Dr. Rob Dillon, Coordinator

Monday, October 9, 2017

What's Out There?

Editor’s Note – This essay was subsequently published as: Dillon, R.T., Jr. (2023c)  What's Out There?  Pp 23 – 28 in The Freshwater Gastropods of North America Volume 7, Collected in Turn One, and Other Essays.  FWGNA Project, Charleston, SC.

Last summer an interesting paper came across my desk – so interesting, in fact, that I filed it in my stack of promising subject matter for the FWGNA blog.  Where it was promptly buried.  But from which this week it has been exhumed, to stimulate my imagination afresh.

The paper is entitled, “Molluscs for sale: Assessment of freshwater gastropods and bivalves in the ornamental pet trade [1].”  The senior author, Ting Hui Ng, and her six coauthors hail almost exclusively from Singapore, the primary exporter of ornamental fishes to the global marketplace.  Ting Hui and her colleagues surveyed seven local retail shops and major aquarium-stock exporters between 2008 and 2014, documenting an impressive 47 species of freshwater gastropods and 12 species of bivalves available for sale to the hobby worldwide.  Of the gastropods, the authors considered 26 to be oriental in their origin, 7 Australasian, 4 Neotropical, and 1 North American, leaving 9 classified as “cosmopolitan.” 
Figure 1 of Ng et al. (2016)
The entire malacological zoo is depicted in the author’s Figure 1 reproduced above, with their (extensive) caption copied at note [2] below.  I hate to fuss [3], but the scales are completely messed up in this figure – not just the unmarked scale bars (which are supposed to be 10 mm) but the scale bars “indicated differently.”  With regard to the actual sizes of any of these 59 creatures, you are on your own.

The single North American species was identified as “Physa sp.”  I would have bet dollars-to-donuts that this must be Physa acuta, the world’s most cosmopolitan freshwater snail [4], which is indeed widely introduced into the waters of Singapore, until I swapped emails with Ting Hui. She called my attention to her 2015 paper reporting the discovery of a single individual physid in a Singapore aquarium shop, and a second individual in Malaysia, 22% different from anything in GenBank [5].  I'm going to fight the temptation to digress here, but see note [6] below.

The main point is that Physa (and three other gastropod species) were classified as “hitchhikers” by Ting Hui and colleagues, which means that they were discovered in Singapore incidentally transported with aquatic plants, ornamental fish or other freshwater mollusks, and not literally “For Sale,” because nobody wants them.  But at least North America wasn’t skunked entirely.  USA!  USA! USA!

The four Neotropical species were all (you guessed it) ampullariids – Pomacea canaliculata, Pomacea maculata, Pomacea diffusa, and Marisa cornuarietis.  All these various South American ampullariids are also quite familiar to us here in the USA, introduced years ago into Florida and Texas, recently knocking on our doorsteps in South Carolina [9].

Among the remainder of the gastropods for sale in Singapore, I myself was most interested by the strange and exotic-looking pachychilids and thiarids.  We are certainly familiar with #48 Melanoides tuberculata here in the USA [10].  But I was surprised not to see the oriental Tarebia granifera in the gallery compiled by Ting Hui and her colleagues.  Tarebia has been introduced to several southern US states and is sporadically common in Central America and on many Caribbean islands [11].

And speaking of noteworthy absences.  Neither Bellamya japonica nor B. chinensis appears on Ting Hui's list of 59.  I’m sure their mothers must love them, but perhaps Bellamya are too plain and clunky to find a market among aquarium hobbyists?  Or has Bellamya become so common here in North America [12] that nobody buys them out of Singapore?  It has always been my impression that they are marketed here in the USA primarily by suppliers to the backyard “water garden” trade, along with koi, water lilies, and little tumbling fountains.  Not the (indoor) aquarium shops supplied by Singapore.

The research interests of Ting Hui and colleagues seem to have been motivated by two primary concerns – that exotic freshwater mollusks might spread, and that they might not.  Let’s set aside the former concern for the time being, and focus on the latter.

Eight of the 59 freshwater mollusk species for sale in Singapore were identified by Ting Hui as “narrowly-endemic.”  These included the three spiny species of Brotia, the four Tylomelania species, and the viviparid Celetaia persculpta.  Quoting directly from Ting Hui: 
“These species appeared to fetch higher prices compared to more common species (up to US$10 per individual Tylomelania sp. compared to US$5 per individual Thiaridae or Neritidae, THN pers. obs.). The rarity of the species may drive increased demand, which may ultimately lead to a decline of the species.”
Pachychilid snails of the genus Brotia are ovoviviparous but sexually reproducing – males and females apparently present in equal frequency.  Brotia armata (#34) and B. binodosa (#35) are members of what Glaubrecht & Kohler called a “species flock” in the Kaek River of central Thailand [13]Brotia pagodula (#37) is endemic to the Salween River and its tributaries on the border of Thailand and Myanmar.   Their habitat is given as “Attached to rocks in sectors with swift currents” [14]

The genus Tylomelania (#39 - 42) is another group of ovoviviparous pachychilids, these endemic to particular lakes, rivers, and streams on the Indonesian Island of Sulawesi.  Our friends Thomas von Rintelen and Matthias Glaubrecht recognized 62 species of Tylomelania the last time I checked, but I haven’t been home since morning [15].

So the paper by Ting Hui and colleagues opened with this sentence: “The ornamental pet trade is often considered a key culprit for conservation problems such as the introduction of invasive species (including infectious diseases) and overharvesting of rare species.”  Can a gastropod population – especially a population endemic to some narrowly-circumscribed patch of habitat – indeed be threatened by overharvest?  What is the evidence that hobbyists can drive populations of their favorite organisms to extinction with love?  Tune in next time.


[1] 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

[2] Fig 1. Freshwater molluscs in the ornamental pet trade.
Unless indicated differently, scale bars = 10mm. 1. Batissa similis; 2. Batissa violacea; 3. Corbicula fluminea; 4. Corbicula moltkiana; 5. Hyriopsis bialata; 6. Hyriopsis desowitzi; 7. Parreysia burmana; 8. Parreysia tavoyensis; 9. Pilsbryoconcha exilis; 10. Scabies crispata; 11. Sinanodonta woodiana; 12. Unionetta fabagina; 13. Marisa cornuarietis; 14. Pomacea canaliculata; 15. Pomacea diffusa; 16. Pomacea maculata (photograph by K.A. Hayes); 17. Bithynia sp.; 18. Clea bockii; 19. Clea helena; 20. Radix rubiginosa; 21. Clithon corona; 22. Clithon diadema; 23. Clithon lentiginosum; 24. Clithon mertoniana; 25. Neripteron auriculata; 26. Neritina iris; 27. Neritina juttingae; 28. Neritina violacea; 29. Neritodryas cornea; 30. Septaria porcellana; 31. Vittina coromandeliana; 32. Vittina turrita; 33. Vittina waigiensis; 34. Brotia armata; 35. Brotia binodosa; 36. Brotia herculea; 37. Brotia pagodula; 38. Sulcospira tonkiniana; 39. Tylomelania towutica; 40. Tylomelania sp.; 41. Tylomelania sp.; 42. Tylomelania sp.; 43. Physa sp.; 44. Amerianna carinata; 45. Indoplanorbis exustus; 46. Gyraulus convexiusculus; 47. Semisulcospira sp.; 48. Melanoides tuberculata; 49. Stenomelania offachiensis; 50. Stenomelania plicaria; 51. Stenomelania cf. plicaria; 52. Stenomelania sp.; 53. Thiara cancellata; 54. Celetaia persculpta; 55. Filopaludina cambodjensis; 56. Filopaludina peninsularis; 57. Filopaludina polygramma; 58. Sinotaia guangdungensis; 59. Taia pseudoshanensis.

[3] No, I don’t.

[4] Dillon, R. T., A. R. Wethington, J. M. Rhett and T. P. Smith.  (2002)  Populations of the European freshwater pulmonate Physa acuta are not reproductively isolated from American Physa heterostropha or Physa integra.  Invertebrate Biology 121: 226-234.

[5] Ng, TH, SK Tan, & DCJ Yeo (2015) Clarifying the identity of the long-established, globally-invasive Physa acuta Draparnaud, 1805 (Gastropoda: Physidae) in Singapore.  BioInvasions Records 4: 189 - 194.

[6]  This is fascinating.  It is certainly possible that the 22% CO1 sequence difference reported in Singapore is another case of mitochondrial superheterogeneity, which (we do know) occurs in North American populations of Physa acuta, almost certainly a signature of cytoplasmic male sterility [7].  It is also possible that the "Physa sp" reported by Ng and colleagues in Singapore might indeed be a distinct biological species.  Over and over again, all over the world, we have seen invasions by species mixtures, which have been sorted out only at the site of introduction, and then subsequently corrected in their places of origin.  I do think it is possible that the American Pacific northwest may be inhabited by a species perhaps best identified as Physa concolor, but called Physa natricina in the Snake River, morphologically very similar to Physa acuta, but reproductively isolated from it [8].  Is this Physa concolor in Singapore?  Wild speculation here, but who knows?  Science is lots of fun, isn't it?

[7] For more on mtSH in Physa, see:
  • Cytoplasmic Male Sterility in Physa! [9June22]
[8] For more on what I called "Hypothesis #2 (of 3) See:
  • The Mystery of the SRALP: A bidding [5Feb13]
[9] I’ve published quite a few essays on the North American Pomacea invasion.  See:
  • Pomacea spreads to South Carolina [15May08]
  • Two dispatches from the Pomacea front [14Aug08]
  • Pomacea news [25July13]
[10] For more on the North American populations of Melanoides, see:
  • To only know invasives [16Oct15]
  • The many invasions of Hilton Head [16Dec15]
[11] Pointier, J-P., R. Incani, C. Balzan, P. Chrosciechowski, and S. Prypchan (1994)  Invasion of the rivers of the littoral control region of Venezuela by Thiara granifera and Melanoides tuberculata and the absence of Biomphalaria glabrata, snail host of Schistosoma mansoni.  Nautilus 107: 124 – 128.

[12] For more on Bellamya in North America, see:
  • Bellamya News [6Oct05]
  • Community consequences of Bellamya invasion [18Dec09]
  • Just before the bust [5Aug14]
[13] Ng and colleagues recorded only 0.5% uncorrected pairwise 16S sequence difference between their individual B. armata and B. binodosa analyzed.  Hmm…

[14] Kohler, F, and M. Glaubrecht (2006)  A systematic revision of the southeast Asian freshwater gastropod Brotia (Cerithioidea: Pachychilidae)  Malacologia 48: 159 – 251.

[15] von Rintelen T., B. Stelbrink, R. M. Marwoto, & M. Glaubrecht (2014) "A Snail Perspective on the Biogeography of Sulawesi, Indonesia: Origin and Intra-Island Dispersal of the Viviparous Freshwater Gastropod Tylomelania". PLoS ONE 9(6): e98917.