INVESTIGATING HOW ONTOGENY AND ENVIRONMENT INFLUENCE MORPHOLOGICAL VARIATION WITHIN SPECIES OF THE BIVALVE GENUS CUCULLAEA

Throughout its 200 million-year history, the bivalve genus Cucullaea has remained morphologically conservative at a synoptic level, despite exhibiting significant temporal variation in its biogeographic and environmental distribution. However, individual species often exhibit significant variation, which has had an impact on species-level taxonomic classifications and the evolutionary inferences drawn from them. A major goal of this research is to address two important causes of morphological variation within Cucullaea species: ontogenetic changes exhibited by individuals during their lifetimes, and variations among individuals as a function of environment. To accomplish these objectives museum type and bulk collections for well-preserved populations of various cucullaeid species are being digitized, allowing these morphological relationships to be investigated in detail.

Morphometric techniques will be used to quantify differences in shell shape, as well as the positions of internal homologous features, which are divided between those that are morphologically fixed and those that are more easily altered in response to environmental changes. Shell features relevant to examining cucullaeid morphology include: i) axial and concentric ribbing, one dominating much more than the other in some species; ii) ventral crenulations, serving to buffer the shell against stress parallel to the plane of commisure, iii) the posterior muscle scar and associated flange, serving a function similar to ventral crenulations and important in defining the genus; and iv) hinge plate and teeth, perhaps analyzed separately from other features because of the amount of internal complexity and variation seen within this area. In addition, features such as posterior flattening of the shell margin and the presence of a byssal notch are being investigated as evidence for semi-infaunal life positions of individuals during juvenile and adult stages.

By understanding the causes and degree of morphological variation seen within species of Cucullaea, variations between species and the features that define these taxonomic units may be defined more clearly, permitting their assessment within a biogeographic, ontogenetic and environmental framework for an improved understanding of Cucullaea's evolutionary history.