The ecology of macrobrachium species (Decapoda, Palaemonidae) in a coastal stream in North Queensland

Five species of Macrobrachium occur in the study area, Bluewater Creek, 27 km northwest of Townsville. They are M. Latidactylus (Thallwitz, 1891), M. tamerum (Riek, 1951), M. australiense (Holthuis, 1950), M. novae-hollandiae (de Man, 1908) and M. species A, probably an undescribed species. A multivariate morphometrics study gave support to the separation of M. sp. A and M. tolmerum which are morphologically very similar.\ud \ud Field studies were undertaken in order to infer causal relationships between environmental factors and distribution patterns. These were complemented by laboratory studies of responses to selected environmental factors.\ud \ud Sampling of adults using baited funnel traps at sixteen sites on thirteen monthly occasions suggested salinity to be a major environmental factor. The downstream limit of distribution of each species appeared to be controlled by salinity, but a range of salinity tolerance was suggested, increasing in the order M. australiense, M. tolmerum, M. sp. A, M. latidactylus, novae-hollandiae. Individuals of M. novae-hollandiae and M. Latidactylus were not found above the zone of tidal influence. Given suitable local conditions, M. tamerum and M. sp. A could be found the full length of the study area and M. australiense the full length of the freshwater reach. In the freshwater reach, riffle areas were dominated by M. tolmerum and large pools by M. australiense. Substrate, current and oxygen regime were suggested as being important factors in this pattern.\ud \ud Larvae in plankton samples were almost exclusively stage I. Those of M. tolmerum and M. Latidactylus could not be distinguished. Larvae of M. australiense were rare in plankton samples. Peak production of M. tolmerum larvae was in the period January to April, during highest stream flow. Estimated densities reached a maximum of 345 m³ in sites where larvae could be assigned to M. tolmerum. In sites further downstream where M. latidactylus larvae could also have been present, maximum estimated density was 578 m⁻³.\ud \ud Laboratory studies of temperature/salinity tolerance of adult M. australiense, M. tolmerum and M. Latidactylus demonstrated increasing salinity tolerance in that order. From response surface analysis, predicted survival time at optimal temperatures and 320/00 salinity was greater than 20 days for M. Latidactylus, 20 days for M. tolmerum and two days for M. australiense.\ud \ud Optimum salinities for larval development were 10.10/00 for M. australiense, 17.9°/oo for M. Latidactylus and 25.5°/oo for M. tolmerum. M. australiense was the only species of these three whose larvae completed development in fresh water. At optimum temperature and salinity, the minimum time for larval development was four days in M. australiense, 21 days in M. latidactylus and 38 days in M. tolmerum.\ud \ud Salinity requirements of larvae do not appear to limit upstream dispersal of M. latidactylus or M. tolmerum in the study area, but could do so in longer water courses.\ud \ud Laboratory studies of substrate preference indicated that both M. tolmerum and M. australiense preferred a substrate which provided shelter. In a short-term competitive situation, M. tolmerum was able to partially displace M. australiense from the preferred substrate. Preference for moving water over still water was shown by M. tolmerum. The relative substrate and current responses of M. tolmerum and M. australiense are considered the main determinants of their distribution patterns in the freshwater reaches of the study area. Laboratory studies demonstrated little difference in the tolerance of low levels of dissolved oxygen by M. tolmerum and M. australiense. However, competition for the more favourable riffle area oxygen regime would tend to produce the observed distribution patterns since M. tolmerum was the dominant species.\ud \ud M. australiense has abbreviated larval development and a small number of large eggs. It is the most freshwater of the species studied, in both larval and adult tolerances. M. tolmerum and M. latidactylus which have higher salinity tolerance as adults and a requirement for saline water as larvae, have much longer larval development and produce many more, smaller eggs. These three species therefore fall into the established pattern of life cycle changes concomitant with the evolution of the ability of decapod crustaceans to inhabit fresh waters. Examples from the Macrobrachium suggest that evolution of large body size of larvae and abbreviation of larval development are important in the colonization of fresh water, the former reducing osmotic stress and the latter reducing displacement of larvae by flowing water. A paucity of planktonic food in flowing fresh waters may also provide some selective pressure for abbreviation of larval development and for lecithotrophy in the planktonic stages that remain.\ud \ud It is concluded that distribution patterns of Macrobrachium species in the study area are largely determined by temperature, salinity, substrate and current whose effects are interconnected with competition between species and adaptation of the life cycle to fresh water