The feeding and reproductive strategies of Acartidae in Southampton Water

This work looked at some of the environmental and biological aspects of the reproductive effort in Southampton Water Acartidae. Adult Acartia congeners A. bifilosa, A. clausi, A. discaudata and A. tonsa have distinct seasonal and spatial distribution patterns in Southampton Water, governed by temperature and salinity respectively. The effect of these factors on other life stages, hatch success and naupliar survival, was investigated by exposing the congeners to a range of salinity (15.5-33.3) and temperature (5-20°C). Acartia clausi is known to prefer more saline waters (Lance 1963, 1964) and showed highest hatch success at 33.3 salinity. Acartia tonsa is more tolerant to dilution and at 15.5 salinity it had the highest hatch success of all of the congeners. Hatch success in both Acartia bifilosa and A. discaudata was similar over the range of salinities investigated, confirming that they are intermediate species in terms of spatial distribution. However, nauplii of all species survived well at the higher salinities and best at 33.3, which allows for differential transport of the poor-swimming nauplii to the mouth of the estuary until size and swimming ability increase when they can then return to regions of preferred salinity (Villante et al. 1993). The summer species, A. clausi and A. tonsa showed higher hatch success at 20°C, whereas A. discaudata which is present in the water column all year round showed no significant differences in hatch success at the three temperatures. Acartia bifilosa, which diapauses over summer showed significantly (P<0.05) higher hatch success at 10°C than 20°C. The physiological relationship between temperature and development time was clear with naupliar survival of all species highest at 20°C and all congeners reached CI significantly faster at 20°C, but no consistent pattern was seen for salinity. Reflecting hatch and nauplii parameters, Acartia bifilosa Scope for Growth (SfG) at 10°C is twice that at 5°C and 20°C and since there is less energy available to it at 20°C the ultimate reason behind its oversummering in the egg phase is to avoid higher temperatures which decrease its competitive ability.