Seasonal reproductive changes in several populations of the echinoid Echinometra mathaei near both the head and mouth of the Gulf of Suez were followed for more than a year. Well-defined reproductive periodicities occurred near the head of the Gulf; gametogenesis began in different
individuals in the spring and early summer, and spawning occurred in the summer and fall. At other times of the year the gonads were completely quiescent reproductively. Nutritive phagocytic tissue in the gonads accumulated mainly in the fall and early winter, and decreased mainly in the late
spring and summer. Much less well-defined reproductive periodicities occurred near the mouth of the Gulf, and in the adjacent Red Sea and farther south there was no synchronized periodicity within the populations. It is suggested that both a critical minimal temperature and a critical minimal
level of nutrient reserves are important for the regulation of the reproductive periodicities.
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