The environmental light to which most animals are exposed has a major influence in their physiology. Over the decades attempts are being made to understand the physiological mechanism of the processing of environmental light information in the regulation of various body functions in general and reproduction in particular. Light as a source of environmental cue in the use of regulation of reproduction has also been considered for fish, the study on which has gained momentum because of its tremendous economic importance as food. Thus for obvious reasons efforts are continued to develop technologies for increasing fecundity and growth of fish in culture. As a result, photoperiodic manipulation has emerged as an effective tool of reproductive management in culture fisheries, and understanding the physiology of photoperiodic regulation of fish reproduction became the priority topic of research. Precisely, this investigative treatise embodies original findings on the identification of photoreceptor cells, demonstration of the photoreceptor molecules and their diurnal and seasonal profiles in the pineal organ in relation to the reproductive status of the free-living carp.