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MISSION DISPATCH 5 June 6, 2005 | H. J. Wagner | San Clemente Basin (off the coast of S. California) INTERNAL CLOCKS IN MESOPELAGIC FISH Each day, vast populations of the mesopelagic fauna undertake vertical migrations from their daytime depths of about 600m to about 200 m at night, allowing them to feed in the nutrient-rich upper water layers with less risk of predation. To date, it is unclear whether this migration is controlled by a strategy where the fish choose to remain at a constant level of illumination, or whether an internal clock controls this behavior; it is also possible that both mechanisms are used in conjunction. 
In order to study the presence of an internal diel clock (oscillator) we study the
level of melatonin in the pineal organ and the retina of the eyes. Melatonin (also
marketed as anti-jet-lag drug) is a neurohormone present in all vertebrates (including humans)
and acts as a signaling molecule, telling all peripheral organs the time as measured
by the photoreceptors in the eye and the pineal organ. Pineal organs and retinae are
isolated and frozen on board ship and biochemical assays will be run in the home lab on
shore. The expectation is that the presence of an internal clock will lead to low levels
of melatonin during the day and high levels at night.
PROCESSING VISUAL INFORMATION IN ALL ROD RETINAE In order to maximize sensitivity, most mesopelagic fish have only rods (no cones) in their retinae. They are therefore not capable of color vision, but are optimally adapted for the detection of bioluminescent stimuli. We have previously shown that the number of nerve cell types in the retinae of these fishes is much reduced and therefore the neural processing less complex than in surface fish, which as a rule, have very well developed color vision systems. In particular, we have focused on the retinal ganglion cells, which function to relay the output signal of retinal processing to the brain, and more specifically the main optic centre, the optic tectum. To date, there are no data on the morphological and functional organization of the optic tectum in deep sea fish. We propose to use mesopelagic fish as a model system to elucidate the function of the optic tectum arguing that the simple organization of the retina must somehow be reflected in the "wiring" of the tectum. Our approach to this problem consists of labeling individual ganglion cells by microinjection of a fluorescent tracer molecule into the axon in the optic nerve. The dye is taken up and fills the cell body plus dendrites in the retina, and also the axon terminal in the optic tectum. This requires a preparation of isolated brain and eyes (retinae) which need to be cultured for 2-3 days. Subsequently, the tissue is fixed, transferred to the home lab and prepared for visualization in the confocal microscope. Additional staining with various antibodies will reveal their connectivity in the retina and the optic tectum. 
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