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Worms that ain't... by Ron Shimek August 1997 Aquarium.Net

This month Ron Shimek discusses Vermetid Snails, Aquarium Net has numerous articles written by the leading authors for the advanced aquarist

Worms that ain't..

Vermetid Snails In Marine Aquaria

by Ron Shimek

One of the greatest groups of animals on Earth is the Class Gastropoda of the Phylum Mollusca. Commonly known as snails, outside of insects this class is the most diverse discrete array of animals. Estimates of the number of 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 a simple lack of knowledge.

Vermetid (possibly Vermetus adansonii ) amongst coral branches. The snail tube is about the diameter of a pencil. The snail's operculum covering the shell's opening is visible.

One of the interesting peculiarities of animal diversity is that within the huge group of insects, they really do not look all that different. Once you have really looked good at an insect, it is unlikely that you would ever confuse an adult insect with an adult animal of any other group. Not so with the snails...

There is only one characteristic that separates snails from all other groups of mollusks, and that is that they 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 rotates 180 degrees, bringing the anus to lie pretty much right above the head. Now, this process is bizarre in the extreme, and is probably related to the locomotion of the animal and the way it must carry its shell. By the way, torsion does not equal coiling. All snails with uncoiled shells, and all snails that lack shells also have undergone torsion (Kozloff, 1990; Ruppert and Barnes, 1994).

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 a notoriously variable shape. They can look like just about anything.

A large vermetid (possibly Vermetus maxima ) photographed on the reef flats of Palau. The animal is about 4 cm in diameter. The shell tube is closed by a brown proteinaceous operculum.

There are three major, and many minor grades of snail structural organization. Within such a grade of structure, the animals share some common attributes. Although grades of structure often have groups of closely related animals in them, kinship is not implied by the 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.

This individual was about 3 m from the individual photographed in the previous figure, and is the same species. It is feeding and the feeding mucus is visible coming from the aperture.

The basic, and presumed primitive, organizational level is called the "archaeogastropod" grade of structure. These are the snails that have shells similar to the earliest snails in the fossil record. Presumably their soft parts are similar, too, however that is not certain as the soft parts don't fossilize while the shells do. There are a reasonably large number of living archaeogastropods, estimates range from a few hundred to a few thousand species, and they are important to aquarists as most of the grazing snails that we utilize for the control of algae are archaeogastropods. This group includes the abalones, keyhole limpets, many other limpets, and the "turbo" grazers such as animals in the genera Astraea , Turbo , Trochus , and Calliostoma . With all of that, however, the archaeogastropods are not really too diverse 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 shelled marine gastropods, each of which also comprises a distinct functional grade. 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 there are literally thousands of species to tens of thousands of species. However, as in the insects, the diversity of this group is not manifested in widely differing shapes and structures. Rather they have diverse specializations within 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 rather than on their innovative external forms.

The third structural grade is referred to as the "meso- (or middle)" gastropods, because in many ways they are morphologically intermediate between the primitive archaeogastropods and the advanced neogastropods. Gastropod evolution and development can be viewed as a sequence of advances, each overcoming an obstacle that allowed for the exploitation of new habitats or food sources. Archaeogastropods, for example, have many different combinations of gills and excretory systems. 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, in the earliest snails, as reflected in the extant archaeogastropods, there appears to have been many ways to for the respective plumbing systems to be interconnected and related. Most of these were not very efficient.

At about the same time as 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 adaptive radiations known. As all aquarists should be able to figure out, archaeogastropods 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).

Worm shells are so-called because their shells look superficially like the shells of some feather-duster tube worms. These particular tube worms have the head modified to act as a filter feeding organ. This filter feeding organ is constructed of a lot of tentacles that are finely and pinnately branched, giving the appearance of tiny feathers on a feather duster. There are several different, but related, groups of worms in the Phylum Annelida that have such a structure. One particular family, the Family Serpulidae of the Class Polychaeta deposits a tube of constructed of calcium carbonate and these are the ones that the worm snails look like (Kozloff, 1990; Ruppert and Barnes, 1994).

The worm snails start out as a small, rather normal-looking, shell. 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 like a serpulid tube worm shell. However, the tube worm produces a shell that generally dull-surfaced on both the inside and outside, while the snail's tube is glossy inside. The worm's tube begins at 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 some other covering. 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, the 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. Although many of these animals also have white shells, many more have shells that are colored in some manner.

Morphologically of course, these are have all the internal characteristics thatdefine 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 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.

A group of the N. E. Pacific vermetid, Petaloconchus montereyensis . The snail tubes are about 3-4 mm across.

The snails' tubes may be closed by a concave, proteinaceous door or operculum. Reflecting their immobile status, they have a reduced foot which 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 are contain digestive enzymes (mostly enzymes that break down sugars) embedded in the mucus. This 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).

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 in to the surrounding water by the use of 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 a vermetid about 3 mm (1/8th inch) across, can project mucus strands about 60 mm (2.5 inches). I have seen some large vermetids were over 50 mm (2 inches) in diameter on reef flats in Palau. The mucus strands from these animals extended over 2 m (about 6.5 feet).

Mucus is sticky, and planktonic materials will 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 it as well, and all get to share in the catch (Hyman, 1967).

Ciliary-mucous suspension-feeding isn't the only feeding mode for vermetids, though. They have also 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.

This vermetid was buried in a coral head. Some species can excavate a trench as they grow, and then the coral grows over them protecting them. The feeding mucus is visible extending up over the coral polyps.

Vermetids seem well-designed to reproduce in aquaria. Unlike most mobile mesogastropods, they do not copulate. However, the males produce packets of sperm called "spermatophores" and these are transferred to the female's mucous nets either by the use of a pedal tentacle by just expelling the spermatophore into the water and "hoping" she will catch it. This is not a forlorn hope; the gregarious nature of the animals often mean that someone of the opposite gender is nearby. The females collect the spermatophores and store the sperm to fertilize their eggs. Embryos develop inside tube of the female 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, often an hour or less before they cement themselves to a substrate (Strathmann, 1987).

Typically, the apertures of the tubes extend upward, probably as an adaptation to facilitate spreading of the feeding web. As the animals grow, they tend 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).

Vermetid snails are relatively diverse, over a hundred species have been described, and some of them may be commonly found in aquaria. A small species with a brown, reddish, or purple shell is probably most common. This may be a species of the genus Dendropoma (Abbott and Dance, 1982). Individuals of this species form a small mound of shell about 2-3 mm (1/8 inch) on a side with a vertical tube extending upward about 4-5 mm (1/5 inch). The tubes are quite sharp and actually can give an unwary person a nasty cut. These animals proliferate very in our systems, and seem to prefer areas with turbulent flow and high currents. Where they are common, they can significantly reduce pump efficiency by increasing water resistance in pipes and tubing. This species probably was Caribbean in origin, but now is endemic in many dealers tanks and may be found on rocks and other hard substrata (they grow well on snail shells, bare coral skeletons, and other hard biogenic substrates).

For an illustration of a different small vermetid follow this link:

Other species may be found in aquaria as well. 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, but given the appropriate conditions it is likely they will proliferate as well.

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

Although I have concentrated on the worm snails of the family Vermetidae, there are two other families which 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 slit running along its entire length. Siliquaria species can sometimes be found embedded in sponges. Turritellids have a rather normal-looking coiled snail shell with a high spire initially, but then uncoil and look rather like vermetids. The turritellid worm snails of the genus Vermicularia can be found embedded in colonial ascidiaceans or sponges. Some species grow attached to gorgonians as well. Little is known these two types of worm snails, either about their natural history in general, or their feeding habits, in particular.

Should you have any questions or comments about worm snails or other invertebrates, I may be contacted at: .

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.

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

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

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

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Last modified 2006-11-19 01:25