ABSTRACT
The phylum Chaetognatha consists of some 52 species arranged in seven genera (Alvariño, 1965). Though fairly similar to one another, chaetognaths are phylogenetically isolated from other animals and their origin is obscure (Hyman, 1959). All species are either marine or estuarine (Alvariño, 1965) and are strictly carnivorous (Fig. 1). There are both benthic and planktonic chaetognaths, but it is the latter which are important in marine food webs. The benthic species, all of the genus Spadella, are small and minor constituents of their ecosystems. Interest in Spadella lies principally in the fact that they are easily maintained and studied in the laboratory (e.g., John, 1933; Parry, 1944; Feigenbaum, 1976) and can help us understand the behaviour and physiology of their more delicate planktonic relatives. In the plankton, chaetognaths are often second only to copepods in numerical abundance. By weight their contribution is even greater (Reeve, 1970a). Chaetognaths are food for a wide variety of larger organisms (Busch, 1851; Bigelow, 1924; David, 1955; Reeve, 1966; Reeve & Walter, 1972a; many others) and, therefore, occupy a central position in planktonic food webs. Some species are highly cannibalistic and function at more than one trophic level (Pearre, 1982).
