Tidal vertical migration of two estuarine copepods: Naupliar migration and position-dependent migration

Abstract

Tidal vertical migration (TVM) of the calanoid copepods Pseudodiaptomus inopinus and Sinocalanus sinensis was investigated three times in August 2003 and March and August 2004 in the Chikugo River estuary, Japan, from the viewpoints of life history and position within the estuary. On each cruise, vertical samples were collected four or five times a day from stations along the estuary by filtering water samples through a 37-m mesh. Stage ratios did not vary extremely during copepodid stages in P. inopinus, whereas in S. sinensis the ratios abruptly decreased with the development of early copepodid stages. The highest densities occurred mostly within the oligohaline zones but did not coincide with the turbidity maxima. TVM that would increase population retention in the estuary was observed in both copepodids and nauplii of both species during the day, while the night vertical distribution of P. inopinus was the reverse of the retention-associated pattern, indicating that diel vertical migration prevailed at night. Sinocalanus sinensis exhibited adaptive position-dependent TVM especially in daytime, in which the scales of TVM were larger in the seaward populations. The interspecific differences in the stage ratios and TVM patterns may be explained by species-specific trade-offs between adaptive strategies for predator avoidance and retention in the estuary. © The Author 2010.