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

Abstract

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.

Correlates of snail shell variation along a unidirectional freshwater gradient in Lithasia geniculata (Haldeman, 1840) (Caenogastropoda: Pleuroceridae) from the Duck River, Tennessee, USA

“…In Duck River L. geniculata, we observed that smaller shells were narrower than larger shells, and that more globose shells were associated with greater discharge volumes and the presence of freshwater drum, river redhorse, and saddleback darter. Our allometric trend was opposite of that seen by Minton et al (2008), who suggested smaller shells were more globose. We feel this discrepancy may be due to sample size; we used 300 shells compared to over 1,100 shells used previously, and Minton et al (2008) used more shells of each morphotype, including many that were intermediate between forms.…”

“…Our allometric trend was opposite of that seen by Minton et al (2008), who suggested smaller shells were more globose. We feel this discrepancy may be due to sample size; we used 300 shells compared to over 1,100 shells used previously, and Minton et al (2008) used more shells of each morphotype, including many that were intermediate between forms. We feel that, taken together, the data suggest that allometry across different morphotypes may not be meaningful relative to analysing each form separately.…”

“…One species, Lithasia geniculata (Haldeman, 1840) from the Duck River in Tennessee, exhibits shell plasticity in an upstream-downstream fashion. Multiple authors have confirmed that L. geniculata has four diagnosable shell forms; once treated as nominal species, they represent one taxon exhibiting continuous variation throughout the river (davis 1974, stein & stansBeRy 1984, Minton & lydeaRd 2003, Minton et al 2008. The smooth more globose-shelled L. geniculata form pinguis occupies the headwaters and is replaced by the more conical duttoniana and fuliginosa forms mid-river.…”

“…stein & stansBeRy (1984) additionally showed gene flow between populations in the river, suggesting L. geniculata may exhibit polyphenic variation (steaRns 1989). Minton et al (2008) were the first to quantify shell shape plasticity in Duck River L. geniculata. They showed that shells displayed continuous variation from upstream to downstream, becoming more globose and obese in lower reaches.…”

Efeito do fotoperíodo sobre o ciclo de vida e a morfometria da concha de Dysopeas muibum (Mollusca, Subulinidae) em condições de laboratório

“…Os moluscos subulinídeos, como D. muibum, foram, em grande maioria, descritos com base apenas na morfologia da concha, porém nem sempre isso é possível devido às semelhanças entre espécies (choh et al, 2006) e variações entre populações de uma mesma espécie (caLDeira et al, 2000;minton et al, 2008). Estudos de variações nas medidas da concha podem resultar na constatação de diferentes padrões morfométricos, permitindo inferir sobre características do animal, como forma do corpo e padrão de crescimento (chiu et al, 2002).…”