The ecology of Australatya striolata (McCulloch and McNeill) (Decapoda: Atyidae)

Abstract

The ecology of the protandrous freshwater shrimp, Australatya striolata (McCulloch and McNeill), was studied in two stream systems in north Queensland, in the context of its distribution in eastern Australia. The size frequency distributions of the populations studied were found to be bimodal, with the left mode consisting primarily of males, and the right mode of females. The modal size classes did not vary through time, although recruitment was found to be seasonal. The most likely mechanism for the maintenance of this bimodal distribution was a two-stage growth curve of an initial phase of relatively rapid growth of Juveniles followed by very slow growth of mature males, then a relatively rapid transition from male to female size followed by very slow growth of mature females. This explanation was well supported by a sigmoidal growth curve for individuals between the modal size classes, low levels of recruitment and negligible growth of tagged and caged males and females. Analysis of length frequency distributions through time also indicated very slow growth for mature males and females (1 mm in 870 days) , and rapid growth of Juveniles (2-3 mm in 61 days). This growth pattern is possibly unique in the recorded literature. Breeding was seasonal and coincided with the summer wet season. Eggs were brooded and larvae released by the females in freshwater. The first larval stage was a zoea which was lecithotrophic and required salinities in excess of 20%. for further development. In lower salinities they could survive for up to 30 days on their yolk supplies. This was shown to be more than would be required for drift to the estuary (about 17 days in Yuccabine Creek and 8 days in Douglas Creek). The subsequent upstream migration of Juveniles required 12 to 18 months in the stream systems studied. A. striolata was shown to be primarily a filter feeder, but sweeping the substratum with the cheliped fan was used in periods of low stream flow. The predominant food was particulate organic matter, the abundance of which was not limiting for naturally occurring densities of shrimp. Predation and disease were shown to be minimal sources of mortality in the populations of adults studied, and the maintenance of near 1:1 sex ratios, despite the greater age of females, indicated high survival. The species was shown to be distributed in coastal streams from the Claudie River, Cape York Peninsular (12'45' S, 14312' E), to the Genoa River, Victoria (37°29' S, 149°35' E). The lowest observed altitude for adult populations tended to decrease with increasing latitude. The northern limit to the distribution was shown to be restricted by habitat availability. It is suggested that the observed differences in the size frequency distributions over the geographic range were due to different rates of recruitment. The adaptive significance of the life history of the species is discussed, with particular reference to the preference of adults for low order streams, and the advantages of the marine Larval