from the upper surface, which in other snails is covered by a shell. Consequently, as the shells have been shed, these animals have "naked gills" exposed to the environment.

See The Slugs, Er, Nudibranchs, Er, Sea Slugs…. What Are These Things, Anyway?

Humans look at the world and from this looking, they make decisions. We are predominantly visual animals and we share, with the other higher primates, a visual system that is probably the best in the natural world. So, it stands to reason that we often make sweeping generalizations based on exterior appearances. For example, politicians look like humans, close enough to fool some experts, but examination of their internal morphology will disclose they belong to a different sort of species. Snails without shells all really look alike. Basically, there is this blobby body which may or may not have any permanent appendages on it, and underneath is a muscular foot. And they secrete mucus. Always. Lots of it.

We call such animals, slugs, and recognize they all basically look alike. Unfortunately, the appearances of similarity are only skin deep, and actually in many cases, not even that, as their skins may be different, too. If all snails are in one group, the group we call the Class Gastropoda of the Phylum Mollusca, nudibranchs and all marine slugs, belong to a subdivision of that large group. They are put in the sub-grouping called the opisthobranchs. Branch, pronounced "brank" which rhymes with "bank," is a derivative of Branchia, a Greek word for the gills of fishes. We ignore the niceties that snails are not fishes and use the term anyway. Opistho- is a derivative of opisthen, meaning "at the back" or "behind," so this group of marine snails, the Opisthobranchia, are those snails with the gills behind, or at the back. Actually, they are the snails with their gills behind or to the rear of their hearts. Terrestrial slugs, by the by, don't belong to this group. They are all air breathers and have a lung. However, in their favor, even if they aren't as colorful as the nudibranchs, some terrestrial slug species are close to being the champion mucus producers of the animal kingdom.

Not all opisthobranchs are slugs; many of them have shells. As an aquarium example, the Pyramidellid snails that parasitize Tridacna, are also opisthobranchs, as are the "bubble shells." The groups of snails characterized by the pyrams and bubble shells have representatives that may have good shells. However, in the bubble shell group, the "head-shield snails" or Cephalaspidiceans, there appears to be a trend toward the loss of the shell, with some species having only a small internal shell, and looking quite sluggish.

So, what is a nudibranch, and what isn't, and how can you tell? More importantly, why should you care?

I will address the second question first. Many sea slugs (notice I was careful not to use the term "nudibranchs") are quite beneficial, as they may be predators on some of the algae that become problems in aquaria. An example of one such animal is the lettuce slug, Elysia cristata, which may eat various green algal pests. A similar situation is found with the sea hares. These beneficial animals all look "nudibranchish," but they are not, belonging to different groups, the Sacoglossans and the Aplysiids, respectively.

Nudibranchs, fortunately, are grouped together in what taxonomists call, the "Order Nudibranchia." Among some other things, this means that all of them have some characteristics in common that will help in the determination of whether or not a newly found slug becomes a pest or a pet. Unfortunately, the Order Nudibranchia is huge, with several thousand species having been scientifically described, and more being found regularly that are not yet described. Some of these undescribed species may be quite uncommon or very cryptic. I have personally found an example of one undescribed species, which has been subsequently described and named by a nudibranch taxonomist using the specimens I sent to her. I found this species in a very well-studied locality near a marine laboratory, and it has subsequently been found elsewhere, but all of these other collections have only been through environmental grab samples. As far as I know, I am still the only person to have seen a living example in its natural environment. It is a small, very cryptic, species, and looks like a small piece of organic leaf litter. It may well be common, but if so, it is commonly overlooked. Other scientifically undescribed or "new" species may be quite large and often very strikingly colored. During my stay, in the early 1970's, as a graduate student at the University of Washington Laboratories, several large and obvious species were found and named from localities around that marine station. That is the largest marine biological laboratory on the west coast of North America, and has been operational since early in the twentieth century, and yet new species of nudibranchs are still being found in that locality.

So, what can an aquarium hobbyist do when confronted with a sluggish blob? How can such a person identify it? Well, identification to species is likely impossible, even for the experts, as in many cases you have to know where it was found in nature to be able to separate it from dozens of other look-alike species. However, to identify the unknown critter as a nudibranch or some other snail, and then (if it is a nudibranch) to be able to identify it to a major group is certainly something that an observant aquarist should be able to do.

Probably the first thing you should do if you find an animal like this in your aquarium is to remove it from the system and put it in a clean container with some tank water. Once it relaxes enough to move around, use a good digital camera to take some pictures. Don't bother taking pictures with a camera that has less than about 3 Mpixels if the slug is small, as the enlarged image will be too blurry to decipher. You can often send good pictures to an expert and get a valid name for the animal.

Once the animal is moving around, examine it. Does it look like it has a hard lump in the middle or does the middle of the body appear swollen? If so, gently touch it. If you feel any hardness or resistance to your touch, it is likely a shell, and the animal isn't a nudibranch, and you probably have a bubble shell. Check references on the Sea Slug forum for more precise Cephalaspidacea identifications.

Figure 1. A diagram of a bubble shell or cephalaspidean.


Figure 2. A small bubble shell with an exposed shell. In most bubble shells, the shell is completely internal.

Figure 3. A small, 1/8th inch long, bubble shell found on a reef in Palau.

If the animal lacks an internal shell, or you don't feel one, examine the head as it is moving. There will be two large tentacles or projections pointing up and, often, forward from the front end. These tentacles are typically larger than other tentacles in the "head" region, if there are any. These are the main sensory organs for the animals, and are called rhinophores (in nudibranchs) or cephalic tentacles (in most other groups). Use a magnifying glass to examine them, and note their structure. If they appear to consist of a fold of tissue rolled into a cylindrical shape, the animal is a sacoglossan, such as Elysia, the lettuce slug, or a sea hare, such as Aplysia. Check other references for these animals.

Figure 4. Most sacoglossans are green and eat algae. I wanted to show you something different. This tiny slug, Olea hansinensis, about 1/50th of an inch long, finds its way to bubble shell egg masses and eats the eggs. I took this photo in nature using a 35 mm camera, and the image on the slide is in crisp focus, but even scanned at 4500 dpi. I couldn't get a crisp image here. The faint dots on the back of the slug are individual cells in the epidermis.

If the back of the animal is covered by a more-or-less smooth surface (there may be small bumps on it), and if the two sensory tentacles arise from under the front of this surface, not through it, you may have either a side-gill slug, or a nudibranch. Use a medicine dropper tip and gently lift up the dorsal surface on the right side of the animal. If there is an elongate structure covered under the flap of the dorsal surface, the animal is a side-gill slug, or Notaspidean. Remove the animal to another aquarium and consult other references for the biology of these animals. Most of them are predatory, and if bothered some will secrete sulfuric acid as a defensive agent. If there is no elongate structure along the right side, although there may be warts, bumps or circular depressions present there, you have an arminid nudibranch. Arminids eat soft corals, and will not be reef safe.

Figure 5. Berthella californica, a side-gill slug, or Notaspidean. The gill, peeking out from under the dorsal mantle, is normally invisible, being covered by the flap of tissue on the animal's right side.


Figure 6. An arminid nudibranch, Armina californica. Animals essentially identical to this individual, except for slight color differences, are found in the tropics.

If your specimen is unlike the examples covered so far, gently touch the tentacles with the dropper tip or some other glass probe. They should retract into sheaths of one sort or another. If so, you have one of the several other types of nudibranchs. None of them are generally reef safe, although a few are useful as they may prey on undesirable reef-aquarium animals such as Aiptasia.

Examine your critter again, this time concentrating at the general shape of the beast, and the number and kind of projections off of the top of the animal. If the animal looks rather like the slug version of a small brick, more-or-less rectangular in cross-section and with defined sharp right-angled corners at the front end, it is likely a "Dendronotacean." These often have two rows of projections called "cerata" on the back. Each row is found on the edge of the back where it meets the side of the animal, and these projections generally have small branches off of them. Dendronotaceans eat soft corals, sea pens, sea anemones, and jellyfish polyps. They are definitely not reef safe, and they appear in aquaria with some frequency. The largest dendronotaceans are found off the Pacific Coast of North America, where Tokuina tetraquetra may reach wet weights exceeding 20 pounds, and Dendronotus iris is commonly seen at lengths exceeding 16 inches. The Tritonia festiva in the slide show in this issue of Reefkeeping Magazine is also a dendronotacean.

Figure 7. Tokuina tetraquetra, the largest nudibranch in the world, is a dendronotacean. Note the small frilly gills along the sides of the flattened dorsal surface. This animal, a small individual, was about 15 inches long, and what is not readily apparent is that the top of the animal is about three inches above the substrate.


Figure 8. An unidentified dendronotacean, possibly a species of Doto, eating an aquarium coral. Photo courtesy of Skip Attix.

Upon examination, if the body is not rectangular in cross-section, and if any projections along the back are not found in linear rows along the back, then you have one of the other kinds of nudibranchs, most likely either a dorid, or an aeolid. Examples of each of these kinds of nudibranchs are found rather frequently, actually, in marine reef aquaria.

Aeolid nudibranchs are a bane for some aquarists, particularly those involved with stony coral culture, as they eat the corals. Identifying them is, fortunately, an easy process. The top surface of aeolids is covered by unbranched projections, also, as in the dendronotaceans, called cerata. These cerata may be arranged in linear rows, running from the front to the back of the animal, or in paired groups or clusters, one of each cluster on each side of the midline on the back of the animal. The smallest aeolids may have only one or two cerata, and are found living on, or between, sand grains. The largest ones cruise tropical and temperate seas in search of their prey. Aeolid cerata, or gills, are distinctive. If you have a "furry" nudibranch, you almost certainly have an aeolid. Examine one of the cerata, and you will truly see one of the wonders of nature.

The cerata have at their center a brown, tan, or beige colored tubular core. Occasionally, in some species a different color is found, but only rarely. This is an extension of the digestive gland in the gut running up through the gill to its tip. The digestive gland is the part of the molluscan gut which actually digests and assimilates food. In some ways it is an analog of the human small intestine. Aeolids are generally predators on corals, anemones, hydroids, and other cnidarians. Each nudibranch species is probably specialized to eat only one or a few species of prey. When they eat their prey, they typically bite off rather large chunks; large relative to the nudibranch, that is. In their digestive process, all the flesh is moved to the digestive gland where it is digested and assimilated. That is, with the exception of the stinging capsules or nematocysts, which are moved up the digestive gland tube in the center of each gill and come to reside in the tissue under the bright white tip, in what is called the cnidosac. The tips of the gills actually are usually the first thing one sees, when an aeolid is noticed.

Figure 9. Flabellina rufobranchialis, a common aeolid from the Northeastern Pacific. Note the brown digestive gland cores in the cerata, which are also tipped with the bright white cnidosacs.

The nematocysts in the cnidosac remain fully capable of discharging and are oriented so that their threads will fire through the epidermis covering the cerata. The nematocysts are provided with a nutritive environment, and immature nematocysts will mature in the cnidosac. Old nematocysts, those incapable of extending firing, are extruded from the cnidosac tips. Nematocysts in the cnidosac remain capable of firing for several days to a couple of weeks after the animal that secreted them has been eaten. If the aeolid is threatened by some potential predator, the nematocysts can be discharged, stinging the potential predator, and presumably deterring predation. As a result of this, relatively few animals will eat aeolids. Consequently, the prey of an aeolid not only nourishes it, but also protects its killer by providing, in the form of nematocysts, protection against predation.

Over the past couple of years, aeolid nudibranchs have been increasingly found in aquaria on Montipora colonies and other small-mouthed stony corals. These tiny aeolids are white or translucent gray, small, about one millimeter long to maybe about one centimeter, and are easily overlooked. They appear to rapidly pass totally through their larval stages in the egg mass, and hatch as crawl-away slugs. As such, once they are found infesting a tank, it may be very difficult to get rid of them, and if they become established in some dealer's tanks, they may be spread far and wide very rapidly. Others have been found just "roaming" tanks. Other aeolids, such as species of Berghia, have been used with varying success in aquaria to eradicate pest anemones such as Aiptasia. These species die when individuals of their prey become so rare that the slugs starve before finding more food. If this starvation occurs when they have eaten all of the pests, then they have succeeded. More often, it simply means they can't find the last of the pests, and with the last nudibranch gone, the pests return.

The remaining nudibranchs typically found in aquaria are in the group called the Doridacea, or the dorid nudibranchs. These animals are characterized by having their anus on the top midline of the body, about two-thirds to three-fourths of the distance from the front to the back. The anus is surrounded by a tuft of gills, which when extended look rather like a feather duster (the implement, not the worm). The rest of the top surface of the dorid may be either smooth, often to the point of "glossiness," or bumpy like the surface of a raspberry, or with appendages that look like the cerata of an aeolid. In these latter species, the cerata are not typically arranged in rows or clusters, but rather are scattered randomly over the back of the dorid. In all cases, however, the anus will be on the mid-line of the dorsal surface, and if the animal is "relaxed" will be surrounded by its tuft of gills.

Figure 10. Archidoris montereyensis, a typical dorid nudibranch. The front of the animal is at the lower right, and the two rhinophore tentacles are visible. The tuft of gills at the upper left surrounds the anus. All dorids have this basic gill and tentacle structure.

Dorids are often the most colorful of the nudibranchs and are often purchased by well-meaning aquarists who bring them home, only to watch them die. Dorids generally eat one of three different types of sessile or non-moving animals: sponge, tunicates, or bryozoans. Often the nudibranch species can only eat one or two closely related species of prey, so it is effectively impossible to keep dorid nudibranchs alive for extended periods. Not because aquarists can't provide the physical conditions necessary, but simply because the food is impossible to get. Few tropical dorids have been well studied, and in most cases we don't even know what they eat, so it would impossible to import food for them, even if we wished to.

Once you have a dorid in a tank, however, you really often want to keep it alive, if for no other reason, that they are often so filled with toxic chemicals that they can kill a tank upon their death. This is especially true for some of the beautiful brightly patterned blue, black and gold Phyllidia. (Check these out on the Sea Slug Forum). The bright, beautiful colors of nudibranchs are some of the best-known examples of aposematic or warning coloration. Warning coloration patterns are found on animals that in some way have a good defense against predators that hunt using vision, such as fishes or crabs. These bright colors and striking patterns are present to be visually obvious and thus to warn the predators away.

The system of dorid nudibranchs and their prey forms an interesting "double warning system." Sponges, tunicates, and bryozoans, the foods of dorid nudibranchs, are also often quite toxic, and brightly colored, providing a signal to fend off their own visually oriented predators. This is the first warning component, and in most cases, these poisons and bright colors work. Very few predators will eat these animals. However, one group of predators that has species that do eat them is the Doridacea, which specializes on these prey. Coincidently, the nudibranchs are blind and can't see the warning color on their prey. But that doesn't matter, as the poisons don't have any effect upon them.

When nudibranchs eat their toxic prey, they often modify the prey's own internal toxins. In many cases this makes them even more toxic. Dorid nudibranchs are typically animals that marine predators such as fish sample only once in their life. If they survive, they never eat or attempt to eat a nudibranch again, and the nudibranch's bright colors are there to tell them what to avoid eating, so this is the second warning in this particular system of predators and prey. As many marine fish live several years to several decades, this is a lesson that, once learned, is of significant benefit to the snails.

Aquarists often see some beautiful nudibranchs in their local fish stores, where they have been starving, and bring them home as an addition to the tank. Generally, the slugs continue to starve in the aquarists' tanks and within a few weeks they die. The toxins that they have accumulated during the life of the nudibranch may be released into the tank water and cause serious problems. These are animals truly best left in the wild, or if inadvertently collected, they are best left for someone else to deal with.


I have tried here to give you a little bit of background on many nudibranchs and how to identify them to their major group. Once identified, you may follow links online or do some research at a library about them and try to determine if you really want to tackle the problems of trying to maintain them. They are truly beautiful and easy to maintain, once you have accepted the cost of that maintenance, which is, of course, the cost of their foods. Although often attractive, they are destructive of other decorative animals such as corals, soft corals, anemones, sponges, tunicates, and bryozoans. They are not reef safe unless your reef is very big indeed.

If you have any questions about this article, please visit my author forum on Reef Central.


If you wish to do some reading about nudibranchs, I refer you to the Sea Slug Forum where many nudibranch references are listed.

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Naked ...Gills on Snails by Ronald L. Shimek, Ph.D. - Reefkeeping.com