Gamogenetic Forms of Evadne tergestina CLAUS (Branchiopoda, Cladocera) of the Inland Sea of Japan

瀬戸内海中央部,備後灘・燧灘における海産枝角類トゲナシエボシミジンコ(Evadne tergestina CLAUS)の有性生殖個体について,従来の報告よりもやや詳しく再記載した。成体オスには1対の精巣があり,輸精管を経て1対の陰茎に終る。第1胸脚内枝末端の刺毛は鋭く反曲した小鉤に変形し,その腹側にある2本の刺毛も強い鎌状剛毛に変形している。有性生殖メスは1個の大きい球形の耐久卵(休眠卵)をもち,尾爪背後方に育房に通ずる生殖孔(膣)の開口がある。これらの特徴はPodonidaeに属する他種について従来記載された点とほぼ類似しているが,若干の差も認められる。1973年9月本水域の数定点における本種の個体群全体中に占める有性生殖個体の出現率は2.7-23.3%の範囲にあり,そのうちオスが2.2-12.5%,耐久卵保有メスは0-12.4%であって,場所によりその出現率にかなりの差のあることが見出された。なお耐久卵の生産は大型のメスによって行なわれる傾向が認められる。

A redescription is given of the gamogenetic forms of Evadne tergestina CLAUS (Branchiopoda,Cladocera) in somewhat more detail than in previous publications, based on the specimens collected on September 11-13, 1973 at several stations in the central Inland Sea of Japan. The adult male is provided with a pair of testes, each lobe of which terminates through a seminal duct in a long, tapering penis situated just behind the last thoracic leg. The terminal seta on the endopodite of the 1st thoracic leg is modified into a sharp, recurved hook, with 2 ventral setae also differentiating into very strong, falciform bristles. The gamogenetic female has a large resting egg, which looks dark in transmitted light, and an opening, the vagina, on the shell surface behind the caudal furca. These characteristics are similar to those in other podonid species. The number of gamogenetic forms in the total population on these dates varied considerably according to the stations, ranging from 2.7% to 23.3%, of which males comprised 2.2-12.5% and females with resting eggs, 0-12.4%. There seems to be a clear tendency for resting eggs to be formed in larger individuals.