Different patterns of stage-specific horizontal distribution between two sympatric oligohaline copepods along a macrotidal estuary (Chikugo River, Japan): Implications for life-history strategies

Abstract

Stage-specific horizontal distribution was compared between two sympatric oligohaline copepods to elucidate their life-history strategies related to reproduction: free-spawning Sinocalanus sinensis and egg-carrying Pseudodiaptomus inopinus. Copepods were sampled by vertical hauls of a 100-μm mesh plankton net along the macrotidal Chikugo River estuary in southwestern Japan from 2005 to 2006. Sinocalanus sinensis dominated throughout the year except for the warm season, when P. inopinus outnumbered S. sinensis. The spatiotemporal correspondence of peak densities with the estuarine turbidity maximum was evident for S. sinensis but ambiguous for P. inopinus. Nauplii of S. sinensis were always distributed on average downstream from copepodids and adults, whereas P. inopinus usually showed the opposite pattern of stage-specific horizontal distribution. During large freshwater discharges, S. sinensis was more subject to washout from the estuary. In addition, the potential vulnerability of S. sinensis to droughts was suggested by low tolerance for high salinities. Consequently, egg-carrying P. inopinus has higher flexibility to live under varying estuarine conditions especially during the warm season. In contrast, ecological specializations, such as tidal vertical migration and detritivorous feeding, are considered to underlie the dominance of the free-spawning S. sinensis under the conditions prevailing in the macrotidal environment. © 2012 The Author.