Characteristics that define the habitat of the leptostracan Paranebalia belizensis Modlin, inhabiting the waters of Twin Cays, a group of Belizian mangrove-covered islands, are described. A primary component of the habitat of this species appears to be Halimeda opuntia. The complex structure of this macroalga creates a filter that traps waterborne detrital particles; thus it collects the food of leptostracans. It also functions as a refuge from predation because its natural defenses discourage

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... erbivory. From field observations and collections made within the habitat, information on the spatial and temporal distribution, population structure, and fecundity of P. belizensis is presented. Females and juveniles predominate the population. Males are rare and primarily occur in immature stages. Mature males were collected only in spring, when population size was largest. Reproduction is highest in early spring and continues throughout the year. Fecundity is not correlated with the total length of a female. Factors affecting the lack of ecological information on the leptostacans are discussed.