Melatonin level in guppy (Poecilia reticulata -: Osteiichthyes, Poeciliidae)

The aim of this study was to monitor the melatonin level in guppy (Poecilia reticulata) under different light conditions: LD 12:12, 16:8, 20:4 (L: 1.00-21.00 and 13.00-9.00 respectively), LL and DD. Melatonin level was detected by radioimmunoanalysis (RIA). Comparison between both sexes and three lines including the original ("wild") form of the used species was accomplished. An average number of 90 adult individuals of both sexes (line A and B together) and 30 embryos were used in each experiment. Fish of original form (Wild) were used in one experiment only. Their number was 84 individuals. For comparison of melatonin levels in brain, eyes and intestine a group of 70 individuals was used. A typical daily melatonin production curve was found in this species under light conditions LD 12:12. All other modifications of daily cycle light conditions caused a destruction of the melatonin level course. Melatonin levels in an aquarium line of standard color (A) under light conditions of LD 12:12 were significantly higher (P < 0.05) than those of melanistic line (B). In all other cases no significant differences in melatonin level values between the lines were found. A statistically significant (P 4 0.05) difference between both sexes (higher in females) was found in LL (permanent light) conditions only. Melatonin levels were also detectable in guppy embryos. Positive correlations between melatonin levels in brain, eyes and gastrointestinal tract (GIT) were found.