Southeastern Naturalist
J. Carter, D. Johnson, and S. Merino
2018 Vol. 17, No. 3
470
2018 SOUTHEASTERN NATURALIST 17(3):470–475
Exotic Invasive Pomacea maculata (Giant Apple Snail)
Will Depredate Eggs of Frog and Toad Species of the
Southeastern US
Jacoby Carter1,*, Darren Johnson2, and Sergio Merino1
Abstract- Pomacea maculata (Giant Apple Snail) is a freshwater snail from South America
that is an invasive species on the Gulf of Mexico coastal plain. A sister species has been
shown to prey on amphibian eggs in Asia. To test whether the Giant Apple Snail will prey
on amphibian eggs, we presented eggs of Lithobates palustris (Pickerel Frog), Lithobates
pipiens (Northern Leopard Frog), and Anaxyrus americanus (American Toad) to Giant
Apple Snails in a laboratory experiment. Giant Apple Snails ate the eggs of all 3 species.
Introduction
Pomacea maculata (Perry) (Giant Apple Snail) is a freshwater snail native to
South America (Hayes et al. 2015) that is an invasive species in the freshwater wetlands
and waterways of the northern Gulf of Mexico, peninsular Florida (Benson
2017, Burks 2017) and globally (Hayes et al. 2015). Karraker and Dudgeon (2014)
found that Pomacea canaliculata (Lamarck) (Channeled Apple Snail) opportunistically
ate frog eggs. The Giant Apple Snail is a sister species to the Channeled Apple
Snail and shares similar life-history attributes (Hayes et al. 2015). However, the literature
indicates that Giant Apple Snail is presumed to be an herbivore (e.g., Burke
et al. 2017, Burlakova et al. 2009). Will Giant Apple Snail eat amphibian eggs? If
they do, they could have a negative impact on anuran populations throughout their
introduced range. In this study, we presented Giant Apple Snails with frog and toad
eggs to determine if they would eat them.
Methods
We purchased the eggs used in this experiment from Carolina Biological Supply
(Burlington, NC). We tested 3 species and 4 egg masses: 1 egg mass of Lithobates
pipiens (Schreber) (Northern Leopard Frog), 2 egg masses of Lithobates palustris
(LeConte) (Pickerel Frog), and 1 egg mass of Anaxyrus americanus (Holbrook)
(American Toad). Availability determined the species, age, and number of egg masses
used. All the species tested are native to the US, and 2 are native to Louisiana.
The snails used for this experiment were from a population maintained in the
US Geological Survey’s Wetland and Aquatic Research Center in Lafayette, LA.
A detailed description of the source and husbandry of this population can be found
1US Geological Survey, Wetland and Aquatic Research Center, 700 Cajundome Boulevard,
Lafayette, LA 70506. 2Cherokee Nation Technologies, 700 Cajundome Boulevard, Lafayette,
LA 70506. *Corresponding author - carterj@usgs.gov.
Manuscript Editor: Scott Markwith
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J. Carter, D. Johnson, and S. Merino
2018 Vol. 17, No. 3
in Sutton et al. (2017). For this experiment, we used Giant Apple Snails that were
born in captivity and are normally fed a diet of mostly green leafy produce including:
apples, cilantro, cob corn, collard greens, cucumbers, iceberg and romaine
lettuce, kale, mustard greens, parsley, spinach, turnip greens, yellow squash, zucchini,
and other items as available. Observation of their eating behavior indicated
that lettuce was a preferred food item. The snails were also given “dog chews” as a
protein supplement. None of the experimental snails had been exposed to frog eggs
before this feeding trial. We selected at random female Giant Apple Snails between
40 g and 60 g in weight. No snails died during or shortly after the experiments. We
weighed all snails again after the experiments, but because several snails laid egg
clutches during the experiments, we did not analyze weight change.
We divided each egg mass into 16 sets with approximately the same number of
eggs for each of 4 treatments and 4 replicates. We randomly assigned the egg sets to
1 of 4 treatments: (C) water alone, (CL) water and 15–16g of lettuce, (S) water with
1 snail but no lettuce, and (SL) water with a snail and ~15–16 g of lettuce (as an
alternative snail-food source). The C and CL were controls and the S and SL were
treatments. There were 4 replicates from the single egg masses of Northern Leopard
Frogs and American Toads and 8 replicates from 2 Pickerel Frog egg masses.
We placed the eggs in a covered 5.7-L plastic container with 3 L of water from the
snail-culture tank and we employed aquarium pumps to aerate the water. The water
in the snail-culture tank came from a well on site. The experimental containers were
housed in the same greenhouse as the snail tank. After 2 or 3 days, we changed
the water and replaced the lettuce in the containers. We initially tried to weigh
the lettuce, but it degraded too much for accurate weighing. We placed the eggs
in the container with the snails for either 6 d, or until most of the eggs developed
into tadpoles, i.e., 2–6 d. In most cases, the experiment was terminated because the
eggs that hadn’t already hatched into tadpoles were clearly dead, as indicated by
discoloration. At the end of the experiment, we counted the number of remaining
live eggs and the number of tadpoles; failed eggs were not used in calculating egg
loss or conversion to tadpoles.
The replicates had unequal numbers of eggs at the start of the experiment due to
differences in the number of eggs in different masses and the difficulty in separating
eggs without damaging the embryos. We exposed the eggs to the snails for different
lengths of incubation time because of differences in the age of the egg masses
at the beginning of the experiment (Table 1). To account for these variables, we
Table 1. Treatment summary-statistics. Each egg mass was divided into 16 sets with approximately
the same number of eggs. The 16 sets of eggs were then randomly assigned to 1 of 4 treatments, with
4 replicates per treatment. We used 1 egg mass each of American Toad and Northern Leopard Frog
and 2 Pickerel Frog egg masses.
Eggs per replicate Days
Species Replicates Average Median SD Min Max incubated
American Toad 4 25.75 25.5 4.54 19 32 2
Northern Leopard Frog 4 50.81 51.0 6.32 44 58 3
Pickerel Frog 8 64.00 64.0 17.36 25 65 3, 6
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calculated the percent of eggs missing. To determine the percent of eggs missing,
we subtracted the number of eggs that died from the number placed in the containers
at the start of the experiment to obtain the “starting” number of eggs, added the
number of eggs and tadpoles at the end of the experiment, divided by the starting
number of eggs, and subtracted that percentage from 1 (Equation 1):
% egg loss = 1 - ([number of eggs + number of tadpoles]t1) / [number of eggs]t0),
where t0 and t1 are the first and last days of the experiment.
We then divided the percent of eggs missing by the number of days of the experimental
run (tl) to calculate percent egg loss per day (Equation 2):
% egg loss per day = (% egg loss) / ( t1)
We used the percent egg loss per day as the response variable; treatment, species,
and treatment x species interaction were independent variables. We were interested
in 12 specific a priori comparisons from the interaction. We performed a Bonferroni
correction (alpha = 0.05/12 = 0.00417) when comparing the means of the species
by treatment and the treatments by species. Our analyses employed a general linear
mixed-model that adjusted for the unequal variances between the controls and snailtreatment
categories. We included unequal variances between the controls and the
treatments because it was biologically obvious that the control would have less
variability. The variances between control and snail treatments were significantly
different (χ1
2 = 98.29, P < 0.0001). Hence, we adjusted the mean comparisons for the
unequal variance. We conducted all analyses using PROC MIXED in SAS 9.4.
Results
Giant Apple Snails consumed the eggs of all 3 anuran species tested (Carter
2018). Percent egg-loss per day for each treatment and species are presented in
Table 2. At the end of the trials, there was an overall average of 12 missing eggs
in the snail treatments (Table 2) with the percentage of eggs lost as high as 28.8%
for 1 of the replicates.
Table 2. Mean percentage of eggs lost per day. The treatments are C = water only, CL = water and
lettuce, S = snail and water, and SL = snail, water, and lettuce.
Average % egg loss per replicate per
day by treatment and species
Replicates Mean initial
Species (# masses) # of eggs C CL S SL
American Toad 4 (1) 25.75 ± 4.50 0.00 ± 0.00 0.00 ± 0.00 10.85 ± 5.62 9.13 ± 6.52
N. Leopard Frog 4 (1) 50.81 ± 6.32 0.43 ± 0.86 0.44 ± 0.57 21.67 ± 10.47 18.76 ± 8.43
Pickerel Frog 8 (2) 64.00 ± 17.36 2.72 ± 5.69 0.19 ± 0.38 7.95 ± 4.62 10.52 ± 3.48
All Species 16 (4) 1.30 ± 0.04 0.26 ± 0.00 12.20 ± 0.08 12.10 ± 0.07
All controls All snail treatments
0.78 ± 0.03 12.14 ± 0.07
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2018 Vol. 17, No. 3
We assumed normality because the residuals of the general linear model were
unimodal and symmetric, and the residuals passed the test for homogeneity after
fitting unequal variances (P-value < 0.0001; Table 3). We found that the overall
model was significant (χ11
2 = 221.4, P < 0.0001) at α = 0.05, the treatment x species
interaction was significant (F6,52 = 3.06, P < 0.0122), and the overall model r2
= 74.71%. All replicates exposed to snails had a significant loss of eggs as compared
to their matched controls, except for the American Toad C vs. S comparison
(Table 3).
Discussion
The current range of the Giant Apple Snail overlaps with the ranges of the
Pickerel Frog and American Toad (Benson 2017, USGS National Amphibian Atlas
2014). Although the current range of the Giant Apple Snail does not overlap the
range of the Northern Leopard Frog, it does overlap the range of the closely related
Lithobates sphenocephalus (Cope) (Southern Leopard Frog). All 4 of these frog
species lay eggs in potential snail habitat—slow-moving or still bodies of water, at
shallow depths on the bottom, or attached to vegetation in the water (AmphbiaWeb
2018). We divided egg masses into treatments and replicates; thus, the number of
eggs encountered by the snails in this study was fewer than what they would have
encountered if they had come upon an egg mass of hundreds to thousands of eggs.
In 3 out of 4 experimental runs, the eggs matured into tadpoles within 3 d, significantly
reducing the time the snails could discover and eat the eggs compared
to the time of exposure to the eggs snails might have in natural field settings. The
incubation time from egg laying to hatching into tadpoles is variable by species—
American Toads = 3–12 d, Northern Leopard Frog = 2–17 d, and Pickerel Frog =
10–24 d (AmphibiaWeb 2018). Therefore, we would expect, accounting for shipping
time, the American Toad eggs should have hatched soon after arrival.
Table 3. Matched comparisons of egg treatments within species. The treatments are C = water only,
CL = water and lettuce, S = snail and water, and SL = snail, water, and lettuce. An asterisk (*) indicates
significance at the 0.05/12 = 0.0041 alpha level.
Comparison Species t1,52 P
C vs. S Northern Leopard Frog -6.81 less than 0.0001*
C vs. S Pickerel Frog -3.52 less than 0.0009*
C vs. S American Toad -2.86 less than 0.0060
CL vs. SL Northern Leopard Frog -5.87 less than 0.0001*
CL vs. SL Pickerel Frog -4.51 less than 0.0001*
CL vs. SL American Toad -3.48 0.0010*
C vs. CL Northern Leopard Frog -0.03 0.9762
C vs. CL Pickerel Frog 0.27 0.7884
C vs. CL American Toad 0.00 1.0000
S vs. SL Northern Leopard Frog 0.66 0.5117
S vs. SL Pickerel Frog 0.68 0.5002
S vs. SL American Toad -0.43 0.6657
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2018 Vol. 17, No. 3
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Even when snails were provided with an alternative preferred food item from
their regular diet, they still ate frog eggs. We have demonstrated that Giant Apple
Snails will eat frog eggs under laboratory conditions. It remains to be demonstrated,
but it is very likely that Giant Apple Snail depredate amphibian egg in natural
settings, just as Karraker and Dudgeon (2014) found in their study on Channeled
Apple Snails in Hong Kong.
Our findings demonstrate that Giant Apple Snails could potentially have a significant
impact on amphibian reproduction. We also suspect that the eggs of other
species of amphibians not tested here may also be susceptible to Giant Apple Snail
predation. More laboratory and field studies are needed to determine if Giant Apple
Snails depredate amphibian eggs in the wild and if so, how might this predation impact
amphibian populations. The results of this study, in concert with the continued
range-expansion of Giant Apple Snails, may cause concern for those interested in
amphibian conservation in the Gulf South.
Acknowledgments
We thank Jeffrey Bernia, of Carolina Biological Supply, for his assistance in obtaining
frog-egg masses. Funding was provided by the US Geological Survey Invasive Species
Program. Any use of trade, firm, or product names is for descriptive purposes only and
does not imply endorsement by the US Government. This study plan was reviewed by the
US Geological Survey’s Wetland and Aquatic Research Center IACUC and was issued a
certificate of compliance number: Carter 2017-1.
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