Knowledge about the development of the nervous system in cirripeds is limited, particularly with regard to the changes that take place during the two metamorphoses their larvae undergo. This study delivers the first detailed description of the development of the nervous system in a cirriped species, Amphibalanus improvisus by using immunohistochemical labeling against acetylated alpha-tubulin, and confocal laser scanning microscopy. The development of the nervous system in the naupliar stages corresponds largely to that in other crustaceans. As development progresses, the protocerebral sensory organs differentiate and the intersegmental nerves forming the complex peripheral nervous system appear, innervating the sensory structures of the cephalic shield. During metamorphosis into a cypris the lateral sides of the cephalic shield fold down into a bilateral carapace, which leads to a reorganization of the peripheral nervous system. The syncerebrum of the cypris exhibits the highest degree of complexity of all developmental stages, innervating the frontal filaments, nauplius eye, compound eyes and the antennules. During settlement, when the second metamorphosis occur, the closely associated frontal filaments and compound eyes are shed together with the cuticle of the carapace and the antennules. In adults, the syncerebral structures are reduced while the ventral nerve cord and the peripheral nervous system increase in complexity. The peripheral nervous system plays an important role in processing sensory input and also in settlement. In summary, through the larval development we observed a structural and thus also functional increase of complexity in favor of the peripheral nervous system and the ventral nerve cord.
Serotonin-immunoreactivity in the ventral nerve cord of Pycnogonida – support for individually identifiable neurons as ancestral feature of the arthropod nervous system
