Light-dark variations in plasma melatonin concentrations in the pot-bellied seahorse Hippocampus abdominalis Lesson, 1827

Abstract

In teleosts, locomotor and physiological activities are controlled by biological synchronizers which can be triggered by endogenous or exogenous cues; the light–dark cycle is one of the most common synchronizers (Vera et al., 2006). The pineal organ processes changes in the light related hormonal signals such as melatonin which has been associated with the mediation of physiological processes (Pavlidis et al., 1999) and the transduction of light information to reproductive and growth processes in fishes (Bayarri et al., 2004). In many vertebrate species blood melatonin concentration is low during the photophase (light phase) and elevated during the scotophase (dark phase). Reiter (1989) defined the three main melatonin synthesis profiles as A, B and C; Type C is characterized by a prolonged peak during most of the scotophase and is the most common melatonin production pattern in fishes. No studies on melatonin levels in seahorses have been reported in the literature. It is known, however, that seahorses (Hippocampus sp.) display a clear diurnal activity pattern (Ouyang, 2005; Karina et al., 2006; Sheng et al., 2006). The aims of this study on adult pot-bellied seahorses Hippocampus abdominalis Lesson, 1827, were to