Ecology of the marine cladoceran Penilia avirostris Dana in Tolo Harbour, Hong Kong

Seasonal abundance of the marine cladoceran Penilia avirostris was studied in the Tolo Harbour, Hong Kong, from November 1987 to December 1989. P. aviroseris occurred in plankton throughout the year, but no clear seasonal pattern could be identified. In general, densities were the highest in the Tolo Channel and decreased progressively towards the inner parts of the Tolo Harbour. P. avirostris showed marked seasonal variations in body size and brood size. Body length of P. avirostris ranged from 0.3 to 1.0 mm. Average body length was greatest on March 1988 and January 1989, and the smallest in May 1988, although no clear seasonal trends could be detected. Brood size of P. avirostris ranged from 1 to 13 over the study period with a mean of 5.6. A significant positive relationship was found between body length and brood size. Diel vertical migration and diel feeding pattern of P. avirostris was followed over a 24 h period from July 31 to August 1.1991 at a station near the southern edge of the dam of the Plover Cove Reservoir. P. avirostris performed typical nocturnal vertical migration. Although animals were seldom found below 10 m during either day or night, a greater fraction of the population was found in the upper 1 to 3 m at night than during the day. A diel cycle in gut pigment content was observed in P. avirostris. Gut pigment contesnt was low during the day and increased sharply during the first 2 h after sunset. Gut pigment content at 21:00 and 03:00 was about 2 times higher than average daytime values. Chlorophyll a concentration was the highest near the surface and decreased towards the bottom, but water collected at 16 m still contained 1.79 ng/cm³ Chl. a.Because P. avirostris were usually found within the upper 10 m, we believe that the observed diel change in gut pigment content was likely the result of a real diel feeding rhythm, and was probably not the result of individual animals moving in and out of the food-rich layer.