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Studies were made of the population structure and life cycle of the sand crab Eme&z anaEoga ( Stimpson).Frequent sampling of the beach and of the plankton during a twoyear period indicates : 1) the population structure of the species differs from beach to beach; 2) an aggregation is not necessarily characteristic of the entire beach population; 3) seasonal changes drastically alter the population structure of sand crabs; and 4) occurrence and abundance of zoeae in the plankton closely parallel the arrival of megalopae on the beaches, but not necessarily on any one beach.The life cycle of this species can now be summarized as follows:The mating season in late spring and summer precedes a continual production and release of young during the summer. These pelagic zoeae pass through at least five molts before returning to shore as megalopae.Megalopae arrive in quantity on our beaches as early as September and continue to arrive during winter and spring. By mid-November the reproductive females that prevailed in late summer apparently disappear.The majority of the beach population during winter and early spring consists of megalopae, juveniles, and males; this suggests an annual cycle for the species.The progressive seasonal incrcasc in the percentage of females in the larger sizes, coupled with the relative absence of females in some of the smaller size classes, suggest a sex reversal ( protandric hermaphroditism ) .

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