This analysis of distribution and abundance of species of Hydromedusae completes a report (Vannucci & Navas, 1973b) on the ecology of Indian Ocean Hydromedusae based on the zooplankton collected during the International Indian Ocean Expedition (IIOE). Distribution and abundance are taken here to be the ecological expression of variability of species in space and time. The aim was to identify the biological signature of below surface water masses that cannot be identified by remote sensing techniques. Selected species were taken as biological units, the oceanic water masses as defined by their T-S and T-O2 diagrammes were taken as the non biological units. Taken together they define different ecosystems of the Indian Ocean. About 45,000 specimens of hydromedusae taken at 480 stations were sorted from 900 plankton and all specimens were determined and counted. Several hauls, mostly stratified, were taken with closing nets, but not all contained hydromedusae. The distribution of each species was studied in relation to water salinity, temperature and dissolved oxygen, the limits of ecological tolerance and preference were defined by the environmental characteristics of the layers sampled by the nets and are given for each species. These can be grouped as follows:
1. Deep water species, cold tolerant, often eurytopic;
2. Antarctic species, cold loving, usually stenothermal with preference for low salinity;
3. Indian Ocean Central Water species, with preference for temperature lower than 19°C and salinity not much higher than 35%, usually found at sub-surface or intermediate depths, they may spread into the Arabian Sea of Bengal in surface layers;
4. Indian Ocean Equatorial System species, warm tolerant, usually prefer comparatively low salinity, high temperature and high oxygen content;
5. Bay of Bengal Surface Water species, found in surface layers of the Bay, with preference for low salinity, high temperature and high oxygen content;
6. Arabian Sea Surface Water species prefer very high salinity and high temperature;
7. Rare species.
Some immigrants from the Mediterranean Sea are described and many species were found to be tolerant of dissolved oxygen content as low as 0.2ml/l. Numerous individuals of many species were found to agglomerate at boundary layers.