Evidence for the presence of serotonin in Mysidacea (Crustacea, Peracarida) as revealed by fluorescence immunohistochemistry

In crustaceans, serotonin (5-HT) exerts a wide range of physiological actions on many tissues. However, 5-HT has not been detected to date in Mysidacea (Crustacea, Peracarida). We have investigated the presence of 5-HT in the brain and the eyestalks of two Mysida (Leptomysis lingvura, Hemimysis margalefi) and one Lophogastrida (Lophogaster typicus) species by using the immunohistofluorescence technique. 5-HT-like immunopositive areas exhibit a similar pattern in the three species. 5-HT-like immunostaining is present in the retinular photosensitive cells, except in the deep-living species L. typicus. 5-HT-like cell bodies and fibres are observed in the lamina ganglionaris and in the three medullae. In the sinus gland, only 5-HT-like endings are detected. In the eyestalks, 5-HT-like fibres detected in the optic tract link with the protocerebrum, in which 5-HT-like somata and their extensions are found. Some neurones are detected in the anterior median cell cluster, in the protocerebral bridge and in the central body. In the deutocerebrum, the paracentral lobes are connected by immunoreactive fibres that run along the deutocerebral commissure. The glomeruli of the olfactory lobes exhibit strong diffuse immunostaining. Beside and in the median part of the deutocerebrum, at least two large serotoninergic neurones project their axons into the olfactory lobe cell cluster. Immunoreactive fibres are also found in the antennular neuropiles. Our results demonstrate the presence of 5-HT-like cell bodies and fibres in Mysidacea. The distribution patterns of the 5-HT immunoreactivity found herein are compared with those of other peracarids and decapods.