Molecular identification of Pomacea canaliculata and P. insularum from rice paddy in different origins in China using mitochondrial adenosine triphosphate subunit 6 gene

A New Species of Apple Snail in the Genus Pomacea (Gastropoda: Caenogastropoda: Ampullariidae)

Pomacea canaliculata and Pomacea maculata, two species of freshwater apple snails native to South America, have been introduced to and become invasive in many tropical wetlands around the world. Among their invaded regions in mainland China, we discovered a new species (Pomacea occulta nov. sp.), which has > 5.8% inter-specific genetic divergence with its closely-related species. The present study provides a morphological description of this new species. This new species can be distinguished from the other recognized species of Pomacea by a combination of the following features: a thick shell with generally smooth, fine axial growth lines and frequent periodic growth arrests, an angulated body whorl shoulder, and a yellow-orange-red inner pallial lip; a flexible operculum, thinning dorsally towards the edge; broad kidney anterior leaflets and posterior leaflets with a deep tapering cleft; a concave rachidian base; a penis sheath with apical and medial glands, with the medial gland located in the sheath channel; and 87 to ~1000 eggs per clutch, an average egg diameter of 2.38 mm, a mean hatching height of 2.23 mm, and a mean first whorl width of 1-day-old hatchlings of 2.11 mm.

Comparative Characterization of the Complete Mitochondrial Genomes of the Three Apple Snails (Gastropoda: Ampullariidae) and the Phylogenetic Analyses

The apple snails Pomacea canaliculata, Pomacea diffusa and Pomacea maculate (Gastropoda: Caenogastropoda: Ampullariidae) are invasive pests causing massive economic losses and ecological damage. We sequenced and characterized the complete mitochondrial genomes of these snails to conduct phylogenetic analyses based on comparisons with the mitochondrial protein coding sequences of 47 Caenogastropoda species. The gene arrangements, distribution and content were canonically identical and consistent with typical Mollusca except for the tRNA-Gln absent in P. diffusa. An identifiable control region (d-loop) was absent. Bayesian phylogenetic analysis indicated that all the Ampullariidae species clustered on the same branch. The genus Pomacea clustered together and then with the genus Marisa. The orders Architaenioglossa and Sorbeoconcha clustered together and then with the order Hypsogastropoda. Furthermore, the intergenic and interspecific taxonomic positions were defined. Unexpectedly, Ceraesignum maximum, Dendropoma gregarium, Eualetes tulipa and Thylacodes squamigerus, traditionally classified in order Hypsogastropoda, were isolated from the order Hypsogastropoda in the most external branch of the Bayesian inference tree. The divergence times of the Caenogastropoda indicated that their evolutionary process covered four geological epochs that included the Quaternary, Neogene, Paleogene and Cretaceous periods. This study will facilitate further investigation of species identification to aid in the implementation of effective management and control strategies of these invasive species.

Comparative mitogenome analysis on species of four apple snails (Ampullariidae: Pomacea)

The genus Pomacea contains most of the economically important Ampullariid apple snails. Five Pomacea species were reported introduced out of their native ranges, including three highly invasive species, i.e. P. maculata, P. canaliculata and an unidentified species, Pomacea sp. In this study, the mitogenome of P. maculata was determined, which carried typical gene set of metazoan mitogenomes and shared the same gene rearrangement of Ampullariidae mitogenomes. The control region of the P. maculata has a 13-bp inverted repeat unit. We compared the mitogenome of P. maculata with P. canaliculata, Pomacea sp. and the aquatic pet species, P. diffusa. The three highly invasive species showed high sequence similarity of PCGs and RNAs. atp8 and nad2 showed the lowest similarity and the highest Ka/Ks ratios, indicating that both genes have potential for studying species identification and populations genetics in apple snails. All PCGs have the Ka/Ks ratios <1, indicating the existence of purifying selection in Pomacea species. We reconstructed phylogenetic trees using 14 Caenogastropda species based on sequences of PCGs and rRNAs using Bayesian inference. Pomacea maculata grouped with other Ampullariids and was most closely related to Pomacea sp. The mitogenome of P. maculata provides useful genetic resource for exploring the genetics and evolution of P. maculata and other apple snails.