In fish, the pineal organ (PO) is the main centre for the circadian and seasonal photoneurohormonal regulation of physiological status and behavioral pattern rhythmicity. This is achieved by the conversion of environmental photosignals into neurohormonal output in PO photoreceptor cells. The function of this organ directly links the physiological profile of the organism with its environment through hormonal attenuation and behavior. Pineal melatonin production is high during the night and low during the day, and is extremely sensitive to ambient radiation.

The role of the PO in circadian organization has received most attention, because it is involved in piloting numerous rhythmic functions and is tightly connected with behavioral strategy choice. The PO is one of the main structures determining the degree and individual characteristics of physiological plasticity and environmental adaptogenesis in lower vertebrates. By applying various types of radiation, it is possible to artificially shift pineal function, thereby achieving functional regulation of various indices.

Over the last seven years, experiments have been undertaken at a trout farm in the Crimean Mountain Reservation. These have demonstrated the efficiency of pineal regulation for piloting spawning, smolts, growth rates, food consumption efficiency and the development of behavioral patterns in rainbow trout. As a species lacking an inner ‘pacemaker’, the salmon is an especially convenient subject for assessing the performance of light dependent functions, because incoming photosignals cause direct neurohormonal effects in its brain, as well as corresponding shifts in physiological indices.

In this study, two sets of experiments were conducted under different illumination regimes: dim light (L:D = 0:24, 10-500 lux) and constant light (L:D = 24:0, 10,000-15,000 lux). Both sets consisted of three experimental groups, each containing 25 specimens. The regime for the control group (20 specimens) was L:D = 12:12, 5,000-7,000 lux. The regime L:D = 0:24 caused growth rates to increase by 7-12% and mortality to decrease by 75-100%, and led to the normalization of feeding behavior. The L:D = 24:0 regime resulted in gonadal maturation increasing by 3-5% but immune status decreasing, as demonstrated by increased numbers of infected specimens and changes in blood cell composition. Schooling and the ability to withstand the current were also shown to depend on the melatonin-piloting illumination regime applied. Considered independently, these changes might not be treated as having a systematic link – but occurring in parallel, they seem far more likely to reflect inner hormonal shifts.

Overall, the entire life cycle of these fish seems to be extremely light sensitive. The physiology of the PO, and pineal regulation of organism activity as a whole, have both been studied extensively – but mainly in higher vertebrates. This study helps to show that they may be more important in lower vertebrates with active locomotion and migratory ethology (e.g. fish). Understanding the neurohormonal structure of the PO is of supreme importance for explaining how behavioral and endocrine strategies are coupled in fish.

Paper presented at Measuring Behavior 2002 , 4^th International Conference on Methods and Techniques in Behavioral Research, 27-30 August 2002, Amsterdam, The Netherlands

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