Russell L. Minton

A review of freshwater gastropod conservation: challenges and opportunities

Abstract North American freshwater gastropods remain an understudied, yet critically imperiled, fauna. As part of a larger discussion on freshwater mollusks in this special issue, we review 4 specific areas of concern regarding freshwater gastropods and discuss how best to address those concerns in the context of conservation. Areas of concern include freshwater gastropod conservation strategies, taxonomy and systematics, ecological research, and conservation challenges. We illustrate how each of these topics relates to conservation efforts and discuss opportunities to improve our baseline knowledge of freshwater gastropod taxonomy, ecology, and conservation. We emphasize throughout that effective conservation strategies require the participation of as many affected and interested groups, from local communities to governmental agencies, as possible for successful implementation and management. We offer suggestions for the direction of cooperative conservation with regard to freshwater gastropods.

Prodigious polyphyly in imperilled freshwater pearly-mussels (Bivalvia: Unionidae): a phylogenetic test of species and generic designations

Abstract Unionid bivalves or freshwater pearly-mussels (Unionoidea: Unionidae) serve as an exemplary system for examining many of the problems facing systematists and conservation biologists today. Most of the species and genera were described in the late 1800s and early 1900s, but few phylogenetic studies have been conducted to test conventional views of species and classification. Pearly-mussels of Gulf Coastal drainages of the southeastern United States from the Escambia (southern Alabama to Florida) to the Suwannee Rivers (Florida) are a unique fauna comprised of approximately 100 species, with about 30 endemic to the region. In this study, mitochondrial cytochrome c oxidase subunit I and 16S rRNA gene sequences were used to test the monophyly and to estimate evolutionary relationships of five unionid species representing three different genera. The molecular phylogenies depict all three genera as polyphyletic. The prodigious polyphyly exhibited within unionids is due to incorrect notions of homology and false assumptions about missing anatomical data. In contrast, the molecular phylogeny provides evidence to support the recognition of all five unionid species as distinct evolutionary entities. Furthermore, molecular genealogical evidence supports the elevation of Quincuncina infucata (Conrad) of the Suwannee River to species level, for which Q. kleiniana (Lea) is available.

Quantifying phenotypic gradients in freshwater snails: a case study in Lithasia (Gastropoda: Pleuroceridae)

Many authors have described a pattern of morphological variation in freshwater bivalves where shells taken from lentic and lotic environments, or headwaters and main stem reaches, appear to exhibit phenotypic gradients in size and shape. For example, mussels taken from headwater reaches tend to possess smooth, less inflated shells compared to the more obese, sculptured individuals downstream. Others observed similar relationships in certain freshwater gastropods, but this variation has not been quantified nor its existence explained in an ecological or evolutionary context. Geometric morphometrics indicated freshwater snails shells from the pleurocerid genus Lithasia from the Duck River, Tennessee, USA, show phenotypic gradients similar to those in freshwater mussels. Shells from upstream areas were narrow and less sculptured on the posterior portions of their body whorls, while downstream shells were more inflated and possessed significantly more sculpture. This phenotypic variation may reduce predation or damage due to dislodging. The nature of the observed plasticity suggests an unidirectional environment similar to that proposed by the river continuum concept.

Practical applications for systematics and taxonomy in North American freshwater gastropod conservation

Abstract The roles of systematics in the field of conservation biology are well understood and accepted for many organisms. However, the role of systematics and taxonomy has not been reviewed in the context of species protection and management of freshwater gastropods. We provide a thorough review of the relevant theoretical literature in systematics and taxonomy and illustrate with recent examples of species delineation and taxonomy in North American freshwater gastropods that these fields play key roles in the practical designation of conservation management units. We summarize some aspects of the biology of freshwater gastropods that can confound taxonomic and management efforts. Based on our review, we recommend that effective conservation plans include the systematic research necessary to recognize unique organismal lineages as primary conservation management units. This strategy must be combined with consistent and rigorous nomenclature, taxonomy, and dissemination of research findings so that all parties have access to the highest quality information.

The genome of the nematode Pristionchus pacificus encodes putative homologs of RXR/Usp and EcR.

Ecdysteroid signaling is an important regulator of arthropod development and reproduction. However, the role of ecdysteroid signaling in another Ecdysozoan animal, the nematode, remains unclear. We report here the identification, cloning, and temporal expression of genes encoding putative homologs of the two nuclear receptor components of the ecdysone receptor, RXR/Usp (NR2B) and EcR (NR1H), in the nematode Pristionchus pacificus. The P. pacificus genes Ppa-pnhr-1 and Ppa-pnhr-2 encode nuclear receptors with strong sequence similarity to RXR/Usp and EcR, respectively. Maximum likelihood analysis incorporating both DNA-binding and ligand-binding domains places the two proteins in the NR2B and NR1H groups with strong bootstrap support. RT-PCR analysis reveals that both Ppa-pnhr-1 and Ppa-pnhr-2 are expressed during larval development and that Ppa-pnhr-1 expression oscillates with the molting cycle. The identification of a putative ecdysone receptor in a nematode amenable to genetic analysis provides a powerful system to investigate the function and evolution of ecdysone receptor signaling in the Nematoda.

Elimia Comalensis (Gastropoda: Pleuroceridae) from the Edwards Plateau, Texas: Multiple Unrecognized Endemics or Native Exotic?

ABSTRACT This research explored the distribution, morphological variation, and molecular systematics of Elimia comalensis (Gastropoda: Pleuroceridae) using geometric morphometrics (n  =  565) and mitochondrial DNA sequences (n  =  15). Elimia comalensis was originally documented as endemic to Comal Springs, Comal County, Texas, but recent collections found this species in multiple springs and drainages in Central Texas. Morphometric analyses showed a high amount of morphological overlap with no clear geographic patterning. Phylogenetic analysis of mt COI sequence data indicated that E. comalensis represents a single species, with no genetic divergence among isolated populations. We conclude that E. comalensis may be an unrecognized native exotic, a species endemic to one area that has been spread by humans and assumed to be part of the natural fauna.

Molecular and morphological divergence in the inland silverside (Menidia beryllina) along a freshwater-estuarine interface

Populations that vary across ecological gradients or that have invaded novel habitats are important to elucidate the association between adaptive divergence and gene flow, factors that may play an important role in speciation of silverside fishes. The inland silverside, Menidia beryllina, is an ideal organism for this kind of research, displaying a great diversity in morphology among freshwater and coastal brackish habitats. Using a combination of geometric morphometrics and mitochondrial (mt) DNA, we investigated patterns of variation within and among the nominal freshwater Menidia audens and coastal M. beryllina, spanning the transition from freshwater to tidally influenced semi-brackish waters of the lower Mississippi River to brackish waters of the Lake Pontchartrain estuary. Although we found no evidence for a phylogenetic split between M. audens and M. beryllina, our results indicate that significant genetic divergence corresponds with body shape differences among the two, with a clear distinction at the interface of freshwater and brackish water. Patterns in mtDNA suggest that freshwater populations referred to as M. audens are of recent origin with evidence for habitat-based divergence compared to coastal populations referred to as M. beryllina. Our findings add to a growing body of evidence that ecological shifts, following colonization of novel habitats, may promote rapid adaptive divergence and reduced gene flow among silverside populations in adjacent environmental regimes.

Molecular phylogeny, evolution of shell shape, and DNA barcoding in Polygyridae (Gastropoda: Pulmonata), an endemic North American clade of land snails

Abstract: A hypothesis of relationships among subfamilies, tribes, genera, and species of Polygyridae was established by Ken Emberton in 1995, using shell, behavioral, allozyme, and soft anatomical characters. We tested this hypothesis using four mitochondrial and two nuclear loci. We present data from 418 polygyrid individuals sequenced for one to six loci, including 110 named species (out of 294 nominal taxa) from 21 of the 24 recognized genera. We carried out phylogenetic and DNA barcoding analyses to examine relationships at the family, genus, and species-level. In our analyses, the subfamilies are not supported as monophyletic groups. The tribes Mesodontini, Ashmunellini and Vespericolini were recovered as monophyletic, while all other tribes were paraphyletic. Regardless of analysis method, we found a close, well-supported relationship between Mesodontini and Triodopsini, two tribes that were distantly related in Embertons hypothesis. Most genera were recovered as monophyletic with the notable exceptions of Cryptomastix Pilsbry, 1839, Mesodon Rafinesque in Férussac, 1821, and Neohelix von Ihering, 1892. Of the species for which we had multiple individuals, populations of 27 formed monophyletic groups on our phylogenies, while 47 did not, indicating an urgent need for revisionary taxonomy at all levels of classification in this family.

CHANGES IN SHELL MORPHOLOGY OF ELIMIA COMALENSIS (GASTROPODA: PLEUROCERIDAE) FROM THE EDWARDS PLATEAU, TEXAS

Abstract Species in the pleurocerid genus Elimia are important components of freshwater systems in the eastern United States, but little is known about their natural history. Using dual-simultaneous linear regression and principal components analyses, we show patterns of morphological change in two populations of E. comalensis from the Edwards Plateau, Texas. Both populations had similar rates of change for all shell measurements analyzed, and both showed similar decreases in growth as total length of shell increased. However, the size at which growth slowed was different between populations, and small shells tended to have a consistent shape, while larger shells were more variable.

Diversity and distribution of freshwater gastropods in the Bayou Bartholomew drainage, Arkansas, U.S.A.

Abstract: Bayou Bartholomew is a low-gradient river system that drains much of southeastern Arkansas and northeastern Louisiana, U.S.A. As one of the few southeastern streams remaining un-impounded, the Arkansas reach of the bayou harbors a rich freshwater molluscan fauna. Collecting efforts have historically focused on documenting freshwater mussel and fish diversity, and there was no prior survey focusing on freshwater gastropods. This survey of the drainage yielded 13 gastropod species representing three orders and seven genera. Pulmonates were most abundant in low-order reaches of the drainage, while gill-breathing snails dominated higher-order reaches. Co-occurrence analyses indicated that pulmonates occurred significantly more often with other pulmonates than they did with gill-breathers; this trend was also observed in gill-breathers. Both stream order and predominant substrate influenced species richness and abundance. Our findings were consistent with other published studies on freshwater snail distribution but may be confounded by drought conditions experienced during the survey.