Variation in density, size, and morphology of the pulmonate limpet Siphonaria lessonii along the Southwestern Atlantic

The density, size, and morphology of coastal invertebrate species often predictably vary with latitude. In this study we evaluate whether the density, size, and morphology of the pulmonate limpet, Siphonaria lessonii, in the Southwestern Atlantic varies in agreement to the predictions of the abundance-centre hypothesis (i.e., higher densities by the center of the species range), Bergmann's rule (i.e., increasing body size with latitude), and Allen's rule (i.e., decreasing body surface to volume ratio with latitude). Our results indicate that the upper density bounds of S. lessonii decrease with latitude across our study range, which may either contradict expectations of the abundant-centre model (i.e, if assuming a Temperate Southwestern Atlantic/Magellanic distribution; 27°S in the Atlantic to 42°S in the Pacific) or represent a partial trend within a broader scale, abundant-centre pattern (i.e., when considering a panmictic distribution from the Temperate Southwestern Atlantic to the Temperate Southeastern Pacific; 27°S in the Atlantic to 5°S in the Pacific). Limpet shell length (i.e., a surrogate of body size) was not significantly related to latitude (cf. Bergmann's rule). The two principal components of morphological variation in limpet shells – which increased with decreasing shell surface to volume ratio – showed positive and negative relationships with latitude (i.e., contradictory support to Allen's rule). From these analyses, we conclude (a) that the size and morphology of S. lessonii are primarily controlled by local conditions rather than broad-scale climatic or environmental gradients and, (b) that geographic marginality does not means reduced performance in this species at least as it concerns to abundance and maximum size.