Seasonal and geographic adaptation to temperature and salinity in the harpacticoid copepod Zaus spinatus spinatus Goodsir

Abstract

Ovigerous females of the phytal harpacticoid Zaus spinatus spinatus Goodsir have been tested in the laboratory to determine seasonal and geographic differences in temperature-salinity ( T- S) adaptation. Animals collected from a ‘summer’ and ‘winter’ population at Robin Hood's Bay (northeast coast of England) and a ‘winter’ population from the Danish Øresund were submitted to factorial experiments utilizing up to 30 combinations of T and S. Irrespective of temperature, survival at Robin Hood's Bay (RHB) was highest at 34 and 24‰ S; winter survival at 16‰ S increased beyond that in summer. In the Øresund best salinity tolerance at all temperatures was between 8 to 24‰ The optimum survival temperature is about 5°C and does not appear to shift seasonally or between populations; thus ‘low-low, high-low’ salinity-temperature relationships point to a cold-water phytogeny of the species. Furthermore, the percentage of nauplii hatching.in Øresund specimens is greatest at optimal (16‰) salinities, and the range of salinities over which this rate is best is at 5°C. Analyses of variance indicated that in both seasonally and geographically different populations T and S affect survival independently, but that in combination there results a significant interaction emphasizing the interdependence of these two factors on survival. Means difference test shows significantly different salinity tolerances between RHB summer and winter emphasizing the existence of efficient compensatory mechanisms, and between the Øresund suggesting possible genetic adaptation. Failure to penetrate into the Baltic and establish a seasonally permanent population further south than the northwest Øresund is a consequence of its inability to tolerate high (>24‰) or low (<8‰) salinities at high temperatures (>15°C). It is proposed that the Californian subspecies Z. s. hopkinsi Lang in being physiologically and geographically isolated from the cold-water nominate subspecies is undergoing allopatric speciation in response to an altered temperature environment.