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Related FAQs: Mollusks, Sea Slugs

Related Articles: Gastropods/Snails, Sea Slugs, Abalone, Cephalopods (Squid, Cuttlefish, Octopus)Bivalves (Giant Clams, etc.), Water Flow, How Much is Enough, Chitons

The Conscientious Marine Aquarist

Mollusks: An Overview

Bob Fenner A Cyphoma gibbosum feeding on a fan

Name another group of organisms that have done so much for humans as the mollusks. They've been food, decoration, tools, even medicine & money; and nowadays, aquarium specimens.

The myriad mix of animals we call mollusks; clams really big and small, Chitons, octopus, snails, abalone, oysters, nudibranchs, sea hares, nautilus, squid, tusk shells and many more, are finally starting to get their due with marine aquarists. With improvements in water quality, foods, and collecting, transportation and handling technology what were once sure losers are being kept for months to years.

I am amazed at the parallel worlds of aquarium and "shell-collecting" interests. In late 1994, I was privileged to present my views to the Southern California Marine Aquarium Conference on issues of the ethics, and government control of marine livestock trade practices. One weekend later, I gave almost the identical talk to the annual meeting of the Conchologists of America, Inc. (a national organization of shell collectors). Our concerns are the same; no one wants to see the natural resource/environment negatively impacted by their participation; neither group sees the real use of government intervention.

You may have a total disinterest in mollusks up to this point, but should be aware of them as a group. If for no other reasoning than they may "show up" in your system as "extras" on live rock, macro-algae, other invertebrates or foods. Many are predatory on other invertebrates; anemones, sea urchins, other mollusks, and even fishes. A few can be fatally dangerous to the aquarist.

Soft-Bodied Diversity:

The Phylum Mollusca is Greek for "soft-bodied". It is the second largest invertebrate group (after the arthropods); some 100,000 valid living species have been described, plus another 35,000 fossil ones.

This diverse group is distinguished by having a muscular foot, a calcareous shell secreted by the individual's underlying soft mantle, and a rasping feeding organ, the radula. In some sub-groups the shell and radula may be secondarily modified, sometimes radically.

All types of nutrition are employed, from vicious suction, drilling and poison darting of prey, parasitism; to relatively unexciting algal scraping and filter feeding.

Modes of reproduction vary as well. Most mollusks release their gametes into the water given environmental cues, with the young going through larval stages as plankton before settling. Some have internal fertilization releasing offspring at different degrees of development depending on the species.

We'll discuss three of the six living Classes, the Gastropoda (snails, nudibranchs...), Pelecypoda (or Bivalvia, the bivalves; clams, mussels...) and Cephalopoda (octopuses, cuttlefishes...) having members commonly kept by aquarists in some detail here, then we'll focus on nudibranchs and abalones and re-emphasize key points on keeping all mollusks in the two following Sections.

A pet-fish bibliography/further reading section is offered at the end for references on the most hardy, available, and interesting groups/species.

Class Gastropoda, the "Stomach-Footed Mollusks

This is the largest and by many measures, most successful of Mollusca classes; with about 35,000 existing species and another 15,000 fossil forms. Gastropods (snails) are found in freshwater, and have conquered the land by converting their mantle cavity into a lung (hence their name pulmonate snails) eliminating their primitive gills. Marine species exploit all types of bottoms, in addition to "free-swimming" in the pelagic environment.

The whole group is characterized by the evolutionary step of changing body shape; a twisting or torsion correlated with the development of spiral shells, allowing these animals to pull back within covered safety. Let's leave their taxonomy here, and direct you to Barnes for particulars, for those who want to know more. By example, let's have a say about:

Cone snails these beautiful shaped, often exquisitely colored snails may be dangerous to your fishes, and your health. They are the poison-harpoon equipped hunters you've seen on television nature shows. There are other cone species that are not so predaceous, or dangerous to their owners. Be careful around these animals; watch where you place your hands if beach-combing/diving/collecting in their environments.

A few other predaceous snail groups you ought to be aware of should you encounter them are the murexes (Family Muricidae) that eat other snails; the coralliophilids (Family Coralliophilidae) that as you might guess from their name, love to eat live coral, thiads (Thiadidae), olives (Olividae), auger shells (Terebridae) and turrid shells (Turridae). I would avoid all these snails.

Limpets as in "the incredible Mister", are the Chinese hat looking herbivores that are found on rocky beaches denuding them of fine algae. Deeper water brethren are the keyhole limpets (Family Fissurellidae) with a hole on top of their shell; found in deeper water. Closely related abalone (Family Haliotidae) we'll discuss two Sections hence. 

Top shells, Family Trochidae are macro-algae eaters that are often shipped from cold-water climates. A related family, the Turbinidae, or turban snails, includes the popular Turbo (yes, it's a genus and a common name) snails, excellently adapted for feeding on hair algae and films.

Tower shells, Family Turritelidae should be employed more by marine aquarists with "invertebrate" tolerant systems. Their long spiral shape and burrowing behavior serves to keep the substrate loose as they feed on substrate detritus.

Conchs, Family Strombidae we'll mention because they're such an important group as human food, and due to their occasional offering in the trade. Be aware that these are active, predaceous animals. Besides algae, they are given to eating other invertebrates, particularly sea stars and urchins. Another downside is their conch-ing themselves out, knocking over and digging up everything in the system.

Other "Good" Gastropods as cleaner-uppers who leave other life alone are the whelks (Family Buccinidae) and basket whelks (Nassariidae) that feed on meaty left-overs and wastes. The highly prized cowries (family Cypraeidae) make gorgeous feeders on algal deposits, but require clean, well-oxygenated water. as cleaner-uppers who leave other life alone are the whelks (Family Buccinidae) and basket whelks (Nassariidae) that feed on meaty left overs and wastes. The highly prized cowries (family Cypraeidae) make gorgeous feeders on algal deposits, but require clean, well-oxygenated water.

Take care in utilizing the services of marine snails as livestock and scavengers to pay attention to the same concerns as you would for freshwater. Quarantine them to eliminate the introduction of infectious disease, limit their number and biomass, and be aware of the species you are dealing with and their life habits. There are many that can make beautiful aquarium additions, as well as the several that can be a pain, literally and figuratively.

Class Pelecypoda

The "hatchet-footed" mollusks; clams, scallops, mussels, oysters. Here's another instance where I wish that biological classification could stick with the best name, outside of existing rules. I like the older name for this class, the Bivalvia, the (guess) bivalves; it's descriptive enough... Back to your 'roots', do you remember the freshwater days? The family of tetras (Gastropelecidae) we call hatchet fish has the same root word, for "hatchet"; in their case for the "hatchet-looking stomach" shape of their bodies. Oh well; most pelecypods (let's call them bivalves) have a fleshy foot that serves to get them around and for anchoring.

Most of this group are attached forms or found within the substrate. A few, notably the file shells, aka flame scallops are free-roaming. Almost all are filter-feeders, with some (the "giant" and not so big Tridacnas and Hippopus receiving considerable nutrition through photosynthetic symbiosis with zooxanthellae. More about them below.

Due to two factors, their mode of feeding, and difficulty in assessing whether they're alive or not, bivalves are not often intentionally kept. This is a real shame. For feeding, they require very little. In systems sparsely populated with filter-feeding organisms, no or very little auxiliary feeding may be called for, with the bivalves simply sieving out what they may. Otherwise, a simple mash of dried, fresh or frozen food may be periodically applied to the water while particulate and skimmer filtration is temporarily shut down.

The latter issue whether they're healthy/alive/dead is a little tougher. These animals should be quarantined a good few weeks, to give you confidence that they're going to "make it" in your main system. Sometimes when they die, they'll take everything else with them.

The swimming members, variously sold as flame or flaming scallops (Lima scabra, actually not a scallop, but a file shell) I find are particularly notorious for dying easily. Keep them only with delicate fishes (gobies, seahorses) and no stinging-celled animals, or predatory sea stars. They have a bad habit of snapping their last snap into these.

A much happier story is that of the tridacnid (giant) clams. These bivalves have become a popular item among sophisticated marine aquarists worldwide. Recent advances that allow the keeping of these animals include a ready and growing source of cultured individuals/species; improvement in the awareness of hobbyists re lighting (note, I did not state improvement in lighting technology); and strides in filtration. Because this is an important group and one whose examples can be carried into all aspects of marine aquarium keeping, let's expand on the above and present a separate bibliographic section.

Animal availability. Alas, there are still some people that deal in "wild-caught" giant clams. Please, don't encourage this practice by buying them. There are aquacultured specimens available of almost all species, Tridacna gigas, T. squamosa, T. derasa, T. maxima, Hippopus hippopus, H. porcellanus in all mantle colors; blue, purple, red, yellow, brown and green. Let's elaborate:

Lighting 

You likely know my position from previous writings. At least four watts per gallon of full-spectrum fluorescents per watt are suggested for these animals; and at that, I'd place mine near the surface to get the full intensity. Stay away from cool-white, Gro-lux, and other pet industry "color" lamps. These are a bad joke spectrum-wise. Actinics and halides may be used as well. Whatever lighting mode you use, you can judge it's performance on the size and growth of your Tridacnids mantle (the colorful fleshy lip that sticks out). you know my position from previous writings. At least four watts per gallon of full-spectrum fluorescents per watt are suggested for these animals; and at that, I'd place mine near the surface to get the full intensity. Stay away from cool-white, Gro-lux, and other pet industry "color" lamps. These are a bad joke spectrum-wise. Actinics and halides may be used as well. Whatever lighting mode you use, you can judge it's performance on the size and growth of your Tridacnids mantle (the colorful fleshy lip that sticks out).

Filtration: 

Must be of sufficient quality to remove all detectable levels of ammonia and nitrite. Though giant clams can, and are used to remove nitrates, their chemical precursors are toxic. "Reef" or real skimmer filtration in addition to other biological and mechanical means are called for. must be of sufficient quality to remove all detectable levels of ammonia and nitrite. Though giant clams can, and are used to remove nitrates, their chemical precursors are toxic. "Reef" or real skimmer filtration in addition to other biological and mechanical means are called for.

Once again, don't purchase specimens collected in the wild. As my friends will testify, I've spent tens of years/thousands of dollars at the "sushi-bar-experience". Never once, have I ordered or eaten Mirugai, giant clam. It bums me out to think that a human would support the killing of a giant clam, likely more than a hundred years old and maybe a thousand pounds in weight, for a lousy tasty morsel.

Buy cultured giant clams for your aquarium. Don't know how to tell if they are? Ask your supplier, demand to see the shipping manifest with the CITES permit number on it. See the bibliography for the Micronesian Mariculture Demonstration Center in Palau for supply locations, if you can't get them locally.

Class Cephalopoda; the "Head-Footed Mollusks

These are the squids, octopuses, cuttlefish and nautiloids; the most advanced mollusks, and quite possibly the most intelligent invertebrates. They possess eyes with a focusing lens, an efficient closed circulatory system, relatively large brains, and are capable of demonstrable learning.

Thinking of keeping an octopus? Consider their positive attributes; interesting behavior, color changing ability, splendid appetites. But then remember the following problems; inking, super-aggressive predation, and Houdini-like escape potential. They eat or will try to consume most everything, will get out of any opening big enough to squeeze their beak through (or push the top off of), and will unknowingly commit hara-kiri if spooked into inking their environment. Other than that, cephalopods make good to suitable specialized aquarium specimens.

Squids, cuttlefish and chambered nautilus have been kept from time to time; see bibliography. Do I have to mention the venomous blue-ring octopus, Hapalochlaena lunulata? If you're bitten by this little beauty, most likely you will die. Who would be so ignorant to keep it in captivity? Ever heard, "accidents will happen?"

Other Mollusk Classes

Include the Monoplacophora, the Tusk Shells, Scaphopora, and the Chitons ("kite-ons"), Polyplacophora; the first two will skip over as the likelihood of ever encountering them is extremely rare. Chitons however you might find stuck to shells and live rock. They are bilaterally symmetrical, ovoid shaped and flattened top to bottom; and as their name infers bear eight articulating plates dorsally. These innocuous creatures spend their days hiding from the light, adhering to a solid surface. They are algal scrapers who will do no harm to anything.

Unidentified Chiton, Class Polyplacophora. Aquarium image from Atlantis Aquarium, Long Island, NY. 

Summary

With knowledge comes power; mollusks are not all bad, nor impossible to keep. They require adequate care like any other living thing. The group, as you have seen is very diverse. Study up on the organisms you want or have ended up with to determine their particular needs.

Lastly, beware of metal and dye medications around these animals; they are amongst the most sensitive to such poisons.

Bibliography/Further Reading:

Mollusks in General

Marine Hitchhiker/Critter ID (Maughmer, Toonen, Tompkins)

American Conchologist, the official publication of the Conchologists of America, Inc. Quarterly. P.O. Box 1226, New Albany, In 47150

Barnes, Robert D. 1987. Invertebrate Zoology, 5th ed., Saunders, Fort Worth.

Kerstitch, Alex. 1990. Mollusca touch. SeaScope Vol. 7, Summer 90.

Kerstitch, Alex. 1991. Living mollusks. FAMA 12/91

Kerstitch, Alex. 1992. Living jewels of the sea. FAMA 1/92.

Gastropods:

Baker, Donald E. 1987. Empire of the cone shells. TFH 9/87.

Burgess, Warren E. 1986. Book critique: Cowries of the World, by C.M. Burgess, Cape Town, Seacomber Publications. TFH 5/86

Berman, Gerald. 1979. The flamingo tongue, Cyphoma gibbosum. TFH 9/79.

Hoff, Frank H. 1988. Conch, the living aquarium cleaner. FAMA 3/88.

Kerstitch, Alex. 1981. A killer in the tank (cones). FAMA 2/81.

Kloth, Thomas. The original mobile home. FAMA 6/80.

Lamberton, Ken. 1993. Some perfect marine gastropods. FAMA 11/93.

Magerlein, Gunter. 1975. The raising of murex snails. Aquarium Digest International 3(4):75.

Nybakken, James. 1978. Carnivorous Conus. TFH 12/78.

Paradise, Paul R. 1975. The shell game. TFH 12/75.

Spitz, Dennis L. Muricidae (Murex, rock shells). Marine Aquarist 2(2):71

Tuskes, Paul M. 1982. Flamingo tongue snails (cowries). FAMA 6/82.

Walls, jerry G. Conchs in the aquarium. TFH 3/81.

Walls, Jerry G. 1981. Snails for marine aquaria. TFH 7/81.

Wilson, John M. 1983. The tulip shell in the home aquarium. TFH 6/83.

Gastropod Graphics: All local Southern Californian.

Row 1 A keyhole limpet, the Volcano, Fissurella volcano

2) "And a one and a two" it's Lawrence Whelk, one intertidal, the other obverse, showing the closing operculum.

3) Permanently attached "tube snails", Serpulorbis squamigerus.

4) The lovely California cowry (shell), Cypraea.

5) A top shell, Calliostoma munching away on macro-algae.

Pelecypoda/Bivalvia References:

Kerstitch, Alex. 1994. Farmers of the sea; the blue revolution, part 4, Scallops, Mussels, Abalone. FAMA 9/94.

Volkart, Bill. 1989. Fascinating filter-feeders: the bivalves. TFH 12/89.

Wilkens, Peter. 1976. Fileshells. Marine Aquarist 7(6):76.

Walls, Jerry G. 1980. Scallops in the aquarium. TFH 2/80.

Wilkens, Peter. Keeping flame scallops. TFH 2/91.

Tridacnid Clams:

Anon. 1991. Tridacnid clam culture. SeaScope, vol. 8, Winter 91.

Abbott, Robert R. 1990. Retailing the giant clams. Pets Supplies Marketing. 10/95.

Carlson, Bruce A. 1991. Tridacna clams; true giants in their field. FAMA 4/91.

Delbeek, Charles J. 1989. Tridacna clams in the home aquarium. SeaScope vol. 6, Winter 89.

Kerstitch, Alex. 1991. Living mollusks (giant clams). FAMA 12/91.

Micronesian Mariculture Demonstration Center. P.O. Box 359 Koror, Republic of Palau 96940. (680) 266 or 1475, fax (680) 1725.

Weingarten, Robert A. 1991. Tridacna- the giant clam. FAMA 2/91.

Romano, Dennis. 1982. The care and breeding of the giant clam. FAMA 7/82.

Pelecypod/Bivalve Graphics:

1) Your favorite flame (fileshell) scallop, Lima scabra

2) Off of California, the mussel namesake, Mytilus californianus.

3) A giant clam shell, T. porcellanus; note uniform peach color.

4) The most popular T. squamosa, the giant clam of shell collectors.

Cephalopods::

Burgess, Lourdes A. 1991. Nautilus: the pearly chambered nautilus. TFH 5/91.

Burgess, Warren E. Octopuses in the aquarium. TFH 2/77.

Carlson, Bruce A. 1979. Chambered Nautilus, a new challenge for aquarists. FAMA 8/79.

Donovan, Paul. 1992. Cuttlefish. FAMA 1/92.

Friese, U. Erich. 1972. Death in a small package (the blue-ring octopus). Marine Aquarist 3(4):72.

Friese, U.Erich. 1973. The blue-ringed octopus. TFH 2/73.

Glodek, Garrett S. 1989. Octopus biology, part one: an examination of natural history; part two: water quality requirements, filtration needs and feeding. FAMA 10/89.

Jackson, L.A. 1981. An introduction to the anatomy and physiology of the octopus. FAMA 8/81.

Johnston, Elizabeth S. & John Forsythe. 1993. An octopus in your house? AFM 8/93.

Kerstitch, Alex. 1988. Primates of the sea (octopuses). FAMA 4/88.

LaRoe, Edward. 1974. Maintaining squid. Marine Aquarist 5(1):74.

Lippe, Kathi. 1984. The care and feeding of Joubin's octopus. FAMA 10/84.

Monnig. (no other name listed). 1977. Marine aquariums; an aquarium inhabitant who sits up and begs (octopuses). Aquarium Digest International #17, 3(77)

Murphy, Geri. 1993. Chambered Nautilus; nature's deep submersible. Skin Diver Magazine. 6/93.

Refano, Joe. 1980. The care and feeding of octopuses; the much maligned octopus is actually one of the sea's most intelligent inhabitants. FAMA 2/80.

Schroeder, Werner. 1973. Cuttlefish. Marine Aquarist 4(1):73.

Slager, Christina J. 1986. Cuttlefish: dazzlers of the deep. FAMA 8/86.

Spencer, Gary C. 1975. Octopus. Marine Aquarist 6(4):75.

Stolzenburg, William. The naturalist; the familiar stranger; in many ways, the alien octopuses act almost like humans. Sea Frontiers 7-8/93.

Volkart, Bill. 1990. The cephalopods. TFH 9/90.

Walker, John. 1987. The discovery: Octopus horridus. FAMA 2/87.

Wood, James B. 1994. Don't fear the raptor; an octopus in the home aquarium. FAMA 4/94.

 



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