The central nervous system of Oweniidae (Annelida) and its implications for the structure of the ancestral annelid brain

Patrick Beckers*, Conrad Helm, Günter Purschke, Katrine Worsaae, Pat Hutchings, Thomas Bartolomaeus

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

12 Citations (Scopus)
9 Downloads (Pure)

Abstract

Background: Recent phylogenomic analyses congruently reveal a basal clade which consists of Oweniidae and Mageloniidae as sister group to the remaining Annelida. These results indicate that the last common ancestor of Annelida was a tube-dwelling organism. They also challenge traditional evolutionary hypotheses of different organ systems, among them the nervous system. In textbooks the central nervous system is described as consisting of a ganglionic ventral nervous system and a dorsally located brain with different tracts that connect certain parts of the brain to each other. Only limited information on the fine structure, however, is available for Oweniidae, which constitute the sister group (possibly together with Magelonidae) to all remaining annelids. Results: The brain of Oweniidae is ring- shaped and basiepidermal. Ganglia, higher brain centers or complex sensory organs do not exist; instead the central nervous system is medullary. Posterior to the brain the ventral medullary cord arises directly from the ventral region of the brain in Myriowenia sp. while in Owenia fusiformis two medullary cords arise perpendicular to the brain ring, extend caudally and fuse posterior. The central nervous system is composed of a central neuropil and surrounding somata of the neurons. According to ultrastructural and histological data only one type of neuron is present in the central nervous system. Conclusion: The central nervous system of Oweniidae is the simplest in terms of enlargement of the dorsal part of the brain and neuron distribution found among Annelida. Our investigation suggests that neither ganglia nor commissures inside the brain neuropil or clusters of polymorphic neurons were present in the annelid stem species. These structures evolved later within Annelida, most likely in the stem lineage of Amphinomidae, Sipuncula and Pleistoannelida. Palps were supposedly present in the last common ancestor of annelids and innervated by two nerves originating in the dorsal part of the brain. A broader comparison with species of each major spiralian clade shows the medullary nervous system to be a common feature and thus possibly representing the ancestral state of the spiralian nervous system. Moreover, ganglia and clusters of polymorphic neurons seemingly evolved independently in the compared taxa of Spiralia and Annelida.

Original languageEnglish
Article number6
Pages (from-to)1-21
Number of pages21
JournalFrontiers in Zoology
Volume16
Issue number1
DOIs
Publication statusPublished - 12 Mar 2019

Bibliographical note

Copyright the Author(s) 2019. Version archived for private and non-commercial use with the permission of the author/s and according to publisher conditions. For further rights please contact the publisher.

Keywords

  • Annelida
  • Brain
  • Central nervous system
  • Glia
  • Neuroanatomy
  • Spiralia
  • Ultrastructure

Fingerprint

Dive into the research topics of 'The central nervous system of Oweniidae (Annelida) and its implications for the structure of the ancestral annelid brain'. Together they form a unique fingerprint.

Cite this