Anomalous walking events of the mangrove crab Ucides cordatus on the beaches of the central coast of São Paulo State, Brazil: A 20-year analysis

During the reproductive period, the crab Ucides cordatus (Linnaeus, 1763) exhibits a migratory behavior known as “andada”. In these events, individuals roam mangroves and sometimes aggregate in large numbers on beaches, a phenomenon referred to here as Anomalous Walking Events (AWEs). This study documents AWEs of the species on beaches within the Marine Protected Area of the Central Coast (APA–MLC), with data on population and reproductive biology from an event on Guaratuba Beach (Bertioga, SP, Brazil). AWEs were recorded through a 20-year bibliometric survey (2000–2020) and media reports. A case study in the Guaratuba Estuarine System was conducted on February 17–18, 2018, during one of the events, and estimated the population density of U. cordatus in mangrove and beach environments. Carapace width (CW) and gonadal maturation were also analyzed. Six AWEs were identified and distributed across two beaches: Itaguaré (66.6 %) and Guaratuba (33.4 %). These events occurred between December and February, predominantly during quarter moons (waning or waxing), with no link to significant rainfall increases. Crab density in mangroves was significantly higher than on the beach (0.82 ± 0.30 ind./m² vs. 0.32 ± 0.38 ind./m²; t = 8.09; p < 0.001). Furthermore, individuals from the mangrove exhibited larger body sizes (CW) compared to those from the beach (73.2 ± 13.4 mm vs. 45.3 ± 7.2 mm; W = 92.73; p < 0.001). Although rare and anomalous, AWEs are directly associated with the reproduction of the species. Several hypotheses are proposed to explain the recorded events, but the reasons for their occurrence being restricted to only a few beaches remains unknown.