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Molluscs for Small Marine Systems

By Bob Fenner

Small Marine Aquariums
Book 1:
Invertebrates, Algae
New Print and eBook on Amazon:
by Robert (Bob) Fenner
Small Marine Aquariums
ook 2:

New Print and eBook on Amazon: by Robert (Bob) Fenner
Small Marine Aquariums
Book 3:

New Print and eBook on Amazon:
by Robert (Bob) Fenner

After small worms, crustaceans and even smaller groups of life, the molluscs are the next most ubiquitous assemblage of life in the world’s seas. Think about substrates… live and abiotic… sand, rock, corals… where have you not seen snails and more. These “soft bodied” (meaning of the word mollusk) animals do have their place in small systems; as usual, in right/small numbers and the “right species”. There are ones that are inappropriate due to size and temperament. Here we’ll give a brief review of the better choices in species and how to sort out the best specimens.


Bivalves: Including Not-So Giant Clams:

            Most folks don’t realize just how many small mollusk species there are, nor how numerous they can be; mainly bivalves and gastropods (snails); a bunch of these are interstitial fauna that are nearly microscopic. There are places in the world where their biomass and empty shell et al. parts constitute an appreciable percentage of the shallow substrate. For aquarists, there are often unintentionally acquired molluscs that “come in”, hitchhiking on and in live rock, live sand and/or with other hard-bodied livestock. Most of these are innocuous, and even if large, should they die (usually from lack of nutrition or some aspect of varying water quality) their passing is of little consequence.

      Paradise and Mollusc Found! Shown here is an example of a common genus (Arca) with more than 200 species found in tropical seas (and a few temperate). A live and dead A. ventricosa in Hawai’i. At WetWebMedia.com we have had folks write in with concerns that their “rocks were moving”, having purchased a sizable clam as part of their LR.


            All but “the” Giant Clam, Tridacna gigas, have been kept as smaller specimens in little marine aquariums successfully. As with computer hard drives, displacement of motorcycle engines and bank account balances, “the bigger/larger the better”; but smaller individuals can be maintained for a period of time, even years, in tanks of a few tens of gallons. There is a BUNCH to state regarding Tridacnid selection in care; particularly in tinier tanks; but one fact I want to make sure and state emphatically: Beware of trying too tiny specimens of these animals! Two inch and less diameter shell width individuals have far less chance of surviving the rigors of shipping and adapting. I advise you to start with three plus inch across choices for any/all species, but two that get too big.



Gigas as in Giga-watt? As in gi-gan-tic! At right “the” giant clam Tridacna gigas. Here at right a Four foot long individual off of Cairns in Australia’s GBR. Way too big!

     Much better (below) are Hippopus hippopus, Tridacna crocea (at 15 cm., 6 inches across, the smallest giant clam), T. maxima, and T. squamosa







T. derasa (right) really get too large (Up to 24 inches across) too quickly for small aquarium use.



Gastropods of Use: Limited Numbers:
     As with Hermit Crabs, I am decidedly NOT a fan of stocking/having too many snails… they just don’t “do enough” for me; looks or function (cleaner upper) wise to warrant having more than a few. Let me add a few negative facts here; snails in numbers are bad because they can die en masse polluting your water, killing your livestock; they can “cross the line” and consume encrusting et al. “good” algae and more that you’d rather keep; they are vectors for many parasites of fishes and other invertebrates. Okay, done.

            IF you still wish to stock a few of the better “stomach footed Mollusca” there are some better snail choices:

Here’s my take on the best of the “clean up crew” snails on offer. Again, the fewer the better in my opinion; based on long experience. I would place no more than a “couple per five gallons”. At right a fave sand sifter, (Super Tongan) Nassarius distortus. Below: the Mexican Turbo Snail, Turbo fluctuosa, a group of Trochus sp., some Nerite sp. , Cerithium sp.,



            Archaeogastropods: About Abalones & Limpets:


There are tropical Abalone (family Haliotidae) species on the world’s reefs, and these are occasionally sold in the trade. Mmm, how to put this? They don’t do much… will not come out, scour your glass, remove blue green algae. Actually, they’re smart; and only consume leafy tasty algal species that you’ll have to supply. What’s even worse? The many times/places that you’ll find cold water species like the Black (Haliotis cracherodii) sold as a tropical. Won’t live.



Are limpets a good idea? Oh yes; of little notice and beneficial for cleaning noisome diatom and Dinoflagellates… If your system spontaneously produces them, leave them be. A nice batch down in Costa Rica’s Pacific.

            Cowries; If You Can Get Them:

Okay! You’ve got me; I’m a bonafide Cypraeid fan! Cowries are gorgeous and functional! Especially smaller species you can buy or if you’re super-lucky get for free as live rock hitchhikers. Am surprised that these have not gained huge popularity with marine hobbyists… as they’re abundant in the wild, easy to collect, and very nice for looks and more in small systems.

The snake-head cowry, Cyprea caputserpentis at right. I’ve seen money cowries offered for sale, but there are many more.



Cephalopods; Squids, Cuttlefishes, Octopi: Not:

            There are no “miniature” octopuses, or squids that are suitable for forty or less gallon systems. This Class of Molluscs are all too active, smart and “dirty eaters/mess makers” to do well in such limited worlds. Go visit them at public aquariums, or even better, if/when you can get out to do a bit of dive-adventure travel, and look for them while out snorkeling, diving.  


About Opisthobranchs & Small Systems:

            Nudibranchs, Sea-, Side-Gill & Sap-Sucking Slugs are fab; but really require their own specialized “for them and their host/food” set ups. Don’t try mixing them with other livestock in little tanks.

I should mention Sea Hares… as they are Molluscs, and Opisthobranchs (“gills at the rear”); AND offered in the trade from time to time (too often). There actually are small, tropical species that exist, but I’ve never seen these offered to hobbyists. The ones that are sold get way too large and die way too easily; taking most everybody else with them in a slimy mess. Oh, as James Lawrence would say; “a statement w/o an example is hollow. So, please see my pic of an Aplysia sp. at right; being offered in a LFS, retail store. To a foot plus in length. Yes; even cool to coldwater species are sold as tropicals at times. Skip the whole group.



Chitons & More: A Brief Mention

            These dorso-ventrally flattened, multi-plated shelled molluscs show up quite often as hitchhikers hidden on better quality live rock. They are harmless and a delight, moving slowly about your system, slowly rasping small soft algae from rock. Enjoy them if they show up.


Popular/Offered Molluscs to Avoid:

            We’ll mention just a few groups here; the Pen Shells called Flame Scallops and some of the cold water snails mis-sold as tropicals at times, and the several conchs, whelks, helmet shells, duns….


Flame scallops, flaming out. Oh how I wish a few of the more commonly offered species sold in our interest would be taken off the market. A key example are the Pen Shells (Family Limidae) genera Lima and Limaria mostly, sold as flame scallops. These rarely live for any period of time (days, weeks) in captivity; starving from want of suitable foods, being placed in unsuitable settings. Avoid them. Shown; the most common species in the trade Lima scabra; here in the wild on a reef in the Bahamas, and languishing in a wholesaler’s sterile holding tanks.


Some examples of cool to cold water molluscs that you may well see offered for sale; that are NOT tropical. At right, the California or Chestnut Cowry; Cypraea spadicea; this one laying ribbons of eggs. Other notable chiller-needing snails sold include Bubbles (Bulla sp.), Olives (Oliveta biplicata) and Moons (Polinices et al. sp.). Unless you have a chiller, chilled system (room temperature won’t do) look for real tropical species in their stead.



“And a one, and a two”; ah not the Lawrence W(h)elk! The bigger conchs, cowries spider shells… all are too rambunctious, messy eaters, renovating specialists for small systems. Stick with smaller species; yes; even IF the large ones are on offer as small specimens. The too-often offered/sold Tiger Cowry (Cypraea tigris). Not a good choice for even large systems. Largest member of the family; should be left in the sea.


Mollusc Specimen Selection Period:

            A good mollusk is easy to find! With just a few criteria to consider, you too can be a long-term keeper. What to look and look-out for:

1)      Time on hand; don’t buy just arrived specimens… Most deaths of these animals occur well, on arrival; but the second time slot for incidental mortality is the next twenty four hours. And the one after that? The next day… Leave newbies at your dealers for days before picking.

2)      Stinky? Not you; the tank. Dead and dying snails really smell bad… give the system a whiff, and if it reeks, give the purchase a pass.

3)      On that same issue; IS the one, are the ones you’re looking at “very” alive? Am reminded of Max in the “Princess Bride”, Billy Crystal’s character stating “So much you know, he’s only mostly dead”. You want molluscs that are mostly alive. Look for ones that are moving, feeding… leave the ones having private parties in the corner.

4)      Lastly; look for empty shells… a bad sign. Unless you’re shopping for new homes for hermits, you want to avoid tanks that have lots of evidence of neglect.



            Ah yes; “a chicken for every pot”, and some, but not all (by a long shot) mollusk for every tank; even very small volumes. Take your time in sorting out what’s available in the mollusk department; some species, source countries are far better for our smaller systems; diverse others, though get way too large, too disruptive, and too eager eaters for our intended uses. Use your references to make a desired list to show your stockist, and adhere to a moderate number of these animals.



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