Serotonergic and SCPb-like innervation of the atrial complex in Gyratrix hermaphroditus (Platyhelminthes, Kalyptorhynchia) revealed with CLSM

Abstract

The platyhelminth reproductive system is a complex series of canals, glands, and sclerotic components that Wgure prominently in our understanding of reproductive physiology, taxonomy, and evolution of the Platyhelminthes. Yet, there is limited information on its innervation, especially for free-living species of Rhabdocoela, the most speciose and ecologically diverse assemblage. Here, innervation of the reproductive system in the common marine kalyptorhynch, Gyratrix hermaphroditus, is studied using confocal laser scanning microscopy (CLSM), Xuorescent phalloidin, and antibodies to serotonin (5HT) and small cardioactive peptide b (SCP b ). Results show that portions of the female atrial system (uterus, female gonopore) are innervated by peptidergic (SCP b ) neurons that may function to control muscles involved in egg movement. In contrast, portions of the male atrial system (male atrium, male gonopore) are innervated by both peptidergic and serotonergic neurons. These neurons form a complex series of hoops around the musculature of the male atrium that houses prostate stylet type II. It is hypothesized that 5HT is the primary myoexcitatory neurotransmitter, and that it acts either synergistically with SCP b to trigger muscle contractions of the male atrium and protract the copulatory stylet, or that SCP b plays an inhibitory role during contraction of the male atrium. A comparison of the distribution of peptidergic and serotonergic neurons in G. hermaphroditus with other free-living species and parasitic rhabdocoels (Neodermata) reveals similarities in innervation of comparable (though probably not homologous) reproductive organs. These results suggest that the atrial systems of phylogenetically diverse species may share a common neuronal physiology despite their structural diVerences and potential independent evolutionary origins within the Platyhelminthes.

Comparative morphology of serotonergic‐like immunoreactive elements in the central nervous system of kinorhynchs (kinorhyncha, cyclorhagida)

“…Very similar results were obtained with specimens of A. paulae and Z. brightae . Because it is hypothesized that serotonin is the primary myoexcitatory neurotransmitter in several invertebrate groups (Hochberg,2009), it is reasonable to assume that the ring complex of kinorhynchs may be involved in locomotory processes. However, serotonin may also be activating 5‐HT receptors on the membranes of different cell types within or outside of the central nervous system.…”