Dimorphilus gyrociliatus (Annelida: Dinophiliformia) Dwarf Male Nervous System Represents a Common Pattern for Lophotrochozoa

SIMPLE SUMMARY: The dwarf male of D. gyrociliatus is the smallest known annelid, at only 28 μm × 40 μm in size. Data on its morphology are limited. This study provides a description of the external ciliation and the morphology of the serotonin-like immunoreactive nervous system of the dwarf male. Serotonin (5-HT) is known to play a key role in controlling reproduction, ciliary locomotion, and sensing in Annelida. Immunochemical labeling with anti-acetylated tubulin, anti-5-HT antibodies, and confocal microscopy were used to visualize the ciliary structures and individual neurons. The male has three ciliary fields. The nervous system of the male includes frontal ganglia, five commissures, two ventral and two dorsal bundles, and penial nerves. A total of 57 neurons were found, of which only five were 5-HT-like immunoreactive. There may be homology between the male and female ciliary structures. The dwarf male may also possess a compact apical organ consisting of two sensory neurons and one 5-HT-like immunoreactive cell. ABSTRACT: Dinophiliformia is a newly revealed clade within Annelida that is a sister group to Pleistoannelida. Dimorphilus gyrociliatus is a representative of this clade that has fascinated scientists with its high degree of sexual dimorphism. Both males and females are small, worm-like creatures that have specific ciliary structures: anterior ventral, posterior ventral, and dorsal ciliary fields in males, and prototroch, metatroch, and ventral ciliary fields in females. There are data on the morphology and development of the nervous system in Oweniidae, Sipunculida, Pleistoannelida, and even Dinophiliformia. However, data on the neuromorphology and development of D. gyrociliatus dwarf males are limited. Here, we present data on the distribution of cilia, sensory neurons, and the 5-HT-like immunoreactive system in 3D reconstructions and cross-sections. Immunochemical labeling with anti-acetylated tubulin and anti-5-HT antibodies and confocal microscopy were used to visualize the ciliary structures and neurons. The male has three ciliary fields: anterior ventral, posterior ventral, and dorsal. These include frontal ganglia, five commissures, two ventral and two dorsal bundles, and penial nerves. A total of fifty-seven neurons and only five 5-HT-like immunoreactive cells were described. Although the sensory neurons were not 5-HT-like immunoreactive, they had 5-HT innervation, which may indicate the role of 5-HT in perception. There may be homology between male and female ciliary structures. The dwarf male of D. gyrociliatus may have a reduced apical organ consisting of two sensory neurons and a 5-HT-like immunoreactive cell.