Dr. Rob Dillon, Coordinator





Tuesday, June 6, 2023

The Mystery Snail Color Genetics Project

Editor’s Note – This essay was subsequently published as: Dillon, R.T., Jr. (2023c)  The Mystery Snail Color Genetics Project.  Pp 63 – 74 in The Freshwater Gastropods of North America Volume 7, Collected in Turn One, and Other EssaysFWGNA Project, Charleston, SC.

Who among you, my vast and far-flung readership, has not at least once in your life lingered at the face of a hobbyist aquarium to enjoy a “Mystery Snail?”  Show of hands?  One or two of you in the back?

From Ms. Rachel Voss
OK, a bit of introduction may be helpful before we get down to business this month.  The common name “mystery snail” is today almost universally [1] applied to Pomacea diffusa, the most popular gastropod in the home aquarium worldwide, a (relatively) small South American ampullariid domesticated in the late 1960s [3] for the international pet trade.  Such snails were generally identified as Pomacea bridgesii (Reeve 1856) until around 2007, when a team of our colleagues [4] suggested that the nomen diffusa, proposed as a subspecies of P. bridgesii by the German biologist Werner Blume in 1957 [5], might merit recognition at the full species level.

Although small bodied by the standards of the Ampullariidae, and hence not as voracious of aquarium plants, nor as dangerous as pests upon escape into the wild [6], Pomacea diffusa are still large enough to have a personality, which is, in the eyes of many an adoring enthusiast, charming.  Open my essay of [21Dec17] in a new window for more context, and a bit of additional biological background on mystery snails in the home aquarium.

Some not-insubstantial fraction of the popularity of mystery snails derives from their color polymorphism.  Although I have seen at least 15 – 20 named color varieties on the market, consensus seems to suggest eight distinct phenotypes, some with multiple names.  These are depicted in Ms. Brookana Ashley Patton’s colorful figure below, with a standard name suggested for each: gold, jade, ivory, blue, chestnut, brown/black, magenta, and purple.

A simple Mendelian hypothesis suggests itself immediately, does it not?  Three loci, each with two alleles, would yield eight phenotypic categories quite splendidly.  One of those loci must obviously control body color, with albinism (a) recessive under pigmented (A), if the pattern seen almost everywhere [7] throughout the remainder of the animal kingdom is followed in ampullariid snails.

The other two loci seem to control the coloration of the shell: background color (Y) and striping or color banding overtop of it (S).  A peculiarity of this system is that in most other animals, typically, a mutation at the locus assigned to albinism blocks all color production, everywhere.  Three of the eight varieties of mystery snails, however (gold, chestnut, and magenta), demonstrate colorless bodies but colored shells.

The invasive pests Pomacea canaliculata and P. maculata have well-known “golden” variants with a colorless body and plain yellow shell.  The shell and the body phenotypes seem to be inherited together, as a simple Mendelian recessive trait [8].  The “Giant Columbian Rams Horn” Marisa cornuarietis also has a “golden” variant, inherited as a simple Mendelian recessive, but in this case the plain, unstriped yellow shell is born by a snail with a pigmented body [9].

In the mystery snail Pomacea diffusa, the rainbow of phenotypes commonly observed in the aquarium suggests one locus analogous to that seen in P. canaliculata and M. cornuarietis, the dominant wildtype allele (S) encoding dark stripes or bands of pigmentation covering the shell.  The recessive (s) seems to encode no banding or striping, uncovering the yellow or “golden” background shell color.  I am reminded of the banding locus in the European land snail, Cepaea, which became such an important model organism in the early development of population genetics [10].

As for that second shell color locus, variation in background color independent of banding above it or body pigmentation below is unique to Pomacea diffusa, in my experience.  I have never seen anything like it anywhere in The Mollusca.  But two alleles again seem to be involved: let’s suggest a dominant yellow background (Y) and a recessive colorless (y).  The implication of the model is that as many as three separate biochemical pathways seem to control mystery snail coloration, yielding eight phenotypes, such that the wildtype brown/black phenotype is encoded A_Y_S_ and the completely colorless ivory variety aayyss.  If the model holds.

The three-locus model I have outlined above is not new or original with me.  It seems to have been a part of the lore of mystery snail husbandry for many years [11].  And there are most certainly some very clever and resourceful snail breeders somewhere in the world who could confirm it in a heartbeat.  But to this day, no formal test has ever been published.  The entire, lovely system – all those exciting phenotypes and the intellectually-gratifying hypotheses that go with them – remain anecdotal, untested, and undocumented in the world at large.

Because, in all fairness, those clever and resourceful snail breeders who developed all those lovely gold and purple and jade and ivory snail varieties have reaped some not-insignificant financial reward for their time and effort, would like to reap more, and consider the genetics behind their product a trade secret.  I understand that.

But doggone it, I myself am a professional mollusk geneticist, retired and bored, with nothing better to do with my life than science.  I have a testable hypothesis about the inheritance of color polymorphism in a promising freshwater gastropod model, and I want to see it tested.  And if that hypothesis could be confirmed, all those striking genetic markers could be powerful tools to answer all sorts of additional questions about the biology of the Ampullariidae, a fascinating family of God’s critters, like for example, the consequences of multiple insemination, the potential for sperm competition, and the capacity for sperm storage, see five paragraphs further down.

The experiments necessary to confirm our three-locus model for the inheritance of color polymorphism in Pomacea diffusa are not complicated.  But they require time, patience, attention to detail, and some significant experience with a very specialized little corner of animal husbandry.  The ampullariid diet is non-trivial.  The egg laying, the hatching, and the rearing of juveniles all require special techniques.  And the experiment will require space.  Mystery snails are substantial animals – their culture requires relatively large volumes of water.

Kyra's basement, May 2020

I cultured Physa and a variety of other pulmonates in my lab for many years, and a couple prosobranch species as well [12], and was repeatedly surprised, perhaps even dismayed, by the requirement of simple space – just open, flat space on shelf and lab bench.  There were times in my career that I was covering hundreds of square feet culturing snails with a maximum adult size of 30 mg.  Adult mystery snails are two orders of magnitude larger than Physa.  Am I going to go hat in hand and ask my wife for thousands of square feet in our suburban home to culture Pomacea?

So back in 2017, doing research for a six-part series on freshwater gastropods in the home aquarium [14], I started lurking about in [15] Facebook groups formed by mystery snail enthusiasts.  There were at that time 1,818 members of a Facebook group called “Mystery Snail Addiction,” 3,166 members of a group called “Mystery Snails and Aquatic Lovers,” and 5,442 members of a group called “Snails, Snails, Snails.”  And an idea dawned on me.

The idea is called “crowdsourcing.”  Paging through screenfuls of excited chatter about mystery snails, with images and videos about every conceivable aspect of their life habit and zany antics, it occurred to me that I might be able to attract a large number of talented and enthusiastic volunteers for what I decided to call “The Mystery Snail Color Genetics Project.”

So, I whipped up a modest website of eleven pages, including biological background, genetic model, and three pages of experimental design.  In broad outline, I guided potential collaborators through a trihybrid testcross, starting with an ivory line (aayyss) and a brown/black (AAYYSS), backcrossing the F1 to the ivory to test for linkage.  The biggest challenge was that, absent any data on the question, I felt as though we needed to assume that females store sperm for life, and hence virgin females would be required.  Hit the link below in a new window to see the website if you are curious.  Or a pdf of the entire website is available as FWGNA Circular Number 7 from footnote [16] below.

MSCGP

Notice in particular that step #1 for all new recruits to the MSCGP army was to email me.  I really wanted to establish a direct relationship with every volunteer and try to learn a bit about each of them personally.  I didn’t want to discourage anybody, but I did want to make it clear that what we are doing together is real science.  We will have standards.  No screwing around.

I announced the MSCGP to the three Facebook groups on November 8, 2018, appealing for volunteers, pointing interested parties to the website, and inviting email inquiries.  And braced myself at my laptop for the excited torrent of likes, loves, emojis, comments, questions and wisecracks I felt sure would pour forth.

And in fact, my posts on those three FB sites did reap 6 likes and 4 comments.  Zero shares, but I don’t know what a “share” is, in this context, so that doesn’t matter, does it?  How many followers do I have, I wondered?  Am I an influencer yet?

But I will admit to considerable disappointment when just one of those 6 likers and 4 commenters emailed me directly, a nice young man in Australia, who said he would love to become involved, but no ivory mystery snails were available in his country.  And my posts disappeared off the bottom of the feeds of all three of those FB sites in a matter of hours, gone without a trace.

I received a second inquiry in January of 2019, a third in August, and then two more in November of 2019, a year after I posted my appeal for volunteers.  This was not the response that I was expecting, either in quantity or in quality.

All of these inquiries came from folks who had googled up the MSCGP website and were curious to hear any results we might have obtained.  All were keeping mystery snails as pets, and had some success in reproduction, and were intrigued by color variation in the offspring.  No results so far, I always replied cheerily, but would you like to volunteer?  And one of them did.

Ms. Kyra Hall, who was one of the two who emailed me in November of 2019, was the first serious volunteer recruited into the Mystery Snail Color Genetics Project.  She ultimately pushed the effort further than any other collaborator to the present day, hatching out quite a few pure ivory sibships and attempting to rear them in isolation.  She actually got to the point that she needed true breeding wildtype females, to lay clutches of brown/black offspring with which to cross her ivories.  And it was not until this late date, well into the winter of 2019, that it occurred to me that developing a true-breeding dominant line might be more difficult than a true-breeding recessive.

Beach Blvd, Jacksonville.
We have absolutely no idea on the genetic background of the mystery snails we find for retail sale in aquarium stores.  What would make anybody assume that a snail demonstrating the dominant phenotype was true-breeding?  In fact, it might be more fun if they weren’t, so that dominant mothers might lay interestingly diverse and colorful F1 sibships.  The more I thought about it, I could see some argument for the mysterious snail breeders at the Mystery Snail Factory to purposefully outcross their brown/black stocks before retail sale, if for no other reason than to prevent competitors from developing such stocks on their own.

Might a few generations in the wild select out any weirdo genes bred into Pomacea diffusa stocks for commercial purposes?  Might a naturalized population be the best source of AAYYSS broodstock?  Hmmm.  Pomacea diffusa populations are not commonly reported in the wild [17].

I had known the late Bill Frank for many years, primarily as the steward of the quirky and entertaining jaxshells.org website.  Bill discovered a naturalized population of Pomacea diffusa inhabiting drainage ditches by Beach Boulevard in East Jacksonville in 2006, and posted the discovery on jaxshells.org, and it is probably my browsing across his black, blue, and wisteria-hued webpage [html] that brought the phenomenon to my attention.

So, I emailed my buddy Bill, and asked him if he had ever seen any color polymorphism in his Beach Blvd population of P. diffusa.  And he said no.  And so it came to pass that on December 5, 2019, I pulled into the Denny’s parking lot for my rendezvous with Bill.  The man was as colorful as the palate of his webpages.  I cannot remember the last time I met anybody, at any station young or old, so dedicated, so enthusiastic, so enthralled by trash snails in a weedy ditch.  This world needs more Bill Franks [18].

And the English thesaurus needs more choices under the noun, “ditch.”  Here at home in the Carolina Lowcountry, our drainage ditches are dug to carry away stormwater.  They are almost always dry.  But down in Florida, it is my impression that most ditches are mostly wet, and often transmit significant volumes of groundwater.  The water in the ditches that Bill showed me that morning by Beach Blvd was demonstrating some non-negligible flow, even though it had not rained recently, the water clear and coolish [19].

Bill and I were able to find a dozen adult P. diffusa in about two hours’ effort, combing through the ditches on both sides of the road.  Against the background of his long-term observations of the population dynamics, Bill considered this result a very good catch.  I carried our fresh wildstock back to Charleston that evening and sent a subset by overnight express to Kyra the next day.

The females among them were, in fact, very healthy and fecund, and Kyra was able to hatch quite a few nice clutches of 100% wildtype brown/black progeny.  But alas, her efforts to affect an ivory x brown/black cross did not come to fruition.

Kyra had to move into an apartment in late 2020, effectively bringing to a close her budding career in gastropod genetics.  “Life got in the way,” she explained.  But as annus horribilis 2020 unfolded, from worldwide coronavirus panic to nationwide political ignominy, I was somewhat surprised and really quite gratified to receive an additional nine email inquiries from potential volunteers, a broad assortment of hobbyists who had googled for information on the inheritance of color polymorphism in mystery snails and happened upon the MSCGP website.

And again, although most of those nine did not ultimately initiate experiments, as far as I know, several of them did, at least one or two of whom made serious efforts.  And in 2021 I received inquiries from another ten potential volunteers, a subset of whom initiated experiments, a subset of whom made serious efforts.  And in 2022 I received seven additional inquiries.  So that, as 2022 came to a close, I had exchanged at least some correspondence with a total of 31 potential volunteers for the mystery snail color genetics project.  I should at this point acknowledge Joanna Walthuis, Leigh Charest, John Hynes, and Rachel Voss for significant contributions of time and effort to the MSCGP at intervals during our history.

On February 16, 2023 I sent out a group email to the entire list of N=31 in my MSCGP address book, just to see what the current status of our project might be.  I received ten replies, four of which came from volunteers who still have active experiments underway.  The six respondents who have retired from the field were evenly split, three citing technical difficulties and three reporting, like Kyra, that “life got in the way.”

Kyra's Library, February 2020
The bottom line remains, however, blank.  Here about halfway through the fifth year of the Mystery Snail Color Genetics Project, we have yet to record our first datum.  The consensus from the rank and file seems to reinforce a concern I myself have harbored since I first outlined the experimental design back in 2018.  Rearing pairs of snails in cups, even large (20 oz ones) may require an impractical frequency of water change, and scaling up to one-gallon containers may require an impractical amount of space.  I am just not sure that the pretty experiment I have outlined on the chalk board can be conducted in a private residence, by a private citizen with a job other than snail farming.

Would anybody like to prove me wrong, by proving me right?  Shoot me an email at DillonR@fwgna.org.  The MSCGP is still looking for volunteers!

Notes

[1] In yet another demonstration of the folly of legislating common names, the official, AFS-sanctioned common name for Pomacea diffusa/bridgesii is “spike-topped apple snail” [2].  But I suspect that the generic term “apple snail” has become too closely associated with invasive pests for the comfort of the aquarium trade.  For whatever reason, for quite a few years now the breeders and retailers have been marking “mystery snails” on their aquaria of Pomacea diffusa.  And if that’s what it says on your sales receipt, that’s what it is when you get home.

[2] Turgeon, D.D., J.F. Quinn, A.E. Bogan, E.V. Coan, F.G. Hochberg, W.G. Lyons, P.M. Mikkelson, R.J. Neves, C.F.E. Roper, G. Rosenberg, B. Roth, A. Scheltema, F.G. Thompson, M. Vecchione, and G.D. Williams (1998) Common and scientific names of aquatic invertebrates from the United States and Canada: Mollusks (second edition), American Fisheries Society Special Publication 26, Bethesda, Maryland, 526 pp.

[3]  I am quoting the “late 1960s” domestication date for P. diffusa from the 1996 book by Perera & Walls, page 36.  Whether all eight color varieties were available at that early date, I do not know.  All varieties were certainly on the market by 1996, as evidenced by their photos in:

  • Perera, G. and J.G. Walls (1996) Apple Snails in The Aquarium.  T.F.H. Publications, Neptune City, NJ

[4] 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

[5] Blume, W. (1957) Eine bis heute unbekannte Unterart von Pomacea bridgesii Rve.  Opuscula Zoologica 1: 1 – 2.

[6] We have directed at least ten or twelve posts on the blog to invasive apple snails of the P. canaliculata/maculata type over the last 20 years.  For an entry into their extensive literature, see:

  • REVIEW: Global Advances in Apple Snails [24May07]
  • Two dispatches from the Pomacea front [14Aug08]
  • Pomacea news [25July13]
  • Invasive species updates [13June18]

[7] Actually, we have documented two complementing albinism loci in our own favorite experimental animal, Physa acuta.  See:

  • Albinism and sex allocation in Physa [2Nov18]

[8] Yusa, Y. (2004) Inheritance of colour polymorphism and the pattern of sperm competition in the apple snail Pomacea canaliculata (Gastropoda: Ampullariidae). Journal of Molluscan Studies 70: 43 – 48.

[9] Dillon, R. T.  (1998-99)  The inheritance of golden, a shell color variant of Marisa cornuarietis. Malacological Review 31/32: 155-157. [PDF]

[10] Cain, A.J., P.M. Sheppard, and J.M.B. King (1968) The genetics of some morphs and varieties of Cepaea nemoralis (L).  Philosophical Transactions of the Royal Society B 253: 383 – 396.  For more, see

[11] In 2005 Stijn Ghesquiere added a “Genetics” page to his world-renowned applesnail.net website, without attribution, featuring a three-locus, eight phenotype Mendelian model very clearly developed and demonstrated.

[12] In addition to the Marisa I reared for my 1999 paper [9], I also had one extended experience with Pleurocera proxima [13], which was a huge pain in the ass.  I know, however, that I’m exaggerating about the “thousands of square feet” estimate up above.  But give it to me – I’m trying to avoid an argument with my wife here.

[13] Dillon, R.T. (1986) Inheritance of isozyme phenotype at three loci in the freshwater snail, Goniobasis proxima: Mother-offspring analysis and an artificial introduction. Biochemical Genetics 24: 281-290.  [PDF]

[14] Here’s my entire series on freshwater gastropods in the home aquarium:

  • What’s Out There? [9Oct17]
  • Loved To Death? [6Nov17]
  • Pet Shop Malacology [21Dec17]
  • Snails By Mail [24Jan18]
  • Freshwater Gastropods and Social Media [14Feb17]
  • Psst, Buddy!  Wanna Buy An Apple Snail? [16Mar18]

[15] Here's a funny demonstration of how unlearned I am in the ways of social media.  In my original essay of 6June23 I used the verb "ghosting" here, to describe reading FB posts without commenting or participating in any way.  On 8June23 my MSCGP colleague "Dylan" sent me an email correcting my verb choice to "lurking."

[16] Dillon, R. T., Jr. (2018) Welcome to the mystery snail color genetics project!  FWGNA Circular 7: 1 - 13.  [pdf]

[17] The USGS Nonindigineous aquatic species database lists 42 records of P. bridgesii/diffusa, as compared to 2,948 records for “Pomacea cf. canaliculata/maculata.”

[18] Here is Bill’s obituary: William Michael Frank (18Sep47 – 16Sept22)

[19] In addition to Pomacea diffusa, the "ditches" along Jacksonville's Beach Blvd are inhabited by Pomacea paludosa, Melanoides tuberculata, and Hebetancylus excentricus.

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