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

Snails That Worm Their Way Into Tanks


Life With A Twist

Success. What is success, and how do you measure it? We all know that there are different definitions of success in the various fields of human endeavor. For some people, success is more-or-less equated to power. For others, it may mean being content with their lot in life. For others, it may translate in to how much good they have done for their fellow humans. It is obvious that the definition of success is arbitrary and depends on the individual and the context of the discussion.

To some extent, the same holds true when discussing the biological world. Within a given group or taxon, success may equate to the number of species found throughout the world. Within other contexts, it may simply mean survival. If life evolved but once, then we are all co-descendants of that one event, and any living organism is monument to some sort of successful longevity. By most estimates, easily 99.999% of all the different types of organisms that have ever lived have come and gone and are extinct (Tasch, 1973; Raup, 1976a,b). Success, in this case, would simply mean persistence.

Other definitions of success, though, relate to the number of evolutionary descendants. By this measure, one of the greatest success stories within that portion of life we call the "Animal Kingdom" is the group termed by taxonomists, the Class Gastropoda of the Phylum Mollusca. Commonly known as "snails," only the great arthropod group we describe as insects has a greater number of scientifically described species. Estimates of the number of living snail species range from well over 100,000 to somewhat less than 30,000. The wide latitude in these numbers indicates the large degree of uncertainty due to a simple lack of knowledge. No matter how you cut it, however, there are a LOT of snail species. Considering this vast number of species, one of the more interesting questions a person could ask is, "Why? What is it about being a snail that confers such a good chance of evolutionary success?" The answer may be in the way snails have partitioned the world into functional units, and to a very great extent this is based on their own structural peculiarities.

Looking beyond the basic differences between mollusks and arthropods, some obvious factors differentiate the degrees of variance between the snails and the insects. One of the interesting oddities of animal diversity is that within the huge group of insects, they really do not look all that different. After having taken a really good look at an insect, it is unlikely that an adult insect would ever be confused with an adult animal of any other group. Not so with the snails. Only one characteristic separates snails from all other groups of mollusks, and that is that snails undergo torsion. Torsion is an internal twist that generally occurs late in the snail's larval life. During torsion, the animal's viscera behind the head and above the foot rotate 180 degrees, bringing the anus to a position pretty much right above the head. Now, this process is bizarre in the extreme, and is probably related to the animal’s locomotion and the way it must carry its shell. Incidentally, torsion does not equal coiling. All snails have undergone torsion whether or not they have coiled or uncoiled shells, or whether or not they even have shells at all. However, the point is that the only thing that absolutely defines snails is internal and invisible to the casual observer. Furthermore, unlike the insects, whose morphology is confined and defined by their exoskeleton, the snails have notoriously variable shapes. They can look like just about anything.

Snails come in a wide variety of shapes and forms; however, there appears to be only a finite number of basic ways to make a snail shell (Raup, 1966; Raup & Graus, 1972). Within such groupings of snails with similar shells, the animals often exhibit some common attributes. Although grades of structure often include groups of closely related animals, kinship is not implied by their structural similarity. Many groups look alike and have similar structures due to convergent evolution. In effect they get to about the same endpoint, but by different pathways, and from different origins.

One such structural level is called the "archaeogastropod" grade of structure. These are the snails that have shells similar in some regards to the earliest snails in the fossil record. Anywhere from a few hundred to a few thousand species can be termed archaeogastropods. They are important to aquarists as this group includes most of the grazing snails that are used to control algae. These are animals such as the abalones, keyhole limpets, many other limpets, and the "turbo" grazers such as animals in the genera Astraea, Turbo, Trochus, and Tectus. With all of that, however, the archaeogastropod structure is relatively consistent and most people can recognize one once they have seen a few of them (Abbott, 1974; Abbott and Haderlie, 1980).

There are two other large groups of marine shelled snails, each of which also comprises animals with distinct basic appearance. One of these, the "neogastropods," comprises the largest array of snail species. These are the whelks, the venomous snails, such as Conus, and their kin and literally tens of thousands of species are put into this grouping. As in the insects, however, this group’s diversity is not manifested in widely differing shapes and structures. Rather, they have diverse internal specializations reflecting their modes of predation. Most of these snails are active predators, and their immense diversity is probably related more to their ability to specialize on specific prey than to their innovative external forms.

The remaining structural group has been referred to as the "meso- (or middle)" gastropods, because in many ways they are morphologically intermediate between the archaeogastropods and the neogastropods. Evolution and development, in general, and gastropod evolution and development, in particular, may be viewed as a sequence of advances, each overcoming an obstacle that allowed for the exploitation of new habitats or food sources. Although the archaeogastropods have similar shells, the many different species possess almost as many different combinations of gills and excretory systems. Consequently, hidden within the similarity of shell shapes are a great many different arrangements of internal plumbing. In snails, unlike vertebrates, the respiratory and excretory systems are integrated to a great degree; blood flows directly from the gill into the kidney and then to the heart. This means that the heart is pumping not only freshly oxygenated, but also freshly cleansed, blood to the tissues. However, there are many ways for the respective plumbing systems to be interconnected and related. Most of these appear to have some deficiencies and are not very efficient.

At about the same time that reptiles started to become common on land, gastropods that were recognizable as mesogastropods started to occur in the fossil record. This marked the beginning of one of the greatest documented evolutionary radiations known. Archaeogastropods, in general, are limited to crawling and feeding on hard substrates. The mesogastropod grade of respiratory and excretory plumbing was apparently all that was needed to start to exploit most of the other marine habitats and methods of feeding.

So, while all of the archaeogastropods have a fairly limited and simple repertoire of shapes, all related to grazing on hard substrata, the mesogastropods literally exploded into hundreds of shapes, sizes, and habitats. Today, the mesogastropod grade of structure is found in about 200 distinct and not closely related lineages. These include such diverse groups as cowries, predatory moon snails, mud-dwelling cerithiids, eulimids which are parasitic inside of sea cucumbers, and, of course, today's main course of this vast gastropod meal, the worm shells or vermetid gastropods (Abbott, 1974).

How They Do It

Worm shells are so-called because their shells look superficially like the tubes of the calcareous feather-duster tube worms known as serpulids. The Serpulids’ head is modified to act as a filter-feeding organ. As with all feather duster worms, this filter-feeding organ is constructed of a lot of finely and pinnately branched tentacles, giving the appearance of tiny feathers on a feather duster. This characteristic structure, however, has nothing to do with their shells, which are often simple straight or coiled tubes. Because the animals construct a small tube when they are small that gets larger as they grow, the tube has the shape of a long, narrow cone that is straight or meanders across the substrate. However, some of them are coiled around their base, very much in the manner of a coiled snail. On the other hand, worm snails start out life as a small, rather normal-looking, snail with a coiled shell, albeit of only one or two whorls. After a short period of free-living life, they cement their shell to a hard substrate. As they grow, the shells may coil or meander over the substrate producing a tube that looks quite similar to a serpulid tube worm shell. However, the tube worm produces a shell that is generally dull-surfaced on both the inside and outside, while the snail's tube is glossy inside. The worm's tube begins as a simple tubular chamber containing the recently settled juvenile worm. The worm tubes are generally white, although they often become colored with coralline algae or other epibiotic growth. In contrast, the small snails start out with larval shells that are tightly and spirally coiled. After the juvenile snail cements itself to the substrate, its shell begins to grow, generally in loose coils, at right angles to the spiral of the larval shell (Keen, 1971). The shells can form quite large entwined masses that are effectively impossible to separate, containing dozens to thousands of snails. Enough snails may become cemented together so that they may actually form reefs, although these reefs are never particularly large.

Figure 1. The shell of a small vermetid from my tank. Note the several normal snail whorls
to the left. This whole animal was about ¼" from top (left) to bottom (right).

Morphologically, of course, these animals have all the internal characteristics that define snails. Unlike the worms, their body is not divided into segments. They have undergone torsion, which in their case is a decided advantage as it places the anus at the front of the tube-shell. Consequently, they can easily defecate undigested foods out of the shell. Most feather duster worms have their anus at the back of the tube and have special morphological adaptations, such as ciliated grooves that serve to transfer their feces out of the tube.

The snails' tubes may be closed by a concave, proteinaceous door or operculum. Reflecting their immobile status, they have a reduced foot that is used mostly in feeding. They possess a pair of relatively large tentacles on the foot, each with an inner groove. A large mucus-producing gland is located in the foot near the tentacles and discharges through the tentacular grooves. Their gut is somewhat peculiar for a snail in that the stomach contains a large rod of hardened mucus called a "crystalline style." Crystalline styles are more typical of bivalves, and contain digestive enzymes (mostly enzymes that break down sugars) embedded in the mucus. The style sits in a sac off the stomach and is secreted at one end of that sac. Cilia in the sac and stomach rotate the style at high speed (in some mollusks the style can rotate at several hundred RPM). The rotating tip of the rod is held against an abrasive area in the stomach, which wears the tip off, liberating the enzymes and mixing them with food that is brought into the stomach in a mucus strand. This particular structure seems to be most commonly found in herbivorous and plankton-feeding mollusks (Hyman 1967).

Figure 2. Vermetus sp. photographed in about 1 m of water on the reef flat in Palau. The animal's aperture was about 5 cm (2") in diameter. The brownish operculum plugging the aperture and the mucous feeding strands are evident.

The feeding methodology of these animals is rather bizarre and interesting. The animals use the mucous gland in their foot to produce a large of amount of mucus. The mucus is extended up into to the surrounding water by the tentacles on the feet (Hyman, 1967; Kohn, 1983). The strands can extend quite some distance depending on the water flow and the size of the animals. In my aquarium, vermetids about 3 mm (1/8th inch) across can project mucous strands over 60 mm (2.5 inches). I have seen some large vermetids that were over 50 mm (2 inches) in diameter on reef flats in Palau. The strands of mucus from these animals extended over 2 m (about 6.5 feet).

Mucus is sticky, and planktonic materials adhere to it. After a short time the animal "reels in" the strand with its catch stuck to it and eats it. Some species have been documented to feed together. When one individual starts to put out mucus, all of its neighbors do too, producing a mucus sheet that seems especially good at collecting plankton. Once one individual starts to withdraw the strand, all of the contributors do as well, and all get to share in the catch (Hyman, 1967). This ciliary-mucous suspension-feeding isn't the vermetids’ only feeding mode, though. They also have been documented to extend from the tube and catch small planktonic animals, and they seem especially responsive to crustaceans (Hyman 1967). In aquaria, they are probably quite able to feed on baby brine shrimp, as well as other small planktonic animals.

T'ank You For The Good Habitat

Vermetids seem well-designed to reproduce in aquaria. Unlike most mobile mesogastropods, they do not copulate. The males, however, produce packets of sperm called "spermatophores" which are transferred to the female's mucous nets by a pedal tentacle, expelling the spermatophore into the water and "hoping" she will catch it. This is not a forlorn hope; the animals’ gregarious nature often means that someone of the opposite gender is nearby. The females collect the spermatophores and store the sperm to fertilize their eggs. Embryos develop inside the female’s tube and are maintained there until they have passed through the larval stages and have metamorphosed into little juvenile snails. They then leave the female and crawl around briefly, usually for an hour or less, before they cement themselves to a substrate (Strathmann, 1987).

Typically, the tubes’ apertures extend upward, probably as an adaptation to facilitate spreading of the feeding web. As the animals grow, they tend to erode a hole through the side of the tube fastened to the substrate and grow a new extension out of it; as they do so, they seal off the old aperture with shell material. At the end of the new extension, they construct another slightly larger vertical extension with the aperture at its end (Keen, 1971).

Figure 3. Small vermetids about 3-4 mm (1/8th inch) high photographed
in my system's refuge tank. They look like small calcareous tube worms.

Vermetid snails are relatively diverse; over a hundred species have been described, and some of them are commonly found in aquaria. Although several species are found occasionally in our systems, generally entering on live rock, one variety in particular may become very abundant, and be a serious nuisance in some systems. This species, probably the most common, is small, with a brown, reddish, or purple shell. Interestingly, the animal is difficult to identify, although that has not stopped numerous reef aquarists from doing so. It probably is Spiroglyphus annulatus, which is a small vermetid originally from the Caribbean. However, similar small species live elsewhere in the world, and they all look pretty much alike. It will probably take genetic testing to verify the identity of our aquarium friends. Whatever species it is, this particular one has small individuals. The tube seldom is over one or two millimeters wide. The shells are typically reddish or reddish-brown; sometimes they are even tinged with violet. The animal forms a small, calcareous shell mound and then sends up a short, three to five millimeter long, vertical stalk. The upper edge of this tube may be razor sharp, and may inflict rather nasty cuts. A few of these would be no real problem; however, this animal reproduces very well in marine aquaria. Left unchecked, it can reach populations of over several thousand in a few months. They prefer high current areas, and will infest and clog plumbing, significantly reducing water flow. In severe infestations they can clog and shut down pumps. The only solution in cases like these is physical removal of the animals using whatever method is easiest (a muriatic acid bath works well).

Fortunately, some fishes such as Copperband butterfly fishes, seem to eat them, and some hermit crabs will eat them as well. Eating these worm snails may well be the only truly beneficial effect of hermit crabs in aquaria.

The larger vermetids found in reef tanks are probably in the genera Dendropoma and Serpulorbis (Abbott and Dance, 1982). They do not seem to reproduce well in our systems and never obtain the plague proportions of their smaller cousins. These larger species tend to enter our systems on live rock or in coral, and are more interesting curiosities than any kind of pest. For some reason the larger species don't seem to proliferate as rapidly, though, and often remain as relatively solitary animals. The larger species seem to be more likely on Indo-Pacific live rock. A moderately large vermetid in the genus Petaloconchus is common in the Caribbean, and makes its way into aquaria now and then on aquacultured live rock. Given the appropriate conditions it is likely it will proliferate as well.

Figure 4. Several vermetids, possibly Dendropoma sp., photographed in Palau, at a depth
of 15 m. These animals were about the diameter of a pencil. The operculum and the
snail's tentacles are visible.

It is unlikely that even a large number of vermetids is directly deleterious to any other aquarium life. The mucus they produce may be used as food by many other animals as well as by the producer. Large masses might produce enough mucus to cause some local disruption in water currents or they may foul some other animal, but the mucus is generally very diffuse and most animals can easily remove it.

Figure 5. The feeding strands of a vermetid embedded in a coral in Yap. If vermetids
become abundant in a reef tank, the copious production of mucus strands may
irritate some corals. Generally, however, they are harmless.

Although I have concentrated on the worm snails of the family Vermetidae, two other families can have shells with a similar appearance. I consider it rather unlikely that specimens of either of these two families would appear in marine aquaria; however, for completeness, they are included here. These are the Family Siliquaridae and the Family Turritellidae (Abbott, 1974; Abbott and Dance, 1982). Siliquarids look quite like vermetids; however, the shell has a slit running along its entire length. Tenagodus species can sometimes be found embedded in sponges. Most turritellids have a rather normal-looking coiled snail shell with a high spire. The oddities, in the genus Vermicularia, look like normal turritellids initially, but then uncoil and look rather like vermetids. They may be found embedded in colonial ascidiaceans or sponges. Some species grow attached to gorgonians as well. Little is known about these two types of worm snails, either about their natural history in general, or their feeding habits, in particular.

Conclusion:

In many reef tanks, some of the most abundant animals are these small snails that often appear to be calcareous tube worms. The larger species are rather rare in aquaria, but the abundance of the larger vermetids on some Indo-Pacific reef flats is truly striking, and gives an indication of the amount of the appropriate detrital or particulate food available. Similarly well-adapted for reef aquarium life, the smaller species are sometimes prolific to the point of being nuisances. However, in most tanks, they simply remain an example of a small, but highly successful, component of reef biodiversity.



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

References Cited:

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

Abbott, R. T. 1974. American Seashells. Van Nostrand Reinhold Company. New York. 663 pp.

Abbott, R. T. and S. P. Dance. 1982. Compendium of Seashells. A full color guide to more than 4,200 of the world's marine shells. E. P. Dutton, Inc. New York. 411 pp.

Hyman, L. H. 1967. The Invertebrates: Mollusca I. Volume VI. McGraw-Hill Book Company. New York. 792 pp.

Keen, A. M. 1971. Sea Shells of Tropical West America. Marine Mollusks from Baja California to Peru. 2ed. Stanford University Press, Palo Alto. 1064 pp.

Kohn, A. J. 1983. Feeding biology of Gastropods. In: Wilbur, K. M. (ed.) The Mollusca. Volume 5, Physiology (2). pp. 1- 63. Academic Press, New York.

Raup, D. 1966. Geometric analysis of shell coiling: general problems. Journal of Paleontology. 40:1178-1190.

Raup, D. 1976a. Species diversity in the Phanerozoic: a tabulation. Paleobiology. 2:279-288.

Raup, D. 1976b. Species diversity in the Phanerozoic: an interpretation. Paleobiology. 2:289-297.

Raup, D. M. and R. R. Graus. 1972. General equations for volume and shell surface area of a logarithmically coiled shell. Mathematical Geology. 4:307-316.

Strathmann, M. F. 1987. Reproduction and development of marine invertebrates of the Northern Pacific coast. University of Washington Press. Seattle. 670 pp.

Tasch, P. 1973. Paleobiology of the invertebrates. John Wiley and Sons. New York and London.




Reefkeeping Magazine™ Reef Central, LLC-Copyright © 2008

Snails That Worm Their Way Into Tanks by Ronald L. Shimek, Ph.D. - Reefkeeping.com