Seasonal Occurrence of a Branchiopod Crustacean, Limnadia stanleyana King (Conchostraca) in Eastern Australia

Abstract

Most species of the primitive orders of branchiopod Crustacea, the Anostraca, Notostraca and Conchostraca, occur only in temporary fresh-water pools. It was not known how these animals survive in this environment so the mechanisms that enable the species Limnadia stanleyana King to exist in the habitat were studied. L. stanleyana is a conchostracan that inhabits some temporary, rain-water pools which form in depressions on sandstone rock near Sydney, Australia. Its eggs can withstand long periods of drought though its free-swimming stages are killed by desiccation (Bishop 1967). Post-embryonic development can only occur when pools contain water, but the distribution of rainfall throughout the year near Sydney is highly irregular and it is impossible to predict when a pool will fill or for how long it will contain water. Desiccation frequently kills the free-swimming stages and pools are not recolonized when this happens (Bishop 1967). The species has to cushion the effects of repeated 'population crashes' to survive. This is achieved by several mechanisms that maintain a reserve of resistant eggs in the mud of pools which L. stanleyana inhabits. The present paper describes how eggs are prevented from hatching during the cool months of the year when they have least opportunity of giving rise to sexually mature conchostracans. The weather influencing an animal has cyclical and aperiodic components. Adaptations of L. stanleyana to the aperiodic components that result in drying and filling of the pools will be described in a later paper. Such adaptations are essentially different from those discussed here. Many animals with a relatively short generation time show fluctuations in abundance that are correlated with season and some species are tided over an unfavourable season by a diapause stage. Shelford's (1929) use of the term 'diapause' has been adopted by many reviewers. Andrewartha (1952), for example, followed Shelford's suggestion 'that the term be restricted to cases where development is arrested spontaneously and does not respond to any ordinary amelioration of the external environment'. Both Andrewartha and Lees (1955) qualified this statement and noted that diapause may be 'traced back to the action of the environment although it is spontaneous in the sense that the response may later become independent of the primary stimulus'. Andrewartha and Lees consider that diapause is a timing mechanism. It synchronizes the life-history so that the active stages of the organism occur when food is plentiful and temperature and moisture are most suitable for development. Diapause stages are frequently more resistant to unfavourable conditions than non-diapause stages. Students of Crustacea have used the word 'diapause' loosely. Earlier workers, who