Assessing Morphological Variability in Silversides along the Mid-Atlantic Bight

Abstract

Inland silversides (Menidia beryllina) and Atlantic silversides (Menidia menidia) are ubiquitous fishes in the estuaries on the east coast of the United States. These species are very similar in appearance and may share many morphological traits. Many different biological processes have the potential to produce dissimilarities in shape between individuals or their characteristics and such shape distinctions can be indicative of varying selective pressures. Morphometrics, a branch of mathematical shape analysis, is a quantitative approach to understanding the diverse causes of variation and morphological transformation. In this study, I analyzed conspecific silverside populations to test for the presence of potential morphological variance within and between species through geometric analysis of landmark coordinates. Preserved specimens were pinned on dissection trays and photographed. These photographs were imported into tpsDIG2, a computer program for digitizing landmarks and outlines for geometric morphometric analyses. Eight landmarks were plotted on each fish and superimposed using the Generalized Procrustes Analysis (GPA) method. Two covariance functions were employed in data analysis, Procrustes ANOVA/regression for shape data (procD.lm) and pairwise group comparisons (pairwiseD.test). Significant differences were observed between the Atlantic silversides from Maryland and all other species specific location groups. Maryland populations in reference to North Carolina populations exhibited the greatest morphological differences. Most variation was observed in the first and second dorsal fins, the pelvic fin, and the anal fin although it varied by location. The selective pressures and environmental conditions of these locations must be studied further to determine the primary driver(s) of this observed morphological variation.