On the differences between the two ‘indicator’ species of chaetognath,Sagitta setosaandS. elegans

The suggestion that the two chaetognath species Sagitta setosa and S. elegans were ‘indicators’ of different water masses was first made by Meek (1928), working off Northumberland, and subsequently developed by Russell (1933, 1935, 1939) in his classical work on their distribution as indicators of ‘Channel’ and ‘Western’ water off Plymouth. The alternation in dominance of the two species during the well-known changes in the Western Channel off Plymouth during the past 60 years (which have been termed the ‘Russell’ cycle; Cushing & Dickson, 1976) have most recently been reviewed and discussed by Southward (1984).