Editor’s Note – This essay was subsequently published as:
Dillon, R.T., Jr. (2019d) Seven dispatches from the Pomacea front. Pp 19 - 28 in The Freshwater Gastropods of North America Volume 4, essays on Ecology and
Biogeography. FWGNA Press, Charleston.
The rate of scientific advance on the invasive species front always seems to far outstrip the (rather languid) research progress we seem to log on any of our native freshwater gastropod fauna. Thus it has become our occasional custom to publish batches of news to catch us up on particular invasive species, bundled for convenience. So here, for the general edification of the readership, are four news items featuring Pomacea.
Clarifying the Identities
A laurel, and hearty thank-you are due to our friend Ken
Hayes and his colleagues at the University of Hawaii (and elsewhere) for their contribution
published late last year in the Zoological Journal of the Linnean Society, “clarifying
the identities of two highly invasive Neotropical Ampullariidae” [1]. This is research from the old school,
emphasizing comparative anatomy, histology, biogeography, and traditional taxonomic
scholarship to resolve a problem that has bedeviled the Pomacea community for
200 years, both North American and South.
I am persuaded that the most common invasive species here in the
southeastern United States is best identified as Pomacea maculata (Perry 1810),
rather than P. insularum (d’Orbigny 1835) as most workers have previously
supposed, and have updated the FWGNA page accordingly [2].
Ken’s primary focus is the distinction between P. maculata (previously
insularum) sampled from Florida and the other common invasive species of global
import, Pomacea canaliculata, sampled from Hawaii. If I have any quibble with his study [3], it
would be that no mention is made of P. canaliculata in Florida, which leaves me
wondering about the reliability of reports by Florida natural resource agencies
[4] that both species co-occur widely in The Sunshine State.
And do P. maculata and P. canaliculata hybridize? In the final paragraph of their discussion
Hayes and colleagues mention “possible hybridization events,” although we are
offered no details. Then what is the
basis of the reproductive isolation between them? There have been several nice descriptions of
mating behavior and sex pheromones in Pomacea [5] – mate choice tests would
appear to be low-hanging fruit. Any
students out there looking for a good thesis research topic?
Modeling the Invasion
Kudos are also due to Jeb
Byers and his colleagues at the University of Georgia for their paper published
in PloS ONE this past February, “Climate and pH predict the potential range of
the invasive apple snail (P. insularum) in the southeastern USA” [6].
Jeb based his model on Pomacea records from the USGS
Nonindiginous Aquatic Species Database (accessed in late 2009) cast onto a
presence/absence grid of 1x1 km plots.
The (N=68) such plots where invasive Pomacea was present were primarily
Floridian, but extended into six other states: TX, LA, MS, AL, GA and SC. Jeb and colleagues also pre-screened 19
climate variables from BioClim to pick two of temperature and three of
rainfall, and downloaded pH data from the USEPA Storet database.
The weakness of inferences based on any such model are
fairly obvious – if the vast majority of the 1x1 km plots are suitable, but
still have no invasive Pomacea, how strong can be the inference on
absence? But I do think the author’s
prediction that the pH may prove too low for Pomacea in the Okefenokee Swamp
should be robust, which is good news.
Dispatches from the Front
And indeed, our good friend Maj. Alan Covich has just
returned from a scouting expedition to the Okefenokee Swamp at the head of a
large detachment from the Society for Freshwater Science. No enemy activity is reported in the area.
Meanwhile, reports from the Department of South Carolina are
mixed. On the positive side, the
northernmost population of Pomacea does not seem to have advanced beyond its
beachhead at Socastee [7]. A colleague
and I canoed the Intracoastal Waterway downstream from the site of introduction
last summer, finding no evidence of Pomacea at any point in our trip. It was our impression that the habitat may
prove unsuitable.
The tidal fluctuation in the Intracoastal Waterway at
Socastee is at least two feet (as witnessed from the photo below) to which one
must add over a foot of slosh for the wake of passing motorboats. The water is quite darkly stained and the
cypress canopy generally dense throughout the shallows, both of which tend to keep
aquatic macrophytes at negligible densities.
We simply did not see much food or habitat for Pomacea in that particular
part of the world.
On the negative side, however, the Socastee population
itself still appears quite healthy in the weedy ponds and ditches where it was
first reported in 2008. This despite the
fact that the winter of 2009-10 was one of the coldest recently experienced in
South Carolina [8], with a low of 18 degrees F recorded on January 11 at
Conway, 10 miles to the north. From the
standpoint of climate, I am afraid that Jeb Byers’ map may prove a robust predictor,
as well.
And also on the negative side, in August of 2010 word
reached us of a second introduction in South Carolina, this at Lake Marion just south of the I-95 bridge. Our friend
Larry McCord of Santee Cooper (the utility that administers the lake) reported
that “the population has been treated with a low level of copper in hopes to
avoid spread to other areas.”
With large populations of invasive Viviparus georgianus, V. subpurpureus [9], and Bellamya japonica jostling each other for grazing room on its
dense beds of Corbicula, Lake Marion was already wearing the yellow jersey in
this year’s “Tour de Malacological Infestation.” Now Pomacea is threatening to eat all the
invasive waterweed as well. Bummer.
Poisonous eggs
I long ago ceased trying to understand why particular odd
little snippets of scientific trivia find their way into the popular
press. But in early June a PLoS ONE
paper by M. S. Dreon and an international team on toxic Pomacea eggs [10] was
selected for feature by the AAAS online organ “Science Now" [11] and then
propagated around the world by e!Science News, Reddit, Facebook, and what have
you.
The “hook” used by Science Now was that the composition of
Pomacea egg neurotoxin is “unusual for animals” and that “the apple snail
creates it in an unprecedented way.” But
were I the editor of some popular science outlet, or indeed one of the authors
trolling for popular attention and looking for bait, I would have emphasized
the weirdness that any toxin should be found in any egg at all.
Embryos are edible. Somewhat counter to the statement of Dreon and colleagues that “many invertebrates
defend their eggs by endowing them with deterrent chemicals,” across all of
evolutionary biology, plants and animals alike, the phenomenon is stunningly
rare [12]. It is easy to observe that a mother-ampullariid
must value her (relatively large) eggs much more highly than a mother-physid or
a mother-pleurocerid, but when we expand the comparison to a mother-hen, a
simple adaptationist explanation becomes more difficult to sustain. Although Orians & Janzen punted their
original 1974 article with “it is possible to be toxic if one is an embryo, but
under most circumstances it isn’t worth the price,” they ultimately concluded,
“We think that refining this answer is worth the price.” Are we any closer to refining this answer if,
now 40 years later, we have forgotten the question?
[1] Hayes, K. A., R. H. Cowie, S. C. Thiengo, and E. E.
Strong (2012) Comparing apples with
apples: clarifying the identities of two highly invasive Neotropical
Ampullariidae (Caenogastropoda). Zool.
J. Linn. Soc. 166: 723-753.
[3] Actually, it would appear that I have listed more than
one quibble above. And add this nasty clinker from page 744, “Molecular data confirm that P. maculata and P.
canaliculata are two distinct species.”
Hayes and colleagues are referring to mtDNA sequence divergence here! Good grief.
[5] Takeichi, M., Y. Hirai & Y. Yusa (2007) A water-borne sex pheromone and trail
following in the apple snail, Pomacea canaliculata. J. Moll. Stud. 73: 275-278.
Burela, S. & P. R. Martin (2011) Evolutionary and functional significance of
lengthy copulations in a promiscuous apple snail, Pomacea canaliculata
(Caenogastropoda: Ampullariidae). J.
Moll. Stud. 77: 54-64.
[6] Byers, J., W. McDowell, S. Dodd, R. Haynie, L. Pintor,
and S. Wilde. 2013. Climate and pH predict the potential range of the invasive
apple snail (Pomacea insularum) in the southeastern United States. PLoS ONE 8:
e56812. [html]
[7] Previous posts:
[8] See Figure 1 in this (weirdly related!) paper:
Dorcas, M. E., J. D. Willson & J. W. Gibbons (2011) Can
Burmese pythons inhabit temperate regions of the southeastern United
States? Biol. Invasions 13: 793-802.
[10] Dreon, M., M. Frassa, M. Ceolin, S. Ituarte, J-W. Qiu,
J. Sun, P. Fernandez, and H. Heras. 2013. Novel animal defenses against
predation: a snail egg neurotoxin combining lectin and pore-forming chains that
resembles plant defense and bacteria attack toxins. PLoS ONE 8: e63782. [html]
[11] ScienceShot: Invasive snails protect their young with
odd poison. [html]