Distribution of Aquatic Oligochaetes

Abstract

About 700 limicolous and 100 true marine species are known so far. The passive ways of their distribution are more important than active migration. Watersheds and the current are the main mechanical barriers while salinity, oxygen content and temperature of the water are the ecological ones. Most of the species need hibernation in cold for successful sexual reproduction and therefore cannot colonize the torrid zone. Only 4.5% of the species (most of them antropochorous) are cosmopolitan while 72% are distributed in one region or subregion only, and 42% are known from a single locality. Nine of the 15 families surely originate from the northern temperate zone. Probably the class arose from its polychaete ancestors in marsh biotopes and differentiated during the dry and cold Perm period, which may be reflected in the thermophobe character of their sexual reproduction. After the dissociation of Pangaea, the ranges of families were distributed between the new continents. The evolution of some groups became quicker in the Pleistocene again.

The inland waters are divided by the author into 6 zoogeographical regions, some of them into subregions. There are some subfamilies, genera and separate species secondarily and independently adapted to marine life. An attempt is made to describe the changes in the limicolous oligochaete fauna of the northwestern part of the Soviet Union in the Holocene.

About 1,300 species of aquatic oligochaetes in the wider meaning of the word (including Aeolosomatidae and Branchiobdellidae) have been observed. Discarding amphibious forms and those casually fallen into water or those populating only the sea littoral (from among the Enchytraeidae and Megadrili), about 700 limicolous and 100 genuine marine species remain. During the recent decades, abundant data from all over the world became available on their distribution. Contours of regional peculiarities are appearing. Therefore it is time to sum up the data on the zoogeography of aquatic oligochaetes.

The material of the present report includes the distribution charts of all known aquatic oligochaetes drawn on the basis of data from literature and the samples collected by the author from the water bodies of the north-western part of the Soviet Union.