This study investigated the diversity and spatial arrangement of fiddler crabs in the Nizampatnam mangroves of Andhra Pradesh by integrating field-based ecological observations with cytochrome oxidase I (COI) gene evidence. Four distinct species were recorded: Austruca annulipes, Austruca variegata, Gelasimus hesperiae, and Tubuca alcocki. A total of 245 specimens were documented, with the highest concentration in the lower intertidal zone (123 individuals), followed by the middle (71) and upper zones (51). Diversity analysis yielded a Shannon–Wiener index of approximately 1.36 and an evenness of 0.98, indicating a moderately diverse and highly balanced community. COI-based phylogenetic interpretation supported species-level differentiation and ecological partitioning, with broad-front and narrow-front lineages reflecting distinct evolutionary histories. The findings classify the Nizampatnam mangrove ecosystem as moderately disturbed but resilient, with fiddler crabs serving as effective bioindicators of habitat health.