This article is a summarization of a series of studies concerning the ecology of marine cladocerans in Bingo- and Hiuchi-Nada in the central part of the Inland Sea of Japan (the Seto Inland Sea) during a period from 1966 through 1972. The purpose of these studies is to clarify some aspects of the ecology of these animals, their importance in the coastal zooplankton having been overlooked rather largely, as well as to obtain certain fundamental biological data needed for their future utilization as food of fish larvae in marine fish farming.
The results obtained in these studies can be summarized as follows:
1) The following 5 species of marine cladocerans belonging to 3 genera have been found to occur in this sea area: Evadne nordmanni LOVÉN, Evadne tergestina CLAUS, Podon leuckarti G.O. SARS, Podon polyphemoides (LEUCKART), and Penilia avirostris DANA.
2) All these cladocerans appear during the time from spring through autumn. Each species has its own season of occurrence which repeats itself regularly every year. Temperature is the leading environmental factor limiting the seasonal distribution of each species.
3) All the species are found to be distributed throughout the area investigated. Podon polyphemoides is most densely distributed in the northern and north-eastern regions where waters are of a most embaymental character. On the contrary, Evadne tergestina and Penilia avirostris are observed to be most abundant in the central and south-western regions where waters are of a most offshore character within this sea area.
4) Restriction to surface layers between 0 and 10 m deep is a characteristic feature of the vertical distribution of marine cladocerans in the area during the daytime. This tendency is especially marked in the 2 species of Evadne: E. nordmanni and E. tergestina. Diurnal changes in the vertical distribution have been observed in E. tergestina and P. polyphemoides, which appear most abundantly in surface layers during the day and migrate downward at night.
5) No marked seasonal change in size frequency distribution is observed in each species. The mean number of parthenogenetic eggs or embryos per batch is found to be highest when the population first appears in the plankton and gradually decreases thereafter, in the following 3 species examined: E. nordmanni, P. polyphemoides and P. avirostris. Bisexual reproduction becomes most intense when the mean number of parthenogenetic eggs or embryos per batch reaches its minimum. With the onset of sexual reproduction, population decreases drastically until complete disappearance from the plankton.
6) An analysis of the population of P. avirostris containing a high-rate of sexual individuals (females bearing resting eggs, and males) has revealed obvious differences in the composition of sexual types among the samples taken at different stations on almost the same day within this area. The maximum number of females carrying resting eggs reaches as many as 13.5% of the total population.
7) Paedogenesis or neoteny has been observed in all the podonids of this study area. The parthenogenetic females of E. nordmanni and E. tergestina liberate their young (strictly speaking, miniature adults) only during the dark period of the 24-hour day: from midnight to early morning before dawn. The rate of growth of the parthenogenetic embryos within the brood pouch seems to be very rapid in summer. At temperatures of 26-27℃, the young are liberated from the mother animal within 48 hours.
8) Many resting eggs of E. tergestina and P. avirostris have been obtained in laboratory tanks. The resting egg of E. tergestina is spherical in shape with a mean diameter of 204µ. The egg membrane is very thick and stout. In reflex light, it is brownish in color. The resting egg of P. avirostris is ovoid, 250µ long and 180µ wide, and is compressed dorso-ventrally with a mean thickness of 100µ. It is concaved characteristically on its ventral surface. In transmitted light, it looks dark, but greyish in reflex light.
9) A search through the bottom mud of Bingo- and Hiuchi-Nada has revealed an affluent presence of resting eggs of marine cladocerans. The eggs of P. avirostris attain 6.7-7.9 ×104/m2 and those of Evadne and/or Podon, 1.5-1.8×104/m2, for the samples taken at 9 stations. On the contrary, much smaller number of eggs has been recovered from the bottom of Tachibana Bay, Tokushima Prefecture.
10) The resting eggs of marine cladocerans obtained in the laboratory tanks and those from the sea bottom mud have been both able to hatch under laboratory conditions. In the eggs of P. avirostris, high hatching rates have been obtained at temperatures of 17-20℃, which coincide with the sea temperatures at the time of the first appearance of this species in the plankton samples. They also hatch under a wide range of low salinities down to 4‰ chlorinity. The resting eggs of Evadne and/or Podon show higher rates of hatching at 12-17℃. As in the former species, hatching is observed in salinities down to 4‰ chlorinity.
11) Some resting eggs of P. avirostris, isolated from the sea bottom mud which had been placed in the laboratory for 13 months, hatched normally.
12) Developmental and hatching processes of the resting eggs of P. avirostris and E. tergestina are described and figured, with morphological characteristics of the newly hatched young (including that of P. polyphemoides). The incubation period of the resting eggs of P. avirostris is about 100 hours at 19.4℃.
13) Some aspects of the problems relating to the cultivation of marine cladocerans are discussed from the viewpoint of reproductive biology, feeding habits, population density, etc., of each species.