The expression pattern of serotonin neurotransmitter (5-HT) was investigated in the optic lobes of the marine cave mysid Hemimysis margalefi (Crustacea, Mysidacea) during the dark to light and light to dark transitions by fluorescence immunohistochemistry. The intensity of the expression was evaluated according to the number of immunoreactive elements. Experiments were carried out under laboratory controlled conditions of illumination. The results show that some structures as the X-organ and medulla terminalis are always immunoreactive during light or dark periods. Other ones as the rhabdom of the retinular cells (rh), the distal pigment cells (dpc), the internal medulla (im), the external medulla (em), and the sinus gland (sg) are immunoreactive during light exposure. In these latter structures, immunoreactivity is quickly detected after illumination (30 min) while, back to darkness, it takes longer to disappear (1 h for rh and dpc, 2 h for im and more for em and sg). The 5-HT expression pattern in the lamina ganglionaris is different. During the dark to light transition, immunoreactive cells are already detected after 30 min of light exposure; after 1 h of exposure, the lamina ganglionaris expresses many immunoreactive cells, then, they are less numerous after 2 h; finally, the detection limit is reached after 3 h of exposure. During the light to dark transition, 5-HT is expressed in the lamina ganglionaris only after 1 h of darkness. The number of 5-HT neurones was increased after 2 h of darkness but no reactivity is detected after one night. To go further, same experiments were carried out in an other mysid species, Leptomysis lingvura that lives at same depth but in a shallow lagoon. In contrast to H. margalefi, L. lingvura is not subject to nycthemeral migrations. A different 5-HT expression pattern in the lamina ganglionaris has been observed: immunolabelling was detected as soon as 30 min, whatever the considered transition phase, dark to light or light to dark. These results are discussed considering the behaviour of each species. We suggest that the expression pattern of 5-HT in the lamina ganglionaris can play the role of a photoreception signal integrator during the light–dark transition phases and that cavernicolous mysid crustaceans can be considered as a good model to study the mechanisms of determination and regulation of circadian migrations of zooplankton.