Two large-bodied apple snails (Pomacea spp.) inhabit freshwater wetlands in Florida: the invasive Pomacea maculata, and the native Pomacea paludosa. P. maculata produces large masses of eggs which yield 1.5-2 mm shell length (SL) hatchlings, while P. paludosa produces fewer eggs that yield 3-4 mm SL hatchlings. Both species are vulnerable as juveniles to predation by crayfish (Procambarus fallax), and mesocosm experiments indicate that crayfish have an 8x greater effect size on P. maculata hatchling mortality when compared to hatchling P. paludosa. We hypothesized that differences in survivorship were largely a result of hatchling size differences.
We conducted a series of feeding trials to quantify how predation rate scales with juvenile snail size. Crayfish of varying sizes were individually presented with an excess of P. maculata at one of two sizes: 1.65 ± .03 mm (mean ± SE; hatchling size), and 3.56 ± .08 mm (~ size of P. paludosa hatchlings), and allowed to feed for 16 hours. We recorded the number of snails consumed by each crayfish and calculated feeding rates as the kill rate (snails consumed/h) and biomass consumed per hour and compared them between the two size categories as a function of crayfish size.
Results/Conclusions
Kill rate was positively related to crayfish size for both size classes of snails, and the effect of crayfish size on kill rate was 6.6x greater for small snails than large snails. Small crayfish (~10 mm carapace length; CL) consumed approximately 1.9x more small snails than large snails, but larger crayfish (~23 mm CL) consumed 4.6x more small snails than larger snails. Thus, as crayfish grow larger, the threat they pose to smaller snails increases, however even the smallest crayfish used (~10 mm CL) were still capable of consuming larger snails at very low rates (<0.6 snail/h). Despite large differences in kill rates between snail size classes, snail biomass consumed per hour was similar regardless of snail size.
Our experimental findings do not completely explain the observed differences in mortality between hatchling P. paludosa and hatchling P. maculata from a prior experiment, however it is likely that hatchling P. maculata spend a greater period of time at highly vulnerable juvenile stages than hatchling P. paludosa until they reach sizes that crayfish cannot handle. Additionally, P. paludosa and P. maculata hatchlings may differ in other traits (behavior, shell strength) that could contribute to differences in mortality rates.