Diluted rotifer, Brachionus plicatilis (Muller), extract as culture medium stimulates hatching of red sea bream, Pagrus major (Temminck & Schlegel)

Hatching rates and hatching times (50% hatch) of red sea bream, Pagrus major (Temminck & Schlegel), eggs in culture media that were prepared to dilute rotifer Brachionus plicatilis Muller extract at 4000-, 6000-. 8000- and 10 000-fold with artificial sea water (σ₁₅, = 24.0) were investigated. Although differences in hatching rate between the culture media and control artificial sea water were negligible, hatching time was significantly shorter in media at 4000- and 6000-fold dilutions than in media at 8000- and 10 000-fold dilutions and in the control artificial sea water, when the eggs immediately post spawning were incubated. The hatching rates of eggs at three developmental stages, immediately post-spawning, gastrula stage and heart-beat initiation stage, incubated respectively in the medium at 6000-fold dilution and the control, did not differ either, yet the hatching times at each egg stage in the medium were significantly shorter than that in the control. These results suggest that hatching enzyme secretion from the hatching glands is promoted to perceive chemical stimuli in the water-soluble fraction of rotifer extract via the embryo's chemoreceptors, from the heart-beat initiation stage to immediately before hatching, thus shortening the hatching time.