Julia Pizá

Allopatry and Anatomical Distinctiveness of Two Puzzling Land Snails in Genus Plagiodontes, from Argentina (Gastropoda: Orthalicidae, Odontostominae)

ABSTRACT Plagiodontes daedaleus (Deshayes, 1851) is the most widespread species in the genus Plagiodontes Doering, 1877. Although some 19th century authors recorded some differences in shell shape between the populations living to the east or to the west of the Pampean Sierras, Argentina, such differences were ignored by later authors. A multivariate statistical analysis on 608 adult shells collected at 29 localities in and around these mountains was used to test if such narratively described shell differences were statistically significant, irrespectively of their taxonomic status. Shells classified as collected from the east or the west of the sierras were correctly identified in 91.9% of the cases by Multivariate Discriminant Analysis, leading to recognizition of two allopatric shell morphologies. These external divergences were found to correspond with anatomical differences in the reproductive system. We conclude that two different, allopatric species were merged under the name Plagiodontes daedaleus in the literature and raise the subspecies Plagiodontes daedaleus strobelii (Doering, 1877) to the specific level. Another subspecies living to west of the sierras, Plagiodontes daedaleus salinicola (Doering, 1877), also showed a simple penis papilla and a non-swollen vagina, which shows that salinicola is not a member of P. daedaleus but rather a population of P. strobelii that does not deserve subspecific status. The taxonomic significance of the presence of either a simple penial papilla or a papilla bearing an accessory lobe was also confirmed for these Plagiodontes species.

Barrels and pagodas: clarifying the identity and variability of two nominal taxa of carinated land snails in the genus Plagiodontes (Gastropoda: Orthalicidae: Odontostominae)

Plagiodontes weyenberghii (Doering, 1877) has a shell with a strong suprasutural keel; the species inhabits a restricted area in Central Argentina. Plagiodontes weyenberghii minor (Parodiz, 1939) was described as a subspecies with obese shells and broader geographic distribution; some populations were sympatric with Plagiodontes daedaleus (Deshayes, 1851). A discriminant analysis on 244 adult shells combined with original information on the internal anatomy confirmed the distinctiveness of P. weyenberghii weyenberghii, whereas P. weyenberghii minor showed a broad overlap with P. daedaleus and no evidence of anatomical differences. As a consequence, P. weyenberghii minor is considered a junior subjective synonym of P. daedaleus daedaleus (Deshayes, 1851). Plagiodontes weyenberghii (Doering, 1877) has unique characteristics such as the weak vascularization of the pallial system, the relative size of the penial papilla and its accessory lobe, the markedly swollen distal portion of the bursa copulatrix duct, the spermoviduct with two whorls, and the vagina with a medial depression.

Intensity of parasitic mite infection decreases with hibernation duration of the host snail

Temperature can be a limiting factor on parasite development. Riccardoella limacum, a haematophagous mite, lives in the mantle cavity of helicid land snails. The prevalence of infection by R. limacum in populations of the land snail Arianta arbustorum is highly variable (0-78%) in Switzerland. However, parasitic mites do not occur in host populations at altitudes of 1290 m or higher. It has been hypothesized that the hosts hibernation period might be too long at high elevations for mites and their eggs to survive. To test this hypothesis, we experimentally infected snails and allowed them to hibernate at 4°C for periods of 4-7 months. Winter survival of host snails was negatively affected by R. limacum. The intensity of mite infection decreased with increasing hibernation duration. Another experiment with shorter recording intervals revealed that mites do not leave the host when it buries in the soil at the beginning of hibernation. The number of mites decreased after 24 days of hibernation, whereas the number of eggs attached to the lung tissue remained constant throughout hibernation. Thus, R. limacum survives the winter in the egg stage in the host. Low temperature at high altitudes may limit the occurrence of R. limacum.

Breeding system, shell size and age at sexual maturity affect sperm length in stylommatophoran gastropods

BackgroundSperm size and quality are key factors for fertilization success. There is increasing empirical evidence demonstrating that sperm form and function are influenced by selective pressures. Theoretical models predict that sperm competition could favour the evolution of longer sperm. In hermaphrodites, self-fertilizing species are expected to have shorter sperm than cross-fertilizing species, which use sperm stored from several mating partners for the fertilization of their eggs and thus are exposed to intense sperm competition. We tested this hypothesis by comparing original data on sperm length in 57 species of simultaneously hermaphroditic stylommatophoran gastropods from Europe and South America with respect to the species’ breeding system. We used 28S rRNA nuclear and COI mitochondrial sequence data to construct a molecular phylogeny. Phylogenetic generalized linear models were applied to examine the potential influence of morphological and life-history characters.ResultsThe best-fit model revealed that the breeding system and age at sexual maturity influence sperm length in gastropods. In general, species with predominant cross-fertilization had longer sperm than species with predominant self-fertilization or a mixed breeding system. Across species with shells (snails), sperm length also increased with shell size.ConclusionsOur study provides evidence that sperm length in stylommatophoran gastropods is influenced by the risk of sperm competition, as well as by age at sexual maturity and shell size. This finding extends present knowledge of sperm evolution to a group of so far poorly studied simultaneous hermaphrodites.