Population structure of the Spanish sardine Sardinella aurita: natural morphological variation in a genetically homogeneous population

Abstract

The population structure of the Spanish sardine Sardirella aurita in the coastal waters of Florida, USA, was examined using protein electrophoresis (one sample from Charleston, South Carolina, was analyzed as a geographic outlier), morphometrics, and meristics. Electrophoresis of proteins coded by 37 presumed genetic loci revealed low levels of genetic variation, little allele frequency variation among samples, and low genetic distances between samples. Gene flow was high and effectively homogenized genetic variation among sample locations, indicating that a single, panmictic population of Spanish sardines exists at least from South Carolina to the Florida panhandle. Size-corrected principal-components analyses performed on the morphometric data (collected in the form of a truss network) revealed regional allometric patterns. These patterns were most apparent in small fish and less obvious in larger fish, implying that the regional morphological patterns may diminish as the fish grow. Regressions of gill-raker number on fork length demonstrated regional patterns similar to those seen in the morphometric analyses. The absence of genetic evidence for geographic populational structuring and the apparent ontogenetic plasticity of body shape suggests that the morphological variation may be ecophenotypic. Alternatively, the electrophoretic analyses may not have detected substructuring that exists. Both the morphometric and meristic data tended to group samples collected from embayments separately from samples collected from more oceanic environments, implying that proximity to embayments may influence some of the observed morphological variation. It appears that for Spanish sardines, as for other clupeids, protein electrophoresis is useful in determining the evolutionary patterns of population structuring, and morphological analyses are of merit in studying short-term, environmentally induced variation.