Oviposition of non-calcareous or thinly shelled eggs represents an important life stage of many insects, amphibians, and several gastropods. A recently identified invasive species of apple snail, Pomacea insularum, exhibits alarming invasive characteristics of high reproductive rates and generalist consumption patterns. This snail takes the opposite approach to egg laying compared to most aquatic insects as adult snails crawl out of the water to place clutches on emergent, or terrestrial, substrates. As fecundity best indicates invasive potential for mollusks, control or management efforts need to understand reproductive behavior in P. insularum to predict, and hopefully impede, its spread throughout the Gulf Coast of the United States. Specific characteristics of wetlands and shallow lakes may facilitate the invasion process of P. insularum by providing females with conditions that permit successful oviposition. In order to investigate this possibility, we studied P. insularum oviposition behavior in an invasive population at two times during the reproductive season in Texas, USA. Based on a subsequent survey (August 2009), plants comprised 78% of the available habitat. Wild taro (Colocasia esculenta) and alligator weed (Alternanthera philoxeroides) represented 48 and 43% of that proportion, respectively. During 2008-2009, no new concrete or metal structures appeared in our sampling reach and consistent amounts of plant stands and woody debris remained dominant. Given this distribution, P. insularum laid disproportionately more clutches on wild taro compared to its availability and less on alligator weed and bulrush (Schoenoplectus californicus) than expected. Owing to limited metal and concrete substrates, we found a higher proportion of clutches on these artificial substrates than expected in both May and August 2008. However, artificial substrates comprised less than 2% of available substrates in the bayou. Our results suggest that wetlands and shallow lakes surrounded by large emergent macrophytes, particularly wild taro, likely provide ideal oviposition sites for P. insularum, promote egg supply, and possibly facilitate invasion into new aquatic ecosystems.

Winning the war against invasive species requires early detection of invasions. Compared to terrestrial invaders, aquatic species often thrive undetected under water and do not garner notice until too late for early action. However, fortunately for managers, apple snails (Family Ampullariidae, Genus Pomacea) provide their own conspicuous sign of invasion in the form of vibrantly colored egg clutches. Managers can potentially use egg clutches laid in the riparian zone as a means of early detection and species identification. To facilitate such efforts, we quantified differences in characteristics (length, width, depth, mass, egg number) of field-laid clutches for the two most common invasive species of apple snail, P. canaliculata and P. maculata, in native and non-native populations. Pomacea canaliculata native and non-native populations differed noticeably only in width. Native P. maculata clutches possessed significantly greater width, mass and eggs numbers compared with native P. canaliculata. Non-native P. maculata clutches significantly exceeded all other populations in all measured characteristics. Consequently, these traits may successfully distinguish between species. Fecundity data also allowed us to develop models that accurately estimated the number of eggs per clutch for each species based on clutch dimensions. We tested one, two and three dimensional models of clutches, including rendering a clutch as either a complete ellipsoid or an ellipsoid intersected by a cylinder to represent the oviposition site. Model comparisons found the product of length and depth, with a different function for each population, best predicted egg number for both species. Comparisons of egg number to clutch volume and mass implied non-native P. canaliculata may be food limited, while non-native P. maculata appeared to produce such enormous clutches by having access to greater nutrients than the native population. With these new tools, researchers and managers can quickly identify, quantify and begin eradication of new non-native apple snail populations.

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While difficult to prevent introductions, scientific research can help guide control efforts of exotic, invasive species. South American island apple snails Pomacea insularum have quickly spread across the United States Gulf Coast and few control measures exist to delay their spread. Usually occupying cryptic benthic habitats, female apple snails crawl out of the water to deposit large, bright pink egg clutches on emergent objects. To help identify the most likely place to find and remove clutches, we conducted four lab experiments to investigate what specific object qualities (i.e. material; shape and height; plant species; natural and artificial) attracted P. insularum females to lay clutches. In our fourth experiment, we specifically examined the relationship between female size and reproductive output. To further understand reproductive output, we quantified experimental clutch characteristics (height above water, dimensions, mass, approximate volume, number of eggs, hatching efficiency). Pomacea insu-larum females laid more clutches on plant material, chose round over flat surfaces and failed to differentiate between tall and short structures. In comparison to a common native plant in the eastern US, Pontederia cordata, snails clearly preferred to lay clutches on a widely distributed exotic, invasive plant (wild taro, Colocasia esculenta). Unexpectedly, smaller snails showed higher overall total fecundity as well as more eggs per clutch than larger snails. Therefore, hand removal efforts of large females may not be enough to slow down clutch production. Collectively, our results indicate that conservationists and managers should search emergent plants for P. insularum clutches carefully to guard against established populations.