The marine crab Plagusia depressa, despite its broad geographic range, is not well known. Here, we attempt to fill this gap by studying several aspects of its biology. We sampled P. depressa in the reefs of Coruripe, Brazil and investigated the sex ratio, body size, maturity, the allometric growth of the gonopod in males and of the abdomen in females, the reproductive effort, the breeding season and the influence of environmental variables. We found that 1) the sex ratio, overall, does not deviate from 1:1; 2) females (33.71±10.62 mm) are larger than males (30.45±9.94 mm; t_1,764: -4.40, p<0.001); 3) males (23.42±0.24 mm) mature at smaller sizes than females (26.78±0.19 mm); 4) the growth of the gonopod is positive allometric in juveniles (slope = 1.66±0.04), but negative allometric in adolescents (0.93±0.02) and adults (slope = 0.86±0.03). In females, the abdomen undergoes a positive allometry in juveniles (slope = 1.74±0.04) and adolescents (slope = 1.97±0.09), but an isometric growth in adults (slope = 0.99±0.04); 5) the breeding period is continuous and not influenced by rainfall, water temperature and salinity; 6) egg mass strongly increases with body mass (slope = 1.56, R²=0.84, p<0.001).