Redescription of the planktonic calanoid copepodAcartia hudsonica from Atlantic and Pacific waters: A new record from Japanese waters

Abstract

The neritic planktonic copepodAcartia hudsonica is redescribed from the Atlantic and Pacific coasts of North America and newly from Japanese waters. This species formerly has been identified asA. clausi in the Northwest Pacific. The Atlantic and Pacific populations cannot be morphologically separated even in fine ornamentation; the variations found in the two populations overlap each other. Prosome length/width ratio of the female is negatively correlated with the body size and the ratio is not significantly different between Atlantic and Pacific females of the same size. The length/width ratio of the genital segment of Pacific females is positively correlated with their body size, and the mean value of the ratio for Atlantic females is smaller than that expected for Pacific females of the same size. The size-temperature relationship differs among localities but it is uncertain whether there is a genetic difference in copepod size between the Atlantic and Pacific populations.