Two swimming modes in Trachymedusae; bell kinematics and the role of giant axons

Matthew E. Meech, Claudia E. Mills, Steven H.D. Haddock, Robert W. Meech*

*Corresponding author for this work

Research output: Contribution to journalArticle (Academic Journal)peer-review


Although members of the Rhopalonematidae family (Cnidaria, Hydrozoa, Trachymedusae) are known to exhibit unusually powerful jet swimming in addition to their more normal slow swimming behaviour, for the most part, reports are rare and anecdotal. Many species are found globally at depths of 600–2000 m, and so observation and collection depend on using remotely operated submersible vehicles. With a combination of in situ video footage and laboratory measurements, we have quantified kinematic aspects of this dual swimming motion and its electrophysiology. The species included are from two Rhopalonematidae clades; they are Colobonema sericeum, Pantachogon haeckeli, Crossota millsae and two species of Benthocodon. Comparison is made with Aglantha digitale, a species from a third Rhopalonematidae clade brought to the surface by natural water movement. We find that although all Rhopalonematidae appear to have two swimming modes, there are marked differences in their neural anatomy, kinematics and physiology. Giant motor axons, known to conduct impulses during fast swimming in A. digitale, are absent from C. sericeum and P. haeckeli. Slow swimming is also different; in C. sericeum and its relatives it is driven by contractions restricted to the base of the bell, whereas in A. digitale it is driven by contractions in the mid-bell region. These behavioural differences are related to the position of the different clades on a ribosomal DNA-based phylogenetic tree. This finding allows us to pinpoint the phylogenetic branch point leading to the appearance of giant motor axons and escape swimming. They place the remarkable dual swimming behaviour of members of the Rhopalonematidae family into an evolutionary context.

Original languageEnglish
Article numberjeb239830
JournalJournal of Experimental Biology
Issue number10
Publication statusPublished - 15 May 2021

Bibliographical note

Funding Information:
S.H.D.H. is supported by the David and Lucile Packard Foundation. Open access funding provided by the University of Bristol. Deposited in PMC for immediate release.

Publisher Copyright:
© 2021. Published by The Company of Biologists Ltd.


  • Cnidaria
  • Dual swimming
  • Giant motor axon
  • Hydromedusae
  • Kinematics
  • Phylogenetic tree


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