биота российских вод японского моря - Materials of Alexey Shipunov

Biology and ecology. P. avirostris is a typical neritic warm-water species and acommon member of plankton communities in the coastal waters of the subtropical andtropical zones of the ocean. It is most abundant near river estuaries (Goswami & Devassy,1991) and may come to river mouths, too (Egborge et al., 1994). In subtropicaland tropical regions P. avirostris occurs at a depth of 30 m or near the bottom anddoes not make daily migrations (Mullin & Onbe, 1992; Onbe & Ikeda, 1995). In Peterthe Great Bay in the regions 30–70 m deep the species was collected only from warmupper layer, not deeper than 20 m.The species is most abundant usually in the middle of summer. In the Inland Seaof Japan it appears as early as late May, and the density is highest in mid-July (up to10–60 thousand sp./m 3 ); in September-October cladocerans disappear from the plankton(Onbe, 1974; Onbe & Ikeda, 1995). The temperature and salinity ranges of thespecies are wide: in the coastal waters of China, namely near Hong Kong, it was foundat a temperature of 16–32˚C and a salinity of 7.3–37.2‰ (Tang et al., 1995). In thenorth-western part of the Sea of Japan peaks of P. avirostris density have been recordedfor later periods. In the southern part of Amursky Bay (Alekseev Bight of PopovIsland, in 1973) P. avirostris appeared in July at a temperature of 17.9˚C; it was one ofthe most abundant (2245 sp./m 3 ) plankters in August at 21.1˚C; in September at17.9˚C its density was 388 sp./m 3 ; in October its numbers drastically decreased to disappearby the end of the month (Mikulich & Biryulina, 1977). The species was notfound in Peter the Great Bay during the “cold” years of 1962–1967 (Kos, 1969, 1974,1976, 1977). In the south-western part of Peter the Great Bay P. avirostris appearssporadically in mid-July at a temperature of 14–17°C and attains maximum density(up to 560 sp./m 3 ) in late August at 20–22°C. In the eastern part of Peter the Great Baythe species is recorded for periods shorter than in the western part. In Vostok Bay it isusually found when the water is warmest (21.8–22.4°C), in late August and in September,reaching a density of 500 sp./m 3 P. avirostris occurs in Peter the Great Bay tillthe end of September or to October (Shkoldina, 2001).Subitaneous (summer) eggs of P. avirostris are alecithal, they are nourishedthrough the brood pouch. Every female bears from two to nine parthenogenetic embryos,Onbe (1974) recorded the highest (seven-nine) number of such embryos forJune. The process of embryonic development divides into 12 stages (Della Croce &Bettanin, 1965). Generally the eggs develop in the brood pouch for three-four days,but sometimes the period shortens to just 30 hours; during 36–40 days the parthenogeneticfemale produces six generations (Onbe, 1978a). Females with resting eggs appearin the population when its density is highest (for the Inland Sea of Japan – in lateJuly), making 1–2% of the whole number of specimens. Gamogenesis intensifies, andthe number of the females with the resting eggs increases towards the end of warmseason, reaching up to 50% of the population (Onbe, 1974, 1978a). Usually the femaleof P. avirostris has one, rarer two resting eggs, which develop going through nine successivestages (Onbe, 1978a). The further formation of the egg’s membrane, its maturation,and the hatching out of a young cladoceran takes place in the oozy sediments.The length of the resting egg ranges from 0.21 to 0.29 mm (usually 0.25); the width,from 0.14 to 0.20 mm (usually 0.18 mm); the membrane is 0.10 mm thick, and thenewborn cladoceran length is about 0.4 mm (Onbe, 1973, 1985; An illustratedguide…, 1997). The resting eggs are found in the sediments the year round; their maximumquantity is recorded for September (Onbe, 1978a).13