Snails

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http://www.reefkeeping.com/issues/2004-05/rs/index.php

THIS ARTICLE IS AN EXCERT.

The Grazing Snails, Part I - Turbo, Trochus, Astraea, and Kin

Introduction:

Of the common reef aquarium animals, only the polychaete annelids, or so-called bristle worms, are more misunderstood and badly treated than the common snails that many aquarists put into their systems to graze on or remove algae. Relatively little thought generally goes into the choice of these animals and, judging from the questions asked, even less thought goes into their care. This maltreatment often results in significant mortality. But who cares, they're only snails, right?

The moral and ethical issues of treating living organisms as "disposable" aside, I care. Snails are fascinating, complex, and often beautiful animals. The evolutionary radiation of body form in animals that can be called "snail" is one of the life's most successful stories. Depending on which way the taxonomic estimate is made, there are an estimated 35,000 to 150,000 living species of snails. This makes the "lowly" snails the largest animal group other than insects. Reflecting this amazing diversity, they have occupied every terrestrial and aquatic habitat on Earth.

I will briefly discuss some of the major groups of the common grazing snails that are kept in aquaria to assist in the control of algae. I will cover a rather large assemblage of similar-looking snails commonly sold in the aquarium hobby. Their similarity of shell shape and internal structures is an indication that they are closely related and that they share many attributes in common. Their structural similarities allow an author to cover a lot of ground by generalizing and focusing on like attributes. However, in such situations, it is always worth the reader's time to remember that no two species have the same requirements. Each species is special and unique in its own attributes, and discussions of generalities can go only so far. So, read what I have to say with a critical eye. I will also cover several of these animals more specifically. This first major group contains those animals, often referred to by aquarists as Turbo, Trochus and Astraea grazers, is called by the scientists who study these animals, the "Superfamily Trochoidea." A superfamily is a large, inclusive, group containing many smaller groupings of superficially similar animals differing in characteristics, at the family level, that are generally considered to be relatively minor. Within the snails, such minor characters are often manifested in small differences in shell shape, operculum, and shell ornamentation.

The evolutionary interrelationships within mollusks have been the subject of much research within the last twenty or so years, and as a result the taxonomy of the whole phylum has been undergoing a set of major changes. Presently, these animals are taxonomically classified as being in species that taxonomists have placed in the Families Trochidae and Turbinidae of the Superfamily Trochoidea. That Superfamily is, in turn, placed in a larger group called an Order. The Ordinal name is uncertain at the present time as it is undergoing revision, but it is one of several orders within the Superorder Vetigastropoda of the Subclass Orthogastropoda. All of these, of course, are within the Class Gastropoda, which contains all the snails. One can refer to all of the grazers I will discuss in this column as Trochoidean snails.

Trochoidean gastropods include all the Turbo, Astraea, Trochus and related animals such as Margarites, Stomatella, Norrisia, and Tegula. There are many other snails that eat algae in aquaria, consequently the Trochoidea is not all-inclusive of grazers. It does not include the nerites, abalone, limpets, the ceriths, conchs, and some cowries. These latter animals are perfectly good algae-eating animals, but they are only very distantly related to the trochoidean grazers, and I will cover them in future columns.

Most, but not all, of the Trochoideans have shells that look rather similar; as they should if they are all closely related. It is, in a sense, this similarity that prompted this column, as these animals are often difficult for hobbyists to tell apart. Their similar shell shapes may sometimes be deceiving, though. Stomatella varia has a small, flattened, cap-shaped shell and tends to look like a slug. It is, however, a good Trochoidean snail. Nevertheless, the shell is decidedly an odd one for the group.

Malacology, or the scientific study of mollusks, began in the sixteenth century among the shell collections or "conchological cabinets" of the "gentlemen naturalists" of that period. These were individuals who had the considerable wealth necessary to indulge their whims by collecting sea shells and other marine oddities to include in their displays, which were often kept in large mansions. Incidentally, the study of corals started at the same time, as coral skeletons were among the items included in these collections. In time, these collections became the basis for many modern museum collections. At the time of the collection, however, the collectors had no consistent way to describe the shells that made up their collections, and over the course of about a century, malacology arose primarily as a way of identifying these shells. In the process, a rather complex and descriptive terminology developed, which allowed the discussion of the various aspects of snail shell shape. I will try to ignore most of this terminology, but some of it is inescapable in a discussion of closely-related snails, because in general, the most visible differences between species are related to differences in shell features.

Shell Shapes:

I think it is useful to start a discussion of a snail shell by considering the basic animal to be a slug, without a shell. Such an animal is essentially a mound of flesh that creeps along on a broad, flat foot. Now, consider that slug with a shell over the top. The shell would likely be rather shield- or cap-shaped, probably something like that found on a limpet. One of the characteristics of snails is that they grow primarily in a top-to-bottom, rather than a front-to-back or side-to-side, direction. In other words, as they add mass to their body, they get thicker, not longer or wider. This means they become taller, and higher off the substrate. Given such growth, the flat cap shell would change to look something like the shape seen in a flat, broad, ice-cream cone. Further growth would tend to make the shell have a truly conical shape, much like an ice-cream cone. If one pictures a small slug carrying a sharply pointed ice-cream cone shaped shell oriented vertically on its back, it is probably obvious that such a shell would be unstable. The way in which this instability was solved appears to have been by coiling the shell. Coiling makes many snail shells doubly conical. They are cones coiled in a conical manner. This coiling pattern creates a shell that is helically coiled; in other words, it's basically spiral in shape, but occupies three dimensions. Each complete 360° "lap" of the coil is called a whorl. The largest and last whorl contains the aperture into which the animal can withdraw, and is called the body whorl. The relative shape of the animal depends on three main factors: the shape of the aperture (which defines the shape of the whorl itself), the rate at which the whorl gets bigger, and the rate at which the aperture moves away from the coil's center. Varying those three factors together can produce all possible snail shell shapes. Fortunately, in the Trochoidean snails, there is not a lot of diversity in shell shape. In some respects, this is a problem, as they all tend to look a lot alike. On the other hand, it makes recognition of the basic Trochoidean pretty simple.


Turban snails tend to have rounded whorls, while trochids tend to have smooth-sided shells. Either shell type may have an umbilicus, a "belly button" or a pit on the underside of the shell caused by the coiling, or a callus, a calcareous plug filling the hole. The auriform shell is formed when the whorl enlarges very rapidly relative to the coiling rate and is found in animals such as Stomatella.

Internal Anatomy:

It is not for the shells, however, that we aquarists put these animals into our tank. To be succinct, we buy these snails for their grazing ability and dietary preferences. Snails, in general, and Trochoideans, in particular, are called "microphagous" feeders. This means they feed on small particulate materials. They get such materials by sweeping or rasping them off the substrate. This rasping ability is related to, and determined by, a structure found only in mollusks called the "radula." The radula (plural = radulae or radulas) is a projection arising from a pit in the bottom of the front part of the digestive tract, just inside the mouth. One side of the radula is covered with spiny projections. In their simplest forms, these spines are made of cuticular material, basically protein and chitin. Vertebrate teeth are also basically cuticular and are found in the mouth and, because of these similarities, the spines on a mollusk's "tongue" are called "radular teeth."

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The radular teeth (red) sit on a radular ribbon (pink) and both are formed in the inside of a radular sac that projects from the bottom of a cavity just behind the mouth. The teeth and ribbon are secreted in the bottom of the radular sac and used teeth are sloughed off into the mouth. The teeth and ribbon sit on cartilage, and the cartilage may be pushed out and pulled in through the mouth by the action of muscles. The radular teeth are relatively complex structures and there may be several hundred rows. The snails' rasping may wear out several dozen rows of teeth per day.

All of these snails feed by moving along the substrate and periodically "licking" the surface with their radula. The design and number of radular teeth is different in each genus or group of species. This difference in the teeth HAS to be reflected in feeding differences - different species will feed on either different algae or different types of substrate (discussed below). The teeth are probably optimized for some algal types, but the feeding behavior and natural history of the Trochoideans is poorly known. It is presumed that most are herbivores, but evidence suggests that a significant number are at least partially carnivorous.

Acclimation Problems, Or Why Do They Die?

The complexity of internal anatomy in snails is one major reason why some of the Trochoideans seem to die easily, or otherwise not live through their full life spans in our tanks. Although most folks probably never give it a second thought, snails have a complex internal morphology. In many ways, the internal structures that the snails possess have allowed them to be very successful. It is not, however, a particularly "rugged" morphology when subjected to some specific stresses. Snail tissues often seem to be composed of thin layers of tissue that are very filmy and diaphanous. Consequently, they often suffer significant damage during water changes or during transport from a dealer's to an aquarist's tank. The circulatory system of snails may be very complex, and many of its vessels and channels can rupture under stresses caused by changes in salinity. The vessels in the kidney are numerous and delicate, and may rupture if the animal is not slowly acclimated when being moved from one set of water conditions to another. If the acclimation is too fast, the animal will die in a few minutes to a few weeks. If the snails are drip acclimated, the acclimation time may need to be on the order of five to ten hours for maximal survival.

Another factor in their long-term survival, of course, is the appropriate food. The diversity of radular structures ought to convince most folks that these animals are not created equal with regard to their diets. Or to put it another way, they eat different things... This is a real problem for aquarists, who often perceive the various grazing snails as "interchangeable," when in fact they may require decidedly different conditions or foods for good health. Unfortunately, the real problem with most of the various grazers is that we haven't a clue what their requirements really are. There have been precious few studies in the natural world of the reefs that assess the determining factors of their existence, and consequently we are stuck with a "trial-and-error" response. This is further complicated by the fact that as hobbyists we really don't know, in most cases, where the animals originate, what microhabitat they inhabit, or even what species they are.

A Trochoidean Primer

Although determining the genus is not terribly difficult, identification to species will take some expert consultation, as there are often several dozen to several hundred species in each of these genera. In all cases, however, there are several characters critical to identification.

First, note the shape and composition of the operculum, or trapdoor, which plugs the animal's hole when it retracts (and with the exception of Stomatella and its near relatives, they all have one).

Second, notice the shape of the shell,

Is it tall or “squat?â€
When viewed from the side, are the edges of each whorl smoothly rounded, or more angular?

Third, does the animal have any sculpture or ornament on it, such as ridges or lines?

Fourth, does the animal have an open "umbilicus" (a pit in the bottom of the shell), or is that pit partially or completely filled by a calcareous "callus?"

All of the above characters are used in assigning the animal to some of the various genera. Be forewarned, most dealer identifications are wrong.

The animals with either a tall or squat shape. The aperture to the shell has an angulate, wavy or toothed appearance due to folds in the shell at the margin near the center of the shell. If the shell is looked at from the bottom, there is a hole in the center. The operculum is typically protein or horn-like material and is generally amber or brown. These are typically good grazers for reef aquaria. They are sold under several names by various dealers. Surprisingly, on some occasions they actually get named correctly, for Trochus, anyway. These genera are closely related and differ mostly by soft part anatomical and radular differences, and so the average hobbyist, and particularly the dealer, may be unable to distinguish them. Trochus or Tectus species seldom have any pronounced bumps, warts, or ridges on the shell surface.

Turban shells, as their name implies, have a more rounded shape, like that of a turban. They have a large and prominent calcareous operculum. They may or may not have sculpturing. Turban shells may have a small umbilicus in the bottom center of the shell, but in most cases the hole is filled with a calcareous callus.

Astraea (and yes, the spelling includes the second "a") may be thought of as "angular Turbo." The outer edges of the coiled whorls are not rounded or inflated as in Turbo, but are often straight or flattened. When viewed from the aperture side of the shell, Astraea tend to present the appearance of a smooth conical shell without the rounded edges visible in Turbo. Many Astraea have very prominent axial sculpture, so that when the animal is observed from above it appears star-shaped. That shape, in fact, gives the genus its name; "Astraea" means star, and they are commonly called "star shells." However, the Astraea species offered for sale in the aquarium hobby are generally not heavily sculpted, although various species may have small bumps or ridges. Most snails sold as Astraea in the marine reef hobby are trochid snails, although they are seldom from the genus Trochus. Many of the actual individuals of Astraea offered for sale in the reef aquarium hobby come from the cooler waters of Baja California. These animals are not from areas that have sand substrates or, really, much of anything else in the way of flat surfaces. If they get dislodged from rocks, they tend to fall into crevices or crannies where they can reach a rock with their foot and attach to it. Consequently, they have never developed a "righting response." This means that if they fall from the aquarium walls onto the sand, they will not be able to turn over, and will die there unless somebody or something turns them over. Given that they also are not warm water animals, they tend to die young and leave a good-looking corpse in the aquarium. Purchase them if you wish. I would spend my money on tropical species, however.

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Astraea, like Turbo, have a calcareous operculum, and unlike trochids they seldom have an open hole in the bottom center of the shell.

Collonista

Collonista are "mini-Turbo snails." They are seldom purchased by hobbyists, but are relatively common in reef tanks anyway, because they appear to hitchhike in on live rock or in some live sand. When hobbyists first see them, they presume them to be "baby" grazers of some sort. Instead, they have some of these animals. They reach a maximum adult size of about 1/4th inch (6 mm) in height and diameter. They are often tan to white and have mottled brown color patterns on the shell. They can be distinguished from all other Trochoideans by their small size and the presence of a small pit or hole in the center of the calcareous operculum that plugs the aperture. They reproduce well in aquaria, and are quite good grazers. If present in large numbers, they may effectively replace all other grazers in our systems.

Stomatella varia

Almost slug-like, but possessing an auriform, cap-like shell, individuals of Stomatella seem unlikely to be Trochoideans. However odd their appearance, their internal anatomy puts them squarely in this group. Stomatella individuals may reach lengths of about an inch and a quarter (3 cm), and as their specific name of "varia" implies, they may be almost any color, although gray, black, and mottled colors tend to predominate. They are quite good grazers, and also reproduce well in aquaria lacking hermit crabs and shrimp predators. Their shell shape precludes their retraction into the shell, but evolution has given them a rather neat way to escape many predators. The rear portion of their foot can break off, much like some lizards' tails, when they are threatened by a predator. The detached portion then writhes, and twists and otherwise distracts the predator while the snail, presumably, makes good its escape. Additionally, they also tend to be nocturnal and avoid a lot of visually-oriented predators in this way.

A large number of Trochoideans are sold to reef aquarists despite having little or no chance of long-term survival in a reef tank. Most of these are collected from the northern shores of Baja California. Although it's south of the United States, the marine environment of the northern part of this peninsula is anything but tropical. Its Pacific side is bathed in waters that are quite cool, and is the home of several Trochoideans that are collected for the reef aquarium hobby. These animals typically have a tolerance for warm conditions, and they are often intertidal animals and can withstand quite hot water - for a while. Prolonged exposure to warm conditions, however, kills them.

Among their many interesting attributes, many marine snails have impressively long life spans. I have counted over 120 annual growth rings on some specimens of Tegula funebralis, a temperate water species. This species is one of the three or four species of Trochoideans collected from cool water areas of Baja California and unethically sold to gullible, or informed, aquarists as a reef aquarium animal under the delightfully ambiguous name of "margarite or margarita snail. Tegula funebralis has a high thermal tolerance for an animal that lives in cold water areas (it ranges northward from Baja and is common in the British Columbian and Alaskan intertidal zones). They normally live a small fraction of one percent of their normal life span, or only a few months, in reef aquaria. Putting these animals in a reef aquarium is both unethical and immoral.


Tegula, two species; Tegula funebralis, and Tegula brunnea. These cool water species of Tegula are harvested from the coast of either Baja California or California and sold under the name of "margarita" or "margarite" snails. Shells are about ¾ inch (2 cm) in diameter.

This confusion is complicated by various distributors and dealers who just can't seem to grasp "The Phenomenon of a Name," and blissfully attach names seemingly at random to their livestock. A good example of this is in the common name, "Margarite snail." Well folks, I have been studying snails for a long time, and to me a "Margarite snail" is a snail in the genus Margarites. These are small snails, similar in many regards to the grazers we put into our tanks. They are even found in the tropics. However, when they are found in the tropics, they live several thousand feet down in water whose temperature is 39° F. or lower. In other words, there are no snails of the genus Margarites that are found in warm tropical waters. One finds examples of the common North Pacific species, Margarites pupillus offered for sale and this invariably leads to a quick death for this species if kept in reef aquaria. As with the species of Tegula, with which they share the common name "margarite" snail, they have no place in reef tanks. Still vendors persist in selling them. Go figure...

Margarites pupillus. This species is a cool water animal and should not be purchased for a reef aquarium.

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Norrisia norrisi

The red turban or red-footed turban shell, Norrisia, is yet another cold water animal offered for sale to reef aquarists. As with individuals of the various Tegula and Margarites species, Norrisia individuals effectively get cooked slowly to death in reef aquaria. Norrisia individuals generally die faster than those of the various Tegula or Margarites species, however, because they not only get slowly poached, they also starve. This animal lives on the giant kelps off the central and southern California coasts, and grazes on the surface layers of the kelp. It really is not a diatom feeding animal so much as it is a feeder on kelp mucus and surface cells. Such food normally is lacking in reef aquaria, so while Norrisia looks nice in the aquarium, for a while at least, it has nothing to eat there.

Conclusion:

Many of the various Trochoidean snails are effective grazers and thrive in coral reef aquaria. The key to finding the right species, however, is in finding species that are actually tropical, rather than temperate. Almost all the tropical species do well in reef tanks, while virtually none of the temperate forms persist. By and large, all of the species imported for the aquarium hobby feed primarily on diatoms, and their longevity in aquaria is directly dependant upon having sufficient food. Although one often sees advice to add "x number" of these snails per gallon, such advice generally results in the death of most or all of the snails, as they generally exhaust their food source and starve to death. It is better to add them to a tank a few at a time until there are sufficient grazers in the tank so that at the end of the day there is a barely visible film of diatoms on the rocks or aquarium walls. Such a film will be gone by the morning, and the snails will keep the diatoms in check.

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More grazers

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


Introduction:

Above 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 article, 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.

The trochoideans that were the subject of the above article 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 article 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.

Abalone

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.

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Limpets

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.

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.

Nerites

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.

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.

Conclusion:

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.

THIS ARTICLE IS A PARTIAL EXCERT FROM THE COMPLETE ARTICLE.

END

 

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more on snails

The Grazing Snails, Part III: Conchs, Ceriths, Cowries, and Columbellids


Introduction:

The focus of this third article devoted to grazing snails is on some of the less traditional algae-eating snails; specifically, the Conchs, the Ceriths, the Cowries and one species of a group called the Columbellids. Unlike the trochoideans, but like the animals that I discussed last month, these animals are not closely related and are from several distinct and different evolutionary lineages. This means that they don't look or behave alike. Their basic snail features, however, are the same as in the trochoideans, and I refer the reader to May's column for the discussion of anatomy and physiology and how they relate to the necessity of slow acclimation to differing conditions, particularly salinity. As with the other snails, these animals all do best at normal reef physical conditions where salinity remains in the 35 ppt to 37 ppt range, and the temperature is between 80°F and 84°F. The basic method of feeding also remains the same; the animals "lick" the surface of the substrate with a structure which has been called a "rasping tongue," and which biologists call a "radula." The radula in these snails differs significantly from the rasping organ found in the trochoideans. For example, it has a lot fewer teeth; however, the teeth are generally more robust and structurally specialized, and their basic function remains unchanged.

There are a LOT of different snail species, and as I mentioned above, the grazers that are the subject of this column have no similarity of shape. Not only that, but with most of these useful species, similarly appearing animals often have decidedly different habits. Not only are these look-alike species not benign grazers, but instead they may prey on desirable reef aquarium species. So, it pays to closely examine the animals that you intend to purchase or receive from dealers. A few suppliers do a good job of separating these dangerous species, but I have seen a number of them surfacing at various places, and this is mostly due to faulty collection and identification.

As I have mentioned in the previous articles, it is up to the individual hobbyist to be sure of the identification of these beasts. I again list the following three references, which can be very useful in determining which snails have appeared in our tanks. These bookshave the added advantage of being found in many local public and university or college libraries, and so may be readily available for consultation.

These references are: 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.

Ceriths

Individuals of a few species from the immense group that taxonomists refer to as the Superfamily Cerithiacea are becoming relatively commonly found in the reef aquarium hobby. The Cerithiacea consists of several large subgroups, called "Families." The most common of these are the Family Planaxidae, which contains the genera Planaxis, Hinea, and Quoya; the Family Modulidae, consisting of the genus Modulus (1,2) whose species look quite trochoidean; the Family Batillariidae, containing the genus Batillaria; the Family Potamididae containing the genera Rhinocoryne, Pyrazus, Terebralia, Telescopium and Cerithidea; and the Family Cerithiidae, containing the genera Cerithium (1,2,3,4,5), Bittium, Rhinoclavis (1,2,3) Pseudovertagus(1,2), Clypeomorus, Gourmya, Colina, Trochocerithium, and Campanile. The animals constituting the species in all of these genera are relatively similar in basic appearance and are characterized by having a shell with a small, almost circular, aperture which has a small siphonal canal or "spout" which projects at an oblique angle to the long axis of the animal. Additionally, the shell is typically quite long relative to its width, and mature animals often have six or more visible whorls. They typically have a proteinaceous or horny covering, or operculum, that plugs the shell's aperture when the animal is withdrawn. In a few species the shell may be smoothly rounded, but typically the shell has some ridging that is parallel to its long axis or spiraling around it, or both. The ceriths typically found in the aquarium hobby generally come from the genus Cerithium, Clypeomorus, or Batillaria, but really almost any tropical cerithiacean would do well in aquaria provided there is an appropriate substrate, and given the non-specific nature of the collectors in the field it is likely that many others will eventually show up in reef tanks. Most of these species are found in or on unconsolidated sediments, although many others, such as those in the genus Planaxis, are found on rock. In most cases, they have been shown to be sand-swallowing organisms. Such animals eat the sediment and digest its organic material, mostly encrusting bacteria and microalgae. Those found on hard substrata are generally grazers that feed similarly to the trochoideans. Individuals from several species are commonly found in aquaria as part of the sand bed fauna. They will also venture onto rocks and aquarium walls from time to time, and use these sites to lay their rather distinctive egg masses.

Conchs

Conchs, or members of the species in the gastropod group called the Family Strombidae, are strictly herbivorous. Within this family are several genera, including Strombus (1,2,3,4,5) (true conchs), Lambis (1,2,3) (spider conchs), Tibia (1,2) (tibias), Varicospina (beak shells) and Terebellum (terebellum), that differ significantly in shell form and structure. Their internal anatomies, however, are similar. Unfortunately, many of these animals, particularly those in the genus Strombus, have a basic shell shape that most people think is more representative of the carnivorous whelks. This type of shell is tapered at both ends with the front end elongated out into a calcareous spout or siphon. In some cases, ignorant or malicious dealers have sold some of the predatory whelks as various conchs. There is, however, a surefire way of discerning whether you have a real conch. When the animal extends from its shell, look closely at its eyes. In all of the animals in this group, the eyes are large and evident, located at the end of a long stalk, and they possess an evident eyeball with a visible pupil. They are visually oriented animals and will watch you. In the predatory whelks the eyes are typically just a small black dot with no discernable structure and are located at the base of the tentacles coming off the top of the head. The conchs have a long and evident proboscis which they use to bite off chunks of algae. In a very real sense, they are not rasping grazers but biting eaters; however, they are superb at eating algae.

All strombids are animals of the sand and have a small muscular foot which is not adapted to crawling on rocks. They actually move by lurching or jumping, in what has been called "saltutory locomotion" by the researchers who have studied it. Saltutory is a $5.00 word for "jumping." Never let it be said that invertebrate zoologists would use a simple word when an odd or complex one was available. This mode of locomotion is good for sand substrata, but is inefficient and largely ineffective when moving on rocks, so the snails tend to stay on the sand once they have reached a shell length of an inch or so.

A good-sized sand bed is needed to keep these animals successfully. Generally, as a rule of thumb, there should be about one to two square feet of open, algae covered sand per inch of snail shell length. Conchs will move around the bases of the rocks and remove algae from quite high up on the rocks with their amazing proboscides. They are interesting animals, and a valuable addition to a tank that can support them.

Some of them, such as the Queen Conch, Strombus gigas, grow to quite large sizes. Adult queen conchs may reach 16 inches in length and are really too large for any home aquarium. The ones for sale in the aquarium hobby are aquacultured and are small juvenile individuals, often about one or two inches long. However, if given enough algae, they do grow well; I have had one that added an inch of shell length per month for about a year. In doing so, they will get too large for a small tank and will then starve to death. Other smaller conchs are equally good at grazing on algae, and should be purchased instead of Strombus gigas.

(CONT)

 

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(cont)

Several of the smaller conchs, such as the Florida fighting conch, Strombus alatus, have the habit of disappearing under the sand for extended periods. In some cases, they actually will move along under the surface of the sediments, eating algae; in other cases, they will stay in one place but use their proboscis to forage in the sediments up to a couple of inches from the animal. Although they appear to be invisible, they often extend their eyes to or slightly above the sediment's surface, and careful observers can find them by searching the sediment for the watchful snail.

Cowries

Cowries are in the genus Cypraea (1,2,3). This is another of the many huge groups of snails. There are about 500 species described in this genus, and some of them are quite excellent herbivores. Cowries are easily distinguished from all other snails. Their shell is basically ovoid, with the aperture on the bottom. This aperture is a slender opening, and lined on both sides by calcareous bumps or nodules. The shells are highly polished and often brightly colored, but the mantle that comes out and covers the shell when the animal is moving is often even more brightly colored. Simply put, they are beautiful animals worthy of investigation to find some that are appropriate as grazers for our aquaria.

Although seldom seen in the U. S., the money cowrie, Cypraea moneta, is both abundant in nature and an excellent herbivore. A similar species, the ring cowrie, Cypraea annulus, is likewise exceptionally common in many shallow reef environments, and is also herbivorous. These two species are small, about an inch in length, and do very well in marine aquaria. Tiger Cowries, Cypraea tigris, are also good herbivores, but they reach lengths of about four inches and are quite capable of rearranging unstable aquarium rock work. Many of the other cowrie species also may be good herbivores, but unfortunately we have little real data about their diets, and only a few non-herbivorous or very large species are commonly available. Many of the few cowries whose diets have been examined feed on sponges, tunicates, or soft corals, but the diets of most species are not known. These are beautiful animals, and as our information base grows, many of them should be acceptable as herbivores for our systems.

Columbellids

Dove shells, or snails in the family Columbellidae, are common in all marine environments, and they form another exceptionally diverse group. Some of the genera in the family are Amphissa, Alia, Euplica, Strombina, Anachis, Columbella, Parametaria, Pyrene, and Mitrella, among many others. They tend to be small animals and are nearly ubiquitous in shallow water marine ecosystems, including reefs. These are animals that are generally considered to be either predators or scavengers, but as a whole the group is poorly studied, and the natural history of most species is simply unknown. Interestingly enough, at least one species has been marketed quite successfully as a herbivore, and indeed this species is a good algae-eating snail. This species, although sold as the Pacific spotted conch, Strombus maculatus, is not that species, and actually is a columbellid snail, probably in the genus Euplica or Pyrene. The taxonomic status of that particular genus is uncertain, but in any case, it is a columbellid. This particular species reproduces well in aquaria by laying capsules on the walls of the aquarium. Successful reproduction in aquaria is frequent, and these nice little snails are becoming quite commonly found in marine aquaria throughout the United States.

Conclusion:

The number of snail species sold in the reef aquarium hobby as algal grazers has increased dramatically over the last decade. Some of these animals, such as the Norrisia and Tegula species discussed in the May column, are unsuitable for reef aquaria and their continued marketing is really a symbol of the irresponsibility of some dealers and distributors. Additionally, many of the truly subtropical or temperate animals sold in the hobby are also inappropriate for reef aquaria. Unfortunately, these latter species are still commonly available and probably constitute the majority of snails sold as grazers.

On the other hand, the marketing of such true coral-reef animals as Hawaiian Trochus, the money and ring cowries, several species of conchs, and the small columbellid species known as "Strombus maculatus" is quite indicative of either fortunate happenstance or the application of reasonable natural history information to develop alternatives to some of the unsuitable animals often sold as "reef grazers." These true reef animals add a natural component to our aquaria. They also live long and prosper in our systems, while providing a stable means of controlling various types of algae.

THIS IS AN EXCERT OF THE COMPLETE ARTICLE.

END.