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

The Grazing Snails, Part II - Abalones, Limpets and Nerites


In last month's column I wrote about the group of snails generally referred to in the aquarium hobby as "Turbo Grazers." Commonly called "top" or "turban" snails these animals are fundamentally similar in shape and in natural history. In this column, I will be discussing other algae-eating snails, specifically the Abalones, Limpets, and Nerites. Unlike the trochoideans, these animals are not closely related and don't form a nice, coherent group with common characters. Rather, they are from several distinct evolutionary lineages. Their generalized internal anatomy and physiology, however, are the same as in the trochoideans, and I refer the reader to May's column if there are questions about the animals' basic needs and the necessity of slow acclimation to differing conditions, particularly salinity. Feeding in these animals follows the basic invertebrate pattern; the animals "lick" the surface of the substrate with a structure which has been called a "rasping tongue," and which biologists call a "radula." On the other hand, while there are similarities in feeding between all of these various groups, there are also differences. The number and shapes of radular teeth as well as, in some cases, their compositions vary between these groups and are different from the trochoideans as well.

The gastropods, or snails, constitute a huge array of species; there are well over 40,000 named snail species and estimates of species numbers range up to 150,000. The number of actual species may be significantly fewer than that, but nevertheless that number will still be very large. The soft parts of snails are largely hidden out of sight inside their shells, so consequently, snails are distinguished based on differences in the shapes of their shells. So far, so good; their identification sounds like it should be easy. However, the typical snail shell is basically a long slender cone wound into a helical shape, and that is a simple shape. There are fundamentally very few variations that can be made in this structure. This, in turn, means that identification of species may be very difficult as there are not a lot of differentiating characteristics that can be seen. Given the number of different species, it is highly likely that there is a large number of distinct species with exceedingly similar shell shapes. Consequently, to identify them requires examining characteristics that are often tiny, obscure or uncertain. To further complicate matters, given that the shells are made of calcium carbonate, the identifying characters may simply be worn away or used as a substrate by other encrusting forms inextricably attached to the shell. Additionally, even with shells in the best of condition, it may be impossible to distinguish some species for the simple reason that their shells are identical. In one snail group I worked with, one researcher described two species, one with a radula and one without, but the shells of the two species were about as close to being identical as they could be (Smith, 1967). For these reasons, I generally do not recommend that aquarists try to identify snails to the species level; it is sometimes possible, but for a great many species, it is simply not worth the effort.

Figure 1. An abalone, Haliotis kamtschatkana, grazing on some algae. Note the tentacles protruding from the holes in the shells. The head is to the left.

The trochoideans that were the subject of the May column all have a relatively similar appearance. In fact, they are so much alike that most aquarists have problems telling them apart. This ambiguity of identification is an advantage in that it allows collectors to provide us with usable animals without having to be very discerning in their collection. In the aquarium trade, the terms Turbo, Trochus, and Astraea (generally misspelled "Astrea") are all applied to any species of those genera. In other words, if you want to know what Trochacean you have, you are pretty much on your own for identification, because many common names used by dealers are unreliable to the point of uselessness. This is largely immaterial, however, as the animals do tend to have similar requirements and attributes.

Unfortunately, the grazers that are the subject of this column have no such similarity of shape. While the consumption of various types of algae is widespread in the gastropods, the number of those species that are suitable for reef aquaria is really surprisingly small, and with few exceptions there are few similarities in shape to help distinguish them. It is up to the individual hobbyist to be sure of the identification of these beasts. Three references may be of significant help in this regard. As aquarists we can, in this case, benefit from the hobby of shell collecting. Shell collectors greatly outnumber reef aquarists and their hobby supports the publication of many shell identification guides. The following three references, while not written with the aquarium hobbyist in mind, will help with the identification of many mollusks including the snails. They have the added advantage of being in many local public and university or college libraries, and so may be readily available for consultation.

Identification References:

Abbott, R. T. 1974. American Seashells. Van Nostrand Reinhold Company. New York. 663 pp. This reference will allow you to make tentative identifications of about 30% to 50% of the Caribbean reef snails.

Abbott, R. T. and S. P. Dance. 1982. Compendium of Sea Shells, A Color Guide to More than 4,200 of the World's Marine Shells. E. P. Dutton, Inc. New York. 410 pp. This reference is good for identifying snails from marine environments around the world, but is missing a lot of, mostly smaller, species.

Keen, A. M. 1971. Sea shells of tropical west America. Stanford University Press. Palo Alto, Ca. 1064 pp. This reference is excellent for Eastern Pacific and Gulf of California animals, and is good for some other tropical Pacific animals.


Abalones, or snails in the genus, Haliotis, are some of the classic grazing snails. Unfortunately, they tend to be too large for reef aquaria as some of them reach diameters of eight inches (20 cm) or more. When viewed in profile from the side, abalones are basically wedge-shaped with the sharp point of the wedge at the front end. Many of them are the preferred prey of some very diligent and persistent visually-oriented predators such as fishes, birds and sea mammals, so the snails have been forced to evolve the behavior of hiding from their predators in cracks and crevices where they are difficult to see and even more difficult to remove. Practically speaking, this means that in aquaria, they often tend to push their wedge-shaped shell under rocks or between the larger pieces of rock. With their large broad feet and tough shells they can exert a significant amount of force on the rocks. In nature, where the rocks are part of Mother Earth and don't move, this behavior ensures that the animal is well protected. In an aquarium, such behavior may significantly rearrange a tank. A four or five inch long abalone can easily move and shift rocks weighing twenty pounds (9 kg) or more. Few aquaria can withstand this sort of perturbation, and as a consequence not many aquarists want to deal with abalones.

We are fortunate, however, as not all abalones will cause this rearrangement. One of the best types of grazing snails for cleaning diatoms and other adherent algae from smooth surfaces, such as aquarium walls, is a species of tropical abalone, Haliotis asinina, commonly called "the Ass's ear abalone." The common name comes from its elongate shape, and to someone who may have never seen a donkey's ear, it probably looks like one. Abalone shells are coiled, but they don't really look like it as the coil expands so rapidly that the shell looks almost like the bowl of a spoon, or an ear. In fact, the scientific name, Haliotis, means "sea ear." Viewed from the top, the shell can be seen to be in the form of a rapidly expanding coil, with the more pronounced whorls being located at the animal's stern. Additionally, abalone shells may be positively identified by the series of small holes running in a line near the left edge; no other flattened type of snail has such a row of holes. When the animal is grazing, a sensory tentacle will often be seen extending from one or more of these holes. The shell color is typically mottled greens and reds to yellows; good camouflage coloration for a reef animal. Much of this variety of color is the coloration of the shell proper, but a lot of it is due to the growth of various algae on the shell. Haliotis asinina reach a maximum size of about four inches (10 cm), but grow slowly in our systems. The ones available for the aquarium trade are generally from aquacultured stock and are often about one to 1.0 - 1.5 inches (2 - 3 cm) long. One specific caution is necessary for their care: they need to be acclimated very slowly to salinity changes, and it is best to err on the side of caution. As with other snails, they use their radula (scroll down on the linked site) to graze. Generally, they prefer to graze on glass, and often will not go on the rocks at all. They are nocturnally active and often will seek a dark space in which to pass the brightly lit hours. Often, they will return to the same "home" space for many months.


The generalized limpet body form is that of a snail bearing a conical, uncoiled cap-shaped shell and possessing a broad foot. This type of morphology is found in a number of snail groups that are only distantly related. Consequently, it is difficult to generalize about or predict the behavior of any particular limpet species. The limpet shape is well adapted to withstand wave stress and the pounding surf, and most limpets are animals of the intertidal regions. Intertidal tropical limpets are common, and are quite good grazers, but they are not good reef tank inhabitants as they tend to climb out of the tank. Often they climb up and out of the tank and then fasten themselves down, presumably to wait for high tide; which, of course, never comes. This results in the death of the limpet and the concurrent creation of limpet jerky.

There are a number of types of subtidal limpets occasionally available for the coral reef aquarium. Generally, they are not particularly good animals to have in aquaria. Those shallow water forms that graze on algae seem to have a decided tendency to eat coralline algae. These limpets are exceptionally well equipped to eat these algae. They have a radula with teeth made of a mixture of iron salts (primarily hematite) and silica (in the form of opal). This gives them a rasping organ with a value of 7 or 8 on the Mohs scale of mineral hardness. Additionally, the opal is deposited in the teeth as small inclusions that tend to abrade away slightly faster than does the hematite. This turns the tooth into a self-sharpening rasp; the more it is used, the sharper it becomes until the tooth abrades away completely and another replaces it. The limpets possessing such a rasping apparatus can cut through and remove coralline algae like it was butter, and some of them can do the same to acrylic aquarium walls. Unlike sea urchins, which typically eat a small patch of algae and move off some distance until they feed again at some later time, the limpets are pretty much constant grazers. There are some other species of small limpets, occasionally available from some vendors, which are relatively benign and good grazers on microalgae. Unfortunately, these species are seldom offered for sale even though at least some of them appear to reproduce in captivity. It is essentially impossible for a novice to distinguish between these species as some of the identifying characteristics are on the internal surface of the shell, so one has to have a dead shell to examine to determine the identity of the animals. Unless the vendor can vouch for the dietary preferences of the animal that he is selling, it might be best to pass by limpets on the way to the checkout counter.

Additionally, there are some limpets that generally do not graze on algae. The ones that we are primarily concerned about are the keyhole and slit limpets. These are limpet-shaped snails with a perforation on the top of the shell (keyhole limpets) or a slit on the front shell margins (slit limpets). These animals are generally carnivorous, and will eat sponges, soft corals, and other sessile animals. I have not heard of any specific reports of them eating stony corals, but I suspect that they would. Some of them, however, may be very useful for the control of some low growing colonial nuisance hydroids. Unfortunately, little work has investigated the specific diets of tropical keyhole limpets, so we don't know the names of any beneficial species. Consequently, unless you are willing to put up with some predation on coralline algae or some of the animals in your system, I would leave most limpets to the sea.

Figure 2. A keyhole limpet, Diodora aspera. The fuzz on the limpet shell is comprised of hydrozoans, and they are probably as safe as they can be anywhere in the vicinity of the limpet since it can't feed on them up there.

Two types of limpets that appear to be not only beneficial, but attractive, are ones that really don't look like limpets. These are the shield limpets and fleshy limpets. When fully active neither of these types of animals has much of a resemblance to the common limpets, but both are related to them. Both of them have a large fleshy structure, the mantle, which extends up over the shell and largely obscures it; in essence the animal looks like some kind of slug or nudibranch.

Shield limpets, Scutus unguis, are commonly collected on Indo-Pacific live rock. They may reach lengths of an inch or a bit larger (up to about 3 cm). Most of the size is due to the fleshy black mantle that covers the shell. These animals are nocturnal and will hide under and between rocks during the day. At night they come out to graze on microalgae such as diatoms on the rocks. They appear to be blackish blobs, although the white shell can often be seen peeking through the folds of the mantle covering the animal. If disturbed so that the animal retracts the mantle, the ordinarily appearing limpet shell will be seen attached to a rather large body. These are good grazers, and are quite beneficial animals to have in a reef tank. Unfortunately, they don't seem to reproduce in reef tanks, and generally don't seem to persist for more than a few months.

Fleshy limpets, Lucapina species, are commonly found on Caribbean rock, and there are several species of them. The largest are about the size of shield limpets, but many are smaller. They also have a mantle that extends up over the shell, but in this case, the mantle is brightly colored and ranges from yellow to red. These are keyhole limpets, and as such are probably omnivorous, but from the reports I have received, they don't seem to be eating the decorative livestock. Generally, they seem to be grazing on algae. Like the shield limpets, they are largely nocturnal and are generally out of sight and inactive during the day. When the mantle is retracted, they will be seen to have a small shell on their back and its center will be perforated with a hole. As with the shield limpets, they don't appear to reproduce in aquaria and generally only survive a few months.


Numerous species belonging to the genus Nerita make good herbivores for many reef tanks. These snails are recognized by the rounded shell which, although it is coiled, has a low spire. The aperture from which the body extends is basically "D-shaped" with several large calcareous bumps on the inner edge. The outer edge is also often marked by similar calcareous bumps. I have seen at least four species of Nerita in reef tanks, and there are undoubtedly more than that. They tend to be dark; browns and blacks are the predominant colors, but there are several species that are white with rich brown markings.

Figure 3. Diagram of a Nerite shell showing the aperture and some of the diagnostic characters used in identification.

Some of the ones collected for sale in reef tanks are really intertidal marsh animals and have no business in a reef tank. I am not the only one who thinks so, by the way. The snails agree with me, and vote with their feet, moving out of the tank and into the wilds of the adjacent rooms. These animals will live in the tank, but seem to have a physiological need to move above the water line. At least in many cases, unlike some of the limpets, they also move back down to the water. Unfortunately, there is no good way for a hobbyist to determine if the animals offered for sale are intertidal or subtidal as the shells are quite similar.

There are several other Nerita species that don't have the vertical wanderlust, and these make good grazers on the glass and elsewhere. These animals seldom harm other species in our systems, although they may occasionally bulldoze some unattached structures around. Nerita females lay egg masses frequently, but the larvae seldom pass through the larval stage and juveniles are seldom seen in our systems. The largest nerites in our systems are about 2 cm (0.75 in) across, and the smallest are adult at 1-2 mm (1/12th to 1/25th of an inch) across. Well-fed Nerites lay eggs almost continuously on the aquarium rocks and walls. The white eggs are enclosed in a protective covering and develop from egg to larvae within it for about a week or so. The egg capsule then opens and the larvae are released into the tank's water. This feeding larval stage is prolonged and it is unlikely any larvae will survive to settle and metamorphose in a reef aquarium.


With the exception of the Nerita, the snails that I have written about in this column are not often seen in reef aquaria. Haliotis asinina are fine grazers, as well as attractive animals, and even though they are aquacultured and have been marketed to hobbyists, their availability is spotty. Relatively few species of limpets suitable for the hobby are available and not many vendors have them. The shield and fleshy limpets are neat additions to reef tanks, but I know of no vendor that specifically and consistently offers them for sale. Next month, I will conclude this short series on the shelled snails that eat algae with some information on some animals that are readily available: ceriths, strombids, cowries, columbellids, and some bubble shells.

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

Reference Cited:

Smith, E. A. 1967. The proboscis and oesophagus of some British turrids. Trans. Roy. Soc. Edinb. 67:1-22.

Here are some additional references that may provide information of interest, and which have data about snail grazing or grazer morphology and ecology:

Abbott, D. P. and E. C. Haderlie. 1980. Prosobranchia: Marine snails. In: Morris, R. H., D. P. Abbott and E. C. Haderlie. Eds. Intertidal invertebrates of California. Stanford University Press. Stanford, California. pp. 231-307.

Fretter, V. and A. Graham. 1994. British Prosobranch Molluscs. Their functional anatomy and ecology. Ray Society. London. 820 pp.

Kerth, Klaus. 1983a. Radulaapparat und Radulabildung der Mollusken. I. Vergleichende Morphologie und Ultrastruktur. Zool. Jb. Anat. 110: 205-237.

Kerth, K. 1983b. Radulaapparat und Radulabildung der Mollusken. II. Zahnbildung, Abbau und Radulawachstum. Zool. Jb. Anat. 110:239-269.

Kohn, A. J. 1961. Chemoreception in Gastropod Molluscs. American Zoologist. 1:291?308.

Kohn, A. J. 1983. Feeding biology of Gastropods. In: Wilbur, K. M. Ed. Biology of the Mollusca. Physiology (2). Academic Press. New York. pp. 1?63.

Kohn, A. J. 1987. Intertidal ecology of Enewetak Atoll. In: Devaney, D. M., E. S. Reese, B. L. Burch and P. Helfrich. Eds. The Natural History of Enewetak Atoll, Volume I, The Ecosystem, Environments, Biotas, and Processes. United States Department of Energy, Office of Scientific and Technical Information. Oak Ridge, Tennessee. pp. 139?157.

Kozloff, E. N. 1990. Invertebrates. Saunders College Publishing. Philadelphia. 866 pp.

Runham, N. W. 1961. The histochemistry of the radula of Patella vulgata. Quarterly Journal of Microscopic Science 102: 371-380.

Ruppert, E. E. and R. D. Barnes. 1994. Invertebrate Zoology. Saunders College Publishing. Philadelphia. 1056 pp.

Voltzow, J. 1990. The functional morphology of the pedal musculature of the marine gastropods Busycon contrarium and Haliotis kamtschatkana. The Veliger. 33:1-19.

Voltzow, J. 1994. Gastropoda: Prosobranchia. In: Harrison, F. W. and A. J. Kohn. Eds. Mollusca I. Wiley-Liss. New York. pp. 111-252.

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The Grazing Snails, Part II - Abalones, Limpets and Nerites by Ronald L. Shimek, Ph.D. - Reefkeeping.com