Predation by calanoid copepods on the appendicularian Oikopleura

Appendicularians have some of the highest growth rates among metazoans but they are usually outnumbered by the slower growing copepods in mesozooplankton communities. We present experimental evidence that the eggs and juveniles of the appendicularian Oikopleura dioica are actively consumed by various copepod species. Clearance rates of Calanus helgolandicus, Candacia armata, Centropages typicus, Eucalanus crassus, and Temora longicornis on eggs of O. dioica were usually above 300 ml copepod-1 d-1. The ingestion rates of C. helgolandicus on O. dioica eggs did not reach saturation even at the highest egg concentrations (4,560 eggs L-1). Although C. helgolandicus, C. typicus, and C. armata preyed actively on 1-mm-long O. dioica (approximately 0.2 mm in trunk length), predatory pressure decreased with increasing appendicularian size. We suggest that the association of dense appendicularian populations with phytoplankton blooms could be explained by their opportunistic response to beneficial conditions before copepod densities become high enough to cause a decline in appendicularian populations due to predation. Our results indicate that, in addition to the direct shunt of biomass from picoplankton to fish, appendicularians can occupy a similar role to that of microzooplankton in the ocean. They represent an intermediate step in the less efficient, picoplankton–appendicularia–copepod-fish food chain.