FILTRATION OF COLLOIDAL MELANIN FROM SEA-WATER BY PLANKTONIC TUNICATES

PELAGIC tunicates of the genus Oikopleura produce, live in and pump water through mucous structures termed 'houses'. Because these houses contain filters with submicrometre pores, oikopleurids have been proposed as important consumers of particulate organic carbon (POC) in the sea 1. We present new evidence that they also consume 'dissolved' organic carbon (DOC) in the colloidal size range down to about 0.2-mu-m in diameter. Such colloids are more abundant in the sea than previously believed 2,3, and have an important role in the global flux of carbon. Oikopleurid tunicates have the potential to filter a significant fraction of the water mass every day 1,4, and to repack much of the colloidal DOC into their houses, faecal pellets and bodies. Oikopleurids are also known prey for several fish species 5-7. Thus, we speculate the oikopleurid tunicates to mediate a substantial energy flow, from DOC through a two-step 'food-chain', to fish of commercial interest, by-passing energy-consuming respiration by bacteria and small planktonic protozoa.