Does Body Size in Acartia clausi (Crustacea, Copepoda, Calanoida) Respond to Ongoing Climate Change?

Zooplankton in temperate areas show variation in adult body size across seasonally succeeding generations. Adult size could therefore be a useful indicator of environmental change over long time periods. Acartia clausi was chosen as the target species for this purpose, with measurements available from Italy and Tunisia for the period 2004–2021. Adults of the winter generations were larger than those of the summer generations, indicating an inverse correlation between size and environmental temperature. However, size variability also appeared to be affected by additional drivers, such as sex (females larger than males) and eco-geographic location (specimens from the open sea larger than those from confined sites). Specimens from the Gulf of Taranto (Italy) and the Gulf of Gabes (Sfax, Tunisia) showed a complex pattern of body size variation from 2004 to 2021 and from 2008 to 2014, respectively. Adult size was inversely correlated with water temperature on the sampling date and one month before. However, the size of specimens from the same area (Taranto) was also affected by the geographic position of the collection site. Given the position of Calanoida in the marine food web, these changes may also have implications for the growth rate and maximum size of other species, including those of economic importance that feed on copepods.