Diel vertical migration and gut pigment rhythm of Paracalanus parvus,P. crassirostris,Acartia erythraea and Eucalanus subcrassus (Copepoda,Calanoida) in Tolo Harbour,Hong Kong

Diel vertical migration and gut pigment rhythm of calanoid copepods were studied in the highly eutrophic waters of Tolo Harbour, Hong Kong in summer between July 30 and August l, 1991 and in winter between January 24 and 25, 1992. Paracalanus parvus and P. crassirostris showed neither diel vertical migration nor diel gut pigment rhythm during the summer study. Acartia erythraea and Eucalanus subcrassus exhibited prominent diel vertical migration and diel gut pigment rhythm. Diel changes in gut pigment content of surface dwelling individuals and increase of gut pigment content before the onset of nocturnal upward migration suggest that gut pigment rhythm in E. subcrassus was not the result of animals moving in and out of a food-rich surface layer. During the winter study, the harbour was affected by a bloom of Noctiluca scintillans. P. parvus and P. crassirostris resided in deeper waters presumably to avoid the dense populations of N. scintillans at the surface. P. crassirostris remained non-migratory during the winter study. There was a nocturnal increase in the number of P. parvus in the surface waters, although no diel change in the mean depth of the population was observed. Both Paracalanus species showed diel gut pigment rhythm. The presence of diel gut pigment rhythm in the non-migratory P. crassirostris during the winter study suggested the presence of an independent feeding rhythm.