Marine cladocerans in Toyama Bay, southern Japan Sea: seasonal occurrence and day-night vertical distributions

Marine cladocerans in the zooplankton samples collected vertically(0–500 m) with Norpac nets at 2–4 week intervalsfrom February 1990 through January 1991 at a station in ToyamaBay, southern Japan Sea, were examined. From mid-March throughearly December, cladoceran populations exhibited five distinctpeaks. in which seven species occurred and showed a species-specific pattern of seasonal distribution. In mid-July. Peniliaavirostris exhibited a prominent peak in population density.Three species of Evadne, E.nordmanni, E.spinifera and E.tergestina,showed a clear ecological separation in time. The first twospecies had a peak density in April and July. respectively,but the last species demonstrated three distinct peaks betweenJune and September. Two congeneric species of the genus Podon,P.leuckarti and P.schmackeri, also did not co-exist in time.Simultaneous horizontal hauls with MTD nets made in June andSeptember 1986 revealed a strikingly marked surface distributionof all species. Evadne nordmanni occurred in June and demonstrateda clear ‘reverse’ did vertical migration, residingin the 30 m depth during the night time and ascending up tothe uppermost water column of 0–10 m during the daytime.Among the four species which occurred in September, E.spinifera,E.tergestina and P.schmackeri also showed a reverse migrationbetween the 30 m depth during the night and the very surfaceduring the day. A different pattern was observed in P.avirostrzs,which was distributed at a depth of 30 m during both the dayand the night with no sign of diel vertical migration. Amongthe more prevalent parthenogenetic individuals, some gamogeneticforms were found to exist in most species occurring in ToyamaBay.