Journal of the Marine Biological Association of India

Volume 13 Issue 2

Monsoonal fluctuations in the distribution of the hydromedusae in the Cochin Backwater, 1968-1969.

V. Santhakumari and M. Vannucci

Plankton collections were made during the post-monsson period (November, 1968) and during the pre-monsoon period (April,1969). Two sets of samples representing day and night were taken from nine fixed stations from Barmouth to Aryad south (southern end of the backwater system). The relative abundance and monsoonal fluctuations of the hydromedusae in relation to salinity are discussed. The collection contains the following species belonging to eleven genera, namely Euphysora abaxialis, zanclea costata, Podocoryne carnea, Tiaropsidium japonicum, Obelia sp.,  Philidium hemisphericum, P. rangiroae, P. brunescence, Eucheilota menoni, Blaackfordia virginica, Phialucium carolinae, P. taeniogonis, Eirene ceylonensis, E. menoni, Eutime hartlaubi, E. browne, E. neucaledonia, E. japonica and  E. commensalis.

During the post-monsoon period, hydromedusae were apparently absent except at Barmouth and at Aroor (stations 1 and 2). Eight species were found and Obelia  sp. formed  the chief component for this season. Maximum density of hydromeduae recorded was only 12.9 specimens /m3 at Barmouth.

During the pre-monsoon period hydromedusae were present in all stations and a total of eighteen species were present. Blackfordia virginica (96.7 sp/m3) Eutima commensalis (68.9 sp/m3) and Eirene ceylonensis  (35.3 sp/m3) were the dominant forms.

Comparison of the (numerical) density of hydromedusae with the salinity of the ambient water shows that there is a relationship between the abundance of a species and the salinity gradient, different species dominating different stations. In general, the very low saline period is not favourable for the hydromedusae of this region.

Date : 30-12-1971