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The longsnout seahorse (Hippocampus reidi) is a vulnerable species found along most of the Brazilian coastline, such as semi-arid estuaries with strong rainfall seasonality, hypersalinity, and low depth. To evaluate the population structure of H. reidi over time, we monitored the seahorse population in the Pacoti estuary (Brazil) for one year, based on 248 registered specimens. Salinity, water transparency, sex, pregnancy, body height, and holdfast use were registered. Mixed linear models revealed that sampling month, salinity, and transparency had no influence on population density in the lower zone of the estuary. Pregnant individuals were more frequent in the dry season and at higher salinities. Mean body height (12.7 cm) increased in the dry season. Bright colors were predominant. The seahorses employed nine types of holdfasts, most often mangrove roots, and were found to reproduce throughout the year, peaking in the dry season. Salinity and transparency did not impact population density. In Brazilian semi-arid estuaries, the longsnout seahorse is strongly associated with mangrove vegetation, used as holdfast. Therefore, the conservation of seahorse populations depends on the conservation of the local mangrove forests. [ABSTRACT FROM AUTHOR]