Variação espaço-temporal de Donax gemmula (mollusca: donacidae) em praias arenosas de Aracaju, Sergipe, nordeste do Brasil

Abstract

Sandy beaches are highly dynamic environments that dominate continental coastlines. The genus Donax (Bivalvia: Tellinoidea: Donacidae) is one of the most common taxonomic groups on Brazilian beaches, distributed primarily in the intertidal and shallow infralittoral zones of tropical sandy shores. Donax gemmula is the smallest species of the genus in the southwestern Atlantic and was the dominant species on the beaches of Aracaju for several years. However, most ecological knowledge on this species comes from benthic macrofauna studies conducted in temperate and subtropical climates. The present study characterized the spatio-temporal variation in the abundance and maximum shell length (MSL) of D. gemmula in the swash zone of sandy beaches in Aracaju, Sergipe, northeastern Brazil, and assessed whether species abundance tracks the variation of oceanic, meteorological, and morphodynamic environmental variables in a tropical system. Eight sites along ~23 km of Aracaju's beaches were sampled bimonthly over one year (Sep/2024–Jul/2025). Specimens were identified, quantified, and measured through image thresholding. Environmental variables were measured in situ and retrieved from online databases. Spatio-temporal variation in abundance was analyzed using PERMANOVA, followed by PERMDISP and post-hoc analyses. The relationship between abundance and environmental variables was analyzed via LMMs, with date as a random factor. Environmental variables exhibited distinct temporal patterns. A total of 3,482 D. gemmula individuals were recorded. Mean abundance varied spatio-temporally, peaking in January and March, with the lowest values in May and July 2025. Spatial differences in mean abundance were detected only in January and May 2025. MSL ranged from 1.52 to 8.7 mm, with January and March showing the highest MSL values. Juveniles strongly dominated the population (96.5%). The LMM indicated a negative relationship between abundance and mean grain size. The observed abundance and size ranges fell within the values reported in the literature, contrasting with the macroecological pattern of reduction towards the tropics. Species abundance was higher in months characterized by higher temperatures, moderate flow discharge of the Sergipe River, lower oceanic chlorophyll-a (chla), and lower wave height; and was lower in months with higher accumulated rainfall. The negative association with chla contradicts the expected pattern of increased abundance with greater food availability. Overall, our findings suggest that patterns established for subtropical and temperate sandy beaches are not directly generalizable to tropical D. gemmula populations, highlighting the critical role of environmental context in driving the spatial and temporal variation of the species.