Systematics and geographical distribution of Galba species, a group of cryptic and worldwide freshwater snails

Abstract

Cryptic species can present a significant challenge to the application of systematic and biogeographic principles, especially if they are invasive or transmit parasites or pathogens. Detecting cryptic species requires a pluralistic approach in which molecular markers facilitate the detection of coherent taxonomic units that can then be analyzed using various traits (e.g., internal morphology) and crosses. In asexual or self-fertilizing species, the latter criteria are of limited use. We studied a group of cryptic freshwater snails (genus Galba) from the family Lymnaeidae that have invaded almost all continents, reproducing mainly by self-fertilization and transmitting liver flukes to humans and livestock. We aim to clarify the systematics, distribution, and phylogeny of these species with an integrative approach that includes morphology, molecular markers, wide-scale sampling across America, and data retrieved from GenBank (to include Old World samples). Our phylogenetic analysis suggests that the genus Galba originated ca. 22 Myr ago and today comprises six species or species complexes. Four of them show an elongated-shell cryptic phenotype and exhibit wide variation in their genetic diversity, geographic distribution, and invasiveness. The remaining two species have more geographically restricted distributions and exhibit a globose-shell cryptic phenotype, most likely phylogenetically derived from the elongated one. We emphasize that no Galba species should be identified without molecular markers. We also discuss several hypotheses that can explain the origin of cryptic species in Galba, such as convergence and morphological stasis.

An outbreak of canine schistosomiasis in Utah: Acquisition of a new snail host (Galba humilis) by Heterobilharzia americana, a pathogenic parasite on the move

“…Snails provisionally identified as Galba humilis were confirmed as such based on comparisons with cox1 sequences from several relevant lymnaeid taxa [ 13 ] and from our own collections ( Table 1 ). This comparison groups the Moab specimens with 13 other samples identified as G. humilis from different parts of North America ( Fig.…”

“…Galba humilis is one of at least 40 related subspecies and species of “fossarine” lymnaeids, all relatively small (<15 mm in shell height), living at or above the waterline but at times submersed as well [ 14 , 28 ]. Many of these taxa will eventually be synonymized [ 13 ]. Collectively their range covers much of North America, many Caribbean islands, and parts of Central and South America.…”

“…The only snail species thus far found to be naturally infected with H. americana [ 12 ] is the small amphibious lymnaeid Galba cubensis [ 13 ]. It ranges from Florida to Texas with populations known from Oklahoma, Georgia and the Carolinas.…”

Towards the comprehension of fasciolosis (re-)emergence: an integrative overview

“…G . truncatula and P. columella , are spread out of their native range (see Lounnas et al ., 2017; Alda et al ., 2020), definitely boosting fasciolosis transmission all over the world. For example, G. truncatula is known for transmitting F. hepatica at very high altitudes not only in the Andean Altiplano (Esteban et al ., 2002) but also in the highlands of southern Tanzania (Walker et al ., 2008) and in the alpine and subalpine ecosystems of the Pyrenees (Roldán et al ., 2020).…”

Molecular species delimitation refines the taxonomy of native and nonnative physinine snails in North America

“…Robustly defining species among gastropods has been a particular challenge. Many are small and the taxonomy has generally been based on characteristics we can see, i.e., shell or soft-part morphology, which have been shown to vary in response to environmental factors or exhibit extreme conservatism among evolutionary lineages, and thus be of uncertain value for diagnosing species 4 – 7 . Consequently, species hypotheses and their higher-order assignments to genera, families, and orders could generously be described as fluid, and even authorities attempting to categorize extant species have not reached consensus, e.g., contrast 8 with 9 .…”