Freshwater Gastropods of North America

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

. 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

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

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

.

Wild

Pomacea

populations range through the lower latitudes
of the New World, specializing on floating macrobenthic vegetation

. 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

.
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.

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.

Van Bentham Jutting

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

. 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.

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.

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.

I didn’t know what to feed them.

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.

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.

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

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.

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

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:

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.

Several weeks subsequent to the post of this blog, my attention was called to the 2016 publication of an expensive, two-volume set entitled “Neritidae of the World” by Thomas Eichhorst. I understand the work is excellent, but have not seen a copy.