Expression of serotonin and enkephalins in calanoid copepods (Crustacea):: an immunohistochemical study

Copepods are the main metazoan component of zooplankton, in terms of both the number of species and biomass. Thus, they have a key role in pelagic food webs; but curiously, nothing is known of the neuroendocrine substances involved in their physiological processes. In higher crustaceans, especially the Decapoda, the role of such molecules in different physiological processes (motility, feeding, reproduction, day-night migrations and so on) has been well explored; so, we have mapped expression sites of some of these substances to provide a better understanding of copepod physiology. Serotonin and, for the first time, leucine- and methionine-enkephalin were detected in copepods using immunofluorescence techniques. Serotonin has mainly been identified in the central nervous system acting probably as a neurotransmitter, notably in the control of the escape reflex. In contrast, enkephalins are only present in peripheral organs such as the naupliar eye, gut and shell glands. This localization suggests that opioids are involved in visual function and reproductive and digestive processes.