This study aimed to qualitatively and quantitatively analyse the molluscan assemblages associated with a Halodule wrightii seagrass bed in a rarely studied area within a conservation unit in north-eastern Brazil. Seasonal and spatial changes in several seagrass meadow characteristics, including sediment, were evaluated to explain temporal and spatial variations in the molluscs found there. The molluscan community differed in its structure among periods and meadows, as well as in the composition of its infaunal and epifaunal assemblages. The results of this study indicated that molluscs are affected by the particular characteristics of a seagrass meadow, especially by its location in the intertidal zone, more than by the area of the meadow. Molluscs were also affected by other characteristics of the seagrass meadow, such as above-ground biomass and shoot density. Changes in all molluscan assemblages were also mediated by differences among months and seasons in this region of the western equatorial Atlantic, but not by seasonal changes of the meadow. The studied meadow was found to be one of the densest in Brazil, which has considerable importance to its associated fauna.

The sexual system of two caridean shrimps (Processa riveroi and P. bermudensis) was investigated, stimulated by reports of male to female sex change (protandry) in Processa edulis from European waters. Shrimps used in the study were obtained from monthly samples taken from March 1982 to February 1983 in a Thalassia–Syringodium seagrass meadow in Puerto Rico. Observations on size, sex, and reproductive condition were used to construct monthly size–frequency distributions and sex-ratios by size-class (SC). Males were smaller than reproductive females, but there was extensive overlap in size between males and immature females. A sex-ratio (SR) analysis by SC of the overall year-long population sample showed an equal or female-biased SR in the smallest SC, a male-biased SR in 1–3 intermediate SC, with larger SC dominated by females incubating embryos. Such a population structure might result from slower growth in males than females and higher mortality in larger males. Observations on and allometric analysis of sexual characters failed to identify transitional individuals (sex-changers) intermediate in reproductive morphology between males and females. Thus, a hypothesis of protandry is rejected, and that of gonochory (separate sexes) is accepted in these two Processa species, revealing possible variation in sexual systems among Processa species.