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

The Mollis Clans - A Celebration of Molluscan Diversity

Coral reefs are interesting places, but they are wildly misnamed. The most numerous living organisms on coral reefs are bacteria. Other than bacteria, the most abundant organisms on coral reefs in terms of weight or mass of living tissue are algae. If we consider animals, most of the animal biomass on a coral reef is, indeed, coral, but in terms of the number of different types of animals, corals come up short. The animal group with the greatest degree of diversity, determined by the number of species, found on coral reefs or in the oceans, for that matter, is the mollusks. I decided that this column should be a celebration and discussion of the diversity of this wonderful group that contains some of the most beautiful, exciting and interesting of all animals.

All coral reef aquarists know what mollusks are… sort of.., almost.., maybe… Actually, most reef aquarists are only vaguely aware of this group, and what it means to them as aquarists. To most reef aquarists, the total array of mollusks that they may think about includes the grazing snails, the herbivorous strombid conchs, a few specialized whelks such as Nassarius, tridacnid clams, and little else. However, the total array of molluscan species found on any given Indo-Pacific coral reef is likely to exceed 2,000 species (by comparison, only 800 or so species of coral exist in all of the Indo-Pacific), and in some areas is probably five to 10 times that number. Some of these are widespread throughout a region; others are endemic to, or found in, only small areas such as a single archipelago or atoll. On a coral reef, they fill every conceivable ecological niche, from detritivore to top carnivore, and in terms of their weights, individuals of Tridacna gigas may be the largest individual animals found living their lives on the reef proper. Some massive coral heads will weigh more than the big clams, but the mass of living tissue on these corals is relatively small. It is worth remembering that in the late 1700's, when shipwrecked on the Great Barrier Reef, Captain Cook fed all the men in his expedition for two days on the flesh of one Tridacna; these clams may be huge. Mollusks are found everywhere on or near a reef; they are found in the waters over it, crawling on it, burrowing in it, and crawling under it.

Molluscan Characteristics

The mollusks form a very cohesive and identifiable group, called the Phylum Mollusca by taxonomists. They are not simple or uncomplicated; in fact, they are amongst the most sophisticated of animals. It is generally considered that only three major groups are at the pinnacle of complexity. These are: the chordates, such as fishes, mammals and birds; the arthropods, such as insects, crustaceans and arachnids; and, of course, the mollusks. All of these animals have complex tissues and organs, arranged in organ-systems.

Mollusks have numerous well-defined characteristics setting them apart from all other animals.

  • They are unsegmented; in no mollusk is the body subdivided into discrete segments as it is in such animals as the Annelids (bristleworms), Arthropods (shrimp), or Chordates (fishes). Mollusk bodies sometimes have replicated gills or shells, but these replications are not reflected in the internal anatomy. In the segmented annelids, for example, the rings seen on the surface of the animal divide the animal into a replicated series of discrete and similar internal compartments. Similarly, the muscular and skeletal system of fishes, and humans, is made of a series of repeated similar components.

  • Mollusks have a specialized gill that is probably the most consistent structural character found throughout the phylum. This gill is called a "ctenidium", and no structure like it is found in any other group of animals. These gills come in pairs, one on each side of the animal, and are elongate structures, looking a bit like fleshy feathers. In primitive mollusks they are found at the rear of the animal in a cavity, called the mantle cavity, under the shell.

Follow this link to see where the gills are on a squid. The link leads to a diagram of a squid partially dissected. The gills are labeled "B" (for branchium, which means gill in Latin).

  • Mollusks have an exceptionally complicated anatomy that is probably best evident in the circulatory system. They have at least one heart (squids and octopuses have three), arteries, and blood chambers called sinuses or lacunae. Only the cephalopods have capillaries and veins. The heart typically consists of one ventricle and one to four auricles. They are true "blue bloods;"the respiratory pigment is hemocyanin, a copper-protein complex that is colorless when deoxygenated and blue when oxygenated. It is not a very good respiratory pigment compared to the hemoglobin found in such lowly animals as earthworms and their (very) distant cousins, the humans. Blood is pumped through the gills prior to being sent to the rest of the body, ensuring that the tissues all receive freshly oxygenated blood. The blood leaves the vessels near the tissues and bathes them directly instead of passing through them in capillaries. Although such a system sounds "sloppy"and "inefficient,"in small animals such as most mollusks, it is an exceptionally efficient system. For them, it is economical since the animal doesn't need to form capillaries. Many mollusks take an additional advantage of this "vessel-less"nature of their circulatory system, and the blood may effectively be used as a hydraulic fluid to pump out or inflate various portions of their bodies, such as a tentacle or an eyestalk. After the blood bathes the target tissue or organ, it flows into a sinus and then back to the heart.

  • Mollusks typically have a well-developed pair of kidneys which, unlike those found in humans, are associated directly with the heart. As in the kidneys of humans, molluscan kidneys are capable of high pressure ultrafiltration, selective reabsorption and active secretion. These processes combine to produce a true urine as the carrier for nitrogenous waste, not just the simple secretion of ammonia found in many invertebrates.

  • Mollusks possess a food-gathering structure called a "radula."This is an organ found in the mouth cavity of most mollusks (except in clams; that have "lost their heads", along with all the related head structures, including the radula) that is used to collect food from the environment. The radula has been called a "toothed tongue"and that name is apt, as the position roughly corresponds to the position of tongues in humans. The surface of the radula is covered with teeth arranged in rows. There may be as few as one tooth per row or as many as sixty. In some cases, the teeth are hardened and the animal uses them as a rasp. In other cases, such as in the grazing snails common in the aquarium hobby, these teeth may be fairly soft, and act more like a "leaf rake"to gather in their food. These grazers actually apply very little pressure to the substrate, and many researchers say the snails just "lick"their food off the substrate.

Do a web search on the term "radula"to find a veritable plethora of images of this remarkable structure.

  • Mollusks typically have at least one calcareous shell, generally found dorsally, or on top of the animal. This is probably the most variable of the basic characteristics for the phylum, as the number of shells may vary from none, in several groups, to eight, in the chitons. The shell is secreted by the mantle, a specialized structure that covers the dorsal or upper part of the animal. Mollusk shells vary greatly in composition throughout the various molluscan groups, but they typically contain protein and calcareous components. Both calcite and aragonite are common, often in the same shell. All of these shell parts are secreted by the mantle. The outermost layer of the shell, called the periostracum, is often a wholly proteinaceous covering forming a varnish-like layer or elaborated into hairs. The protein continues throughout the shell as the fiber matrix that the calcareous mineral is deposited upon. Initial shell secretion occurs at the outer, or growing edge, of the animal's shell. Subsequent deposition by the underlying mantle thickens the shell. In large benthic mollusks, mostly clams and some snails, the shell may get quite thick and heavy. Those mollusks found in the deep abyss, where calcium carbonate is very soluble, often have shells made totally of protein.

  • Many mollusks move on a creeping ventral foot. This foot is well supplied with mucous glands, and they move by muscular action of the foot using the mucus as an adhesive, or by the action of fine microscopic cilia using the mucus as a lubricant. Other mollusks, particularly some snails, have modified the foot into a swimming organ, and live some or their entire lives swimming in the plankton.

  • The complex molluscan brain consists of a ring of ganglia surrounding the esophagus. In the more advanced mollusks, 30 or more separate ganglia or large aggregations of nerve cells, may be found in the brain. Additionally, there are connecting nerves and tracts that run across and between ganglia. Many mollusks are generally quite capable of learning, and in some, such as the larger octopuses, the sophistication of the brain is secondary only to that found in the higher vertebrates.

  • The mollusks have a characteristic embryological development with several features peculiar only to them, such as an arrangement of cells in the early embryo called the "molluscan cross."Primitive mollusks have a larva called a trochophore, similar to that found in several invertebrate groups including annelid worms. More derived animals have a larva called a trochophore, found only in the mollusks.

There are few other animal groups as economically important as mollusks. We harvest many of them as food, others compete with us for different foods, and introduced species are often ecologically disastrous pests in areas as diverse as Yellowstone National Park or tropical islands. Others are vectors or secondary hosts for some of the most debilitating human parasites. Their effects and relationships serve to structure and maintain many marine ecological communities and habitats, including coral reef assemblages, as well as large areas of the temperate marine environment.

Major Variations on the Molluscan Theme

The number of mollusks is estimated at between 50,000 and 150,000 species. The lower numbers were in vogue in the 1980's and 1990's and represent a reaction to such practices as the naming of different snail color forms and minor shell variations as separate species by nineteenth century scientists. For example, William Healy Dall (his name rhymes with "gal") for whom the Dall sheep and the Dall porpoise were named, was the first curator of mollusks at the Smithsonian. He named over 5,000 species, many of them mollusks. Some of his species descriptions are just one or two sentences long, such as: "It is a small white snail, found living in Alaska."He would then show a drawing of the animal. His next description might be, "It is a white, small snail from Alaska" with another drawing. As there are a lot of small white snails from Alaska, such names or descriptions are essentially useless.

Most of the workers in the latter part of the twentieth century thought that most of these color forms could be "lumped" together into a few species. Recently, however, genetic investigations are starting to show that many species that were considered to be cohesive "good" species, may actually consist of as many as five to ten reproductively isolated, essentially identical "sister species."For example, the common small blue mussel found intertidally on beaches throughout the Northern Hemisphere used to be considered to be one, widespread species, Mytilus edulis. It is now apparent that this "species" is actually a group of at least four species found intermixed throughout the range of the" lumped" Mytilus edulis. I think it is likely that when all of this shakes out, probably by the turn of the next century, that molluscan diversity will be shown to be far in excess of 150,000 species. For the present, however, it is best to be aware of the variation in the estimates.

Much recent research has lead to a complete revision of molluscan taxonomy in the last few years. About 30 years ago, we considered that there were about five major subgroupings, called "Classes", of mollusks. Present taxonomy, based largely on molecular genetic studies as well as a better understanding of the fossil record, recognizes at least nine classes. In the following list, the key characters - those that can help you distinguish what kind of mollusk you may hold in your hand - are in bold font. The other characters are important, too, but are secondary in the identification process. The molluscan groups follow, and those found in aquaria are indicated in blue:

Class Solenogastres - estimated to have about 250 species.
o They lack a shell, and instead are covered in calcareous "slivers" called spicules.
o They are hermaphroditic.
o They lack gills.
o Solenogastres have a ventral ciliated groove that is used to grip gorgonians as they climb on them. This is thought to be their "foot".
o They climb on and eat gorgonians and hydroids, but there have been no detailed ecological studies.
o Occasionally they are found in aquaria, coming in on uncured live rock.

Figure 1. This non-prepossessing creature is a Solenogastre or Neomeniid mollusk found living on the spines of a pencil urchin collected in the Bahamas. The pencil urchin's spines were covered with zoanthids, which this animal was eating. The mouth is extended at the left end of the animal. It was about an inch long. Solenogastres found in aquaria are longer, tend to be more "wormlike" and eat gorgonians.

Class Caudofoveata (or Aplacophora) - estimated to have about 250 species.
o They lack a shell, and are covered in calcareous "slivers" called spicules.
o They have separate sexes.
o They have gills.
o They are without any remnant of the foot, and they look and act like worms.
o Caudofoveatans live in sediments and oozes, where they prey upon and eat foraminiferans and other things in the sediments. Detailed ecological studies are lacking.
o They can be found in the sediments around coral reefs, but are rare in those areas.
o They are not reported from aquaria.

Figure 2. This is a Caudofoveatan or Chaetodermid mollusk, Chaetoderma. The mouth is at the bulbous right end, and the gills and anus at the left. The body is covered in calcareous spicules, although they are best seen near the left end. Chaetodermids move through sediments and eat foraminiferans and other small animals.

Class Monoplacophora - estimated to have about 25 to 50 species.
o They have a single limpet-like shell; similar to fossils found at the beginning of the Paleozoic Era, 450 million years ago.
o These are classic living fossils; almost all the living mollusks are thought to have descended from Monoplacophoran ancestors.
o Their major distinguishing characteristic is that virtually all organ systems are found in multiples, unlike all other mollusks.
For example in Neoplina:

  • 10 pair of lateral pedal connectives in the nervous system.
  • 8 pair of pedal retractor muscles.
  • 6 pair of nephridia.
  • 6 pair of gills.
  • 2 pair of gonads.
  • 2 pair of auricles.
  • 1 pair of ventricles

o Living examples are small, deep-water limpets, and their ecology is unknown.
o They are not found on coral reefs or in reef tanks.

Figure 3. This is the fossil shell of a monoplacophoran mollusk, Helcionella walcotti, that lived almost 500,000,000 years ago. Such animals are thought to be very similar to the ancestors of most shelled mollusks. Living monoplacophorans have smooth shells, and tend to be found in deep water.


Class Polyplacophora - Chitons - estimated to have about 500 species.
o They have eight shells embedded in a broad mantle, called a girdle. o They have a broad, strong foot, specialized for gripping in areas of breaking waves.
o The head is reduced and lacks eyes; although they all have photoreceptors that pierce the shells.
o The radula has 17 teeth per row, and some of these teeth are highly mineralized with iron salts.
o Their gills are replicated and lie in grooves beside the posterior half of the foot. o Most chitons are shallow water animals specialized to eat coralline algae.
o Generally, they have separate sexes.
o They are commonly found in reef tanks.

Figure 4. This chiton, Tonicella lineata, is more colorful than are most other chitons. Many chitons found in aquaria tend to have the shells largely covered by the tough tissue girdle on the sides. The shells often appear as a series of diamond-shaped white spots or plates on the upper surface.


Class Gastropoda - Snails, slugs, nudibranchs - estimated to have between 40,000 and 150,000 species.
o All gastropods have one shell as a larva, but the adults may have one, two, or no shells.
o They have undergone torsion (the body and guts rotate 180° during embryological development), so that the anus comes to lie over the head.
o The shell shows coiling. This coiling is independent of, and not related to, torsion.
o There is extreme diversification of shapes and ecological roles within this immense class. This is the most diverse group of non-insect animals, and they have come to occupy every habitat on the Earth's surface and are dominant animals in many of them.
o They are very common on coral reefs and commonly found in reef tanks.

Click Here for a Gastropod Slide Show

Class Cephalopoda - Nautilus, Squids, Octopuses - estimated to have about 500 species.
o Typically, cephalopods have one shell, but a shell is totally lacking in octopuses.
o The head is elaborated into 8 to 90 tentacles.
o They have a closed, high-pressure circulatory system.
o The cephalopod respiratory system is the most efficient respiratory system found in the marine environment.
o They have well-developed sensory structures. Their eyes, like those of the vertebrates, are based on the principal of the pin-hole camera. Essentially, as in the human eye, such eyes are spherical fluid filled chambers with a front lens and posterior retina. Such eyes are capable of forming excellent images. The cephalopods have the best eye of this design in the invertebrates, and it is as good as any vertebrate eye of the same size.
o They possess an exceptionally well-developed brain, particularly in the largest octopodes. They are capable of learning, have long-term memory, and have been shown to play using criteria developed for mammals.
o Squids are the largest of the invertebrates. The deep-sea giant squid, Architeuthis lux may be over 60 feet long and weigh over 2 tons. Enteroctopus dolfleini of the Pacific Northwest Coast of North America may have a tentacle span of about 40 feet and weigh as much as 500 pounds.
o Most cephalopods are predators, generally on fishes or crustaceans.
o Small species may be kept successfully in reef aquaria.

Click here for a Cephalopod Slide Show

Class Rostroconchia (all fossil) - estimated to have 1,000 or more species. All rostroconchs are extinct; they are the probable ancestors of living bivalves.
o They had a shell that was similar to the shells of a clam, in that the rostroconch shell grew down over the sides of the animal. Unlike the shells of a clam, there was no hinge connecting the two sides. Instead, a solid mass of calcareous shell material connected the two sides. The shell appears to have grown by repeated breakage at the juncture between the shells.
o Their ecology is unknown, but they are presumed to be bivalve-like.
o Some species were found in fossil coral reefs.

For an image of a couple of Rostroconch fossils, follow this link.

Class Bivalvia - Clams, Mussels, Scallops - estimated to have about 10,000 species.
o All have two shells.
o All lack a head, or any of the associated structures such as a radula or brain.
o They have reduced sensory and locomotory capabilities compared to most other mollusks.
o The shells are connected together dorsally by a rubber-like hinge that is a derivation of the protein of the shells.
o The shells enclose the body in a hydrodynamic space used for suspension feeding in most of them.
o The gills are elaborated into large organs used for respiration and feeding.
o One group of predatory species is common in the deep sea and rare elsewhere. o Most bivalves are specialized to live in sediments or in rocks. In coral reef areas they commonly burrow into coral heads. A few species, such as mussels, scallops, and tridacnids are found on the surface of substrata. Hermaphroditism is common.
o They are common on coral reefs and common in reef tanks, although most species cannot be easily or successfully kept for the long term.

Click here for a Bivalve Slide Show

Class Scaphopoda - Tooth or Tusk shells - estimated to have about 600 species.
o All have a singular tubular shell open at both ends.
o They lack gills.
o They possess several hundred specialized feeding tentacles used to capture their prey.
o The head is present, but reduced, and cannot extend from the shell.
o They have a large radula used as a grinding organ.
o The foot is large and used to pull the animal through the sediment.
o They have separate sexes.
o They are specialized to live in sediments.
o All are predators, and most seem to specialize as predators upon foraminiferans.
o They are found in coral reef areas and some have symbiotic relationships with solitary corals.
o They are not reported from reef tanks.

Much information about scaphopods may be found at this most excellent site: http://www.rshimek.com/Scaph1.htm

Figure 5. This scaphopod, Fissidentalium actiniophorum, generally carries a brownish sea anemone on the top of the shell (animal "F" lacks the sea anemone, and the yellow arrows point to the retracted mouth opening of two of the anemones). Some scaphopods found in sediment patches around coral reefs carry solitary corals in a similar manner but, unfortunately, I don't have any photos of those. Scaphopods have a conical tubular shell open at both ends, and normally live oriented horizontally in sediments. These animals are about three inches long and were collected from sediments under water 13,500 feet deep.

Mollusks with names of note:

Abra cadabra: a clam.
Bufonaria borisbeckeri (marine snail); named after the tennis player.
Amaurotoma zappa; Anomphalus jaggerius (fossil snails); named after the musicians.
Aaadonta and Zyzzyxdonta (terrestrial snails) both named by the late Alan Solem with the idea of being the first and the last entries in any list of endodontoid snails. He also named, for you fans of Charles Dickens, Ba humbugi; a terrestrial snail from Mba island, Fiji.)

And last, but not least…. Trivia (a genus of marine snails).

More Information:

The abundance, diversity, and economic importance of mollusks is reflected in the amount of information about them available to the casual searcher and the serious student. Quite literally, there are libraries devoted to them. I hope I have given enough information to pique interest about them. I have listed some easy to find treatments discussing mollusks in the Suggested References section that follows. In addition to the links listed in this article, there are literally thousands of others leading to various molluscan topics.

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


General Links

By their nature, the links included in articles such as this are subjective in character and reflect the author's interests as well as what is available. I did a search on the terms "mollusk, mollusc, and mollusca" and got 124,000 hits. Try looking around, you will likely find something of interest. Here are a few to get you started.

Lots of Information:

Endangered mollusks

General - Easy Reading:

Overall Treatment:

Godan, A. P. 1998. Molluscs: Their Influence in Science, Medicine, Trade and Culture.
Blackwell Science, London. 220 pp

Identification (The Americas only):

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

Keen, A. M. 1971 Sea shells of tropical west America. Stanford University Press. Palo Alto, Ca. 1064 pp.

Rehder, H.A. 1990. National Audubon Society Field Guide to North American Seashells. Alfred A. Knopf, Inc., New York. 894 pp.

Technical - Maybe Not So Easy Reading:

Taxonomy: The best treatment of modern molluscan taxonomy is in the following reference.

Beesley, P. L., J. B. Graham, R. Wells, and A. Wells. Eds. 1998. Mollusca: the Southern Synthesis. Fauna of Australia vol. 5. 1250 pages, 2 volumes. CSIRO Publishing, Melbourne.

The following volumes are probably the best treatment of molluscan anatomy and physiology and, in some cases, ecology.

Harrison, F. W. and A. J. Kohn. Eds. 1994. Mollusca I, Microscopic Anatomy of Invertebrates, Volume 5. Wiley-Liss, New York. 404 pp.

Harrison, F. W. and A. J. Kohn. Eds. 1997. Mollusca II, Microscopic Anatomy of Invertebrates, Volume 6. Wiley-Liss, New York. 828 pp.

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The Mollis Clans - A Celebration of Molluscan Diversity by Ronald L. Shimek, Ph.D. - Reefkeeping.com