Ecophenotypic variation in the Flat Tree Oyster, Isognomon alatus (Bivalvia: Isognomonidae), across a tidal microhabitat gradient

Abstract

Isognomon alatus is a sessile intertidal bivalve species that attaches to hard substrata. Within the Florida Keys, where tidal ranges are usually less than 1 m, settlement sites only a few centimetres apart experience drastically different tidal microhabitats. These varying conditions may create morphological gradients that can complicate taxonomic designations. To quantify and characterize the conchological effects of this microhabitat gradient and estimate the degree to which these effects can confuse species identification, I. alatus was studied in relation to tidal position using geometric morphometrics. Isognomon alatus was only found at positions 0.4 m below the highest tide mark, positions exposed to air less than 80% of the time. Below this limit specimen size, immersion time, and area available all covary. Shape differences due to ontogeny were removed using regression and the remaining phenotypic variation within these specimens is significantly correlated with the measured habitat conditions, immersion time being the more robust predictor. The observed phenotype effects are small but can influence species identification. Species assignment tests between I. alatus and the morphologically similar Indo-Pacific I. ephippium ranged in accuracy from 98 to 90% across microhabitat conditions.