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Reproduction in the widely distributed tropical sea urchinEchinometra mathaei(de Blainville) was examined in three Kenyan reef lagoons that differed in substrate cover andE.mathaeipopulation density. Histological examination of the gonads and gonad index measurements for 3 years showed a seasonal reproductive cycle with gametogenesis commencing in July, when temperature and light are at their lowest, and spawning commencing in December and peaking in February to May, when temperature and light reach their annual maxima. Monthly gonad indices correlated significantly with seawater temperature and light intensity. Male urchins had mature gametes for a longer period (8 months) than females (6 months), possibly an adaptive strategy that increases the probability of fertilization. Lunar periodicity was not observed, as male and female gonads were full of gametes on all days of the lunar cycle during the period of spawning. The peak in spawning activity coincides with the peak in phytoplankton abundance, which could ensure high food availability for the planktonic larvae. Gonad weights were significantly higher relative to urchin weight at the reef having the largest mean sizes and lowest population of urchins, indicating availability of food resources for growth and reproduction.E.mathaeiat the reef with the highest density had the smallest urchins and high relative gonad sizes, indicative of a tradeoff between growth and reproduction when food is limited. [ABSTRACT FROM AUTHOR]