| Summary: | Calanoid copepods are abundant planktonic crustaceans that drift around the pelagic zone of the ocean. Despite being of major ecological importance in marine food webs, their minute body sizes allow them to be easily overlooked, and hence their nervous systems are understudied. Recently, immunohistochemical techniques and confocal microscopy have been used to map the aminergic cells in one key species, Calanus finmarchicus. However, the mapping has been without the aid of a well-delineated nervous system labeled by the same technique. The number of aminergic cells found was small, opening the possibility that this approach could be used to make quantitative as well as qualitative comparisons of particular biogenic amine-containing cells across species. Because of the important position of copepods in crustacean phylogeny, such an approach might provide a good method for gaining insights into the evolution of the crustacean nervous system. To develop this approach into a tool to understand the evolutionary relationships within the copepod order Calanoida, the nervous systems of two species, Calanus finmarchicus and Metridia lucens, were fluorescently labeled and their anatomical features quantitatively mapped. Then a second fluorescent antibody was used to label histaminergic cells against a contrastingly labeled nervous system to provide a better visualization and more accurate localization of the positions of these cells. Histamine was used as a marker, and the histaminergic cells were compared by characterizing their numbers, pairings, sizes, and locations in the nervous system between the two species. This in turn permitted a quantitative comparison of morphologies in these two groups to help in the understanding of their evolutionary relationships.
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