Cycliophora is one of the most recently described metazoan phyla and hitherto
includes only two species: Symbion pandora and Symbion americanus. With a very complex
life cycle, cycliophorans are regarded as an enigmatic group with an uncertain phylogenetic
position, although they are commonly considered lophotrochozoan protostomes. In order to
extend the database concerning the distribution of immunoreactive substances in the freeswimming
chordoid larva of S. pandora, we investigated synapsin immunoreactivity using
fluorescence-coupled antibodies in combination with confocal laserscanning microscopy.
Moreover, we analyzed the co-localization patterns of synapsin, serotonin, and RFamidelike
immunoreactivity in the chordoid larva by 3D imaging technology based on the confocal
microscopy image stacks. Synapsin is expressed in large parts of the bilobed anterior cerebral
ganglion including anterior and dorsal projections. Two pairs of ventral neurites run longitudinally
into the larval body of which the inner pair shows only weak, scattered synapsin
immunoreactivity. In addition, a lateral synapsin immunoreactive projection emerges posteriorly
from each ventral longitudinal axon. Double immunostaining shows co-localization of
synapsin and serotonin in the cerebral ganglion, the outer and the inner ventral neurites, and
the anterior projections. Synapsin and RFamide-like immunoreactivity co-occur in the cerebral
ganglion, the outer ventral neurites, and the dorsal projections. Accordingly, the cerebral
ganglion and the outer ventral neurites are the only neural structures that co-express the two
neurotransmitters and synapsin. The overall neuroanatomical condition of the cycliophoran
chordoid larva resembles much more the situation of adult rather than larval life cycle stages
of a number of spiralian taxa