The effects of emersion on ammonia efflux of three Hong Kong Nodilittorina species

Abstract

On moderately-exposed Hong Kong shores, the nodilittorinids, Nodilittorina trochoides and N. radiata occur in the high-splash zone and N. vidua slightly below this level, extending into the eulittoral. These species experience long emersion times and high rock and air temperatures. After exposure on the natural rock for 0 min (awash and active, control animals) or 1, 4 or 22 h emersed, groups of animals (n = 6) were re-immersed and their ammonia effluxes (weight-normalized for fresh tissue mass) measured after 0.5, 1.0 and 2.0 h. Re-immersed groups had initial (30 min) effluxes of 3.91, 6.01 and 3.53 μmoles NH 4 × g -1 × h -1 for N. trochoides, N. radiata and N. vidua respectively, which were high compared with the final values of 1.66, 2.02 and 0.32 μmoles NH 4 × g -1 × h -1 for the same species after 2 h of re-immersion. There were clear inter-specific efflux differences and the handling procedure evoked enhanced ammonia excretion rates possibly as a stress response. Re-immersed animals had lower effluxes than control groups and, generally, such rates were negatively related to the duration of the preceding emersion period. Effluxes of N. vidua, measured 30 rain after re-immersion were always higher than those measured at 1 or 2 h but this occurred with the other species only after they had been emersed for 1 h. Rates at all three sampling times following 4 or 22 h of emersion were very similar. The more eulittoral N. vidua may continue to produce ammonia during emersion periods of >1 h but the high-shore species do not. The energetic cost implications of this difference in post-emersion effluxes may be that emersion tolerance, hence vertical height on the shore, is limited for N. vidua. No evidence of a switch from ammonotely to uricotely was found for any species, but the production of non-excreted uric acid is not precluded.