VITAL ROLE OF DRIFT ALGAE IN THE LIFE-HISTORY OF THE PELAGIC HARPACTICOID PARATHALESTRIS-CRONI IN THE NORTHERN NORTH-ATLANTIC

The harpacticoid copepod Parathalestris croni, a widespread pelagic sp ecies of the northern North Atlantic, was found to be closely associat ed with seaweed originating in the intertidal zone floating on the sur face of the sea around Iceland. The abundance of the copepod increased with distance from the shore and it showed a special affinity for the brown alga Ascophyllum nodosum. The copepods in the macroalgal clumps were mostly ovigerous females, and the seaweed clearly serves a nest function. The eggs were deposited on the seaweed where the nauplii, wh ich are unable to swim, crawl on the algae until they metamorphose int o the first copepodite stage (CI). The CI copepodites leave the floati ng weed, and the juvenile stages may eventually disperse widely in the water column until floating weed is colonized, probably by adults or juveniles in the last one or two copepodite stages. Parathalestris cro ni is one of a group of very few species known or suspected to be depe ndent for survival on the extremely unpredictable habitat of floating weed in the northern North Atlantic.