Organisms Diversity & Evolution

, Volume 17, Issue 2, pp 447–475 | Cite as

Morphology and evolution of the nervous system in Gnathostomulida (Gnathifera, Spiralia)

  • Ludwik Gąsiorowski
  • Nicolas Bekkouche
  • Katrine Worsaae
Original Article

Abstract

Within Spiralia, Gnathifera may represent the deepest branching lineage comprising the jaw worms Gnathostomulida and their sister group Micrognathozoa + Syndermata. Yet, very few nervous system studies have been conducted on this lineage of microscopic, jaw-bearing worms, limiting our understanding of the evolution of this organ system in Spiralia. The nervous system of representatives from all major groups of Gnathostomulida was here mapped using confocal laser scanning microscopy and immunohistochemistry. Their intra-epidermal, unsegmented nervous systems comprise an anterior brain and three to five ventral and two to four dorsal longitudinal nerves, connected by few transverse commissures. Neurites of the stomatogastric nervous system were found lining the pharynx and connecting to a prominent buccal ganglion. Supposedly, sensory ciliated cells in the pharynx and the gut were documented for the first time. Based on these morphological results, primary homologies of neural structures in Gnathostomulida and other Gnathifera were hypothesized and thereafter tested using parsimony. This first neurophylogeny of Gnathostomulida resulted in a topology congruent with molecular data, supporting the monophyly of Bursovaginoidea, Conophoralia, and Scleroperalia. From this topology, the evolution of the gnathostomulid nervous system was reconstructed. It suggests a specialization and diversification of cords and serotonin-like immunoreactive cell patterns from a plesiomorphic neuroarchitecture of three unsegmented nerve cords and a compact anterior brain and buccal ganglion. These plesiomorphic states resemble the nervous system of Micrognathozoa, and possibly the ancestral states of Spiralia.

Keywords

Neurobiology Meiofauna Lophotrochozoa Neurotransmitters Phylogeny 

Supplementary material

13127_2017_324_MOESM1_ESM.pdf (157 kb)
ESM 1(PDF 156 kb)
13127_2017_324_MOESM2_ESM.pdf (935 kb)
ESM 2(PDF 935 kb)

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Copyright information

© Gesellschaft für Biologische Systematik 2017

Authors and Affiliations

  1. 1.Marine Biological Section, Department of BiologyUniversity of CopenhagenCopenhagen ØDenmark

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