Distribution and abundance of cladocerans in the eastern Arabian Sea

Of the two species, Penilia avirostris Dana and Evadne tergestina Claus encountered, 86% of the population of the former occurred within the 50 m depth zone and 75% of the total population of the latter in the 50 m-100 m depth zone. Striking day-night variations in the population densities of Penilia and Evadne were observed. The samples collected at night contained 99.7% and 88% of the populations of the former and latter species respectively. Swarming of Evadne and Penilia is a regular phenomenon during the southwest monsoon season.The highest concentration of the former species was observed in August and that of the latter in September. High concentrations of Penilia and Evadne were observed in the 30 m depth zone off Cochin and in the 40 m depth zone off Karwar. The abundance of Penilia observed in the 30 m depth zone of Wadge Bank off Cape Comorin coincided with its maximum recorded in August off Cochin. The length frequency studies on Penilia and Evadne showed that different size classes dominated in the different regions from 07°N to 19°N and that an increase in size of the individuals was evident from south to north in both Penilia and Evadne. Fecundity of Penilia was the highest in the southernmost region between 07°00'N and 09°00'N while that of Evadne was in the northern region, 15°N- 19°N. Gamogenetic females of both Penilia and Evadne were very few in number. Occurrence of large numbers of parthenogenetic females of Penilia and Evadne with well developed advanced embroys in their brood pouches in the night samples probably suggest nocturnal brood maturation.