Resting eggs of the rotifers Brachionus plicatilis and B. rotundiformis are generally found in the bottom sediments in the nature. Here, environmental conditions tend to inhibit the hatching of resting eggs because oxygen concentrations and light levels are low. However, several workers have also reported the appearance of floating resting eggs in these species. These are characterized by a black ring inside of the extra-embryonic space. If this floating behavior can be utilized to avoid the inhibitory environmental effects which reduce the hatching rate of these eggs, then this mechanism clearly has important ecological significance. In this study, the factors which induce flotation in resting eggs were investigated in the laboratory. Changes in temperature, salinity, photoperiod, dissolved oxygen and ammonium sulfate concentration did not increase the frequency of floating eggs in either species, irrespective of the developmental stage of the egg. Floating eggs appeared only when resting eggs were exposed to air. These results suggested that flotation of resting eggs may not be due to a biological process in the normal life cycle of the rotifer. It seems more likely that it occurs as an artifact derived from the methods used in handling mud samples or managing of rotifer cultures.海産ツボワムシの休眠卵はふつう沈降性を示し,天然では汽水域の底泥中に分布するが,孵化直前の休眠卵の中には,卵空室部にガスがたまって浮遊性を示す例が報告されている。休眠卵の孵化には光照射が必要なので,前述の現象を孵化に阻害的な底質環境を回避する手段と捉えることができるかもしれない。その形成機構を2種の海産ツボワムシ(Brachionus Plicatilis, B. rotundiformis)の休眠卵を材料として調べた。環境因子(水温,塩分,光条件,溶存酸素量,硫化アンモニウム濃度)を様々に調節して,卵の保存とインキュベートを行ったが,浮遊卵の出現は全く観察されなかった。一方,乾燥処理や培養槽の瀑気など,休眠卵と空気の接触の影響をみたところ,卵空室部内にガスが取り込まれ,浮遊卵が高率で出現した。この現象は2種のワムシに共通にみられた。したがって,この現象を,孵化に必要な光照射の機会を増大するための手段として考えることは難しい。過去の浮遊卵の観察例は,休眠卵を分離する際に採取した底泥を乾燥したり,培養槽に通気を施すなどの人工的な操作に基づくものと考える。既往の知見と併せると,底泥に分布する休眠卵は光の直接照射よりも光化学的な作用機構によって孵化する可能性が高い
