Hydroxyapatite (HAp) prepared from baked livestock bone as fine solid powder efficiently inhibited the release of iron ions from the metallic iron plate in a 3% sodium chloride solution. The effects of fine powder HAp on Tafel measurement and anodic polarization using iron electrodes were examined. The response current density in the active region significantly increased depending on the potential on the iron electrode surface without HAp. On the other hand, on the iron electrode surface installed with fine powder HAp, the response current density increased in the active range up to about -0.2 V, and then, the increase in response current density was completely suppressed up to about 0.5V. Next, the surface of the iron electrode contacted with the fine powder HAp after the anodic polarization measurement was analyzed by TEM. As a result, the formation of iron hydroxide layer mainly containing Fe(OH)₃ with a thickness of approximately 1.3 μm between the iron electrode and HAp was confirmed. It was suggested that fine powder HAp strongly suppressed the release of iron ions and electrons with protection of the passivation iron hydroxide layer.

The sea urchin Mesocentrotus nudus is a valuable commercial species in Hokkaido and northern Honshu, Japan, where artificial seeds produced in the hatcheries are released on fishery grounds for stock enhancement program. To develop a method for promoting broodstock maturation, we investigated the effects of photoperiod manipulation on the gonadal maturation of this species. Adult sea urchins were reared from December for 9-11 months under manipulated photoperiods where the annual change in day length between the two winter solstices was shortened to 6, 8, and 10 months, or advanced 2 months ahead of the natural photoperiod. Maturation progress was evaluated by observing the wet-squash gonad preparations and spawning reaction tests, which involved observing gamete release from the gonopore after removing the coelomic fluid. Experimental urchins reared under any of the accelerated and advanced photoperiods matured around August, one month earlier than those reared under the ambient photoperiod and the wild urchins collected from the coastal water. We conclude that the maturation of M. nudus can be promoted for early gamete collection in the hatcheries using the artificially accelerated or advanced photoperiods.