Aspects of freshwater mollusc ecological biogeography

Abstract

Freshwater molluscs include a variety of groups derived independently from marine ancestry, and varying in range of habitat, reproduction, and distribution. The fossil record shows the great age of most families, that likely differentiated in the Paleozoic. Present-day distributions of species and higher groups are rationalized as due to different ranges of habitat, life history, and means of dispersion, with a strong historical component. At the global level distribution of 42 principal families of freshwater and marginally marine molluscs is summarized. Most are widespread, the others representing only a few types of distributiion: groups characteristic of the southern continents, Tethyan, or localized. Localized families are mostly prosobranchs, and are concentrated in areas otherwise noted for endemism. The Atlantic and Indo-Pacific Oceans have vastly different relationss to the ranges of freshwater molluscs. The Atlantic cuts across genera and sets of genera, as if it came into being after the evolution of those groups. The Pacific is not so crossed, but bordered with primitive and local families, as if development of the Pacific basin were earlier and accompanied origin of living families of pulmonates. The fossil record in North America is reviewed briefly. Already in the Cretaceous a faunistic division is recognizable. Some groups in the west are related to those of east Asia, others in the east are of trans-Atlantic relationships. Withdrawal of the epeiric sea from the mid-continent exposed what can be viewed as an enormous land bridge linking different faunistic regions. These regions retain distinctiveness today: no large scale faunal migrations have occurred. Supposed migrations to South American and Australia are therefore dismissed. Freshwater mussels have often been employed in zoogeographic studies. The region west of the Rocky Mountains in North America has a sparse fauna of Asiatic affinities. Throughout all of Cenozoic time these features have been characteristic, emphasizing the hisorical value of mussel distribution. The historical roots of some zoogeographic thoughts are traced back to Charles Lyell, who introduced uniformitarianism in geology. The principle that the distribution of different groups of freshwater molluscs is carried out in different ways, determined by range of habitat and life history, is uniformitarian. It is based on the action of presently observable processes, and on their result over long periods of time. Attention to details of modern ranges, and recognition of polytypic and vicariant groups, is in full keeping with Lyell's urging “patiently to explore the realities of the present”. Lyell's belief in a center of origin of species captivated the mind of Charles Darwin. Believing in evolution, he nevertheless was compelled to deal with geographic distribution in a Lyellian, non-evolutionary way. Efforts to deal with animal geography by numbers of immigrants, flying birds, moving plates, and floating rafts go back beyond Darwin to Lyell's special creation. The uniformitarian interpretation of geographic distribution is linked causally to the uniformitarian approach of geology. Both are corollaries of a more general principle: earth and life evolve together. This also provides the premises for interpretations of the past.