The pineal gland of vertebrates is important because it produces and releases the time-keeping hormone melatonin (Klein et al. 1981). In most species investigated, the rhythm in melatonin production results from the activity of circadian clocks synchronized by photoperiod. However, the mechanisms through which this is achieved have been profoundly modified in the course of vertebrate evolution (Collin 1971, Collin and Oksche 1981). In aquatic species, cone-like photoreceptor cells concentrate a photoreceptive unit, clock machinery and melatonin-producing unit (Falcón et al. 2007a, 2010).