Some Adaptations of Limnadia stanleyana King (Crustacea: Branchiopoda: Conchostraca) to a Temporary Freshwater Environment

Abstract

Limnadia stanleyana King (Crustacea: Conchostraca) which inhabits temporary rainpools near Sydney, Australia, was studied to determine how it survives in this severe environment. Drying of the pools in which the species lives frequently kills its freeswimming stages, though eggs are drought-resistant (Bishop 1968). Eggs enter diapause in autumn and do not hatch during the cooler months of the year when the animals have a reduced chance of attaining sexual maturity before pools dry (Bishop 1967). Climate has further, more profound effects as the water level of pools is controlled by the local precipitation/evaporation ratio. Near Sydney the rainfall is irregularly distributed throughout the year and it is impossible to predict when a pool will fill or for how long it will contain water. This paper discusses some adaptations that enable the species to counteract this unpredictability of the environment. Conchostracans in a pool are frequently killed by drying before they can reproduce and the species has to withstand such population crashes if it is to survive in the pool. The following strategies are theoretical possibilities: (a) The species may recolonize the pool when this has dried and then refilled. (b) Some part of the population may be kept as a drought-resistant reserve in case the rest of the animals are killed by desiccation. (c) The species may evolve mechanisms by which it increases its chances of reproducing before the pool dries. Longhurst (1955) attributed the wide distribution of some species of Notostraca to the effectiveness of their eggs as dispersive agents. Proctor (1964) found that eggs of several branchiopods survived passage through the gut of waterfowl and were dispersed by them. Dispersal of eggs could result in recolonization of pools by L. stanleyana after drying had killed previous populations. Gillett (1955a, b) investigated a genetic mechanism that restrained some eggs of mosquitoes (genus Aedes) from hatching to form a reserve in case larvae were destroyed by premature drying of the habitat. (Aedes inhabit temporary pools and the eggs of some species are drought-resistant (Clements 1963).) Some branchiopods hatch only soon after their pool habitat fills with rain-water. This has obvious adaptive value as it maximizes the period over which post-embryonic stages can grow and reproduce thereby enabling the species to make the most effective use of the time the habitat is favourable. Hall (1959, 1961) concluded that development of eggs of Chirocephalus diaphanus Prevost (Anostraca) was inhibited in water deeper than 20 cm. Moore (1963) did not find that this was the case in Streptocephalus seali Ryder and Eubranchipus holmani (Ryder) (Anostraca). He suggested that eggs of these species