Fig. 1. Scheme showing the discovery of periodicity in the disappearance of spermatophores in Litopenaeus vannamei. (Left) Long-term observations on three different types of shrimp population. (Middle) The percentage of males found without spermatophores. (Right) A flow chart summarizing the logical flow of thought that led to our hypothesis. (A) Field observations twice a month on maturing populations of shrimp in aquaculture ponds showed the females to be arrested in a pre-pubertal state. The percentage of juvenile males found without spermatophores - initially 100% - decreased steadily to an average of 9.7±8.6% during a 5-month period as the animals matured to become sub-adults. The remainder of the males were found to carry mature spermatophores containing fully developed spermatozoa. (B) In an experimental adult population, males were found with mature spermatophores most of the time during which the females were not receptive, and the average percentage of males found without spermatophores during this time was 5.6±3.2% (Bi). When females were going through vitellogenic cycles and were receptive (Bii, red font), the average percentage of males found without spermatophores was 12.4±11.3. (C) In an experimental all-male population, 5.0±1.4% of the males were found without spermatophores.