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

Abstract

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 inconsis-tency 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.