Melanomacrophage centers in Gymnotus carapo as environmental bioindicators: histological and morphometric analysis in kidney, liver, and spleen.

This study analyzes the morphology of melanomacrophage centers (MMCs) in the kidney, liver, and spleen of Gymnotus carapo across two different environmental settings with varying degrees of anthropogenic impact, aiming to link habitat conditions to fish health and validate MMCs as bioindicators for environmental monitoring. A total of 28 specimens, captured and humanely sacrificed under anesthesia, were processed using conventional histological techniques. Morphometric measurements were recorded, and tissue samples were examined on an Olympus BX 41 microscope, selecting 10 random 10x fields per organ to count MMCs. Macroscopic and histological examination of the liver, kidney, and spleen highlighted MMCs distribution patterns and notable differences across sex and age groups. Adult and juvenile males showed major MMCs counts in the kidney and spleen compared to females, while liver samples from both sexes contained only pigmented cells without significant MMCs formation. Differences in MMCs quantity and structure were also observed between environments: natural habitats displayed lower MMCs counts in both sexes compared to samples from locations with higher anthropogenic impact, where MMCs frequency and pigmentation intensity were notably elevated. These findings suggest that MMCs variations in G. carapo may reflect environmental conditions and stressors, supporting the potential application of MMCs as health indicators for aquatic organisms and ecosystem monitoring. This work provides comprehensive morphometric and histological data on MMCs distribution in G. carapo, emphasizing its potential as a bioindicator species, particularly valuable in regions with varying pollution levels.