An ecophysiological study of the effects of changes in salinity and temperature on the distribution of Macrobrachium Petersi (Hilgendorf) in the Keiskamma river and estuary

摘要

Summary: The distribution of adult, juvenile, post larval and larval Macrobrachium petersi (Hilgendorf) was studied in relation to temperature and salinity in the Keiskamma river and estuary from May 1979 to May 1981. M. petersi is a subtropical species, which confined activity of all stages in the field to the summer months. Variable freshwater discharge and tidal effects determined the salinity and temperature profiles in the middle and upper reaches of the Keiskamma estuary. Thus, a dry 1979/80 summer and a wet 1980/81 summer markedly influenced the abundance and distribution of adult M. petersi in the river and estuary respectively. Adult M. petersi migrate to the estuary under flooding conditions and upstream in response to elevated salinities. Although M. petersi has nine larval stages only a preponderance of stage I were caught in the field. These portray a distinct nocturnal and diurnal distribution pattern which is influenced by salinity, especially under stratified conditions. After flooding the larvae show an affinity for salt front regions and reach these by remaining in the water column on the ebbing tide. A substantial drop in abundance downstream from the salt front suggests that the larvae loose their planktonic phase which is an effective retention mechanism, and confines larval development to the middle and upper reaches of the estuary. Post larvae were caught towards the end of the 1981 breeding season which indicates that complete larval development takes place in the estuary. A post larval migration to freshwater, which reaches a peak in February and March, was monitored. Thus both the freshwater and estuarine environments form an inseparable link in the life cycle of M. petersi. The distribution of stage I larvae in the Keiskamma estuary suggested that salinity played a role in development. The fact that other larval stages were not found emphasised the necessity for a quantitative laboratory investigation to determine the importance of salinity in the developmental history of M. petersi larvae and post larvae. However, the modifying influence of temperature could not be ignored so a multivariable approach was adopted. This, together with a surface response technique, aided the interpretation of the effect of a variety of combinations of salinity and temperature on ecdysis to stage II, larval survival and requirements for metamorphosis to post larvae. It was estimated that the minimum salinity requirement for complete larval development, within a temperature range from 18 to 30⁰C was 8%₀, although ecdysis to stage II and metamorphosis to post larvae could occur in salinities less than this value. Despite the euryhalinity of the larvae, the behaviour of adult M. petersi to an increase in salinity and the affinity of stage I larvae to salt front regions restricted development to the upper reaches of the estuary. This is discussed as an adaptation which not only ensures retention within the estuary but favours recruitment to the adult population in freshwater. The osmoregulatory patterns of larval, post larval, juvenile and adult M. petersi correlated with their distribution. These were approximated by a cubic polynomial which enabled the different patterns to be compared. The larval stages investigated (I, II, V & IX) displayed a remarkable capacity to regulate which was strongest in stage I as these could regulate in both freshwater and 35%₀. The ability to regulate in freshwater was lost hereafter but regained in the post larvae, which also regulated in 35%₀. Juveniles (caught at the ebb and flow) displayed a similar regulatory pattern to the adults and "hyposmoconformed" in salinities beyond the isosmotic point as the need to regulate in 35%₀ was no longer necessary. The osmoregulatory capacity of M. petersi larvae in relation to other decapod larvae is discussed. Marine transport of the euryhaline larval and post larval stages accounts for the distribution of M. petersi along the South African coastline. However, south of 31°S latitude the sea-surface temperature decreases abruptly. This region coincides with the southern limit of the distribution of M. petersi. Although larval M. petersi can tolerate high salinity (35%₀) in combination with low temperature, the post larvae cannot, which is likely to account for their restricted southern distribution. The genus Macrobrachium are in the process of invading freshwater. The possible course that this might have taken has been discussed in the light of available evidence as well as the findings of this study.