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.

Keywords:

• Geometric morphometrics
• multivariate regression
• ontogeny
• phenotype
• shell morphology

Published in collaboration with the University of Bergen and the Institute of Marine Research, Norway, and the Marine Biological Laboratory, University of Copenhagen, Denmark

Published in collaboration with the University of Bergen and the Institute of Marine Research, Norway, and the Marine Biological Laboratory, University of Copenhagen, Denmark

Acknowledgements

Partial funding for this project was provided by the U.S. National Science Foundation PEET program (DEB-9978119 to RB and Paula M. Mikkelsen). Research on this chosen exemplar species in the Bivalve Tree-of-Life project (www.bivatol.org) was supported by the U.S. National Science Foundation AToL program (DEB-0732854/0732903/0732860). Field collecting was partially supported by FMNH Department of Zoology's Marshall Field Fund, with Petra Sierwald (FMNH) helping RB during sampling. Roy Plotnick (UIC) advised on statistical methods. Lab space and computing facilities for JW were supplied by Mary Ashley (UIC), who also provided, together with the Ashley lab research group, suggestions and advice on the manuscript. Undergraduate laboratory assistance was rendered by Adam Blufield. The constructive reviews of the manuscript by Fred Wells and an anonymous reviewer are greatly appreciated.

Notes

Published in collaboration with the University of Bergen and the Institute of Marine Research, Norway, and the Marine Biological Laboratory, University of Copenhagen, Denmark