The Cladocera of the Zanzibar Area of the Indian Ocean, With a Note on the Comparative Catches of Two Plankton Nets

Abstract

Three species of Cladocera are recorded from the Zanzibar area of the Indian Ocean, Pen ilia avirostris, Evadne tergestina and Podon polyphemoides; this last species was rarely taken. Two nets were worked at three stations for 13 months. Figs. 1 and 2 show the incidence of Pen ilia in the plankton, Figs. 4 and 5 the incidence of E. tergestina. Fig. 6 shows these two species as a percentage of the total plankton numbers during their periods of abundance. Fig. 3 shows the temperature conditions at the three stations. "Explosive" populations of Penilia are discussed, with the suggestion that a combination of the 0 0 content of the water and inherent progressive degeneration of parthenogenetic females are the limiting factors of a population. The coincidence of heavy diatom and Penilia populations is noted, with the suggestion that the former supplies 0" for the latter. Catches of two nets are compared. The CF 70 catches about 10 per cent less Pen ilia and about 20 per cent less E. tergestina per unit volume of water than the J.C.S.N., this latter being therefore the more efficient net under the conditions in which they were used.