SUMMARY
The Purple Mangrove Crab, Goniopsis pelii is one of the more elusive crab species inhabiting most of the microhabitats in the mangrove ecosystem of Southern Nigeria. This study was carried out to investigate the size composition, growth pattern, diets, and reproductive biology of G. pelii from Abule-Eledu mangrove swamp in Lagos Lagoon, Nigeria. A total of 116 specimens were studied. Data revealed that the carapace length and total weight ranged from 2.5 to 4.9 cm (3.72 ± 0.39) and from 11 to 37 g (23.90 ± 4.21) respectively. The crab exhibited negative allometric growth pattern (b<3), and the carapace length-weight relationship was positively correlated (r = 0.577-0.900). The condition factor (K) ranged from 3.5-8.0 (male), 3.2-7.5 (female) and 3.4-8.0 (sexes combined). The highest K-values were recorded for the smaller size group (2.5 - 2.9 cm). In terms of sex, the male has the higher K-value (8.0). Identified food particles included plant materials, algae, detritus, diatoms, and protozoa. The sex ratio was 1:0.57 (male: female), and was significantly different from the theoretical ratio of 1:1. Fecundity ranged between 1.9 and 2.8 million eggs with egg diameter ranging from 0.26 to 0.38 mm. Crabs with partial gonad development were predominant in the mangrove, while adults with fully ripe gonads were encountered occasionally throughout the study period. In this study, the reproductive indices and body conditions are indicators of a viable population.Keywords: allometric growth, crab feeding, reproductive biology, Lagos Lagoon.