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(1) Gravid females representing fourteen species of marine snakes were collected from four localities along the coast of Malaysia in 1975. The average clutch size ranged from 2.9 to 17.8 but most species had between three and seven young. The number of young produced and the size of gravid Lapemis hardwickii differed significantly at two sites in the Straits of Malacca. Eight species showed a significantly positive relationship between the number of oviducal eggs and female weight. The rate of infertility among ten species ranged from 4.5 to 20%.
(2) Sex ratios in the collections varied significantly through time and between localities. Enhydrina schistosa and other species collected at Muar showed tightly synchronized annual reproductive cycles. Species at other localities appeared to have more loosely synchronized cycles.
(3) A comparison of species in terms of clutch size, female weight and estimated birth weight revealed several distinct reproductive patterns that are considered in the context of r and K-selection. Hydrophis fasciatus is a small species that produced small clutches of medium sized young and it puts out the highest relative effort per embryo of any marine snake, 11 %. Lapemis hardwickii and Thalassophina viperina are large snakes which have three or four very large young. Their relative effort per embryo is high, too. Enhydrina schistosa is the largest species studied and has many medium sized offspring. Its relative effort per embryo is the lowest of any marine snakes studied (2%). The remaining species make up a loose group that falls between the group T. viperina and L. hardwickii and E. schistosa in terms of effort per embryo. In general, these species have modest sized clutches and small female weights.