Switch from production of subitaneous to delayed-hatching and diapause eggs in Acartia japonica Mori, 1940 (Copepoda: Calanoida) from Sagami Bay, Japan

Abstract

To clarify temporal variation of in situ egg production of the copepods, Acartia japonica Mori, 1940, in Sagami Bay, central Japan, the species’ abundance, egg morphology and hatching patterns were studied during 2015. Adults of A. japonica were observed in the water column from June to October. Results showed that adults reared in the laboratory produced three types of eggs: subitaneous, delayed-hatching and diapause. All egg types had a surface covered with short spines; diapause eggs had a thicker chorion than subitaneous eggs. During the early period of the species occurrence in the bay, most females produced only subitaneous eggs, which hatched within 72 h. From the end of September, a large proportion of females began to produce diapause eggs. Linear regression analysis showed that the ratio of diapause eggs was negatively correlated with day length and water temperature (P < 0.05). Delayed-hatching eggs generally hatched over a broad time span and were also produced in conjunction with diapause eggs. This is the evidence that individual females of a marine copepod could produce all three-egg types simultaneously. Diapause eggs of A. japonica had the capacity to survive at least 300 days at ambient temperatures in the laboratory. The production of diapause eggs in this species probably functions to connect temporally separate populations in each year. Production of all three-egg types likely insures the species’ viability as a copepod that is well adapted to seasonal and sudden environmental changes.