Presence or absence of carinae in closely related populations of Leptoxis ampla (Anthony, 1855) (Gastropoda: Cerithioidea: Pleuroceridae) is not the result of ecophenotypic plasticity

Abstract

Ecophenotypic plasticity, or nonheritable variation of the phenotype with local conditions, has been invoked in order to partly explain conchological differences within freshwater snail species as responses to fluvial conditions and stream size (Adams, 1915; Lam & Calow, 1988; Minton, Norwood & Hayes, 2008; Minton et al., 2011), river substrate (Urabe, 1998, 2000) and presence or absence of predators (Holomuzki & Biggs, 2006; Hoverman & Relyea, 2007; Lakowitz, Bronmark & Nystrom, 2008). In the freshwater gastropod family Pleuroceridae, Dillon (2011) and Dillon & Robinson (2011) used similarities in allozyme loci to conclude that conchological differences (e.g. presence or absence of carinae, slenderness or robustness of shells) in multiple species of Pleurocera and Elimia were attributable to intraspecific ecophenotypic plasticity. However, no analysis of shell morphology was performed by Dillon (2011) or Dillon & Robinson (2011) to test for either an ecophenotypic or genetic component of the observed variation. In order to understand the effects of ecophenotypic plasticity on the presence or absence of carinae on the shell of one species of Pleuroceridae, we cultured two populations of Leptoxis ampla (Anthony, 1855), one with a carinate shell morphology and one with a smooth shell morphology (Fig. 1). In culture, these snails were exposed to uniform environmental conditions. Therefore, observed morphological differences among juveniles raised from egg to adult were not affected by the environment. Culturing methods are ideal for analysing variation in shell morphology in the Pleuroceridae because not only can environmental conditions be controlled, but complications arising from translocation experiments (such as transfer of disease vectors and the potential for mating with native populations) are avoided. Although culturing methods have not previously been used for studying ecophenotypic plasticity in Pleuroceridae, they have been successfully applied to other freshwater gastropod families such as Planorbidae (Hoverman & Relyea, 2007) and Lymnaeidae (Lam & Calow, 1988; Lakowitz et al., 2008). Leptoxis ampla is a federally threatened pleurocerid restricted to the upper Cahaba River and certain tributaries in central Alabama. Throughout most of its range the shell morphology is smooth (Fig. 1), but one population from the lower Little Cahaba River in Bibb County, AL, USA, possesses a distinct shell phenotype characterized by 3–4 carinae on the upper part of each whorl (Fig. 1). Based on analyses of L. ampla populations cultured in a uniform environment we show that the presence or absence of this discrete character is the result of heritable genetic differences rather than ecophenotypic plasticity. The carinate form of L. ampla from the Little Cahaba River was described as L. mimica (Goodrich, 1922) based on the presence of its distinctive carinae (Fig. 1). This was later synonymized with L. ampla (Fig. 1), because of similarities in overall shell morphology (Burch & Tottenham, 1980). Populations above and below the confluence of the Little Cahaba River and Cahaba River possess a smooth shell morphology. The carinate form was historically found in both the Little Cahaba River and Six Mile Creek in Bibb County, AL, USA (Goodrich, 1922), but recent survey work indicates that it is now restricted to the lower Little Cahaba River (unpublished ADCNR survey data). The morphological pattern seen in L. ampla is not an example of clinal variation as in, for example, Lithasia geniculata (Minton et al., 2008) or Io fluvialis (Adams, 1915), since there is no intergradation in morphology and the carinate form is restricted to a single tributary near the centre of the species’ historical range. In mid-March 2010, we collected 45 adult individuals of L. ampla from the Cahaba River with smooth shells and 45 individuals from the Little Cahaba River with carinate shells and placed them into artificial culture systems at the Alabama Aquatic Biodiversity Center (AABC) in Marion, AL, USA (see Table 1 for locality information). No differences in body pigmentation or shape of egg clutches were observed between populations. Fifteen snails (three replicates per population) were placed in 20-l acrylic tanks equipped with 0.84-cm bulkhead fittings allowing constant exchange of well water acclimated to ambient outdoor temperature (c. 8 l/h). Each 20-l culture tank had water from a shared source, but independent aeration and drainage. A 15 l/min powerhead attached to each tank lid created a constant flow regime. Females in all treatments attached eggs to tank walls in concentric clutches from late March to early June 2010. Within 90 d of hatching, cultured juveniles from each population showed the same phenotype as their parental population