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



Hidden within a single shell and slimy foot is the secret to one of the most significant evolutionary success stories in all of the Animal Kingdom. If one were to scan the entire array of animals and total up the number of known species, the animal group with the most species would be, without question, the insects. The second most species-rich group, however, would be the gastropods, or snails, in all of their slow, slimy glory. The total number of snail species is not known, but estimates range upwards of 50,000, and sometimes much higher.

Shells indistinguishable in details from modern snails appeared during that part of the Earth's history that geologists term the early "Paleozoic" period, well over 400 million years ago. Although we don't know the anatomy of these animals, we must presume that they were not too dissimilar from some of today's snails, such as the Pleurtomariids. These ancient snails were not particularly rare, but neither were they particularly abundant or ecologically dominant. Tracking forward from this ancient beginning, the snails show several changes in bodily structure, at least as reflected in the changes in shell shapes that appear in the fossil record. In many cases, there appear to be modern descendents of these groups, so it is possible to postulate what the ancient representatives were like. Some of these ancient forms would be passingly familiar to aquarists; they look to be the ancestors of animals such as the trochoidean grazers, sand- or mud-dwelling ceriths or the small, rice-grain sized rissoids found in many aquaria. Most other types, however, would be recognizable only to malacologists, those scientists who, like me, study mollusks.

Figure 1. Perotrochus lucaya, collected from a Bahamian coral reef at a depth of 300 m (1000 ft.), is a Pleurotomariid snail, a true "living fossil" and a remnant of an ancient group once dominant in the Paleozoic era. The slit in its shell is characteristic of these animals, which are thought to be similar to the ancestors of most other snails. This animal was about 2.5 cm (1 inch) in diameter.

Sometime about the beginning of the Mesozoic era, when the dinosaurs' dominance of the terrestrial world was not yet even a dream of the lumbering reptiles of the time, one, or several, gastropod lineages underwent a series of striking changes in form that resonate down to us today. Some of these changes would be obvious to the casual aquarium observer, if that person could choose some ancient snails for his tank. Other, equally important, changes would be less obvious. All of these changes in form, however, came together to produce animals whose shells look like modern whelks. These animals were probably not the first predatory gastropods, but their body's form is supremely adapted for predation upon soft-bodied or sessile prey, and once they appeared, they explosively radiated throughout the world's oceanic habitats.

The Earth's history is replete with advances in animals' bodily form made to exploit hitherto unexploitable environments. Until the late Paleozoic period, it appears that while soft-bodied worms such as those presently found living in a deep sand bed were abundant, they weren't very diverse. Additionally, clams, while present, were rare. If we could see it, the ocean bottom of that time would appear much different from the one we see today, and the diversity of its life would be very limited. At the end of the Permian epoch, 252 million years ago, all of Earth's life almost died. By some estimates, 95% percent of all animal types and 99% of all animal species became extinct. This extinction event is so dramatic that geologists use it to mark the end of Paleozoic period. When life rediversified after this event, many changes occurred. The old groups of animals didn't simply reappear but, rather, their few survivors diversified, radiated and evolved to fill all sorts of "vacant" ecological niches left by the massive extinction event. At this time, animals began to effectively exploit the soft ocean bottoms for the first time. Diverse arrays of clams and worms, among other animals, appeared. This process wasn't rapid or immediate; it took several scores of millions of years, but by the beginning of the Jurassic period, roughly 200 million years ago, the ocean bottoms were again teeming with life and for the first time, it appears that diverse and abundant life was found in the soft sedimentary ocean bottoms. As the animals that populated these regions evolved, so did their predators, and one of the major groups of these predators was comprised of the newly evolved whelk-like snails (Tasch, 1975).

What's a Whelk?

The whelks are predatory snails found in several distinct, but probably closely related, taxonomic groupings. Although it is not terribly difficult to describe a whelk, and I will get to that below, several things must be taken into account when trying to determine if the mystery snail in an aquarium is, or is not, a whelk. First, color doesn't matter. While I haven't seen a chartreuse or bright blue whelk, I have seen just about every other color in their shells from black to white to hot pink. Second, size doesn't matter. The smallest adult whelks are only a few tenths of an inch long, and the largest top out at over two feet in length. Additionally, of course, tiny juveniles of relatively large whelks are among some of the whelks most commonly found in reef tanks. As a consequence of these two factors, illustrations may not be very useful in trying to identify such animals. (Nonetheless, here are some links to images; all of these animals - and many, many, others - may be termed "whelks," 1, 2, 3, 4, 5, 6, 7, 8, 9, 10.)

This latter point, the abundance of juvenile whelks in reef aquaria, causes some further complications. Whelks are, in general, very long-lived animals and estimates of adult ages in the three to four decade range are fairly frequent. Such a long life often presupposes a long juvenile period, and that appears to be the case with many whelks; they may not reach sexual maturity for several years. As in humans and most other animals, they grow to their adult size while sexually immature. Unfortunately for the person interested in a quick and dirty identification, the definitive shell characteristics - those characteristics used to distinguish the animal from its nearest relatives - often don't appear until sexual maturity. In snails, these "definitive" characters often involve thickening of the aperture's outer lip and changes in the aperture's shape. Consequently, while it is often relatively easy to identify an animal as a generalized "whelk," particularly the small juveniles that appear in reef aquaria, it is often very difficult to put a definitive and specific name on it.

Figure 2. Generalized shell characteristics of whelks (modified from Keen and Coan, 1971). Not all whelks have all of these characters, and the characters may occur in just about any combination. Note that the front of the shell, as the animal is crawling on the substrate, is at the bottom in the illustrations.

The generalized whelk shell form (Figure 2) is fusiform or biconical. That is, the shell may be thought of as spindle shaped, or as two cones joined at their wide base with the pointed apical ends facing to the front and rear. Although people generally hold a snail shell with one pointed end up and the other down, the animal crawls with one of these ends, the one ending in a trough, directed forward, and the other, the apical, or the end with the sharp point, directed to the rear. It is conventional to discuss the shell with the orientation shown in Figure 2. The apex is at the top and the siphonal canal, or notch, is at the bottom. If the animal is held in such a manner, its aperture will generally be on the right side when viewed with the aperture facing the observer. If observed from the top, above the apex, the animal coils in a clockwise direction. Such a coiling pattern resulting in the aperture appearing on the animal's right side, is termed "dextral." Most snails exhibit dextral coiling, but a few, including some common whelks found along the Gulf of Mexico's northern coast, do it the other way and coil counter-clockwise. Such coiling is referred to as sinistral coiling. (1, 2 - and here is a dextrally coiled near relative of the illustrated sinistral animal). Sinistral coiling is rare; well over 99 percent of the shelled snails coil to the right. Why they do this is unknown.

The shell characters illustrated in Figure 2 are exceptionally variable. They vary between species, and the various combinations of the characters are used to distinguish species. Unfortunately, they may also vary within a specific species. For example, one researcher categorized over 2400 combinations of color, lamellar height and number, and shell proportions in the common intertidal whelk, Nucella lamellosa, of the Pacific Northwest. Additionally, some of the characters are labile, meaning that while they are genetically determined, the extent of that genetic expression may be triggered by some environmental characteristic. For example, in the frilly dogwhelk (Nucella lamellosa) mentioned earlier, some populations effectively lack the lamellae that characterize the species. If individuals from one of these populations are put into an aquarium that receives the outflow from one of the snail-eating crabs found in the region, Cancer productus, some chemical given off by the crab will induce the production of lamellae as the snail grows. Because snails grow and add shell only around their aperture, we know that the older parts of such animals lack lamellae and the younger parts have them.

Figure 3. Nucella lamellosa, a whelk from the Northeastern Pacific with highly variable shell morphology. Notice the animal on the left is effectively smooth, lacking the lamellae that give the species its name. The shell on the right has the lamellae. Color variations such as seen in these two specimens are common in this species.

From the aspect of the aquarist or the person trying to identify such animals, there is also the problem of the ambiguity of many of the terms used to describe the shells. For example, the difference between a varix and a lamella is really one of degree. There is no discrete difference between a varix and a lamella. In one sense, a varix is a low rounded lamella and a lamella is a flattened tall varix. To make things even more fun, some whelks are considered to have lamellae on top of varices (the plural of "varix" is "varices"). Other terms describe shell characters that are equally "fuzzy." Unfortunately, there is nothing we can do about this delightful situation; practically, it means that it may be very difficult to identify an animal from a written description. One observer's varix is another's lamella and vice versa. For any readers who occasionally read the comments on my online forum, this is one reason I generally ask for images of snails. Likewise, the variability seen in the apertural characters, and their importance in the species descriptions, is the reason that I ask for images that show the apertures in crisp, clear focus.

These problems notwithstanding, it is generally pretty easy to determine if a snail is a whelk. Whelk shells are generally biconical or broadly fusiform. Their aperture is typically oval. The animals typically, but not always, have an operculum on the back of their foot which plugs the aperture when the animal withdraws into its shell. If such an operculum is present, it is made of protein and typically is brown, golden or black; and it is never calcareous and round, but rather oval, crescent-shaped or somewhat "leaf-shaped." Whelks always have a siphonal canal with an anterior siphonal notch. The notch is generally quite distinctive, although the canal may be short and twisted. All other sculpturing is variable and dependent upon at least species, and maybe upon environment.

Whelk Lookalikes

In aquaria, basically two other snail types may be confused with whelks; these are the smaller columbellids and the strombid conchs. Columbellids are small, seldom attaining sizes greater than about four tenths of an inch (one centimeter). They may appear quite whelkish, but although they often have an anterior notch at the front of their shell, they generally lack a true siphonal canal. Additionally, they often have columellar folds, which are uncommon or lacking in most whelks. Columbellids are rare in reef tanks; the only common one is the small species sold as "Strombus maculatus" by several vendors. Actually a columbellid snail in the genus Euplica or Pyrene, its basic shape and aperture without a distinct siphonal canal indicate it is not a whelk (here is an image of an actual Strombus maculatus). Strombids, commonly called "conchs," are quite "whelkish" in shell morphology; however they lack a siphonal canal, although they often have a prominent siphonal notch. Strombids have well-developed eyes, with visible "eyeballs" on the ends of stalks. No whelk has an eye that looks remotely like this. All whelk eyespots are small back dots located at the base of their cephalic tentacles, found on each side of their head. Whelks eat meat, thus are all predatory or scavengers; however, no strombid can eat flesh, and because of this strombids are perfectly good reef aquarium animals.

Figure 4. This small columbellid (Euplica or Pyrene species) is often misidentified as "Strombus maculatus." It is a whelk look-alike that is a highly beneficial grazer found in many reef aquaria. Notice that although it has a siphonal notch and visible siphon, shown in the left image, it really doesn't have a siphonal canal. It grows to a length of up to 8 mm (0.3 in).

Figure 5. Some strombid whelk look-alikes. Top: Strombus gigas, the queen conch. The proboscis with the mouth at its tip is similar to that found in whelks, and the siphonal notch is also present. Bottom: Strombus alatus, the Florida fighting conch. Pictured is the front end of a half-buried animal showing its eye, with a visible "eyeball," which is characteristic of the strombids. All strombids have such eyes, while no whelk does. Thus, the appearance of the eye, alone, can serve to distinguish the beneficial strombids from the predatory whelks.

What Really Makes a Whelk a Whelk?

As with books, one cannot really judge a snail by its cover or, in this case, its shell. Because of the group's vast diversity many snails are bound to look alike that are, at best, distantly related. The true whelks are distinguished from other snails by the characteristic anatomy of both their external and internal soft parts. If the basic ancestral snail was something like the Paleozoic Pleurotomariids discussed previously, numerous changes must have occurred before the animal could be considered to be whelk.

Snails are differentiated and characterized by the process of torsion. Torsion is a 180° rotation of the animal's visceral mass just behind its head. In essence this converts the straight gut of the ancestral mollusk into a "U" shaped gut, with the anus situated, in the "idealized" form, directly above the head. In practice the anus always is some distance off center, and generally lies to the right of the midline, under the shell's right front edge. A snail's body extends behind its head and then rises upward in a mass that contains most of the internal organs or "viscera." This "visceral hump" is what the shell covers. In some snails, such as the limpets, this mass is a simple low cone, and the shell is simply a calcareous conical covering of the hump. In most snails, including the whelks, the hump is much larger and extends upward. It is coiled, and this causes its calcareous cover to turn into the characteristic coiled snail shell. This shell, with its internal mass of guts and other goodies, is relatively heavy and lies on the animal's back as it moves along. In doing so, it creates a space in front of the visceral mass, but under the shell and behind the snail's head. This space is called the mantle cavity, and it is lined by a mass of tissues referred to as the mantle. The mantle lines the snail's aperture and secretes the shell from some folds along its leading edge. It also continues up under the shell and can secrete more shell material to thicken the shell from underneath.

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Figure 6. The process of torsion is THE one unique and defining characteristic of all gastropods or snails. No other types of mollusks undergo torsion. Left: Torsion results in the shell and body mass being turned 180° relative to the head-foot mass. This brings the anus from the rear to the front of the animal. Right: The relationship of the pre- and post-torsional nerves and gut. Torsion occurs in all snails when they are larvae, regardless of whether or not they have shells, and regardless of whether or not the shell, if any, is coiled.

The mantle cavity in the whelks contains a number of important organs and structures. The left front edge of the mantle extends outward to form a fleshy tube that is often quite extensible. This tube, called the siphon, may often extend as much as one shell length in front of the snail and is generally held upward at an angle to the substrate. The animal inhales breathing water through the siphon. In some whelks, the siphon is completely covered by shell material; in others the shell protects only the bottom part of the fully extended tube.

Figure 7. A diagram of a generalized, and hypothetical, whelk as seen by its prey as it approaches. The body's and mantle cavity's structures are labeled.

Figure 8. Nucella lamellosa, a whelk, dissected by one of my students, with its mantle cut open to show some of the structures shown in Figure 7. The snail's body is to the left, out of the image.

As the water from the tube passes into the mantle cavity from the left front, it passes over an organ referred to as the osphradium. This structure is an oval or diamond-shaped mass of pleated ridges lined with chemosensory nervous tissues. In effect, it is the snail's equivalent of a nose or "sniffer." Whelks often have exceptionally good abilities to smell their prey from a distance, and when they are hunting, they move their siphon around to find scent, similar to a bloodhound sniffing while on the trail of its prey. This behavior may be seen in reef aquaria by watching Nassarius snails in search of food. Nassarius are perfectly good whelks, differing from the typical whelks only in that they are not predatory, but wholly scavengers. Nonetheless, they can still follow the scent trail upstream to their food, the delicious, juicy remains of the recently deceased, from several meters away.

After the water has passed the osphradium, it encounters another large mass of pleated ridges, the gill, and gas exchange takes place. Then it passes to the right and first flows over a glandular area that secretes a lot of mucus, called the hypobranchial gland; then over the urinary opening or kidney pore; next it passes by the genital structures, and finally over the anus on the way out of the right side of the mantle cavity.

Unlike the trochoidean grazing snails, such as Turbo and Trochus species, the whelks have well-developed copulatory organs. They also perform internal fertilization, laying eggs enclosed in tough proteinaceous capsules. A few whelk species lay eggs that hatch to produce swimming larvae. Most whelks, however, deposit capsules that hatch to release fully developed juvenile snails, and therein lies one major problem for aquarists. Many of these capsules are amazingly rugged and do a very good job of protecting the developing snails within them. They can easily pass through the collection, cleaning, transportation and curing processes that occur with live rock only to hatch sometime after the rock has been put into an aquarium. The length of time from capsule deposition to hatching for most tropical whelks is not known, but some of their temperate counterparts have impressively long encapsular periods. They may take anywhere from three to 13 months from deposition to hatching. Aquarists who neglect to carefully examine their live rock and remove such egg capsules may acquire "a gift that truly keeps on giving" for a long time. Each female whelk may deposit dozens to hundreds of egg capsules, and these may hatch over a several week period, months after deposition. Each capsule typically releases up to a half dozen or so voracious little whelks. These animals are predatory from the moment they leave the capsule and some may cause significant damage even when very small.

Figure 9. Whelk egg capsules. Left: A Neptunea pribiloffensis female depositing her egg mass. When she is finished, the egg capsule mass, made of hundreds of capsules each containing thousands of eggs, looks like a corn cob. Typically, these eggs take 13 months to hatch from their capsules. Right: The egg capsules of a Ceratostoma foliatum. Here each capsule is separate and contains less than 100 eggs, which will hatch after about three months. Regardless of the number of eggs in the capsule, generally less than six small snails will hatch out of the egg capsules of any given whelk. Those that survive eat the other eggs to nourish their own growth.

Why Whelks are Problems in Aquaria

These animals' internal anatomy is not particularly important except for one thing: compared to the common grazing snails, they have very highly modified foreguts and stomachs. The typical ancestral and primitive mollusk's foregut, the part of the gut found between the mouth and the stomach, contains an organ, the tongue-like radula, which abrades and collects algal cells. Although often likened to a rasp, the radula in these animals is more like a leaf rake; possessing rows of relatively flexible teeth. When the animal grazes, the radula acts like a tongue and "licks" the substrate. During this process, the teeth dislodge and sweep up unicellular algae such as diatoms. The flexible nature of these animals' teeth is the major reason that most molluscan grazers won't eat something such as hair algae: they simply can't cut it loose from the substrate. The radula in these grazers sweeps fine particles of food into their mouth where they are bound in mucus, produced by salivary glands, and conveyed to the stomach. The very complex stomach typical of herbivorous mollusks has internal modifications to ensure that only tiny, and precisely sized, particles of "vegetable" material are sent out of the stomach to the midgut glands to be digested. The stomachs of most clams, including tridacnids, grazing snails such as the trochoideans and strombid conchs all possess the same basic architecture. Such animals are obligatorily herbivorous; they can be nothing but vegetarian in their choice of food.

Whelks, on the other hand, are obligatorily carnivorous. Their radula is no leaf rake! Instead of dozens of modified flexible radular teeth in each row, whelks typically have only three robust teeth with long sharpened scimitar-like cusps in each row and, of course, the radula may contain many hundreds of rows of teeth (here is a beautiful scanning electron microscope image of the "business end" of such a radula, from the whelk Murex brandaris). The teeth are discarded as they are used up and replacements are added as it becomes necessary. These teeth are designed either to lacerate flesh or to first cut through bodily coverings, such as shells, and then to lacerate flesh. These animals' radula is very strong, and is located at the end of a long proboscis that may extend great distances out of the false mouth that exists at the front of their head. The actual mouth is located on the tip of the proboscis. It is not unusual for a whelk only an inch long to be able to extend its proboscis a foot or more. The radula is located near the end of the proboscis just behind the mouth, and by extending the proboscis, the mouth can be pushed into a burrow or tube to tear apart and eat the animal living within. As the radula rips into the food item, salivary glands produce lytic, or digestive, enzymes that start digesting the prey as it is being eaten alive. The slurry of partially digested prey tissues and bodily fluids is pumped into the stomach and transferred directly to the midgut or digestive glands for final digestion. As no sorting of the food is necessary, whelks have lost the complicated primitive stomach and have replaced it with a simple "U" shaped bend with openings to the digestive glands. These animals cannot eat anything but slurries of flesh and blood.

Figure 10. The lacerating radular teeth of two Northeastern Pacific whelks. Each row of teeth contains three teeth, one on each side and one in the center. The radula may contain over 100 rows of teeth. Animals in these two species are predominantly predators on polychaete worms. The scale bars are 1 mm (Shimek, 1984).

One of the other properties of some whelks that makes them such efficient predators is their ability to bore or drill holes completely through calcareous shells such as the plates covering barnacles and clam shells. These specific whelks, which often carry the common name "drills," don't have any particular modification to their radula that allows them to cut holes into a shell. The radular teeth are proteinaceous; by themselves they cannot cut through the calcareous "limestone" that comprises a snail, clam or barnacle shell, no matter how vigorously they are rubbed over it. However, these particular whelks have a gland in their foot that secretes a group of chemicals that act as a chelating agent. Chelating agents are chemicals that have an affinity for some metal, in this case, calcium. The affinity these chemicals have is so strong that they can literally pull the calcium out of the calcium carbonate crystalline matrix of the clam or barnacle shell. The shell basically turns into a pasty substance and the radula just cleans it out of the way. The animal moves its radula in a circular motion on the shell's surface and periodically "wets" the area with the glandular secretion from its foot. In very short order the shell can be perforated. A small whelk is able to cut through a normal clam shell in a couple of hours, and a large whelk could easily drill through the thickest part of a Tridacna shell over the course of one night. Once the shell has been pierced, the whelk extends the proboscis though the hole and the radula rips up the clam's body and ingests it.

Figure 11. Diagram of the side of a whelk showing the proboscis and foregut structures. Top: This shows the proboscis retracted into the cephalic hemocoel, or blood cavity, at the animal's front end. With the proboscis retracted, the true mouth is hidden as the proboscis is pulled completely into the animal.

Figure 12. This whelk, a small Neptunea pribiloffensis, was feeding on a group of polychaete worms living in a hole when I removed it for measurement. The proboscis is seen partially extended and was retracting when I took the picture; fully extended, the proboscis was about twice the shell's length.

Whelks in Reef Tanks

With the exception of one group, whelks are not animals that are, or should be, welcome in a normal reef aquarium. The exceptional whelks that do well in reef aquaria, and which are good neighbors to all animals in the reef tank, are the nassariids. These animals, mostly in the genus Nassarius, but also including a few other small genera, are typical whelks in all regards except their diets. They are specialized to eat only carrion. In reef tanks, they eat excess meaty food before it can rot, and they eat recently deceased or dying organisms. Probably as a result of their specialization upon carrion, which in nature is found on the surface of sediments, nassariids typically have a proportionally shorter proboscis than other whelks. They can't reach deep into spaces to eat worms, nor can they drill holes through clam shells. They can, however, and do clean up excessive meaty foods very efficiently.

Figure 13. Nassarius vibex. Nassarius species are some of the few reef aquarium safe whelks; they are true scavengers and eat only either dead or dying animals.

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Figure 14. Some common Caribbean whelks which may be found in reef tanks, as they are typically imported on live rock. Left: Fasciolaria tulipa, the tulip shell. These animals reach lengths of 20 cm (8 inches), but small juveniles are sometimes found in aquaria. Center: A whelk in the genus Cantharus. These animals reach lengths of 5 cm (2 inches). They are very commonly found in aquaria. Right: The Horse conch, Pleuroploca gigantea. The adult animal's body is black and is visible extending toward the sand from this specimen's aperture. Individuals of this species are some of the largest shelled snails; adults may reach lengths exceeding 60 cm (2 feet). Small juveniles are very attractive, having a bright orange shell and red body. They are quite frequently encountered in aquaria.

The more standard whelks are often long-lived animals. As with many such animals, they would do well in a tank dedicated to their specific needs. Additionally, these animals are often beautiful; not only are their soft parts often brightly colored, the shells of some of the whelks are among the most beautiful and desirable sea shells. The whelks that most often enter aquaria are inadvertent hitchhikers on live rock; juveniles of some of the common Caribbean species. Still others are purchased by unknowing aquarists assuming they are some kind of grazing snail. In either case, their appearance in an aquarium is "bad news." Here is a slide show of what can happen when a whelk, in this case an individual of Cantharus cancellarius, is placed in a typical reef tank. If these attractive animals were not so predatory they likely would be welcome additions to any reef tank. Unfortunately, they are, so they are not. If they cannot be maintained in a system of their own, whelks found in reef tanks should be removed and humanely killed; freezing is a good method. Put the animal in a cup of tank water and put the cup in a freezer overnight. Then clean and examine the shell; with some luck you will have one of the more beautiful sea shells as a curio and reminder of the immense diversity of coral reef animals.

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Figure 15. Not all whelks eat worms, clams or snails. These two individuals of Beringius kennicottii were photographed eating the detached anemone (Urticina crassicornis) shown laying on one of the snails. The anemone had effectively been completely "cored;" only its outer tissue layer remained. The snails are about 10 cm (4 in) long (Shimek, 1980).

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

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