Dr. Rob Dillon, Coordinator

Thursday, May 10, 2018

Dichotomous Dichotomous Keys

Last month [1] we reviewed the Gastropoda chapter contributed by Christopher Rogers to the new Fourth Edition of Thorp & Covich’s Keys to Nearctic Fauna [2].  The bottom line was, “Buy this book.”  And among the several lines of support I offered for this recommendation was a somewhat enigmatic observation to the effect that the new fourth-edition dichotomous key complements, but is in many cases strikingly different from, the old third-edition key.  What did I mean by that?

In two words, Christopher’s new key is evolutionary, whereas the old one was ecological.  Christopher has divided the North American freshwater gastropods phylogenetically, designing his dichotomous key to branch as a phylogenetic tree might branch.  The older key divided the fauna functionally, according to morphological adaptation.

Broad/flat vs. Narrow/filiform, if you're curious...

 So, couplet #1 of the old Third Edition was a choice between “shell coiled” and “shell an uncoiled cone.”  The former choice took the user onward into the body of the key, while the latter choice immediately took the user aside to seven limpet genera.  Those seven limpet genera are assorted into three families: the Ancylidae with four, the Acroloxidae with one, and the Lymnaeidae with two.  This is an ecological distinction – the limpet shape demonstrating superior strength against predation, and superior performance against hydrodynamic drag, given a solid substrate upon which its bearer can graze.

That patelliform shape has, of course, evolved many, many times independently in many, many different gastropod lineages, both freshwater and marine [3].  And it has apparently evolved (at least) three times separately in the freshwater gastropods – in the Acroloxoidea, in the Lymnoidea, and in the ancylid taxa of the Planorboidea.

So Christopher elected to open his new Fourth Edition gastropod key with operculum present/absent as his couplet #1. This is the easiest character by which to distinguish pulmonates from prosobranchs, the primary phylogenetic division in the freshwater gastropod fauna.  All the freshwater limpets are pulmonates, missing an operculum, hence all go together to his couplet #7.  Then at Christopher’s couplet #7, the user finds: 
7(1) Shell not patelliform, or if patelliform, then spire sinistral (apex centered or to right of midline) and blunt, with adult patelliform shell larger than 7 mm … go to #8.
7’ Shell patelliform with spire dextral (apex to left of midline), acute; adult shell less than 7 mm in length … Acroloxidae.
So I understand what Christopher is trying to do here, and I appreciate his effort.  He is beginning to sort the pulmonate families using evolutionary distinctions.  But I’m just not sure it works.

First, as a practical matter, users cannot determine if the spire is sinistral or dextral for 99.999% of all the limpets in the creek – they’re blunt and smooth.  So, users must read over that text to “apex centered or to the right of midline” vs. “apex to left of midline.”  And second, being finely evolutionary, both Acroloxus and the lymnaeid limpets (Lanx and Fisherola) demonstrate shell apexes to the left of midline, at least when young [4].

Now I understand if Christopher does not want to engineer his dichotomous key to the entire freshwater gastropod fauna of North America around an accommodation for juvenile-Lanx-collectors.  So I’ll let it go.  Let’s suppose we have under our scope some non-patelliform pulmonate, like Physa or Helisoma.  And we have dodged through couplet #7 to arrive at couplet #8, undiscouraged.  Here we read: 
8(7) Tentacles narrow, filiform …. 9
8’ Tentacles broad, flat, triangular; haemoglobin absent; coiled shell always dextral, patelliform shell with apex central or sinestral [sic]; never planospiral … Lymnaeidae
For heaven sake!  Now Christopher seems to expect us to have a living animal under our scope, or at least a very well-preserved one, to distinguish narrow tentacles from broad tentacles?  Narrow compared to what?  There’s no tentacle figure.  Haemoglobin, are you serious?  Must I medevac this little speck of coiled brown nothing to England and set up an IV?  Typed and cross-matched, stat?

In the Third Edition key, after I had observed that my Physa was coiled and been directed to couplet #8, I was next asked if the shell was planospiral or “with raised spire.”  If planospiral go to Planorbidae, otherwise go on to couplet #20.  Simple.

I am not going to criticize every couplet in Christopher’s entire 20-page key.  It’s a tremendous effort, and I don’t want to diminish his contribution.  I will simply observe that the evolutionary approach taken in the Fourth Edition is not as user-friendly as the ecological approach taken in the third.

So, here’s the bottom line for this month’s essay.  Don’t throw away your old Third Edition of Thorp & Covich.  Open it up on the lab bench next to your new Fourth Edition and use both simultaneously.  The two works side by side are a dichotomous perspective on the wonderful diversity that marks the North American freshwater gastropod fauna.


[1] REVIEW: Thorp & Covich Fourth Edition [12Apr18]

[2] Thorp, J. H., and D. C. Rogers (2016) Keys to Nearctic Fauna.  Thorp and Covich’s Freshwater Invertebrates, Fourth Edition.  Volume II.

[3] Actually, the “hypothetical ancestral mollusk,” from which all sevenish of the molluscan classes diverged back in the Precambrian, is generally modelled with a limpet-like (or plate-like) shell similar to that borne by the present-day Monoplacophora.

[4] Basch, P. (1963) A review of the recent freshwater limpet snails of North America. Bull. Mus. Comp. Zool. Harvard 129: 399-461.

Thursday, April 12, 2018

REVIEW: Thorp & Covich Fourth Edition

Thorp, J. H., and D. C. Rogers (2016) Keys to Nearctic Fauna.  Thorp and Covich’s Freshwater Invertebrates, Fourth Edition.  Volume II.

“What a marvelous contribution!  I never thought I would live to see the day.”

Both these sentiments oscillated through my mind in rapid succession as I flipped through the pages of Christopher Rogers’ “Class Gastropoda” key in the new edition of Thorp & Covich displayed on the auction table in Raleigh [1] last June.  The taxonomy is complete, thorough, well-researched and modern.  Just three genera of living pleurocerids, check.  Hubendick’s single, broadly-inclusive genus of lymnaeids, check.  Just two genera of physids, check!  Bellamya yes, Planorbella no.  Onward as the pages turned.  What a marvelous contribution!  I never thought I would live to see the day.

This was not the first time such thoughts had flickered through my mind, however.  It had happened twice before.

I imagine that most of my readership will be familiar with the Thorp & Covich tradition.  The first edition of Ecology and Classification of North American Freshwater Invertebrates, as the collaboration was originally called, was published in 1991, with a second edition in 2001 and a third in 2010.  Over this 20-year span Thorp & Covich has become the primary bench reference for biologists working with the (non-insect) freshwater macrobenthic fauna of the United States [2].

Ken Brown was the solo author of the Gastropoda chapter through its first two editions, Chuck Lydeard joining him in 2010 [3].  Ken has an excellent background in freshwater gastropod ecology, which together with anatomy, physiology and (later) gene trees comprised the first 70% of the chapter.  The remaining 30% was allocated to a dichotomous key following the taxonomy of Burch [4] rather closely [5], but I am not criticizing.  That’s all there was.

So the first time that “marvelous-contribution-but-it’ll-never-happen” thought flickered through my brain was in March of 2011, when I opened an email from Jim Thorp.  Jim intimated to me that, even though his third edition was only a single year off the press, he and Christopher were already making plans for a major revision.  He projected that his fourth edition would expand into a series of volumes, the first of which would be dedicated to more ecological themes, the second of which would present dichotomous keys to the Nearctic fauna, the third to the Palaearctic, and so forth [6].  The fourth-edition keys would be much larger and more detailed than those of the third edition, extended down to the species level wherever possible.  And he wanted me to lead the Nearctic Gastropoda team.

I was flattered, of course.  But the series of negotiations into which we entered did not ultimately yield fruit.  I am a big fan of review and synthesis, and I greatly appreciate the value of reference works like Thorp & Covich for biologists in the field today.  But I climb onto the shoulders of giants with a pair of binoculars looking up, not down.  An historian I am not.

So for example, of the roughly 150 nominal species of pleurocerid snails Calvin Goodrich recognized in North American fresh waters [7], forwarded to the present day by Burch, I should estimate that no more than about 30 – 40 are biologically valid.  How am I to construct a dichotomous key to such a biota, I asked Jim, if most of the taxa to be keyed are not legitimately distinguishable?  Could I synonymize?

Jim (quite understandably) demurred.  Christopher suggested that, in cases such as the Pleuroceridae, I simply key down to the lowest firm taxon and drop it, with some language to the effect that “This genus is in need of revision and therefore identifications are left at genus level.”  Well, I thought to myself, all the North American freshwater gastropod genera are in need of revision.  Most have never had a vision in the first place.  Ultimately, I found it impossible to rationalize the expenditure of my own time and effort unless I could move the ball forward [8].

So Volume 1 of the Fourth Edition appeared in 2015, our good friend Mark Pyron’s name appearing above that of Ken Brown as author of Chapter 18, “Introduction to Mollusca and the Class Gastropoda.”  I was pleased to see that the Pyron & Brown contribution extended to 41 pages, up from the 30 pages of the 1991 original.  And sure enough, those 41 pages were focused entirely upon ecology and evolution, no dichotomous key anywhere in evidence.

And the second time that “marvelous-contribution-but-it’ll-never-happen” thought flickered through my brain was later that same year, when Christopher emailed me to request that I review the dichotomous key he himself had prepared for Volume 2.  His draft looked excellent – I really didn’t have a whole lot of suggestions to offer.  In retrospect, I think that from among the entire 3 x 10^8 population of the United States of America, I might have been the worst possible choice to review a freshwater gastropod key.  Because I understood what Christopher was trying to say, and I often skipped ahead, knowing where he was trying to go.

Regarding the problematic groups, I was gratified to see that Christopher had taken the approach he had first advocated during our 2011 negotiations.  Here’s a verbatim quote from his section entitled Limitations: 
“Particularly vexing is the strange confusion in the taxonomic literature, with many taxa accepted or rejected without explanation, and good quantitative taxonomic revisions ignored without explicit justifications.  Because of this issue, the keys are taxonomically conservative, often terminating with species groups rather than species.”
 So Christopher judged such terminations to be necessary for 13 “species groups” in the Nearctic freshwater gastropod fauna all told: the hydrobioid genera Amnicola, Lyogyrus, Clappia and Fluminicola, the hydrobioid families Cochliopidae and Hydrobiidae (ss), assorted prosobranch genera Valvata, Campeloma, Pleurocera/Lithasia, and Leptoxis, and the planorbid genera Helisoma, Carinifex, and Menetus.  Fine.  It’s hard to know which of those thirteen groups is the biggest mess.

The rest of the freshwater gastropods of North America Christopher did, very bravely, key all the way down to the species level.  Among the pulmonates he recognized 10 species of Lymnaea, 9 species of Physa, and 24 species of (non-Helisoma, non-Carinifex, non-Menetus) planorbids [9].  All good estimates – probably pretty close.  From among the entire 3 x 10^8 population of the United States of America, I may well appreciate the effort that Christopher must have put into this contribution the most.

So buy this volume.  Actually, you should back up and buy Volume 1 if you haven’t already, as well as Volume 2.  Do it for three reasons.  First, the gastropod taxonomy is correct, complete, and current.  Second, the new fourth-edition keys complement, but in many cases are strikingly different from, the old third-edition keys.  We’ll develop that theme next time.  And finally, assuage my guilt.  In retrospect, I may have been more a hindrance than a help to Christopher as he labored over this marvelous contribution.  I never thought I would live to see the day.


[1] SFS Raleigh  [4Apr17]

[2] Pennak held that position when I was coming up through the ranks, and is still an excellent reference.  Nothing in the essay above is intended to take anything away from Doug Smith’s (2001) Fourth Edition of “Pennak’s Freshwater Invertebrates of the United States” at all.

[3] Alan Covich and Ken Brown asked me to help with the 2010 edition.  I was flattered, but ultimately declined, for reasons similar to the ones I gave Jim Thorp in 2011.  I can be a difficult person to deal with.

[4] This is a difficult work to cite.  J. B. Burch's North American Freshwater Snails was published in three different ways.  It was initially commissioned as an identification manual by the US EPA and published by the agency in 1982.  It was also serially published in the journal Walkerana (1980, 1982, 1988) and finally as stand-alone volume in 1989 (Malacological Publications, Hamburg, MI). 

[5] The hydrobioid subfamilies were raised to the full family level in the third (2010) edition.

[6] Twelve volumes are now on the drawing board, according to a more recent communication I enjoyed with Jim.  Volume III is now Neotropical Hexapoda, and Volume IV is the Palaearctic Fauna, which was slightly delayed.  But both are currently in press. 

[7] The Legacy of Calvin Goodrich [23Jan07]

[8] My exact words to Jim and Christopher were, “It’s the fourth quarter of my career.  I won’t take a knee on the ten-yard line.”

[9] Ancylid limpets integrated with the Planorbidae.  OK, fine.

Friday, March 16, 2018

Psst, Buddy! Wanna buy an Apple Snail?

Editor’s Note –This is the sixth (and final) installment in my series on the general topic of freshwater snails in the aquarium hobby.  Previous posts have been “What’s Out There?” [9Oct17], “Loved to Death?” [6Nov17], “Pet Shop Malacology,” [21Dec17], “Snails by Mail” [24Jan18], and “Freshwater Gastropods and Social Media” [14Feb18].  It might help you to read (at least) my previous (February) post on this subject before going on to the essay below.

First let us clarify the situation as pertaining to law.  In addition to their broader body of regulations regarding the movement of gastropods generally, the Feds have a set of explicit restrictions regarding the importation and movement of ampullariids [1].  Quoting the USDA-APHIS verbatim: 
“aquatic snails in the family Ampullaridae (e.g., Pomacea canaliculata, channeled apple snail), with one exception, may not be imported or moved interstate except for research purposes into an APHIS inspected containment facility. One species complex in the family Ampullaridae, Pomacea bridgesii (diffusa) may move interstate without a permit because these snails are not known to be agricultural pests but are primarily algae feeders. An import permit is required for aquatic snails in order to verify species and examine shipments for contaminants that are agricultural pests.” [2] 
Note that it is not illegal to own, buy, sell, trade, breed or propagate invasive apple snails of the maculata/insularum/canaliculata type, conventionally abbreviated IAS.  Naturalized populations of IAS are already widespread in certain regions of the United States.  I, living in South Carolina for example, could easily gather Pomacea maculata from any number of local retention ponds in my area and enjoy them in my home aquarium.  I cannot, however, ship them to my friends in North Carolina, nor carry them in a cooler up I-95.  Nor can my Tarheel buddies come down here and fetch any.

So in last month’s [14Feb18] essay I shared my impressions from 30 days of monitoring the conversation on a Facebook group called, “Snails, Snails, Snails.”  I tallied eight mentions of IAS during that month, including four separate appeals for purchase, and concluded: “Without a doubt, significant pent-up demand exists within the community of aquarium hobbyists for large, invasive apple snails.”

"Last round of Peruvian Apple Snails"

I didn’t mention it at the time, but I am mentioning it now, because I think it is especially significant.  Among the eight mentions of IAS I logged during my 30 days of observation on Snails, Snails, Snails was one offer to sell.  A pet supply store in South Dakota named “Woofs & Waves” posted the photo above, simply captioned, “Last round of Peruvian Apple Snails for the season.”

This post generated 10 comments, plus about 25 associated replies.  Comments included, (1) Cooooool!! and (2) Oh I wish I could get those, and (3) I want one so bad!  He’d do great in my 110! and (4) what does he charge for those?  Where is he located?

The reply to the previous query was, “$9.99, Sioux Falls.”  Then this discussion followed: “I am sure they wold ship if you asked nicely” and “Pretty sure they can’t cuz those are illegal in many places” and “Yeah, South Dakota is pretty lax on wildlife stuff unless you’re poaching.”

I think that independent aquarium stores may be the primary agents for the introduction and spread of invasive apple snails around the USA.  The survey I posted in December [21Dec17] satisfied me that the big-box pet stores don’t sell them, and the desultory survey of major online retailers I published in January [24Jan18], including Amazon and Ebay, didn’t turn up any.  Are Mom-and-Pop independents, which in the patois of social media are called “LPS” (local pet stores), the well from which North American populations of invasive apple snails spring?

Following this hunch, last week I made a field trip across town to the only independent aquarium store in the Charleston Area, a really handsome shop with great stock and a knowledgeable staff called, “Tideline Aquatics.”  And in addition to the usual assortment of mystery snails and nerites and rabbit snails [4], I found offered for sale a small batch of “jumbo gold mystery snails,” maybe six or eight head in the lot, crammed timidly into the corner of an aquarium on the bottom rack, behind the filter.  These are clearly not our benign little friend Pomacea diffusa/bridgesii.  These are invasive apple snails.  Click for larger:

NOT Pomacea diffusa
And more than just any random IAS, my local pet store is apparently stocking golden-form apple snails, of a kind widely introduced throughout Asia and the Pacific Islands.  We don’t host any golden morphs at all in the populations of Pomacea maculata naturalized here in South Carolina.  I’ll bet dollars-to-donuts that the stock of “Jumbo gold mystery snails” for sale a few miles from my house originated from Asia, probably from dealers not unlike the ones surveyed by Ting Hui Ng and her colleagues in 2016 [5].  See figure #14 in the Ng et al. plate I shared with you all back in October [9Oct17].

And here we also reprise a theme I developed in December [21Dec17] – the mysterious origins of aquarium stocks worldwide which, like any other commodity I suppose, it behooves suppliers to protect.  And the plasticity of the names attached to such stocks.  Common use and legal precedent has developed such that “apple snails” are bad and “mystery snails” are good.  So the “jumbo mystery snail” has been born, to mysteriously arrive at an independent aquarium-stock retailer near you.

And so we have now come full circle, which means that it is time to sum up.  I am charmed, genuinely charmed, by the widespread interest and heartfelt love often demonstrated by aquarium hobbyists toward our mutual friends, the freshwater gastropods.  And I think such interests should be encouraged, if for no other reason than they might blossom.  I cannot see how harvest of wildstock freshwater gastropod populations for the aquarium trade could endanger such populations, at any imaginable harvest rates.  I can see, however, a problem with the spread of invasive species.

Here a tiny and obscure freedom, escaping the notice of our founding fathers, is associated both with a tiny societal benefit, and a tiny hazard.  I can’t think of any solution to the tiny hazard, beyond what we’re already doing.  Let’s just leave that freedom alone, shall we?


[1] And I should immediately stipulate that some states have their own regulations more restrictive than the Feds.  Our good buddy Joshua Vlach from the Oregon Department of Agriculture informs me that Oregon has a five-page list of invertebrates that are ALLOWED to cross its state lines, and all others are prohibited [3].   The bottom line is the same for IAS, however.  Go home!

[2] US Department of Agriculture, Animal and Plant Health Inspection Service.  Plant Health / Import into the US / Permits / Regulated Organisms and Soil Permits / Snails Slugs.  [html]

[3] This reminds me of a scene from one of the Peanuts videos, where Violet and Lucy tell Charlie Brown, “There were two lists, Charlie Brown.  There was a list to invite, and a list NOT to invite.  And you were on the WRONG LIST!”

[4] The shells of the “Rabbit Snails” were completely encrusted with calcification.  Absolutely unidentifiable.  The ugliest gastropods I have ever seen in captivity.

[5] 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.  https://doi.org/10.1371/journal.pone.0161130

Wednesday, February 14, 2018

Freshwater Gastropods and Social Media

Editor’s Note –This is the fifth installment of my series on the general topic of freshwater snails in the aquarium hobby.  Previous posts have been “What’s Out There?” [9Oct17], “Loved to Death?” [6Nov17], “Pet Shop Malacology,” [21Dec17] and “Snails by Mail” [24Jan18].  But don’t worry.  Full appreciation of Essay #5 is not contingent upon familiarity with Essays #1 – 4.

I am not social, in any medium.  I don’t even text, much less twitter or tweet or insta-chat or whatever it is that the kids are doing these days.  I understand that social media can be effective tools for communicating on a large scale.  I did join Facebook about ten years ago, in order to “like” a political group of which I was serving as an officer [1].  I regretted it at the time, and regret it now.

In any case, about once a week I gather up my courage and log onto Facebook.  And watch in horror as great garbled masses of disconnected conversations and news and opinions and jokes and photos and videos from family and friends and professional colleagues and high school classmates and Sacred Harp Singing Societies are disgorged simultaneously onto my desk in one gigantic, hideous, stupefying dose.

So several months ago, a Facebook friend called my attention to a group called “Snails, Snails, Snails.”  Heart racing with a mixture of curiosity and dread, I clicked over to the homepage for the group, and what to my wondering eyes should appear, but an internet forum for “lovers, keepers, breeders, and sales of freshwater and saltwater snails and slugs,” boasting 5,442 members!

"Gary doesn't smell so good."
It was a closed group.  So I submitted my CV, top five recent publications, and three letters of reference, and was, after some period of deliberation, duly admitted to membership.  And have subsequently been charmed.

What an engaging assortment of odd-lot humanity!  Mostly young, apparently from a wide variety of backgrounds, hailing from all over the world, unified by the love, yes love often and freely confessed, of gastropods.  Most of the members seem to be freshwater aquarium hobbyists.  Posts about marine gastropods are occasional, as are photos of pet land snails, and even peripheral aquarium fauna, like shrimp.  But I would estimate that, of the perhaps 15 – 20 posts per day, at least 80% have to do with somebody’s freshwater aquarium pet.

Fascinated by the social interactions as they unfolded before me, I resolved to log onto Snails, Snails, Snails every day for 30 days, beginning 25Aug17, and monitor all activity.

I recorded 16 different freshwater gastropod categories receiving mention during my month of observation, totaling 375 mentions.  Of that total, 230 mentions (61%) were of Pomacea diffusa/bridgesii, almost universally referred to as “mystery snails,” apparently the most popular gastropod pet in the home aquarium by far.  Indeed, at some point during the month my attention was called to a pair of independently-operating FB groups dedicated exclusively to P. diffusa, “Mystery Snail and Aquatic Lovers” with 3,166 members and “Mystery Snail Addiction” with 1,818 members [2]. 

The discussion seems to focus on husbandry – food, water quality, life history in culture – not too much different from chatter about aquarium fish, I don’t suppose.  One probably reads more of the “How do I tell if Gary is dead” sorts of questions.  One also reads a surprising number of posts sharing “the cute thing I saw Lightning do,” probably very similar to typical social media interactions about cats and dogs.

The next-most-popular category of freshwater snails in social media seems to be the nerites of all species, with 33 mentions on Snails, Snails, Snails for the month.  This is unsurprising, given the results of the survey of big-box pet retailers I reported in December [3].  The remainder of the species with double-digit mentions during my 30 days of monitoring were “Ramshorns” (24), Melanoides tuberculata (19), Physa (17), Assassin snails (14), and “Rabbit Snails” (Tylomelania, all species) with 10 mentions [4].

I tallied eight mentions of “apple snails” during my month of observation, by which I was able to unambiguously confirm that the author was referring to large, invasive Pomacea maculata/insularum/canaliculata types.  I also caught two mentions of the invasive “Columbian Rams Horn” Marisa.

One young lady in Houston shared an article from the Houston Chronicle entitled, “Harvey Floodwaters bring weird pink things to the Houston landscape [5].”  There were 11 comments and replies, most of the “LOL” sort.  But other comments included "I'll take them 😄" and "I wish I could find some of these here," and "So jealous!  I'm in Illinois and haven't been able to get my hands on a pair."

Without a doubt, significant pent-up demand exists within the community of aquarium hobbyists for large, invasive apple snails.  I counted four separate appeals to purchase such animals during my 30 days of observation, generally of the form, “Does anyone have a LARGE (like, baseball sized) apple snail that they would sell? I LOVE snails and I can't find any that large near me.” 

It is impossible to know, of course, to what extent such requests are satisfied through one-on-one “messaging.”  Typical public replies to such solicitations included “You’d have to find locally. So you should post your location. Shipping adults is dangerous.” or “For channeleds you'd have to find a local seller. I believe it's illegal to ship them over state lines.”  Here’s one (especially revealing) reply: “Haha thank you! Yes our aquarium shop gets lucky once in a great moon and they have a personal tank with one literally apple size so I am always checking in.”

I never saw any misgivings expressed by any member of the Snails, Snails, Snails FB Group about the potential for large apple snails to become invasive pests.  Significant qualms were not uncommonly expressed, however, about the potential for large apple snails to destroy valuable aquarium plants.  One member asked, “How do you stop apple snails from eating your expensive plants?”  After several commiserations, condolences, and expressions of despair, the Group Administrator posted this meme, which I do not understand:

Finally.  I leave you on this Valentine’s Day with one woman’s heart-wrenching testimonial to the love she bore for her gastropod friends.  She posted: 
“My son was in a horrific car accident on Tuesday and almost lost his life. He is home now and doing well but, while i was away i had my kids feeding my snails for me. It did not turn out so well. I lost a banjo catfish, a betta, and all but two of my big apple snails. I am praying these two guys will recover but idk.”
Yes, you read that correctly.  Her son was in an automobile accident, and she is praying for her snails.


[1] The South Carolinians for Science Education. [SCSE]  Like us on Facebook!

[2]  The list goes on and on, actually.  A simple search for “snails” within Facebook will also return groups called “Land Snails” (742 members), “Tree and Land Snails” (925 members), “Snail Enthusiasts: USA” (1,400 members) and even (I wish I was kidding) “Giant African Land Snails” with 5,800 members.

[3] Pet Shop Malacology [21Dec17]

[4] Others mentioned included Bellayma (6), “Pagoda snails” (5), Thiara scabra (3), “Devil’s Spike” (1), Lymnaea peregra (1), Gyraulus parvus (1), and New Zealand Mud Snails (1).

[5] Houston Chronicle (7Sept17).  Harvey Floodwaters bring weird pink things to the Houston landscape. [html]

Wednesday, January 24, 2018

Snails by Mail

Last month we surveyed the elements of the freshwater gastropod fauna widely available to hobbyists in the Big Box retail outlets that seem so dominant on the landscape of aquarium supply today [1].  We found two categories of snails reliably offered for sale, strikingly different in their biology but ironically similar in their provenance – the “mystery snails” (Pomacea bridgesii/diffusa) and the nerites.  But, as my readership has already doubtless inferred from my essay of 9Oct17 [2], ampullariids and neritids do not the entire market comprise.  What else might be available online?

Assassin Snail - Aquatic Arts
If one simply enters “freshwater snails” on the subject line of a google search, the first 50 hits include four major retail suppliers – Amazon, eBay, aquaticarts.com, and liveaquaria.com.  Most of the stock available for purchase from these sources are (once again) nerites or mystery snails in their various color varieties.  But below I have compiled a brief review of the remainder, sorted into seven pigeonholes.  The first four taxa or groups of taxa appear to be widely available for purchase online, the next two categories seem to be occasionally available, and the last category is what I would call a “wastebasket.”

Ramshorns – These easy-to-culture snails seem to have remained a perennial favorite of aquarium hobbyists for many years, at least since I was a kid.  All the stocks with which I have any personal experience are North American Helisoma trivolvis, but Ng and colleagues [3] identified Singapore ramshorns as Oriental Indoplanorbis exustus.  I wish I knew more about the relationship between these two taxa.  Most of the offerings for sale online today are “red ramshorns,” which are actually albinos, their absence of body pigmentation allowing that red hemoglobin so characteristic of planorbids to show through.  Stocks with wild pigmentation are marketed as either “brown” or “black.”  There is also a “leopard” variant for sale that has patchy pigmentation on its mantle, and a “blue” that (I think) demonstrates some sort of mutation in shell pigmentation.  I wish I knew more about that, too.

Assassin Snails – Approximately thirty nominal species of the nassariid genus Clea (or Anentome) burrow in the soft bottoms of broad, coastal rivers from southern China and Southeast Asia into The Philippines.  What fascinating creatures!  The group is one of only two neogastropod genera to have successfully invaded fresh waters [4].  As their name implies, assassin snails are predatory – hunting other freshwater snails and sucking them out of their shells.  The little tigers widely marketed to the aquarium hobby today are universally identified as Clea helena, but the excellent recent study by Ellen Strong and colleagues [5] suggests that commercial stocks may represent as many as four species, none of which seems to match topotypic Anentome helena from Java.

Rabbit Snails – Several species of the pachychilid genus Tylomelania are not uncommonly offered for purchase online, variously marketed as “giant” or “orange” or “golden” rabbit snails.  It may be recalled from my October essay that Ng and colleagues [3] identified four Tylomelania species in the Singapore aquarium trade, all of which are apparently endemic to Lake Pozo on the Indonesian Island of Sulawesi.  Some conservation concern has been expressed, but see the follow-up essay I published on this subject in November [6].

Japanese Trapdoor Snails – Yes, our old familiar Bellamya japonica (or maybe B. chinensis?) often seems to be marketed to the indoor aquarium hobby, generally labelled as "Viviparus malleattus.”  The biology of these large oriental viviparids will be well known to my FWGNA readership, but see my species pages [japonica] and [chinensis] for a refresher.
Rabbit Snail - Aquatic Arts

Pagoda Snails – Several nominal species of the pachychilid genus Brotia bearing heavy, strikingly spiny or tuberculate shells are harvested from the rivers of Thailand and occasionally available from online retailers as “Pagoda snails.”  We touched on these back in October as well.

Chopsticks, Spikes, or Long nosed Snails – Occasionally the discriminating freshwater gastropod connoisseur will find thiarids of the genus Stenomelania offered for sale online.  Again, Ng and colleagues [3] identified four Stenomelania species marketed in the Singapore pet trade, although raising no conservation concerns.  The Discover Life website lists 36 nominal species in the genus, ranging throughout Southeast Asia, Australia, and Oceania.  The most common specific nomina mentioned in the pet trade are Stenomelania torulosa and S. plicaria, both distributed widely from India through Indonesia to China.

The Wastebasket – Although (almost) universally reviled, stocks of the “Malaysian Trumpet Snail” (Melanoides tuberculata) are available for purchase on Amazon and eBay.  This invasive thiarid, apparently native to low latitudes throughout the Old World (in various clones), has been widely introduced into the New.  See my FWGNA species page [tuberculata] for more.  And (if you can believe it) hobbyists with a thirst for the small, brown, and mundane can also purchase Physa acuta stocks from Amazon.  I get the impression that both the Physa and the Melanoides are primarily marketed as prey for Assassin snails.  The Physa listing on Amazon advertises, “great natural food for your puffer.”

What I did not find for sale online last week, thank heaven, was any ampullariid stock other than Pomacea bridgesii/diffusa. I remember in years past being able to purchase, at least occasionally through mail order or mom-and-pop aquarium stores, Pomacea insularum/maculata (“Golden Apple Snails”), Pomacea paludosa (“Florida Apple Snails”) and Marisa cornuarietis (“Giant Ramshorns.”)  But I was unable to find, at least upon superficial search, any listing for any such invasive ampullariids through the major online retail outlets today.

So to conclude.  Should we be concerned that any of the freshwater gastropod groups listed above might escape to become pests here in North America, other than the ones already introduced and spreading?  We have reviewed the criteria for invasiveness on quite a few occasions in the past [7], ultimately settling on two ecological qualities which I have called “weedy” and “different.”  So the ramshorns, trapdoors, and wastebaskets are already here.  And the rabbits, pagodas, and chopsticks are not all that ecologically different from North American pleurocerids, in many cases, nor do their life histories seem especially weedy.  That leaves the Assassin snails.

Could an introduction of Clea succeed here in North America?  Some concern has already been expressed [8].  All the range maps I have seen for the genus seem to suggest that their natural distribution is entirely tropical – apparently ranging from the equator to around 20 degrees N latitude.  So our own Key West floats in the Caribbean at latitude 24.5 degrees N, perhaps still a bit too temperate to raise concerns about the threat of gastropod assassination here in the USA.  But you all down in Mexico and Central America might best be on the lookout.


[1] Pet Shop Malacology [21Dec17]

[2] What’s Out There?  [9Oct17]

[3] 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. https://doi.org/10.1371/journal.pone.0161130

[4] The only other neogastropod group to invade fresh waters is the marginellid genus Rivomarginella.

[5] Strong EE, Galindo LA, Kantor YI. (2017) Quid est Clea helena? Evidence for a previously unrecognized radiation of assassin snails (Gastropoda: Buccinoidea: Nassariidae) PeerJ 5:e3638 https://doi.org/10.7717/peerj.3638

[6] Loved to Death?  [6Nov17]

[7] For the biology of freshwater gastropod invasions, see:
  • Invaders Great and Small [19Sept08]
  • Community Consequences of Bellamya Invasion [18Dec09]
  • The Most Improbable Invasion [11Oct12]
  • The Many Invasions of Hilton Head [16Dec15
[8] Mienis HK. 2011. Will the uncontrolled sale of the snail-eating gastropod Anentome helena in aquarium shops in Israel result in another disaster for Israel’s native freshwater mollusc fauna? Ellipsaria 13(3):10-11.

Bogan AE, Hanneman EH. 2013. A carnivorous aquatic gastropod in the pet trade in North America: the next threat to freshwater gastropods. Ellipsaria 15(2):18-19

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, 12].  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 Singapore 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 Singapore 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.

[12] 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.

Monday, November 6, 2017

Loved to Death?

Back in 1976, when I first joined the American Malacological Union, the community of shell collectors played an important role in the day-to-day operations of the society.  It seemed to me that the amateurs actually outnumbered the professionals at our annual meetings.  Shell clubs ran the registration table, operated the A/V equipment, and sponsored the receptions.  Receptions, heck – the Houston Shell Club threw a rip-snorting party, with music, and dancing, and pretty girls.  And (then-Treasurer) Connie Boone came through kissing everybody who held still.  Man, those were the days!

My 1974 edition of a 1966 classic
So the climax of the annual meeting of the American Malacological Union in those golden days was always the shell auction, for the benefit of the student fund.  Dick Petit would get half-tuned and wisecrack from lot to lot of colorful specimen shells, offering drop-dead gorgeous little jewels of nature to the highest bidder, and some of the prices paid were eye-popping.

Although I myself never offered any bids for any of the lovely seashells on auction, I felt a strong connection to those who did.  I was quite the hobbyist myself in my youth.  I did collect a lot of marine gastropods and bivalves for their shells, and traded shells with friends all over the world, and (yes) did indeed purchase specimen shells from shell dealers and shell shops, till the day I left home for college.  Shell collecting gave me my start in malacology.

Alas, in 1996 the American Malacological Union threw it all away.  Citing vague conservation concerns, that summer the AMU Council [1] banned the sale or trade of shells at annual meetings, implying strongly as it did that overcollecting by hobbyists posed a threat to natural mollusk populations.  The hobbyists felt as though they had been insulted, which they had, and they left the AMU and never came back.  It was the stupidest thing ever done by a roomful of mollusk people.

So last month we took a peek into the worldwide trade in living freshwater gastropods for the aquarium hobby [2].  Most of the 47 species at least occasionally available from Singapore, the primary wholesale exporter of aquarium stock to the global marketplace, are widespread and trashy, as one might expect.  But our colleague Ting Hui Ng and her coauthors [3] also identified several freshwater gastropod species as “narrowly endemic,” mostly Tylomelania and other Southeast Asian pachychilids, with a viviparid or two thrown in for diverse measure.  Quoting Ting Hui directly, “The rarity of the species may drive increased demand, which may ultimately lead to a decline of the species.”

So what evidence might there be to suggest that hobbyists or collectors might drive populations of gastropods such as these to extinction by overharvest?

The discipline of fisheries management is generally considered to have been born in the early 1930s, with the development of the concept of maximum sustained yield [4].  Stated simply, MSY = Kr/4, where K is the carrying capacity of the environment, and r the intrinsic rate of natural increase.  These are difficult and near-impossible parameters to estimate, respectively, and so the concept of MSY has rarely seen application, even in the management of the most commercially valuable fisheries.  Much less snails.

But, if you’ll allow Captain Obvious to take the helm here, note that the concept of MSY depends on the assumption of density-dependent population regulation.  And, to be quite frank, as your Captain always is, it is my strong impression that essentially all our colleagues with research interests in the ecology of mollusk populations, or indeed with interests in any aspect of the biology of any invertebrate population whatsoever worldwide, carry with us a (near-universally unstated) assumption of density-independence.  In other words, we assume that the size of our study populations is not a function of r and K, but of extrinsic factors such as weather, or floods, or harvest by Indonesian locals who might want to gather up a few small but exotic-looking snails from the lakeshore to sell to feed their families. 

Well, shame on us all.  The tiny little scraps of evidence available today suggest that the regulation of freshwater gastropod populations is as density-dependent as any population of living things on this earth.  The figure below, reproduced from Chapter 5 of my (2000) book, shows the densities of five freshwater gastropod populations over records of 5 – 10 years.  My PBLR test on the longest record available (the Ancylus data set of Russell-Hunter) returned a value of t significant at the 0.01 level, showing strong evidence of density-dependent regulation [5].

Dillon (2000) Figure 5.11
The grand mean density of the Ancylus population in the stream studied by Russell-Hunter, 260 per square meter, can therefore be taken as a rough estimate of carrying capacity.  The grand means of the other populations (per meter squared) were Physa = 77, Lymnaea = 93, Hydrobioides = 41, and Pomacea = 0.11.  The Pomacea estimate did not include juveniles, however.  Shall we take, as a rough estimate of carrying capacity K for freshwater gastropod populations, about 10 per square meter?

Table 5.1 of my (2000) book offers a big compilation of demographic data for 36 populations of freshwater gastropods [5].  The median value for intrinsic rate of natural increase tabulated was around r = 1.5, but the two prosobranch values were systematically lower, just r = 0.09 for Pomacea and r = 0.24 for Melanoides.  Then if we roughly estimate the value of r for freshwater prosobranch populations as r = 0.1 per generation, and carrying capacity K = 10 per square meter, we find MSY = Kr/4 = 0.25 snails per square meter per generation.

My biological intuition suggests that Tylomelania probably mature at around age one year and reproduce continuously thereafter. And the surface area of Sulawasi’s Lake Pozo, where all the Tylomelania live, is approximately 300 square km.  I realize all the lake bottom is not equally inhabitable by snails, but you get the picture.  Back-of-the-envelope estimates suggest that any subtraction of snails by collectors from the Tylomelania populations of Lake Pozo below roughly 10 million snails per year will not ultimately lower the population size, but will be replaced.  And there is zero evidence suggesting otherwise. 

OK, I understand the justification.  My colleagues want grant funding to gather hard data on the size and demographics of Tylomelania populations, so that an informed decision can be made.  In the meantime, since we don’t know, we must err on the side of caution, yes?  Well, I agree with that first sentiment, but not the second one.  Science does not make recommendations based on the premise, “Since we don’t know.”  I understand the fear, but fear is not reason.  Fear is the opposite of reason.

The chairman of my graduate committee 1977 – 1982 was a prominent ecologist named Dr. Robert E. (Bob) Ricklefs.  At some point in my graduate career, he made a point that has stuck with me for 40 years.  Bob observed that if we assume population size is a function of density-independent effects, populations are like drunks on a subway platform, veering left and right randomly until they either go extinct or cover the earth ass deep.  I realize that sometimes it seems extinction is frighteningly common.  But if the size of freshwater gastropod populations were indeed a function of density-independent factors [6], the two phenomena (extinction and ass-deep-coverage) would be equally common, with the frequency of extinction = frequency of coverage = 0.50.  Since that is not the case, density-dependence must prevail.

Well, we’ll return to that ass-deep-earth-coverage thing next month.  But the bottom line for today is that there is no evidence that natural populations of mollusks can be driven to extinction by the love of hobbyists [7].  And a lot of evidence that the love of hobbyists can be beneficial to the work of science.  The birdwatchers are a tremendous asset to ornithology, and the stargazers a tremendous asset to astronomy.  Indeed, the vibrant amateur communities of birders and gazers are where the professional ornithologists and astronomers are born. 

Meanwhile we malacologists are so wrapped up in the remote likelihood that our study organisms might disappear into extinction that we cannot see the imminent extinction of our own profession.  And I strongly suspect the attrition rate in the halls of malacology has been well above Kr/4 for quite a few generations now.


[1]  I myself was not called to the AMS Council until 1999.  I did campaign heavily for repeal of the shell ban during my 2001-02 presidency, but could not find the votes.  The best I could do was neglect the issue entirely when I redrafted the AMS constitution & bylaws 2002-03.

[2] What’s Out There? [9Oct17]

[3] 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. https://doi.org/10.1371/journal.pone.0161130

[4] Russell, E. S. (1931). Some theoretical Considerations on the "Overfishing" Problem. ICES Journal of Marine Science. 6 (1): 3–20.
Graham, M. (1935). "Modern Theory of Exploiting a Fishery, and Application to North Sea Trawling". ICES Journal of Marine Science. 10 (3): 264–274.

[5]  Dillon, R. T., Jr. (2000) The Ecology of Freshwater Molluscs.  Cambridge University Press.  All the details for the demographic analyses referenced above are found in Chapter 5, with the material on carrying capacity pp 202 – 207, and intrinsic rate of natural increase on pp 172 – 182.

[6] Or if such populations could be driven to extinction by harvest rates very much above Kr/4.

[7] In fact, it seems unlikely to me that even unregulated commercial harvest could drive marine shellfish populations to extinction.  And I don’t think there’s any evidence that the pearl-button industry was responsible for any of the freshwater mussel extinctions.  The only cases of human overharvest I’ve ever heard of, for any mollusk population in any environment whatsoever, are the occasionally-successful handpicking efforts to control land snail pests.  See:

Simberloff, D. 1997. Eradication. Pages 221–228 in D. Simberloff, D. C. Schmitz, and T. C. Brown, editors. Strangers in paradise. Island, Washington, D.C., USA.