Dr. Rob Dillon, Coordinator

Thursday, December 21, 2017

Pet Shop Malacology

Back in the early 1960s, the very first aquarium shop to open its doors in Waynesboro, Virginia, was an aquatic wonderland called “Fin Fair.”  Sometimes, especially in the winter, I prevailed over my father to drive me to their store on West Main Street just so I could stroll among the dozens of tanks filled with glistening little jewels of nature.  Of course, I wanted my own aquarium.  I kept a series of aquaria through my childhood, as I grew up, and the things in them didn’t.

from Brookana Ashley Patton
And of course, any proper aquarium must have snails to scavenge the uneaten food, am I right?  In the 1960s and 1970s, in my personal experience, almost all I ever saw for sale were “ramshorns,” apparently Helisoma trivolvis.  No fancy colors, either.  Just plain, brown, “ramshorns.”

So you may be able to imagine, knowing me as you all do, the impression made by the first “mystery snail” I ever saw [1].  They were Pomacea paludosa, almost certainly wild-collected down in Florida, and they were huge!  The pet shop owner explained to me that they laid eggs out of water, the mystery being that nobody ever saw them do it.  I bought three mystery snails with my hard-earned allowance money, and I don’t think they lasted two weeks [2].

But a couple years later, the Dillons went on a family vacation down to Florida, and among our many adventures, booked passage on a glass bottom boat out of Silver Springs.  I remember the experience being very much like sailing across the top of Fin Fair.  And almost immediately, my eyes were attracted down through the schools of catfish and bream to the bottom of the springs where, to my fascination, lay small piles of mystery snail shells.  My father boosted me over the back fence on the way out to the parking lot, and I was able to snatch a couple empty shells from the marshy margins of the springs.  Watch for gators, he said.  Great father.

It is difficult for me to place myself at age 12 here in Charleston, 2017.  But one thing is certain.  The Charleston area today is blanketed by big-box pet stores - PetSmart (5 outlets) and PetCo (4 outlets).  And the eyes of any kid walking into the well-stocked aquarium departments of any of these giant retail outlets will fall on two types of freshwater gastropods, both spectacular in their own way: modern-day mystery snails [1] and nerites.

The mystery snail of the modern aquarium hobby is Pomacea diffusa, ne bridgesii [3].  They are big enough and active enough to have a personality, and charmingly diverse in coloration, as witnessed by the lovely photo montage above.  I surveyed several of the local big-box retailers, and found Black, Ivory, Blue, and Gold varieties, which for some reason PetCo calls “Gold Inca.”

The inheritance of color polymorphism in P. diffusa is a fascinating topic, to which we may return in a future post.  I cannot find anything published about it in the scientific literature, but somebody, somewhere, really seems to know what he is doing. If any member of my vast readership has any good information on the striking color polymorphisms manifest in commercial P. diffusa stocks, especially where these things are ultimately coming from, please contact me at your earliest convenience [4].

Wild Pomacea populations range through the lower latitudes of the New World, specializing on floating macrobenthic vegetation [5].  They are especially large-bodied as freshwater gastropods go, with even larger mouths with specialized lips to manipulate leafy greens, and even larger teeth.  Their shells are bulbous and surprisingly light, adapted to enfold an air bubble, making their bearers positively buoyant.  The reproductive adaptations of Pomacea are weird and wonderful – climbing up out of the water to lay huge clutches of huge eggs, typically on emergent vegetation.

Now here’s a riddle.  Among the prosobranch fauna of warm freshwaters, what is the opposite of Pomacea?  How about an unspecialized grazer of benthic periphyton with an especially heavy shell adapted to high-energy environments in the Old World?  Laying tiny eggs that go down?  How about the nerites?
Zebra nerite "N. natalensis" from Wikipedia
Nerites are the best known group of freshwater gastropods about which nothing is known [6].  Although much smaller than the mystery snails, the nerites marketed to the aquarium hobby are even more eye-catchingly colorful.  All the big-box stores sell a nice variety.  Our hypothetical twelve-year-old-boy would find “tiger nerites” and “zebra nerites” in the local PetCo, and “black nerites” and “mixed nerites” at the PetSmart.

Both the tigers and the zebras are labelled as “Nerita natalensis” in my local PetCo, but I am just not sure.  I can google around the internet like the best college freshman, and I did (in fact) find a variety of Wikipedia and hobbyist-type references to Neritina (or Nerita) natalensis, depicting the tiger-striped snail sold by PetCo, listing the native habitat as the freshwater-tidal and brackish mangrove-type habitats of East Africa.  The problem is that I pulled my trusty copy of D. S. Brown [7] off the shelf, and the PetCo nerites don’t really match Brown’s figures of Neritina natalensis.  They do match the figures labelled “Vittina coromandeliana” and “Vittina turrita” in the paper by Ting Hui Ng we reviewed back in October [8], both of which are elements of the Oriental / Pacific Islands fauna, not Africa.

Oh, good!  Ng and colleagues got CO1 sequences for their Hong Kong samples of Neritina (Vittina).  That should help us out here, right?  Nope, sequence data are worse than useless in this situation [9].  The individual Vittina turrita sequenced by Ng didn’t match anything in GenBank.  The V. coromandeliana sequence did match a GenBank sequence labelled as turrita, as did the sequence of a third nerite from the Hong Kong pet shops, which Ng identified as V. waigiensis.  Quoting Ng directly: 
“Two individuals identified by morphology as Vittina coromandeliana and Vittina waigiensis were 99–100% matched to two separate submissions on GenBank that were identified as Vittina turrita. Neither study included photographs of the species, nor could the sequenced individuals be located; because the two GenBank sequences for Vittina turrita were separated by a 4.5% uncorrected pairwise distance, we retained our morphology-based identifications.” 
So that brings up Neritina (Vittina) waigiensis, which may be what is lying sullen at the bottom of the tank labelled “mixed nerites” in my local PetSmart.  That’s the impression I got from my google search, anyway.  Almost all neritid populations demonstrate striking shell color polymorphism, but the snails that pet stores tend to call the “red nerite” and the internet usually identifies as Neritina waigiensis beat anything I have ever seen.  The combinations mix a delicious-looking strawberry-red color with brilliant gold and black zig-zags.  In fact, it seems possible to me that the nerites separated out as zebras and tigers in the big-box pet stores are almost within the range of color polymorphism of waigiensis.  I don’t know.
Neritna (Vittina) waigiensis
Van Bentham Jutting [10] gave the range of Neritina (Vittina) waigiensis as “especially in the eastern part of the Malay Archipelago, also in the Philippines.”  I can’t discover anything about its habitat or life history.  The species appears in both freshwater and marine references.  Most of the Oriental / Pacific Island neritids live in rapidly-flowing streams that empty directly into the sea, their eggs hatching into planktonic larvae swept down to develop into marine juveniles, migrating back into fresh water [11].  Other tropical neritid species inhabit tidal, mangrove-type environments, laying eggs that hatch into crawling juveniles, like normal freshwater prosobranchs.  The various popular aquarium nerite species seem to manifest both types of life cycles, as may be judged on YouTube, if you’d like to conduct your own cutting-edge research in freshwater neritid biology.

So we’ll close this month’s essay with one more general observation on the oppositeness of the mystery snails and the nerites, and a final point of ironic similarity.  Pomacea diffusa stocks are all (I feel sure) captive-bred.  But the nerites must be gathered from the wild – I cannot imagine an aquarist completing the life cycle of a neritid in culture.

From Chris Lukhaup
What this means is that whoever is gathering these strikingly colorful tropical nerites, whatever they are, does not want us to know where he is finding them – this is their “trade secret,” in a sense.  In fact, it will actually be to the perceived advantage of the sellers to mislead the buyers about all aspects of their malacological commodity – especially range and habitat, probably even identity.  I wouldn’t be surprised if, at some point in the supply chain between the hunter/gatherer source and your neighborhood Big-box aquarium supply outlet, the East African name “Nerita natalensis” is written in grease pencil on aquaria full oriental nerites fraudulently, in a deliberate effort to mislead.

And the ironic similarity is this.  For all their tremendous popularity in the worldwide aquarium hobby, the colorful varieties of mystery snails are every bit as genetically mysterious as the colorful varieties of nerites are ecologically mysterious.  Both categories of information seem to be jealously-guarded trade secrets.  Such are the challenges of Pet shop malacology, 2017.


[1] Common names make no sense, and there’s no sense in trying to make sense out of them.  Sometime between the 1970s and the 2000s, the name “mystery snail” was transferred to viviparids like Bellamya (Cipangopaludina), the mystery being that nobody ever saw them lay eggs.  And the various Pomacea became known as “Apple Snails.”  So that is the convention followed in both the Perera & Walls (1996) “Apple Snails in the Aquarium,” and the Turgeon et al. (1998) “Common and Scientific Names of Mollusks.”   But I think the bad press suffered by the larger, invasive, pest species of Apple snails, variously identified as Pomacea canaliculata/insularum/maculata, prompted the aquarium trade to move back to writing “mystery snail” on tanks of Pomacea bridgesii/diffusa.  Only the pest Pomacea species are still called “apple snails” by the aquarium hobby.

[2] I didn’t know what to feed them.

[3] Rawlings, T.A., K. A. Hayes, R. H. Cowie, and T. M. Collins (2007)  The identity, distribution, and impacts of non-native apple snails in the continental United States.  BMC Evolutionary Biology 7: 97.

[4] I have seen the (2004) paper by our good friend Yoichi Yusa on the inheritance of body color polymorphism in Pomacea canaliculata.  The situation in P. diffusa is obviously more complicated.

[5] Hayes, K. A. et al. (2015)  Insights from an integrated view of the biology of apple snails (Caenogastropoda: Ampullariidae)  Malacologia 58: 245 – 302.

[6] Well, European Theodoxus is fairly well studied.  There’s lots of general biology in Fretter & Graham’s (1962) “British Prosobranch Molluscs.”  And see my book pp 85 – 86 for diet & habitat.

[7] Brown, D. S. (1994) Freshwater Snails of Africa and their Medical Importance.  London: Taylor & Francis.

[8] Ng, Ting Hui, Tan SK, Wong WH, Meier R, Chan S-Y, Tan HH, Yeo DCJ (2016) Molluscs for Sale: Assessment of Freshwater Gastropods and Bivalves in the Ornamental Pet Trade. PLoS ONE 11(8): e0161130.  Review:
  • What’s Out There? [9Oct17]
 [9] Here’s another vivid demonstration of a point we have made repeatedly on this blog.  Sequence data are a dependent variable, not an independent.  They cannot be used to elucidate the systematics or evolution of an unknown study group.  Only if we have a previous hypothesis about the evolution of a group, from good, hard science, can sequence data be interpreted.  See:
[10] van Benthem Jutting WSS. Systematic studies on the non-marine Mollusca of the Indo-Australian archipelago: V. Critical revision of the Javanese freshwater gastropods. Treubia 1956; 23: 259–477.

[11] Alison Haynes published several papers on the neritid fauna of the Pacific Islands in the 1980s, which although not especially helpful to identify the Malaysian/Philippine species of immediate interest in this essay, are useful for the biology of the family.  See:

Haynes, A. (1988)  Notes on the stream neritids (Gastropoda: Prosobranchia) of Oceania.  Micronesica 21: 93 – 102.  I’ve also heard that her (2001) book is good, but don’t have access to a copy.


  1. Hiyer, Rob!

    I have a copy of Alison Haynes book. I wrote to her to inquire where a copy might be found and she very kindly sent me one with a stack of her reprints. The book is excellent, with keys and photos depicting all the Pacific Island freshwater gastropods known at that time, including a freshwater opisthbranch.

    Merry Christmas,

  2. Hi Rob, I really enjoyed this essay and its earlier cousin. I'm still trying to convince my parents to grab me a few P. floridensis on their Feb. schlep down!

    Again I think the most interesting part to me, as a restive hobbyist and almost-scientist, is what do folks want in a aquaria snail: wonder or function?

    The market could certainly use some carefully managed diversification of fauna. Anything native and more unique than eBay pages full of Physa spp., amirite?


  3. Mike - You and I are absolutely on the same page... I do wish twelve-year-old kids here in North America didn't go to Hong Kong for pets. But I think we've got three strikes against us - our lentic malacofauna is all brown, any snails we've got with any color are lotic, and we have very little fauna adapted to waters as warm as a typical home aquarium.