Phenotypic variability in Sargassum polyceratium (Fucales, Phaeophyta)

Abstract

Local-scale population differentiation of Sargassum polyceratium Montagne from the Content Keys, Florida, was evaluated. Forty-seven phenotypes were recognized based on blade features. The most common phenotype had lanceolate, often undulate, blades with serrated margins. Plants with flat or curly ovate blades or flat linear blades were common. Other phenotypes were rare (< 2% of population). Blade shape occurred over a continuum, ranging from linear to lanceolate to ovate. Intraspecific differences in blade features were extensive; otherwise, morphological traits were usually similar. Unlike previous descriptions of intraspecific variation, divergent morphologies were intermixed, occurring at distances of a few centimeters. There was no evidence to indicate that short-term environmental effects or hybridization between species were responsible for the considerable morphological variation in this population. Furthermore, the failure to recognize the range of phenotypic variability in the genus Sargassum may result in taxonomic inconsistencies when few specimens are collected, when morphological features cross perceived intraspecific boundaries, when phenotypes display different developmental rates, or when species descriptions originate from a single specimen or population. Designating species or intraspecific taxa should be avoided without a complete appreciation of their morphological variability in nature.