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22001199 SOUTHEASTERN NATURALIST 1V8o(3l.) :1486,9 N–4o7. 53
Observations of Pomacea spp. (Apple Snails) Beyond the
Shallow Marsh
Jennifer L. Bernatis*
Abstract - Fluctuating populations of native and non-native Pomacea spp. (apple snails)
pose a particular concern to managers tasked with developing recovery plans for the endangered
Rostrhamus sociabilis plumbeus (Everglades Snail Kite) in Florida because the
snails are the primary food source for the kite. The data presented herein provide observational
records and quantitative evaluation of Pomacea paludosa (Florida Apple Snail) and
P. maculata (Island Apple Snail) occurrences in a variety of Florida aquatic ecosystems.
Qualitative observations documented Florida Apple Snail and Island Apple Snail in water
depths up to 14.6 m and 8.4 m, respectively. In 1 study location, 65% of all apple snail species
were in depths greater than 0.75 m. This data suggests that in areas where apple snails
appear to be rare or absent from shallow habitats (less than 1.0 m), surveys in deeper waters should
also be conducted because apple snails may be found there.
Introduction
Understanding the life-history traits of animals is necessary to develop habitat
and animal management plans. Aquatic ecosystems, for example, are often managed
for water levels, vegetation, and recreational use, which may affect organisms
residing in the system (Blanch 2000, Darby et al. 2008, Deegan et al. 2012, Dreitz
et al. 2001, Warwick and Brock 2003). These manipulations may be designed to
benefit some species, but might be harmful to others and lead to unintended longterm
population or community-level responses (O’Brien and Dawson 2016).
The native Pomacea paludosa (Say) (Florida Apple Snail) was considered
abundant throughout Florida (Heard 1970). Over the last 30–40 y, the populations
appear to be declining in some systems, many of which are managed for 1 or more
of the reasons stated previously (Darby et al. 2009, 2012; Heard 1970; Kushlan
1974). Similar changes in the abundance of the nonindigenous Pomacea maculata
(Perry) (Island Apple Snail) and Pomacea canaliculata (Lamarck) (Golden Apple
Snail) have also been reported in Florida, but quantitative data to evaluate population
changes is lacking (Bernatis and Warren 2014). The 1 confirmed population
of Golden Apple Snail was utilized in a removal study (Bernatis and Warren 2014)
and still had no snails in 2015 (J. Bernatis, pers. observ.). The federally endangered
Rostrhamus sociabilis plumbeus Ridgeway (Everglade Snail Kite, hereafter,
Snail Kite), is a resident of many of these heavily managed systems. Snail Kites
are dependent on apple snails for food, making declines and fluctuations in apple
snail populations an added challenge to the development of Snail Kite recovery
plans (Cattau et al. 2014, Cottam 1939).
*Dodge City Community College, Department of Math and Science, Dodge City, KS 67801;
jbernatis@dc3.edu.
Manuscript Editor: Eugene Turner
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Island Apple Snail and Golden Apple Snail are invasive species that are responsible
for economic impacts on Oryza spp. (rice) and Colocasia esculenta (L.) Schott
(Taro) crops and might have a role in ecosystem alteration (Carlsson et al. 2004). As
a result, numerous life-history studies have been conducted on reproduction, fecundity,
feeding, physiological tolerances, and distribution of these species, with the
primary focus on phylogenetic research (Albrecht et al. 1999, Bernatis et al. 2016,
Burky et al. 1972, Hayes et al. 2009, Seuffert and Martin 2010, Yusa et al. 2006),
but long-term quantitative population studies are unavailable. Life-history studies
on Florida Apple Snail include studies similar to those of the invasive apple snails,
and include some additional work on distribution and population changes over time
(Darby et al. 2009, 2012). Survey methodology for all 3 species has focused on
shallow water (less than 1 m), which has possibly caused a misconception of distribution
within habitats and ignored the snail’s use of deeper waters.
The movement to deeper waters could be a factor in why apple snails have been
reported to “reappear” after 1–2 y of drought conditions. Although Darby et al.
(2002) concluded that Florida Apple Snails are typically absent at depths greater
than 50 cm, Bernatis (2010) reported data from Apopka Spring (Lake County, FL)
in which Florida Apple Snails crawled on spring vent walls at a water depth of
14.6 m. Only 1 study, currently underway by the author, directly investigates the
occurrence of apple snails in deeper waters. Understanding the movement patterns
and habitat use of apple snails, and evaluating potential snail impacts, are important
in the development of Snail Kite management plans. I present here preliminary
results of the ongoing depth study and statewide observations of apple snails to
support the hypothesis that apple snails are not restricted to shallow waters, but also
may utilize deep-water habitats.
Methods
I collected quantitative data from East Lake Tohopekaliga in Osceola County,
FL. This lake is ~5 km in diameter and reaches a maximum depth of ~5 m. Vegetation
occurs around the periphery of the entire lake. Dominant vegetation included
Schoeneoplectus spp. (bulrush), Eleocharis spp. (spikerush), Typha spp. (cattail),
Vallisneria americana Michx. (Tape-grass), and Potamogeton illinoensis Morong
(Illinois Pondweed). The latter 2 species extend at least 1 km from the shore to a
depth of at least 2.25 m. The surface-to-bottom visibility was up to 2 m and the
bottom visibility was 3–5 m.
I used 1.0-m2 quadrats at depths of 0.25 m, 0.75 m, 1.25 m, 1.75 m, and 2.25 m
to collect samples. I collected 4 replicate samples from 18 sites located around the
lake at each depth. I conducted sampling at 0.25 m and 0.75 m by wading to a site,
randomly deploying the quadrat, removing vegetation from the quadrat, visually inspecting
all removed material, and performing a series of alternating hand searches
and dipnet sweeps (10 sweeps/set, repeated 3 times). I determined the direction of
the quadrat by simple replacement-randomization methods that included selecting
a piece of paper from a bag with the direction to throw; each throw direction
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was based on the current sample quadrat to avoid overlapping quadrats. I threw
quadrats as far from the start point as possible, but always a minimum of 3 m from
the staging site. Sampling at 1.25 m combined the use of scientific scuba divers and
wading personnel. The wader removed and inspected vegetation from the quadrat,
while the divers conducted hand searches of the quadrats. Divers conducted sampling
at 1.75 m and 2.25 m. They either removed vegetation for inspection (i.e.,
dense patches of Tape-grass), or inspected and left it intact (i.e., sparse stems of
bulrushes); all snails were returned to the surface for identification. I recorded the
species and size of all live apple snails collected.
I recorded additional data for Island Apple Snail and Florida Apple Snail as part
of efforts to sample invertebrate community structure in central and south Florida
lakes, as well as statewide locations in streams from 2007 to 2016. The majority of
the data are qualitative observations. I occasionally collected snails using quantitative
methods (i.e., ponar, Hess sampler, or suction dredge) but, because the study
focus was not apple snails, the data are reported herein as qualitative observations.
Several sampling events included the use of modified crayfish traps baited with
fruit and Hydrilla (water thyme) to collect apple snail species. This sampling was
part of separate projects where the objectives did not include a depth-distribution
component, but we recorded the depth of deployment as part of the ancillary data.
The first sampling event was in Newnans Lake (Alachua County, FL) in 2007 and
was part of a monitoring effort during a chemical control treatment. We deployed a
total of 22 traps, both before and after treatment, at depths from 0.25 m to 0.75 m,
which were left in place for 48 h. In September and October of 2010, we deployed 3
traps in the Lake Okeechobee rim canal at depths from 3.8 m to 4.4 m, which were
left in place 1–2 h. We set 6 traps at the mouth of the Kissimmee River in November
of 2010 at depths from 5.4 m to 9.7 m. These traps were left in the water for 60 h.
Results and Discussion
The distribution and abundance of apple snails poses a particular concern to
managers tasked with developing recovery plans for the endangered Snail Kite
(Cattau et al. 2014). Apple snails are the kite’s primary food source and must be
available for its success, but numerous authors have reported snail declines or
fluctuations in populations (J.L. Bernatis, pers. observ.; Darby et al. 2009, 2012;
Heard 1970; Kushlan 1974). Long-term studies to quantify these fluctuations are
not available from the literature. Inferences from short-term and laboratory-based
studies suggest that habitat quality and low water levels are the primary cause for
the perceived declines and fluctuations (Bernatis et. al 2016, Darby et al. 2008, Ito
2002, Martin et al. 2001, Pizani et. al 2005, Yusa et al. 2006). The data presented
here provide evidence in support of the alternate hypothesis, that snails utilize
deeper waters during low water levels and are, therefore, not as easily observed.
These results may be beneficial for the development of snail eradication programs
and Snail Kite management plans, and increases our understanding of apple snail
habitat use and distribution within aquatic habitats.
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Native and nonindigenous apple snails routinely occurred at depths greater than
0.5 m throughout the sampling events (Table 1). We collected, trapped, or observed
snails at depths from shoreline (less than 0.1 m) to 14.6 m. We observed egg masses on
emergent vegetation and human-made structures in water depths of less than 0.1–7.1 m.
We recovered only 3 Island Apple Snails from traps at Newnans Lake, all in the
pre-treatment collection; however, we observed egg masses on emergent vegetation
at a depth of 1.1 m. We recovered Island Apple Snails from traps in the Kissimmee
Table 1. Records of Pomacea observations as part of invertebrate community sampling throughout
Florida. Results are provided by species, waterbody (county), date of collection, observation type (EM
= egg mass, LS = live snail), and water depth. The number of observations represents the number of
days during the sample dates that snails or eggs were observed. Observations for Apopka Spring on
April 2010 were from Bernatis (2010).
Species Location (county) Date (# of observations) EM LS Depth (m)
P. paludosa Apopka Spring (Lake) Apr 2010 (2) x 12.2–14.6
P. paludosa Black Creek (Clay) May 2009 (1) x 0.75–1.5
P. paludosa Chipola River (Jackson) Jul 2014, Aug 2015 (3) x x 0.5–2.2
P. paludosa Ichetucknee River (Suwannee) Jul 2006–Mar 2007 (6) x x 0.1–2.0
P. paludosa Julington Creek (Duval) Oct 2006 (1) x 0.9–2.5
P. paludosa Lake Panasoffkee (Sumter) Aug 2010–Apr 2013 (6) x x 0.1–1.2
P. paludosa Lake Okeechobee (Palm Beach) Feb, Aug 2007–2015 (6) x 1.2–2.3
P. paludosa Outlet River (Sumter) Aug 2010–Apr 2013 (6) x x 0.5–2.1
P. paludosa Santa Fe River (Alachua, Jun 2011 (2) x x 1.0–1.25
Columbia)
P. paludosa East Lake Tohopekaliga (Osceola) Apr–Jul 2016 (14) x x 0.5–1.75
P. paludosa Lake Apopka (Orange) Sep 2010 (1) x x 0.2–1.1
P. maculata Lake Apopka (Orange) Sep 2010 (1) x x 0.2–1.1
P. paludosa Lake Harris (Lake) Apr 2015 (2) x x 0.6–1.5
P. maculata Lake Harris (Lake) Apr 2015 (2) x x 0.6–1.5
P. paludosa Lake Okeechobee (Glades) Apr–Mar 2009–2011 (9) x 0.1–1.5
P. maculata Lake Okeechobee (Glades) Apr–Mar 2009–2011 (9) x x 0.1–1.5
P. maculata Carillon Lakes Subdivision (Polk) Apr 2010 (2) x x 0.1–7.1
P. maculata East Lake Tohopekaliga (Osceola) Dec 2015 x x 0.7–1.95
P. maculata East Lake Tohopekaliga (Osceola) Apr – Jul 2016(15) x x 0.7–1.95
P. maculata Harney Pond Canal (Glades) Feb, Aug 2008–2015 (9) x 0.2–5.2
P. maculata Julington Creek (Duval) Oct 2006 (1) x 1.0–2.5
P. maculata Kissimmee River (Okeechobee, May–Oct 2010 (3) x x 0.2–7.9
Glades)
P. maculata Kissimmee River (Okeechobee, Nov 2010 (3) x x 6.8–8.4
Glades)
P. maculata Lake Mirror (Polk) Jan–Apr 2009 (4) x x 0.1–0.9
P. maculata Lake Okeechobee Rim Canal Feb, Aug 2007–2015 (9) x x 0.1–5.2
(Okeechobee, Glades)
P. maculata Lake Okeechobee Rim Canal Sep 2010 (4) x 4.1
(Okeechobee, Glades)
P. maculata Lake Okeechobee Rim Canal Oct 2010 (2) x 4.2–4.4
(Okeechobee, Glades)
P. maculata Lake Panasoffkee (Sumter) Apr 2013 (1) x 1.2
P. maculata Lake Weir (Polk) Jan–Apr 2009 (4) x x 0.1–1.2
P. maculata Newnans Lake (Alachua) November 2007 (3) x x 0.2–1.1
P. maculata Newnans Lake (Alachua) March 2008 (3) x x 0.2–1.1
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River and the Lake Okeechobee rim canal. In the river, we collected 5 Island Apple
Snails (all less than 35 mm in length) in traps at depths of 7.65 m, 6.9 m, and 6.8 m. During
the September 2010 rim canal collections, we collected 3 Island Apple Snails
(all >60 mm) from depths of 4.4 m, 4.2 m, and 4.1 m. We observed more than 40
Island Apple Snail egg masses on human-made structures in the river and canals
(i.e., bridge supports or retaining walls), where the water depth exceeded 5 m, and,
in one case, exceeded 8 m. These observations suggest that Island Apple Snails will
inhabit deeper water.
The results of the East Lake Tohopekaliga survey demonstrated that Island Apple
Snails and Florida Apple Snails are often found in waters >0.5 m deep (Table 2).
We searched a total of 552 one-m2 quadrats and collected 219 snails (Island Apple
Snail = 214, Florida Apple Snail = 5). We collected over half of the snails (n = 140;
65.4%) in water that was in ≥0.75 m deep. We observed egg masses of both species
on emergent vegetation or dock structures in water depths of up to 1.75 m. These
results provide strong support for more detailed surveys in deeper waters because
snails may be utilizing these areas. Population estimates might be underestimated
if deeper areas are not sampled.
The movement patterns of the snails may depend on the system characteristics
(e.g., water flow, availability of food items and egg-laying substrate). Darby et al.
(2002) observed that adult Florida Apple Snails (>30 mm long) tended to stop moving
in waters less than 10 cm deep and moved toward shallow waters when depths reached
50 cm. However, they also reported movements of Florida Apple Snails to deeper
waters to escape drought conditions. Observations of snails reported herein did
not demonstrate any restrictive behavior as a function of water level. Multiple observations
of adult Island Apple Snails indicated that shallow water did not hinder
movement or other behaviors. I observed adults of both species feeding and mating
in locations where water measured less than 10 cm deep. Bernatis and Warren (2014) reported
that Golden Apple Snails were routinely observed in water less than 10 cm and were
often seen at the shoreline consuming vegetation. Water depth may have an impact
Table 2. Number of Island Apple Snail and Florida Apple Snail collected in each treatment area and at
each depth contour. The total number of snails is based on size class (<10 mm or >10 mm in length)
is provided. The total number of replicates per depth is in parentheses.
Depth (# replicates)
Species 0.25 (96) 0.75 (96) 1.25(100) 1.75 (132) 2.25 (132) Totals (552)
P. maculata
<10 mm 39 30 10 8 8 95
>10 mm 38 51 19 7 4 119
Total 77 81 29 15 12 214
P. paludosa
<10 mm 0 0 1 0 0 1
>10 mm 2 1 1 0 0 4
Total 2 1 2 0 0 5
Grand total 79 82 31 15 12 219
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on Florida Apple Snail behavior, but this does not appear to be true for Island Apple
Snail or Golden Apple Snail. The perception that apple snails are encountered more
often in shallow-water habitats may be a function of a lack of sampling effort in
deeper waters.
The assumption that the apple snail population declined because of reduced
snail densities in shallow water should be avoided. Based upon my observations
presented here, surveys should include deeper waters because the snails may be
seeking refuge there. Understanding these movement patterns and habitat use by
apple snails is beneficial to anyone working with an infestation of nonindigenous
apple snails, and knowing where to look for the snails is necessary to develop a
successful plan. Furthermore, understanding the snails’ habitat distribution and
use is critical when developing management plans, particularly those that involve
water-level manipulation or vegetation and animal protection.
Acknowledgments
I thank the anonymous reviewers for their comments and suggestions and Gary Warren
for his edits and comments. I am very grateful to Tom Morris and Mike Dickson for their assistance
with recovering the snails while scuba diving. I thank the Florida Fish and Wildlife
Conservation Commission for funding support of this work.
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