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Monthly zooplankton collections were carried out from January to December 1991 at two sampling sites, Cuenca Norte and Boj6rquez lagoon, in the Nichupt6 lagoon system, a partially enclosed network of interconnected waterways located in the northeastern region of the Yucatan Peninsula (Mexico) adjacent to the Caribbean Sea. Only one species of Chaetognatha, Sagitta hispida Conant, was present and was more abundant at Cuenca Norte (total density 450.6 organisms/m3) than at Boj6rquez (138.6 organisms/m3). The latter site is smaller, more physically isolated, and more environmentally stressed than the former. From monthly gonadal and length-frequency analyses of 1390 specimens, it was found that (1) total length significantly differed among four successive maturity stages, (2) juvenile and immature specimens occurred in greater numbers at Boj6rquez. while more mature specimens comprised a greater percentage of individuals found at Cuenca Norte, and (3) individuals collected at Bojbrquez, where slightly higher temperatures wererecorded, were significantly smaller than those from Cuenca Norte. The latter two findings indicate that Sagitta hispida spawns at a higher frequency at Boj6rquez, possibly due to the cumulative effect of higher temperature.

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