Dr. Rob Dillon, Coordinator

Wednesday, June 13, 2018

Invasive Species Updates

Editor’s Note – This essay was subsequently published as: Dillon, R.T., Jr. (2023c)  Invasive Species Updates.  Pp 15 – 22 in The Freshwater Gastropods of North America Volume 7, Collected in Turn One, and Other EssaysFWGNA Project, Charleston, SC.

Potamopyrgus in Maryland.  Early last September our good buddy Matt Ashton of the Maryland DNR sent me an email with a couple jpegs attached, including the photo below.  Matt was forwarding the report of a concerned citizen with very good reason to think he had discovered New Zealand Mud Snails in the Gunpowder River at the Prettyboy Dam tailwaters, about 30 km north of Baltimore.  See the 21Sept17 article in The Baltimore Sun from note [2] below for more details.

From Matt Ashton, MD-DNR
The Gunpowder is a lovely little river through most of its course, passing through four state parks on its 90 km journey from headwaters in southern Pennsylvania to mouth at the Chesapeake Bay.  In the late 1980s the Prettyboy Dam tailwaters were stocked with brown and rainbow trout, eggs and fingerlings as well as adults, and both populations have now become self-sustaining [3].  Attention from an avid community of fishermen has quite predictably followed.

This is the second population of Potamopyrgus antipodarium reported from a US Atlantic drainage.  My readership will probably remember the first report I posted back in November of 2013, from a small tributary of the Susquehanna River in central Pennsylvania, also heavily fished by trout anglers [4].  Spring Creek is perhaps 50 km north of the Gunpowder headwaters.  The implication would be that the little snails were transported from Pennsylvania to Maryland via muddy waders or bait buckets.

From the mouth of the Gunpowder to the heart of our nation’s capital is but another 80 km.  I understand that legislation has been introduced in Congress to build a big, beautiful six-inch wall from the Delaware River to the West Virginia line, with a three-inch guardhouse on the Baltimore/Washington parkway.

And (less surprisingly) in Syracuse.  The Great Lakes populations of Potamopyrgus are much older than the Atlantic populations; New Zealand mud snails were first reported from Lake Ontario as early as 1991 and Lake Erie in 2005.  So, surfing around in the USGS-NAS Nonindigenous Aquatic Species Database [5], I picked up a couple 2016 reports of Potamopyrgus in tributaries of Onondaga Lake, near Syracuse.

The Erie Canal runs from the eastern end of Lake Erie across the width of New York, intersecting Oneida Lake and the tailwaters of several of the Finger Lakes, as well as Onondaga Lake, to meet the Hudson River just north of Albany.  How long will it be until tired, poor Potamopyrgus are tempest-tossed down the Hudson into New York Harbor, to enter Ellis Island through the back door?  Is that one golden, too?

Pomacea progress.  As threatening as an invasion of 5 mm snails down from the north most certainly must be to homeland security, the threat posed by an army of 5 cm snails advancing up from the south is two orders of magnitude worse.  On the plus side, however, we are getting some very good science out of it.

In late 2015 our colleague Ken Hayes and 32 coauthors published a comprehensive review of the biology of the entire family Ampullariidae [6], broader than their systematic paper of 2012 [7] and better-integrated than the big 2006 book edited by Joshi & Sebastian [8].  More than just the economically-important pests and pets, the authors reviewed the entire scientific literature published on all 117 ampullariid species (by their estimate) in all 7 genera worldwide.

But though Ken and his colleagues cast their nets as broadly as possible, what they drew back was almost entirely Pomacea canaliculata.  They concluded with a call for additional research of a comparative nature, including behavioral studies that might reveal patterns that may have played a role in the evolution of the other 116 species.  We most certainly agree.

PhilRice II.  So speaking of Joshi & Sebastian.  Last year the Philippine Rice Institute published a second big collection of research focused on the Pomacea invasion, this volume with our good friend Rob Cowie among the editors [9].  A free pdf download is available from the link below.  

Fig. 7 of Rama Rao et al. [14]

Maculata/canaliculata hybridization.  Since the first appearance of pest Pomacea in North America, I have continued to be especially curious about reproductive isolation between populations of P. canaliculata and what is now called P. maculata.  So perhaps the most interesting Pomacea news in recent years, from my perspective, was the 2008 discovery that P. maculata has been introduced together with P. canaliculata throughout east and southeast Asia [10].

Apparently published a bit too late to be collected in the nets of Hayes and his 32 colleagues was an excellent 2013 study by Matsukura et al. documenting extensive hybridization between P. canaliculata and P. maculata throughout Japan, Korea, Vietnam and The Philippines [11].  Of the 16 populations sampled, 7 (all Japanese) were apparently pure canaliculata.  The remaining 9 were mixtures, including one population that was primarily maculata with a few hybrids, and one that was primarily hybrid, with a few pure maculata and canaliculata.

Even more interestingly, Matsukura performed a set of no-choice mating experiments between canaliculata and maculata, suggesting that F1 hybrid clutches suffer significant reduction in their hatching rates, from around 80% down to maybe 20% [12].

Pomacea release sex pheromones and demonstrate elaborate courtship behaviors, featuring nuptial gifts [13].  It would be tempting to hypothesize that some sort of prezygotic reproductive isolating mechanisms may have evolved in their native ranges, which have now broken down in Asian rice fields, as lions and tigers mate in zoos.  But in a surprise finish, Matsukura and colleagues also obtained a sample of 17 individual Pomacea from 5 sites in Argentina and discovered evidence of hybridization in ten of them!  Even in the home range.  Fascinating.

If maculata and canaliculata cannot tell each other apart, how can we?  Hayes et al. [7] observed that typical egg masses of P. maculata contain significantly more eggs of a significantly smaller size than P. canaliculata, a result that Matsukura and colleagues confirmed in Japan.  But that’s not terribly helpful with an adult in hand. 

So Hayes et al [7] also suggested a variety of distinguishing shell traits [14], including adult size, ratio of spire height to shell length, shell thickness, umbilicus, shouldering, and lip pigmentation.  But just last month Rama Rao and colleagues [15] published an interesting study of 130 Pomacea sampled from 8 populations in peninsular Malaysia, 5 of which turned out to be canaliculata/maculata mixtures, judging from CO1 sequence. 

Rama Rao and colleagues did not employ nuclear markers, and hence could not identify hybrids [16].  But judging from mtDNA haplotype, they selected 26 canaliculata and 26 maculata and performed an extensive series of shell morphological and morphometric analyses, including simple linear measures and ratios as well as the geometric analysis shown in the figure above.  They did discover a statistically-significant difference between the two groups in ratios of shell height to shell width and aperture height, but of such a fine nature as to be practically useless.  The bottom line is that the Pomacea of Malaysia seem to comprise one single heap of big slimy snails, darn near indivisible.

And meanwhile, in South Carolina.  Our colleague Elizabeth Gooding and six coauthors (including yours truly) have just published a statewide survey [17] for Pomacea at the highest latitudes of its current US range [18].  The Socastee/Myrtle Beach area [19] is on the northern tip of USDA cold-hardiness zone 8b, with average annual extreme minimums around 15 to 20 degrees F, or -9 to -6 degrees C.  We report both copulation and egg laying year-round, even in coldest months of winter [20].

I understand that legislation has been introduced in Washington to build a big, beautiful wall on the North Carolina line from Cape Hatteras to Tennessee, 50 feet tall by back-of-the-envelope calculation [21], Pedro himself manning the I-95 guardhouse just two mucus trails and one gigantic traffic jam North of the Border.


[1] My earlier Pomacea reviews include:
[2] Invasive New Zealand mud snails found in Gunpowder River. The Baltimore Sun 21Sept17.

[3] The situation is actually a bit more complicated.  The first 7.2 miles below the dam are managed as a wild trout stream, catch-and release.  Anglers in the next 4.2 miles can keep two trout/day.  And the next 6.1 miles are stocked in the spring and fall with ordinary hatchery-reared trout, creel limit five trout/day.  See MD-DNR-fisheries.

[4] For more about the discovery of New Zealand Mud Snails in central Pennsylvania:
  • Potamopyrgus in US Atlantic Drainages [19Nov13]
[5] For my reviews of state and national invasive species online databases, see:
  • To Only Know Invasives [16Oct15]
  • To Only Know Invasives in My General Vicinity [9Nov15]
[6] Hayes KA, Burks RL, Castro-Vazquez A, Darby PC, Heras H, Martín PR, et al. (2015) Insights from an integrated view of the biology of apple snails (Caenogastropoda: Ampullariidae). Malacologia 58(1–2):245–302.

[7] Hayes KA, Cowie RH, Thiengo SC, Strong EE (2012) Comparing apples with apples: clarifying the identities of two highly invasive Neotropical Ampullariidae (Caenogastropoda). Zool J Linn Soc. 166(4): 723–753.

[8] Joshi RC, Sebastian LS. (2006) Global advances in ecology and management of golden apple snails: Philippine Rice Research Institute (PhilRice), 600 pp.

[9] Joshi, RC, Cowie RH, Sebastian LS. (2017)  Biology and management of invasive apple snails.  Philippine Rice Research Institute (PhilRice) 406 pp.  [PDF]

[10] Hayes, K. A. et al. (2008) Out of South America: multiple origins of non-native apple snails in Asia.  Diversity and Distributions 14: 701-712.
Matsukura, K. et al. (2008) Genetic divergence of the genus Pomacea (Gastropoda: Ampullariidae) distributed in Japan, and a simple molecular method to distinguish between P. canaliculata and P. maculata.  Appl. Entomol. Zool. 43:535-540.

[11] Matsukura, K et al.  (2013) Genetic exchange between two freshwater apple snails, Pomacea canaliculata and Pomacea maculata invading East and Southeast Asia.  Biol. Invasions 15: 2039-2048.

[12] The hatchability experiment was not as neat as one might hope, and the results a bit ambiguous.  See the paper itself [10] for details.

[13] Burela S, Martin PR. 2009.  Sequential pathways in the mating behavior of the apple snail Pomacea canaliculata (Caenogastropoda: Ampullariidae).  Malacologia 51: 157 – 164.
Burela S, Martín PR. 2011. Evolution and functional significance of lengthy copulations in a promiscuous apple snail, Pomacea canaliculata (Caenogastropoda: Ampullariidae).  Journal of Molluscan Studies 77: 54–64.
Takeichi M, Hirai Y, Yusa Y. 2007. A water-born sex pheromone and trail following in the apple snail, Pomacea canaliculata.  Journal of Molluscan Studies 73: 275–278.

[14] There are also anatomical differences between canaliculata and maculata, most notably a difference in penial sheath.  A study of anatomical hybrids would be most salutary.

[15] Rama Rao S, Liew T-S, Yow Y-Y, Ratnayeke S (2018) Cryptic diversity: Two morphologically similar species of invasive apple snail in Peninsular Malaysia. PLoS ONE 13(5): e0196582.

[16] Back in the allozyme days, I could have solved that problem with one afternoon in the laboratory and $30 for reagents.  It does frustrate me that science often seems to lose good old technology in its restless quest for the new.

[17] This is the same survey of which I made passing mention in my essays of November and December of 2015.  Although we did not discover any Pomacea on Hilton Head Island, we do formally report invasive populations of Bellamya, Biomphalaria, Melanoides and Pyrgophorus in our 2018 paper [17].  To refresh your memory:
  • The Many Invasions of Hilton Head [16Dec15]
[18] Gooding, E. L., A. E. Fowler, D. Knott, R. T. Dillon, T. Brown, M. R. Kendrick, & P. R. Kingsley-Smith (2018) Life history and phenological characteristics of the invasive island apple snail, Pomacea maculata (Perry, 1810) in stormwater retention ponds in coastal South Carolina, USA.  Journal of Shellfish Research 37: 229 - 238. [PDF]

[19] Yes, the introduction of Pomacea we first reported from the Myrtle Beach area in 2008 may even be spreading.  I understand that the initial site of discovery around that trailer park in Socastee was treated with copper sulfate, but the snails seem to have jumped a couple km SE into ditches and stormwater retention ponds around a sprawl of strip malls and big-box retailers.  For ancient history, see:
  • Pomacea spreads to South Carolina [15May08]
  • Two dispatches from the Pomacea front [14Aug08
[20] Although no juveniles seem to hatch in the South Carolina winter.  One presumes that egg masses laid in the winter will hatch in the spring.

[21] If a 6-inch wall is sufficient to stop a 5 mm gastropod coming south between Baltimore and Washington, then to stop a 5 cm gastropod advancing north from the Carolinas we need 100 x 6 inches = 600 inches = 50 feet.