Seasonal occurrence of the cold-water calanoid copepod <i>Acartia omorii</i> and its rare occurrence during midsummer in the Seto Inland Sea

Acartia omorii is a predominant planktonic calanoid copepod during winter and spring in temperate coastal waters of the Northwest Pacific. Seasonal occurrence of A. omorii in warm-temperate bays has been known to be restricted to the period with water temperatures below 20°C. However, we found an adult male of A. omorii in a sample from the eastern Seto Inland Sea in September 2023, when the water temperature exceeded 27°C. Rare summer occurrence of A. omorii in the same region was confirmed by re-examining the samples collected at >20°C in previous studies. Previous experimental studies on the species have shown that their biological functions related to population growth increase with rising temperature at around 20°C, which is inconsistent with the field observation that seasonal occurrence of the planktonic population ends when temperatures exceed 20°C. The reason for this inconsistency could be explained by mass mortality of the female after laying resting eggs, before the high temperature becomes detrimental to their biological functions. Individuals appearing in mid-summer are considered likely those hatched from resting eggs.