Effect of hydrostatic pressure on eggs of Neocalanus copepods during spawning in the deep-layer

We examined the effect of hydrostatic pressure on copepod eggs to test the hypothesis that eggs of calanoid copepods can develop, hatch, and be successfully recruited under the environ- mental pressure at which they are released. To explore this hypothesis, the pressure tolerance of eggs of the deep-sea-spawning copepods Neocalanus cristatus, N. plumchrus, and N. flemingeri was examined and compared with that of Calanus sinicus, which shows egg hatching success that decreases with increasing pressure. The duration of egg development and the hatching success of Neocalanus copepods were measured at 4°C under hydrostatic pressures of 1, 10, 50, and 100 atm. Neither the egg development time nor hatching success rates were affected by pressure. This implies that eggs of Neocalanus spp. are adapted to a wide pressure range even though reproduction occurs in a high-pressure environment. Both Neocalanus spp. and C. sinicus live over wide depth ranges throughout their lifetimes — from the ocean surface to depths of greater than 1000 m — by undergo- ing ontogenetic vertical migration. However, during early life stages such as the egg and nauplius stages, C. sinicus is exposed only to a narrow pressure range, from 1 to about 10 atm. In contrast, Neo- calanus copepods are exposed to a wide pressure range from deep to surface layers during their early life stages. Therefore, the pressure tolerance of the eggs of calanoid copepods appears to be related to the depth range experienced in the early life stages.