The pineal complex of Senegalese sole (Solea senegalensis): Anatomical, histological and immunohistochemical study

Many fish exhibit seasonal reproductive rhythms that are synchronized by environmental cues such as photoperiod and temperature. The pineal organ of teleost fish, a non-visual photosensitive structure, plays an important role in this synchronization because it transduces the photoperiodic and temperature information into neural and neurohormonal messages that reach the brain and the reproductive axis. In this study, we describe the anatomy and histology of the pineal complex of Senegalese sole, Solea senegalensis, a seasonal species of increasing importance for aquaculture. We have also analyzed the presence of photoreceptor and melatonin-producing cells using immunohistochemical techniques. The pineal complex of sole consists of the pineal and parapineal organs, and a highly lobulated dorsal sac. The sole exhibits a real metamorphosis during early developmental stages that induces an asymmetry of the rostral forebrain areas, including the pineal organ that shifts its photosensitive pineal vesicle towards the upper-right pigmented side, where both eyes are also placed. To the best of our knowledge, such a marked asymmetry in the position of the epithalamic pineal organ has never been described in fish. The pineal vesicle is hypertrophied, probably reflecting an adaptation to benthonic life. Melatonin-secreting cells, which were revealed using antisera against its precursor serotonin and against hydroxyindole-O-methyltransferase, appear to be direct photoreceptive cells because they adopt a distribution similar to that of the cone opsin- and rodopsin-immunostained cells, along the pineal stalk and vesicle. This work and other ongoing studies analyzing pineal projections and melatonin targets will give us an important basis for understanding the integration of the photoperiodic signal in this species.