The Fresh-water Invertebrate Fauna: Problems and Solutions for Evolutionary Success

Abstract

The volume of salt waters of the world is more than 10,000 times the highly fragmented volume of inhabitable fresh waters, yet the latter has come to support a rich and euryokous fauna. In making the salt-brackish-fresh transition, the fresh-water (carbonate) fauna has developed clusters of new adaptations. The vast majority of marine species are restricted to a narrow and monotonous chemical environment, but most freshwater species thrive in habitats exhibiting a wide (e.g., 10-fold) range of dissolved salts, featuring high carbonates, low sodium chloride, and low potassium. In comparison with their ancestral marine forms, the temperate fresh-water fauna is generally characterized by: (1) much more efficient osmoregulatory systems, (2) smaller body size, (3) lower reproductive potential, (4) loss of specializedlarval stages, (5) a wide variety of anabiotic devices and strategies, (6) well developed low temperature tolerances, (7) aestivation stages in adults, (8) a remarkable facility for overland geographical migration and ecesis by disseminules, (9) facility and adaptations for withstanding prolonged habitat silting, (10) adaptations to swift unidirectional currents by many lotic species, and (11) a more general ability to live anaerobically. Certain characters of the marine biota have usually been lost during transition and ecesis in the fresh-water environment, such as bioluminescence, bright body coloration, and distinctive bright color patterns. Morphological embellishments in the marine biota are common, including cirri, palps, setation, protuberances, and respiratory devices, but such structures are uncommon among fresh-water species. It is suggested that respiratory stratagems have been developed to an excessive and “unnecessary” degree among many marine forms, as well as in a few fresh-water insects and most Eubranchiopoda.