The Distribution of Gammarus Species in Estuaries. Part I

Abstract

The work of which an account is here given was largely carried out from the autumn of 1937 to the spring of 1940, when it was interrupted by the war. In taking it up again (in July 1945) while, facilities for field work are still limited, the author feels it useful to publish results as they stand and indicate where further work is considered advisable.In examinations of the free-swimming bottom fauna of the Tamar and other estuaries, attention was inevitably drawn to the populations of Gammarus species, which make up the greater bulk of it. Before quantitative observations were planned, some interesting points came to light with regard to the qualitative composition of populations. This aspect lent itself more readily to study and, though byno means a new field for exploration, soon proved worth examining ingreater detail than previous workers had attempted.The broad fact of a replacement of one Gammarus species by another in passing up an estuary was well enough known, though exact knowledge for the British Isles only starts from the time when G. zaddachi Sexton was recognized as a regular member of the upper estuarine fauna of the Tay (Bassindale, 1933; Alexander, Southgate & Bassindale, 1935) and of the Deben (Serventy, 1935). This species proved to be the main brackish-water species overlapping with the marine G. locusta (L.) near the seaward end, and with the fresh-water G. pulex (L.) at the river end (or ‘head’) of the estuary. The status in estuaries of two other brackish-water species, G. duebeni Lillj. and G. chevreuxi Sexton, remained obscure.