Ultra-elongate Freshwater Pearly Mussels (Unionida): Roles for Function and Constraint in Multiple Morphologic Convergences with Marine Taxa

Abstract

Morphologic convergence may arise because natural selection produces an optimal solution for a given set of environmental conditions or because constructional and historical constraints limit available variation, making certain morphologies inevitable. Shell shape in bivalves typically is interpreted as functional, with emphasis placed on substrate preferences and life habits. Freshwater pearly mussels (Order Unionida) represent the most diverse freshwater bivalve clade and, although their life history and related morphologic traits are strikingly divergent from marine bivalves as well as other freshwater bivalve clades, multiple convergences in shell form within and among these groups occur. Ultra-elongate shells (length/height ratios > 3.0) in both marine and unionoid taxa are one such example. At least 13 families, including 4 phylogenetically defined unionoid families, have ultra-elongate representatives. These taxa occur in substrates ranging from soft sediments to hard grounds and a variety of life habits including nonmotile semi-infaunal, active burrowers, and borers; which seems to imply weak functional/adaptive control on morphology. For many of these taxa, however, this shape may reflect functional forces related to direct substrate penetration without major anterior/posterior rotation of the shell, rather than the type of substrate penetrated. Further, shell elongation is achieved through a variety of differential growth patterns, which argues against a strong role for constructional or historical constraint. Clarifying the meaning of a modern analog or proxy is critical for evaluating paleoenvironmental and paleoecological interpretations of extinct ultra-elongate bivalve taxa as well as for informing efforts to protect and restore extinction-vulnerable extant populations.