Delayed response to irreversible non-genetic adaptation to salinity in early development of the brachyuran crabRhithropanopeus harrisii, and some notes on adaptation to temperature

Abstract

Abstract In the first series of experiments the embryos of the estuarine crab Rhithropanopeus harrisii (Gould) were acclimated to either a low (10 %0) or a high (30 %0) salinity at the cyclic temperature 25 to 30°C. After hatch each of the four zoeal stages and the megalopa stage was subjected to either an increase in salinity from 10 to 35 %0 or a decrease from 30 to 5 %0 and reared to the first crab stage. Increase of salinity at each of the zoeal stages did not seem to have any effect on mortality or rime of development during those stages. There was, however, a significant increase of mortality in subsequent pre-metamorphic and metamorphic moults and the developmental time significantly increased between these two moults. Similar effects occurred when 20 %0 Swas used instead of 10 %0 S. The greatest effects were observed when the salinity was increased in the third or fourth zoeal stages. The increase of salinity at the zoeal stages also caused the megalopa to develop abnormally. If the increase of salinity occurred at the megalopa stage it had no obvious effect on the mortality of that stage. A decrease in salinity from 30 to 5 %0 did not have any obvious effect on survival of the larvae during any of the stages. The delayed response to increased salinity is suggested to be caused by a disturbance induced at intermoult stages at the time for salinity change. The first development of systems that should be functional at the megalopa stage occurs during the zoeal stages. The 'delayed response effect', suggests that oceanic water (i.e., high salinity) is a deterrent to successful larval dispersal of R. harrisii between estuaries. In a second series of experiments the crab embryos were acclimated to various salinity and temperature combinations, which in some cases were changed immediately after hatch and then kept constant up to the first crab stage. A cyclic temperature of 30 to 35°C was lethal to almost all larvae at 5 or 35 %0 S, irrespective of whether they were acclimated to a high or low salinity before hatching. In a cyclic temperature of 25 to 30°C, 60 percent or more of the larvae metamorphosed at those same salinities, except those embryos acclimated to a high salinity (30 %0) and transferred to 5 %0 through hourly changes of 5 %o-steps, all of which died in the first zoeal stage. Resistance adaptation of the crab embryos to high temperatures prolonged the survival in high temperatures through later larval stages compared to when the embryos were acclimated in lower temperatures. In high temperatures, as in high salinities, the pre-metamorphic and metamorphic moults were the most critical.