Feeding and reproduction including diapausing egg production as cold-water adaptations for overwintering of Acartia steueri (Copepoda, Calanoida) in Okkirai Bay, Sanriku, northern Japan

Abstract

Feeding experiments with a natural microplankton assemblage and egg production experiments with the neritic copepod Acartia steueri were conducted simultaneously in an inlet on the Sanriku coast during autumn. Diapausing egg production status of A. steueri was also investigated. Further, A. steueri dominated during September to October and then gradually decreased until December, before disappearing after January. The dominant microplankton in natural waters during the study period were dinoflagellates, followed by centric and pennate diatoms and oligotrich ciliates. A. steueri fed only on dinoflagellates and oligotrich ciliates at rates of 87.0–309 and 17.7–71.5 cells ind⁻¹ d⁻¹, respectively. Egg production rate of A. steueri ranged from 1.6 to 15.0 eggs female⁻¹ d⁻¹ and significantly increased as ingestion rates of dinoflagellates and oligotrich ciliates increased. The copepod began producing diapausing eggs in October, and by late December, 86% of the eggs produced were diapausing. The population’s egg production rate was highest in September (18,961 eggs m⁻³ d⁻¹) and gradually decreased through the end of December. These dietary and diapausing egg production seasons of A. steueri in the Sanriku area are significantly different from the results of previous studies in the temperate zone, where the copepod mainly fed on diatoms and produced diapausing eggs during spring. A. steueri can alter its feeding habits and the timing of diapausing egg production in response to changes in habitat. The flexibility of this species to environmental change has likely allowed expansion of its geographical distribution.