Effects of melatonin in connection with the antioxidant defense system in the gills of the estuarine crab Neohelice granulata

Numerous studies have shown that melatonin exerts some influence on the antioxidant defense system (ADS) in vertebrates, but for crustaceans no such effect has been demonstrated till now. However, earlier reports did show a similar profile of daily variations in the ADS of the gills and the melatonin content of the eyestalk in the crab Neohelice granulata and, thus, the aim of this study was to take a closer look at the effects of melatonin in the gill ADS of N. granulata. Gill ADS is to a minor extent modulated by reactive oxygen species (ROS), because only the nonproteic sulfhydryl (NP-SH) content increases (p < 0.05) in the presence of hydrogen peroxide (H₂O₂). No significant differences (p > 0.05) were observed in the melatonin content of the hemolymph between intact and eyestalkless crabs. Gills from intact and eyestalkless crabs injected with physiological saline showed a daily variation in the total peroxyl radical scavenging capacity (TPRSC) (p < 0.05) with two peaks, one at the photophase and another at the scotophase. However, in the gills of eyestalkless crabs injected with melatonin (2 × 10⁻¹² mol crab⁻¹), the daily variation in TPRSC values was abolished (p > 0.05). This molecule did not change the NP-SH content (p > 0.05) in vitro, but decreased (p < 0.05) the oxygen consumption in gills when incubated for 120 min. In the in vivo experiments melatonin also decreased (p < 0.05) the oxygen consumption in eyestalkless crabs after 390 min. The results suggest that melatonin does not act directly on the ADS of the gills of N. granulata, but decreases the aerobic metabolism possibly involved in variations of tissue ADS.