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Observations of Pomacea spp. (Apple Snails) Beyond the Shallow Marsh
Jennifer L. Bernatis

Southeastern Naturalist, Volume 18, Issue 3 (2019): 469–475

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Southeastern Naturalist 469 J.L. Bernatis 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 Southeastern Naturalist J.L. Bernatis 2019 Vol. 18, No. 3 470 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 Southeastern Naturalist 471 J.L. Bernatis 2019 Vol. 18, No. 3 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. Southeastern Naturalist J.L. Bernatis 2019 Vol. 18, No. 3 472 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 Southeastern Naturalist 473 J.L. Bernatis 2019 Vol. 18, No. 3 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 Southeastern Naturalist J.L. Bernatis 2019 Vol. 18, No. 3 474 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. Literature Cited Albrecht, E.A., N.B. Carreno, and C. Vazquez. 1999. 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