Resting eggs of the marine rotifer Brachionus plicatilis Muller: Development, and effect of irradiation on hatching

Diapause and hatching of Brachionus plicatilis Muller resting eggs were examined through histological and optical approaches. Compound microscope observations on 1% toluidine blue-stained embryo sections suggests that the total number of nuclei in an embryo during the internal diapause period increased from 22 on Day 2 to 39 (each n = 1) on Day 6. The outer layer of embryo membrane gradually thickens from 1.2 (Day 0) to 4.0 mu m (Day 8) (each n = 10). Resting eggs that have completed maturation and are in the external diapause period require light for hatching. The threshold of light (halogen lamp) intensity for hatching was estimated to be 4400 lux for 30 min. Hatching rate decreased with longer wavelength irradiation (mercury lamp). Irradiation at more than 350 nm caused 1-25% hatching, but it reached 50-60% at 250-310 nm light. The addition of hydrogen peroxide or prostaglandins (E(1), E(2) or F-2 alpha) caused resting egg hatching even in darkness. The production of peroxide in seawater caused by light as well as the oxidation of fatty acid to prostaglandins inside the embryo is a possible mechanism of resting egg hatching.