Egg size and reproductive adaptations among Arctic deep-sea copepods (Calanoida, Paraeuchaeta)

Abstract

Reproductive strategies of the four congeneric and sympatric calanoid copepods Paraeuchaeta glacialis, P. norvegica, P. barbata, and P. polaris were studied in the Arctic Greenland Sea. Females of all species produce egg sacs and carry their brood attached to the genital opening until the offspring hatch. However, egg size and lipid content as well as clutch size and the fraction of females carrying egg masses show characteristic differences among the four species. P. glacialis and P. norvegica produce large numbers (37 to more than 50) of relatively small eggs, whereas P. barbata and P. polaris rely on small numbers (10 to 19 and 4 to 6, respectively) of large eggs with a high energy content. There is no correlation between female body size and egg size or clutch size, respectively. Females of the smallest species, P. polaris, produce relatively large eggs and show the highest energetic investment per egg. In contrast, energetic investment per clutch is highest in P. glacialis. Reproductive adaptations appear to be strongly related to the depth range inhabited by the respective species. In the central Greenland Sea P. glacialis and P. norvegica occur in the epipelagial and upper mesopelagial, whereas the other two species inhabit lower mesopelagic to bathypelagic depths. Thus, egg size increases with depth of occurrence, whereas clutch size is inversely correlated with depth. This observation leads to the hypothesis that the evolution of large eggs in deep-sea copepods may enable hatchlings to rely on a lecithotrophic development and thus represents a successful adaptation to cope with the limited food supply at great depths, whereas high offspring numbers in epipelagic species compensate for higher predation risks in the euphotic zone.