LIFE HISTORY VARIATION IN INVADING APPLESNAILS (“POMACEA CANALICULATA”) MAY POSE ECOLOGICAL THREAT TO WETLANDS
We would like to thank Bob Howells (TPWD), Mark Kramer and Ann Brinley (Armand Bayou), and Southwestern University for supporting this research. Ecology students Kim Boyd, Matthew Garcia and Cody Freas provided the intraspecific competition data.
Rebecca K. Marfurt, Brandon B.Boland and Dr. Romi L. Burks
Department of Biology, Southwestern University, Georgetown, TX 78626
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Each Results figure refers to
Experiment # in Methods
In native habitats, channeled applesnails (Pomacea canaliculata) graze periphyton. However, casual observations from introduced populations suggest these invaders show variation in feeding ecology, predator response and life history strategies. Attempts to predict this consumer influence on ecosystem function suffer from a lack of basic data. We tested how salinity affected snail mortality. Both adults and hatchlings tolerated salinity levels up to 8 ppt. Adult feeding on lettuce increased significantly at 8 ppt compared to 0 ppt (p = 0.002), while hatchling consumption of algae did not vary (p = 0.284). To see how these consumers responded to predators from the invaded ecosystem, we tested behavioural responses to predatory cues from fish, turtles, crayfish and adult applesnails. Results indicated that fish and crayfish prompted similar predator-avoidance behaviors in hatchlings (p’s < 0.05) and that hatchling response changed over time. Consumption rates of juvenile and redear sunfish did not vary (Χ2, p > 0.05) between native (ramshorn) and exotic applesnails. We recently found that the presence of sunfish enhanced consumption by adult snails when presented alone, but depressed consumption in combination with conspecific cues. Juveniles did show signs of intraspecific competition. Research providing insight into the basic ecology of applesnails can foster management efforts at the ecosystem scale.
- EXPERIMENT 1 – SALINITY IMPACTS:We exposed either 1 adult or 6 hatchling applesnails instantaneously (i.e. no acclimation) to 4 levels of salinity ranging from 0 ppt – 35 ppt (N=4). Adults fed on lettuce and hatchlings consumed algae off tiles. We recorded percent consumption of the resources.
- EXPERIMENT 2 – PREDATOR RESPONSE: We exposed groups of 6 hatchling applesnails (N=6) to 1 of 5 predatory chemical cue treatments (incubated 17-L for 12 hrs): 1) no cues; 2) turtle; 3) sunfish; 4) adult applesnail; or 5) crayfish; and observed alarm responses (burying or crawling out) after 60 minutes.
- EXPERIMENT 3 – PREDATOR AGE-SPECIFIC PREFERENCE:We fed adult or juvenile redear sunfish (Lepomis microlophus) juvenile applesnails, native ramshorn snails (Planorbella) or both types for nine days (N=3) to test each prey choice with fish size (N=3). We then recovered the remaining snails after 2 hrs of predation.
- EXPERIMENT 4 – REFUGE EFFECT:Groups of 6 snails occurred in a 2x2 experimental design (N=4) in the presence or absence of redear sunfish as the predator and water hyacinth (Eichhornia crassipes) as the refuge. We recorded mortality after 24 hrs.
- EXPERIMENT 5a & 5b – RESOURCE CONSUMPTION IN PRESENCE OF PREDATORY CUES: We tested per gram consumption rates of adult applesnails in A) the presence of fish cue (1 L from redear sunfish tank into 9 L distilled) and in B) the presence of fish cue plus cues from crushed-up conspecifics (i.e. other applesnails). In a 2x2 design (N=4), snails fed for 4 hrs on either green leaf lettuce or Myriophyllum spicatum (Eurasian water milfoil, chemically defended) in the presence or absence of predatory cues. We calculated a consumption ratio of grams of plant consumed per grams of applesnails.
- EXPERIMENT 6 – INTRASPECIFIC COMPETITION AND RESOURCE CHOICE: We studied the impacts of snail density (1 vs. 3 snails in containers measuring 36 x 25 x 13 cm) and lighting (constant light vs. dark) on the juvenile applesnail consumption of two resources: lettuce versus purple cabbage. Four trials took place over a span of 7 days and we averaged the per capita consumption rates. We predicted that lettuce would be the preferred food.
- In our experiments, exotic, invasive, channeled applesnails demonstrated their: 1) ability to respond to predators; 2) capability of surviving moderate environmental stress; 3) sensitivity to intraspecific competition; 4) and their susceptibility to predation by native predators.
- With their reproductive capacities, generalist feeding, and genetic diversity, CAS represent a serious ecological threat to estuarine and freshwater systems.
- P. “canaliculata” did not survive salinity levels >14ppt. Thus, storm surges typical of tropical storms or hurricanes could damage applesnail populations in estuarine systems. However, none of our results indicated that the snails would not thrive in freshwater systems as well.
- Environmental and biotic stresses might prompt applesnails to increase their consumption of resources, thus exacerbating their impact on community and ecosystem dynamics.
- Hatchling exhibited predator-specific alarm response to cues. This agrees with literature showing differential responses by other freshwater snails (Physella; Turner et al. 2000).
- Despite high consumption, redear sunfish failed to show a feeding preference between Planorbella or Pomacea, suggesting that sunfish would be unlikely to suppress applesnail populations in the field.
- Applesnails indicated sensitivity to high density and the presence of crushed conspecifics, suggesting perhaps some ecological weaknesses as an invader.
- Eichhornia failed to provide a refuge for applesnails but other macrophytes in the field might enhance their ability to avoid predation.
- Overall, more research on basic ecology of the CAS complex is necessary to predict their impact or curb their spread (Estebenet & Martín 2000).
Control: No cue
- The channeled applesnail (CAS – Photo A;Pomacea “canaliculata”) made the list of the Top 100 invasive species (ISSG 2004) and has restricted status in Texas.
- Species-specific identity within the complex“canaliculata” leaves room for much debate in this group of applesnails (B. Howells, personal communication), making differences in life history all the more interesting.
- CAS exhibit high fecundity rates, laying up to 3000 eggs/clutch (Marfurt, unpublished data). Such high reproduction generally increases success of exotic species (Lodge 1993). To reduce predation, adults lay eggs above the water’s surface usually on plants (Photo B); Hatchlings (Photo C) drop into the water where they confront predators.
- CAS voraciously consumer aquatic macrophytes (Estoy et al. 2002), and possess the potential to invade systems with varying salinity, including freshwater ponds and lakes.
- Seen as a generalist feeder, CAS may be very destructive to wetland habitats and possibly cause a decline in native plants and animals (Lach et al. 2000).
- Recent research, however, calls into question the generalist nature of this highly destructive herbivore (Lach et al. 2000) and more studies that investigate factors that influence feeding may add to our power to predict impacts.
- Exotic, invasive species pose a real threat to biodiversity, especially in small lakes & ponds (Sala et al. 2000). Thus, research focused in this area may help prevent ecosystem degradation or promote restoration.
- Contrasting mechanisms and feeding preferences of exotic (Texas) vs. native (Uruguay) populations of applesnails.
- Investigating plant preference and deterrence in different life history stages of applesnails.
- Examining the successful transition of applesnails from egg to hatchlings to juveniles under different environmental conditions (temperature, salinity, predator cues).
- Developing a predictive regression model for estimating abundance of applesnails in the field based on egg masses.
- Estebenet A. L. & Martín P. R. 2002. Pomacea canaliculata (Gastropoda: Ampullariidae): Life history traits and their plasticity. Biocell 26(1): 83-89.
- Estoy et al. 2002. Effects of food availability and age on the reproductive effort of the apple snail, Pomacea canaliculata. Appl. Ent. Zoology 4: 543-550.
- Invasive Species Specialist Group (ISSG). 2004. 100 of the world’s worst alien species. http://wwww.issg.org/booklet.pdf.
- Lach et al. 2000. Food preference and reproductive plasticity in an invasive freshwater snail. Biological Invasions 2: 279-288.
- Lodge D. M. 1993. Biological invasions: lessons for ecology. TREE 8: 133-137.
- Sala et al. 2000. Biodiversity scenarios for the year 2100. Science 287: 1770-1774.
- Turner et al. 2000. Chemical cues modify species interactions: the ecological consequences of predator avoidance by freshwater snails. Oikos 88(1): 148-158.
Background picture shows students conducting field surveys of applesnails at Horsepen Bayou near Houston.