It is usually very difficult to obtain knowledge of predator-prey relationships in the marine plankton. However, because of the very specialized nature of this relationship, where the copepod spends part of its life as a parasite and part of its life as a predator, detailed understanding of the relationship has been obtained from both laboratory and field observations.

The usual concept of predation involves a predator which is the same size or larger than its prey. This prompted Foxton (1966), in discussion of the large salp Salpa thompsoni, to consider only the gut contents offish, sea birds, seals, etc., when analysing data on predation. He noted that considerable mortality occurred between the embryos and the adult stages which did not appear to be explained by the data on predators which he considered. He concluded that ‘further study is necessary before (the salp's) true role can be defined’.

Copepods and amphipods are often associated with salps in distribution (Hardy, 1936), usually being regarded as parasites (Furuhashi, 1966; Wolfenden, 1911). Amphipods which lay their eggs directly into the living tissue of salps have been described (Haruhiko, 1967), while the use of the salp test as a protective house by the amphipod Phronema is well known (Hardy, 1956).

Dense swarms of Thalia democratica occurring in S.E. Australian waters are often associated with high densities of Sapphirina copepods (Dakin & Colefax, 1940; Thompson, 1948; Sheard, 1965). Observations of these swarms underwater, by the author, show the presence of many Sapphirina copepods inside salps in the undisturbed state, as in Plate I (A).