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 intervals from February 1990 throu gh January 1991 at a station in Toyama Bay, southern Japan Sea, were e xamined. From mid-March through early December, cladoceran populations exhibited five distinct peaks, in which seven species occurred and sh owed a species-specific pattern of seasonal distribution. In mid-July, Penilia avirostris exhibited a prominent peak in population density. Three species of Evadne, E.nordmanni, E.spinifera and E.tergestina, sh owed a clear ecological separation in time. The first two species had a peak density in April and July, respectively, but the last species d emonstrated three distinct peaks between June and September. Two conge neric species of the genus Podon, P.leuckarti and P.schmackeri, also d id not co-exist in time. Simultaneous horizontal hauls with MTD nets m ade in June and September 1986 revealed a strikingly marked surface di stribution of all species. Evadne nordmanni occurred in June and demon strated a clear 'reverse' diel vertical migration, residing in the 30 m depth during the night-time and ascending up to the uppermost water column of 0-10 m during the daytime. Among the four species which occu rred in September, E.spinifera, E.tergestina and P.schmackeri also sho wed a reverse migration between the 30 m depth during the night and th e very surface during the day. A different pattern was observed in P.a virostris, which was distributed at a depth of 30 m during both the da y and the night with no sign of diel vertical migration. Among the mor e prevalent parthenogenetic individuals, some gamogenetic forms were f ound to exist in most species occurring in Toyama Bay.