Summary
The marine and estuarine crab Hymenosoma orbiculare has been found living in fresh water in Lake Sibayi. Experiments have been carried out to determine whether H. orbiculare normally exposed to sea water can live and breed in low dilutions of sea water. The zoeae are the most sensitive stage but they possess a marked acclimation ability. Preliminary experiments indicate that H. orbiculare from subtropical waters have a much higher upper lethal temperature than crabs from warm temperate regions. The size of H. orbiculare is not salinity dependent, as was previously thought. It appears that H. orbiculare living in Lake Sibayi do not constitute a distinct physiological race.