海産枝角類l乙関する研究-I. : Peniliaの生態について

　瀬戸内海備後灘に面する2定点において， 1966-1967年の1年間海産枝角類の調査を行なった.出現した5種のうちウスカワミジンコ，Penilia schmackeri RICHARD，について若干の観察を行ない，次のような結果を得た.
1) Penilia の出現は6月中句にはじまり，その後急激に増加して7月初旬に最高25，000f個体/m3 の密度に達した.以後急速に減少してゆき9月にはみられなくなった(Table 2).
2) 体長の分布は0.38-0.93 mm にわたるが，大別して大小の2群にわかれ，大型群は主として抱卵側体群より成る(Text-fig. 3).
3) Penilia の体長は出現初期から順次僅かながら小さくなるようであるが，雌1尾あたりの抱卵数は出現初期の平均7.5個より時日の経過とともにきわめて顕著に減少する傾向が認められ， 8月末には平均2.0個となった(Text-fig. 4).
4) いわゆる夏卵を有する個体の占める割合は，出現初期にすでに25％を示し，7月初旬に最高69%に達し，以後次第に低下して40%前後に至るが，途中6月末頃より，すなわち群集密度が最高に達する直前に雄個体とともに耐久卵をもっ雌個体が現われ，それら有性生殖{間体群の出現率は群集の消滅前に最高に達し10%に及んだ(Table 3，Text-figg.6).

The population of the marine cladocerans was followed up at the two stations in Bingo-Nada, the Inland Sea of Japan, during the period from November 1966 through October 1967.

By the observations made on Penilia schmackeri RICHARD, one of the five species of cladocerans occurred, the following results were obtained:

1) Penilia first appeared in the middle of June; then a sharp increase in number was observed, reaching the maximum value of about 25,000 individuals per cubic meter in early July. After that, the numbers decreased rapidly to zero towards September.
2) Standard length varied within the range of 0.38 to 0.93 mm. There was a general tendency that groups of two sizes seemed to be seen. The larger group was mainly composed of egg-bearing females.
3) The standard length of females with eggs seemed to become gradually smaller from the time of its appearance to the later period, while the fertility as expressed in quantity of eggs or embryos per brood was highest (7.5) at first, followed by a drastic decrease thereafter.
4) Females with summer eggs took up about 25% of the population in the middle of June just after its first appearance. They attained their maximum (69%) in early July, then gradually decreased in number to about 40%. Sexual individuals, namely females bearing a resting egg and males, occurred already in late June, shortly before the highest population density, and continued to be seen until the end of August when Penilia was going to disappear.