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