The Pomacea Project

Water Chemistry and Quality

Calcium and pH Effects

Florida Apple Snail Growth and Shell Integrity

Observations from field sampling in WCA3A and WCA2B indicated that underlying water chemistry, particularly low pH and calcium concentrations, may limit snail abundance in the Everglades. Water quality data from the South Florida Water Management District (SFWMD) DBHYDRO database indicated that interior WCA1 had significantly lower Ca²+ concentrations and pH levels (5.7 mg/L and 6.1, respectively, see Glass and Darby 2008 for details) compared to WCA3A (78 mg Ca²+/L and pH=7.6). Glass and Darby (2008) reported on lab experiments of the effects of pH and calcium on apple snail shell growth and integrity. They also tested whether snails from low Ca²+/pH waters may be better adapted at growing in these conditions compared to snails taken from high Ca²+/pH conditions. Finally, they examined differences in shell calcium content and the resultant shell crush weights (CWs) (see Figure 1), as affected by treatment conditions.

Figure 1. Crush weight measuring device, designed and constructed at the Univ.of West Florida, consists of a platform and framework fixed to a scale accurate to 0.01 kg. (Photo by N. Glass, published in Glass and Darby 2008).

Two experiments on hatchling shell growth rates were conducted using snails hatched in the lab from eggs collected in the field from Water Conservation Area (WCA) 1 (experiment 2) and 3A (both experiments) (Glass and Darby 2008). The Ca2005 treatments consisted of a range of Ca²+ levels created by adding calcium salts to water (pH≈8.0). In Ca2006 treatments, field water was used from WCA1 (pH maintained ≈8.0) and WCA3A pH≈8.0. In the 2005 experiment, change in shell length (ΔSL) was significantly greater in treatments with ≥ 28 mg Ca²+/L compared to treatments with ≤ 14 mg/L (F4,10 = 15.1, P=0.002). In the second experiment, snails grown in WCA1 water experienced significantly lower growth rates (mean of 1.22 mm/wk) and markedly thinner shells compared to snails in WCA3A water (2.35 mm/wk). Snails taken from high calcium/pH water and placed in water with 3.6 mg Ca²+/L and pH < 6.5 showed signs of shell erosion (see Figure 2). Apple snails from populations existing in low Ca²+/pH waters did not appear better adapted at growing in such conditions. Shell CWs were 2.7-times higher for snails grown in the WCA3A treatment compared to WCA1 water. There were also significant differences between the calcium ion content of snails grown in WCA1 water (15% of weight as Ca²+) compared to WCA3A water (26%).

Figure 2. Pitted apex region of snail shell from WCA1 water (on right), compared to the smooth shell of a snail from WCA3A. (Photo by N. Glass, published in Glass and Darby 2008).

Smaller, thinner shells and lower CWs have implications for predation vulnerability and reproductive success (Glass and Darby 2008). Except for avian predators, most predators of snails (e.g., alligators, turtles, and fish) crush them; thinner shells would render snails easier to crush and consume. The results of this work are consistent with reports associating relatively low snail densities with relatively low Ca²+/pH waters.

Although not published in Glass and Darby (2008), Glass (2007) also examined cation ratios and how such ratios might have affected apple snail growth and survival in the lab. Cation ratios (e.g., calcium:magnesium; calcium:potassium; and calcium:sodium), however, were not studied by design and no definitive conclusions could be made. The cation ratio work was studied further by D.Gueringer (B.S., 2009, Biology Department, UWF). She examined whether a decrease in the Ca:Mg ratio would result in a decrease in growth (ΔSL). She tested twelve Ca:Mg ratios ranging from 0.11-3.22. Ca²+ concentrations ranged from 10-56 mg/L, and Mg concentrations were adjusted to reach the test ratios. She also examined the effect of sodium (89.9-207.6 mg/L) on growth. No significant effects of cation ratio on apple snail growth were observed at the levels studied.

Egg Production

Low Florida apple snail densities in WCA1 (part of the Arthur R. Marshall Loxahatchee National Wildlife Refuge) may be associated with relatively low pH and calcium levels, as indicated by field sampling (Karunaratne et al. 2006) and controlled lab studies (Glass and Darby 2008). Studies of other gastropods show that low pH and calcium (as well as other constituents) affect fecundity. Tiffany Trent (USFWS and Nova Southeastern University) is conducting a field study along a water chemistry gradient in WCA1 to look at potential effects on Florida apple snail fecundity. Her project objectives include looking at different levels of calcium, total phosphorus, and sulfate, and how they affect: 1) apple snail egg number and size, 2) C:N ratios of egg clusters, and 3) snail wet weight at hatching. Her results will be interpreted in the context of the potential effects of rerouting water via restoration efforts that may ultimately cause changes in water chemistry in some areas of the Everglades. Anticipated date of project completion is 2009. For further information, contact Ms. Trent at (Tiffany_Trent@fws.gov).

Literature Cited

Glass, N.H. 2007. Calcium and pH effects on growth and shell integrity of the Florida apple snail (Pomacea paludosa Say). Master’s Thesis, University of West Florida, Pensacola, FL.

Glass, N.H. and P.C. Darby. 2008. The effect of calcium and pH on Florida apple snail, Pomacea paludosa (Gastropoda: Ampullariidae), shell growth, and crush weight. Aquatic Ecology DOI 10.1007/S10452-008-9226-3.

Gueringer, D. 2009. Examining the effects of calcium:magnesium ratios on the change in shell length in Florida apple snails, Pomacea paludosa (SAY). Honors Thesis, University of West Florida, Pensacola, FL.

Water Quality / Effects of Pollutants

Effects of Nitrate on Apple Snails

It has been suggested that increased nitrate levels in ground and surface waters may have led to a decrease in apple snail populations in Florida springs. There was particular concern over Wakulla Springs, where once abundant limpkins and snails (as evident by their eggs) disappeared over the course of a few years. Corrao et al. (2006) looked at two aspects of apple snail-nitrate interactions. First, a field survey was conducted in 2002 to examine potential correlations between nitrate levels and apple snail densities in six spring-fed river systems. Second, a laboratory study in 2003 tested the effects of elevated nitrate levels on the survival and growth of apple snails.

The field survey found no correlation between snail density and nitrate concentration in the springs sampled (Jackson Blue, Rainbow, Alexander, Wakulla, Wacissa, and Ichnetucknee) (Corrao et al. 2006). Nitrate-nitrite levels in these springs ranged from 0.6-3 ppm (see Scott et al. 2002). Snail densities in the springs ranged from 0-5 snails/m². Laboratory studies revealed:

1) adult and juvenile snail survival over a 96 h period was not affected by nitrate
concentrations more than 100-times higher than those typically found in springs (<5 ppm);

2) adult snails experienced swelling of the foot and mantle at nitrate
concentrations of 125-1000 ppm;

3) in two trials, nitrate level impacted juvenile snail growth; EC50 values were 504 and
622 ppm;

4) at very high nitrate values (>500 ppm) juvenile survival during the 14-day growth study was <50%.

Results of the study suggest that the nitrate levels found in Florida springs do not directly impact growth or survival of apple snails, because effects were observed only at nitrate levels orders of magnitude above those found in nitrate-polluted springs. Changes in habitat structure (including from efforts to control hydrilla) may explain trends in apple snail abundance observed in the springs (Corrao et al. 2006).

Mercury Residues

Mercury has been detected in sediments throughout central and south Florida (Atkeson 2005). Mercury bioaccumulates in wetland and lake fauna as methyl mercury (Eisler 1987). Wildlife consuming fish and shellfish with methyl mercury residues may experience a wide range of sublethal effects including impaired reproduction, growth, and development (Eisler 1987). Eisemann et al. (1997) sampled 62 apple snails (shell lengths 19-50 mm) and 66 apple snail egg cluster masses for mercury residues, in part over concern that snail kites may suffer deleterious effects from exposure. Methyl mercury was not detected in apple snail eggs. Mercury residues in snails’ soft tissues were generally low (0.063 ppm on average), with highest concentrations found in the Panther National Wildlife Refuge (0.091 ± 0.035 ppm). No correlation was found between mercury level and snail size. The authors concluded that although snails may serve as an indicator of bioavailable mercury in central and south Florida wetlands, accumulated residues were generally low and presumably would have little impact on kites.

Organochlorine Pesticide Residues

Organochlorine pesticide residues (DDT and its breakdown products, and dieldrin) were found in a dead adult snail kite, as well as in a dead nestling and an egg that had failed to hatch (Lamont and Reichel 1970). These specimens were collected near the Loxahatchee National Wildlife Refuge in 1966-1967, prior to the ban of DDT. Organochlorine tissue concentrations were generally low (~0.1 to 0.4 ppm) and considered representative of background levels. In this same study, a composite sample of 30 apple snails had ≤0.1 ppm of DDT, DDE or DDD.

Snail kite use of wetlands in the vicinity of a solid waste combustor and landfill in Palm Beach County raised concerns regarding exposure of kites to chemical contaminants (Rumbold et al. 1996). From 1989-1991, 6-10 apple snails from two locations were analyzed for organochlorine pesticides (e.g., DDT, DDE, dieldrin and other pesticides). No organochlorine compounds were found in any of the snail samples. [Organochlorine residues were found in the eggs and tissues of anhingas and white ibis.] Rumbold et al. suggest that the generally low lipid content of apple snails (<1%) may explain the lack of fat-soluble chemical residues. In addition, most organochlorine pesticides were banned or restricted in the 1970s.

Effect of Copper on Apple Snails

Imlay and Winger (1983) thought that copper-diquat formulations applied to kill hydrilla in canals surrounding the Loxahatchee National Wildlife Refuge may explain a decline in the refuge’s apple snail population. Copper is a well-known molluscicide, and it continues to be applied throughout the world to control Schistosomiasis-bearing aquatic snails. The applied concentration of copper (1 mg/L) was shown to kill several aquatic snail species, although the authors reported no direct observations of impacts on apple snails. In their brief overview, they conclude copper-based herbicide formulations should not be applied to waterways with apple snails that support foraging kites.

Copper continues to be applied to Florida waterways to control aquatic plants and algae, although with restricted applications (see http://aquat1.ifas.ufl.edu.guide/sup3herb.html). We have seen limited concern expressed regarding potential impacts of herbicide applications on apple snails. However, copper bound to sediments may be an issue as agricultural lands become flooded as part of wetland restoration efforts in south Florida. Copper-containing pesticides applied to these areas will likely persist and accumulate in aquatic biota following the reintroduction of water. Several publications have emerged in the past two years that address this concern in the context of impacts on apple snails (Hoang et al. 2008a and b; Schuler et al. 2008; Rogevich et al. 2008a). Hoang et al. (2008a) found copper (Cu) concentrations up to 300 ug/L in flood water where underlying soils contained up to 205 mg/kg. Hoang et al. (2008b) reported that hatchling snails (<4 d old) exposed for 28 days to water with 14 ug Cu/L concentrated the Cu by 1000-fold relative to the water treatment (i.e., ~1500 ug/kg in whole snails). In this same study, adult snails (defined as 3 mo. old) were exposed to Cu in water, soil, and food (Cu-laden lettuce). Hoang et al. found that water-borne Cu contributed little to body burdens in adult snails. They concluded the aqueous route of exposure of Cu to snails would be of little importance in the Everglades relative to soil and diet, in part because dissolved organic carbon (DOC) in water sequesters the Cu, rendering it unavailable for snails. Adult snails accumulated Cu via the soil (via direct contact and possibly via ingestion), and even more so through contaminated food. Very little Cu accumulated in adult snail shells; bioaccumulation factors in soft tissues (viscera and foot) were 8-10. They concluded that snail predators would be at risk of exposure to snails that colonize agricultural areas that become flooded.

Apple snail populations may be affected by acute and chronic exposure to Cu in flooded soils. Rogevich et al. (2008a,b) exposed apple snails to Cu in water. In their acute toxicity studies, the 96-hr LC50 (concentration that kills 50% of snails in 96-hr) was 34 ug/L for 2-d old snails; toxicity decreased with age up to the oldest snails tested (120-d old snails had an average 96-hr LC50 of 182 ug/L) (Rogevich et al. 2008a). For the chronic studies, designed to run for 9 months, test concentrations were set at 8, 16 and 24 ug/L (plus control tanks) (Rogevich et al. 2008b). The 24 ug/L treatment had to be dropped due to >50% mortality after seven days. The original snails exposed to Cu (starting off at <4-d old) were the F0 generation. Many of these snails grew to adult size and reproduced to create the F1 generation. The Cu-treated water significantly reduced snail growth rates up to 60 d, but after 230 d the sizes of control snails were no different than the Cu-treated snails. Cu significantly reduced the number of egg clusters produced by F0 snails as well as the percentage of those eggs that hatched. The number of eggs in a cluster was not affected by copper. The F1 snails that hatched from these eggs were grown out in clean water. Any deleterious effects of copper exposure to the F0 snails were not passed on to the next generation; growth and survival in F1 snails from Cu-exposed F0 snails were no different from controls.

Literature Cited

Atkeson, T. 2005 (last update). Mercury in Florida’s Environment. Available at http://www.floridadep.org/labs/mercury/docs/flmercury.htm (Last accessed July 31, 2009).

Corrao, N.M., P.C. Darby, and C.M. Pomory. 2006. Nitrate impacts on the Florida apple snail, Pomacea paludosa. Hydrobiologia 568:135-143.

Eisemann, J.D., W.N. Beyer, R.E. Bennetts, and A. Morton. 1997. Mercury residues in south Florida apple snails (Pomacea paludosa). Bulletin of Environmental Contamination and Toxicology 58:739-743.

Eisler, R. 1987. Mercury hazards to fish, wildlife, and invertebrates: a synoptic review. Biological Report 85 (1.1), Contaminant Hazard Reviews Report No. 10. U.S. Fish and Wildlife Service, Patuxent Wildlife Research Center, Laurel, Maryland.

Hoang, T.C., E.C. Rogevich, G.M. Rand, and R. A. Frakes. 2008a. Copper uptake and depuration by juvenile and adult Florida apple snails (Pomacea paludosa). Ecotoxicology 17:605-615.

Hoang, T.C., E.C. Rogevich, G.M. Rand, P.R. Gardinali, R.A. Frakes, and T.A. Bargar. 2008b. Copper desorption in flooded agricultural soils and toxicity to the Florida apple snail (Pomacea paludosa): Implications in Everglades restoration. Environmental Pollution 154:338-347.

Imlay, M.J. and P.V. Winger. 1983. Toxicity of copper to Gastropoda with notes on the relation to the apple snail. Malacological Review 16:11-15.

Lamont, T. and W. Reichel. 1970. Organochlorine pesticides in whooping cranes and Everglade kites. Auk 87:158-159.

Rogevich, E.C., T.C. Hoang, and G.M. Rand. 2008a. The effects of water quality and age on the acute toxicity of copper to the Florida apple snail, Pomacea paludosa. Archives of Environmental Contamination and Toxicology DOI 10.1007/s00244-007-9106-1.

Rogevich, E.C., T.C. Hoang, and G.M. Rand. 2008b. Effects of sublethal chronic copper exposure on the growth and reproductive success of the Florida apple snail (Pomacea paludosa). Archives of Environmental Contamination and Toxicology. DOI 10.1007/s00244-008-9231-5.

Rumbold, D.G., M.C. Bruner, M.B. Mihalik, E.A. Marti, and L.L. White. 1996. Organochlorine pesticides in anhingas, white ibises, and apple snails collected in Florida, 1989-1991. Archives of Environmental Contamination and Toxicology 30:379-383.

Schuler, L.J., T.C. Hoang, and G.M. Rand. 2008. Aquatic risk assessment of copper in freshwater and saltwater ecosystems of south Florida. Ecotoxicology 17:642-659.

Scott, T.M., G.H. Means, R.C. Means, and R.P. Meegan. 2002. First magnitude springs of Florida. Open File Report No. 85. Florida Department of Environmental Protection, Tallahassee, FL.