ULTRA-ELONGATE FRESHWATER BIVALVES: AN EXAMPLE OF MORPHOLOGIC CONVERGENCE NOT REFLECTING ECOLOGIC SIMILARITY

Freshwater pearly mussels (Unionoida) represent the most diverse freshwater bivalve clade, with 6 families, 161 genera and 840 valid nominal species extant globally. Although reproductive strategy, soft anatomy, and shell morphology of unionoids are strikingly divergent from marine bivalves (and other freshwater clades), convergence of shell form with various marine taxa occurs. Extremely elongate unionoid species (length/height ratios > 3.0, here called ultra-elongate taxa) are an example of such putative convergence, as these taxa have been compared to marine razor clams such as Solen, which are infaunal suspension feeders capable of rapid burrowing in soft sediment. Ultra-elongate unionoids also represent multiple convergent episodes within their clade, and are distributed across 4 families (Unionidae in East Asia, Hyriidae in Australia, Iridinidae in Africa, and Mycetopodidae in South America).

The environmental range and ecologic context of these taxa, however, differs among unionoid families and from marine razor clams. The data of Savazzi and Yao (1992: Lethaia. Vol. 25, pp. 195-209) for four unionoid genera in China document a variety of burrowing methods and speeds, and environmental distributions ranging from anoxic lake sediments to well-oxygenated fluvial environments, although most species examined positioned themselves at similarly low angles (<30°) relative to the sediment/water interface. In contrast, Mycetopoda and Mycetopodella represent two ultra-elongate genera in western Amazonia. These taxa typically occur sympatrically in cutbank exposures of semilithified Neogene fluvial deposits in tributaries of the Amazon River in southeastern Peru. Members of both genera excavate permanent nearly vertical dwelling burrows below the average dry-season water line but well above the sandy bedload. In particular, Mycetopoda constructs a smooth-sided burrow (slightly wider and ~1.5 times longer than its body), which it can retreat into and anchor itself within using rapid pedal muscle retraction and simultaneous expansion of its large bulbous foot. In conclusion, general convergence of form between unionoids and marine taxa or even within unionoids does not necessarily indicate convergence of environmental and ecological characteristics.