Gnathostomula paradoxa (Schmidt-Rhaesa, 2016)

Gnathostomula paradoxa View in CoL

Serotonin-like immunoreactive (SLIR), tyrosinated tubulin-like immunoreactive (TyrTLIR), FMRF-amide-like immunoreactive (FLIR), and synapsin 1-like immunoreactive (Syn1LIR) nervous structures as well as DAPI-stained cell nuclei are visualized with CLSM.

The brain occupies most of the rostrum and consists of a central neuropil ( np, Figs. 1b View Fig ; 2b, c View Fig ; 3b, f) surrounded by numerous cell bodies, including 10 SLIR (2 × ldp, 2 × llp, 2 × mp, and 4 × slp, Figs. 1c View Fig , 3c) and nine FLIR perikarya ( bp, Fig. 4h View Fig ) as well as sensory structures ( asc, lsc, s1–4, sn, so, Figs. 1d View Fig , 2 View Fig , 3d–h, 4a). The neuropil contains four SLIR commissures: two dorso-anterior ( dac1 and dac2, Figs. 1c View Fig , 3c), one ventro-anterior ( vac, Figs. 1d View Fig , 3d), and one ventro-posterior ( vpc, Figs. 1d View Fig , 3d). The two latter form a characteristic ring-shaped structure and a SLIR median connective ( mc, Figs. 1d View Fig , 3d) connecting the dorsal part of the neuropil with the nerve cords. Two TyrTLIR commissures were detected in the brain, one equivalent to the SLIR ventro-posterior commissure ( vpc, Figs. 2d View Fig , 3h) and one thick anterior commissure ( ac, Figs. 2d View Fig , 3h) containing both dorso-anterior and ventro-anterior SLIR commissures.

Six longitudinal nerve cords are found: the main ventro-lateral (originating from the ventro-posterior brain commissure; vlc, Figs. 1b, d View Fig ; 2b, d View Fig ; 3b–h; 4j), lateral ( lc, Figs. 1b, d View Fig ; 2b, d View Fig ; 3b, d–h; 4j), and dorso-lateral pairs (originating from the anterior commissure; dc, Figs. 1a, c View Fig ; 2a, c View Fig ; 3a–e; 4j). These three pairs extend from the brain to the posterior commissure ( pc, Figs. 1b View Fig ; 2b View Fig ; 3b, f; 4b), with the dorso-lateral pair continuing as dorsal tail nerves ( dtn, Figs. 1a View Fig , 2a View Fig ); all of them are SLIR, TyrTLIR, and Syn1LIR. Accessory lateral ( aln, Figs. 2b, d View Fig ; 3f, h; 4d, e) and dorso-lateral ( adn, Figs. 2a, c View Fig ; 3e, g) TyrTLIR neurite bundles split from the respective longitudinal cords, giving rise to the peripheral nervous system. Short TyrTLIR and FLIR irregular median processes ( mpr, Figs. 2b, d View Fig ; 4d–g, i View Fig ) extend from the accessory lateral neurite bundles towards the ventral midline. Two transverse commissures, one postpharyngeal (TyrTLIR, ppc, Figs. 2b, d View Fig ; 4d View Fig ) and one posterior (SLIR, TyrTLIR, and Syn1LIR, pc, Figs. 1b View Fig ; 2b View Fig ; 3b, f; 4b), interconnect the nerve cords; the latter associated with two small TyrTLIR posterior clusters of perikarya ( pcc, Figs. 2b View Fig ), which also include SLIR posterior perikarya ( pp, Fig. 1b View Fig , 3a, 4b). A midventral TyrTLIR nerve plexus ( mnp, Figs. 2b, d View Fig ; 4f View Fig ), formed by some of the processes of the accessory lateral neurite bundles and ventro-lateral cords, extends longitudinally from the postpharyngeal commissure to the male opening, also innervating a few ciliary gut receptors ( cgr, Figs. 2b View Fig ; 5a, b). A pair of peripheral SLIR perikarya ( ap, Figs. 1d View Fig , 3d) projects their neurites into the ventro-lateral nerve cords anterior to the pharynx. A single middorsal SLIR and TyrTLIR perikaryon ( mdc, Figs. 1a View Fig , 2a View Fig , 4c View Fig ), which probably plays a role in copulation, is located dorsal to the penis.

The stomatogastric nervous system was visualized with SLI reactivity, TyrTLI reactivity, and Syn1LI reactivity and consists of a buccal ganglion ( bg, Figs. 1a, c View Fig ; 2a, c View Fig ; 3c, g; 4j; 5j), and a pair of buccal nerves ( bn, Figs. 1c View Fig ; 2a, c View Fig ; 3g; 4j,) connecting directly to the ventro-posterior part of the neuropil. The ganglion consists of approximately 40 cells and connects to the paired ciliary pharyngeal receptors ( cpr, Figs. 2c View Fig , 5j), situated in the dorso-posterior part of the pharynx. Additional TyrTLI reactivity was detected in the scattered ciliated glandular cells in the lateral and dorsal walls of the buccal cavity ( pcl, Figs. 2c View Fig ; 5j, k).

The rostral sensory structures consist of four pairs of TyrTLIR compound cirri ( s1–s4, Fig. 2 View Fig , 3e–h, 4a) and up to three pairs of TyrTLIR spiral organs, positioned laterally on the dorso-anterior margin ( so, Figs. 2c View Fig , 3h, 4a). Moreover, four SLIR cells (two anterior and two lateral) send projections to the tip of the rostrum ( asc and lsc, Figs. 1d View Fig , 3d) .