A microscopic morphological feature (eosinophilic granulocytes, eG) was recently detected in the gonads of two species of economically important and broadly distributed, tropical Indo-Pacific eteline snappers, primarily in the ovaries of mature but reproductively inactive female Ruby Snapper Etelis carbunculus. Eosinophilic granulocytes have been known for more than a century to occur in diverse tissues (e.g., liver, kidney, gill, gonad) of a variety of freshwater and marine teleosts and have usually been recognized as bio-markers of immune response to environmental toxins or pathogens. Until now, they have been mostly overlooked as indicators of natural atresia in gonads and evidence of prior reproductive function in healthy fishes. The eG feature is herein suggested as a bio-marker of sexual maturity that might be useful when combined with an existing suite of largely qualitative markers of maturity in mature females that lack direct signs of maturity such as atretic yolked oocytes and post-ovulatory follicles. This marker may reduce the present uncertainty in distinguishing mature but inactively reproducing females from immature females in studies of fish and fisheries biology, specifically when estimating body size at sexual maturity.