Density‐dependent growth and reproduction of the apple snail, Pomacea canaliculata: a density manipulation experiment in a paddy field

Abstract

To examine density dependence in the survival, growth, and reproduction of Pomacea canaliculata, we conducted an experiment in which snail densities were manipulated in a paddy field. We released paint-marked snails of 15-20 mm shell height into 12 enclosures (pens) of 16 m 2 at one of five densities -8, 16, 32, 64, or 128 snails per pen. The survival rate of released snails was 95% and was independent of snail density. The snail density had a significant effect on the growth and egg production of individual snails. This density dependence may have been caused by reduced food availability. The females at high density deposited fewer and smaller egg masses than those at low density, and consequently produced fewer eggs. The females at densities 8 and 16 deposited more than 3000 eggs per female, while the females at density 128 oviposited only 414 eggs. The total egg production per pen was, however, higher at higher snail density. The survival rates of juvenile snails were 21%-37% and were independent of adult density. The juvenile density was positively correlated with the total egg production per pen and hence was higher at higher adult density. However, the density of juveniles larger than 5 mm in shell height, i.e., juveniles that can survive an overwintering period, was not significantly different among density treatments. These results suggest that snail density after the overwintering period is independent of the density in the previous year. Thus, density dependence in growth and reproduction might regulate the population of P. canaliculata in paddies.

Life cycle of the apple snail Pomacea canaliculata (Caenogastropoda: Ampullariidae) inhabiting Japanese paddy fields

“…1). All snails Ն25 mm in shell height were sexed using the shapes of the shell and the operculum (Tanaka et al, 1999), to calculate egg production per female. All snails were released into the original fields after measurement.…”

“…In fact, the variation in egg numbers between different localities or years showed similar trends to those in snail densities: the number of eggs/m 2 in July in Nara was about 1/3 of that in Kumamoto, and it differed Ͻ2-fold between years in Nara; therefore, the key mechanism for population recovery appeared to be oviposition activity. In Nara, snails grew larger than in Kumamoto, probably due to density-dependent effects (Tanaka et al, 1999). Egg masses in Nara tended to be larger, as large snails tend to lay larger egg masses (Miyahara et al, 1986b).…”

“…In particular, none have monitored the snail populations over a long period (Ͼ1 year) on a regular basis (at least once a month), which impedes the elucidation of detailed life cycle patterns and population dynamics in this highly flexible (Estebenet and Cazzaniga, 1992;Martin and Estebenet, 2002) as well as fast-growing (Tanaka et al, 1999) snail.…”

“…The distribution of the snail continues to expand in East Asia, including Japan, China, and Korea (Joshi and Sebastian, 2006), and hence studies on the population dynamics and key regulating factors in cold districts are especially important for the prediction and prevention of further expansion. One key factor in the population dynamics of this snail is den-sity dependence in growth and reproduction (Tanaka et al, 1999). Wada et al (2004) reported a rapid population recovery during summer in paddy fields where snail densities had been low in June.…”

Resaca supports range expansion of invasive apple snails (Pomacea maculata Perry, 1810; Caenogastropoda: Ampullariidae) to the Rio Grande Valley, Texas

“…Limited food availability or density may also explain the range of clutch sizes found at our RGV site. For example, Tamburi & Martín (2011) found egg mass size in P. canaliculata decreased as food availability decreased and Tanaka et al (1999) found that increased density also decreased egg mass size. Pomacea maculata likely exhibit similar responses, and future investigations will hopefully distinguish between these possibilities that our initial data could not.…”