BBC Natural History Unit, Whiteladies Road, Bristol BS8 2LR, UK.
Friday Harbor Laboratories, 620 University Road, Friday Harbor, WA 98250, USA.
J Exp Biol. 2021 May 15;224(10). doi: 10.1242/jeb.239830. Epub 2021 May 25.
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.
虽然 Rhopalonematidae 科（刺胞动物门，水螅纲，Trachymedusae）的成员除了正常的缓慢游动行为外，还以异常强大的喷射游泳而闻名，但大多数情况下，相关报道都很少且只是轶事。许多物种在全球范围内分布于 600-2000 米深处，因此观察和采集依赖于使用远程操作潜水器。通过结合原位视频和实验室测量，我们已经量化了这种双重游动行为及其电生理学的运动学方面。所包括的物种来自两个 Rhopalonematidae 进化枝；它们是 Colobonema sericeum、Pantachogon haeckeli、Crossota millsae 和两种 Benthocodon。将它们与从第三个 Rhopalonematidae 进化枝带来的自然水运动带到水面的 Aglantha digitale 进行比较。我们发现，尽管所有的 Rhopalonematidae 似乎都有两种游动模式，但它们的神经解剖结构、运动学和生理学存在明显差异。在 A. digitale 中，快速游动时传导冲动的巨大运动轴突在 C. sericeum 和 P. haeckeli 中不存在。缓慢游动也不同；在 C. sericeum 及其近亲中，它是由仅限于钟基部的收缩驱动的，而在 A. digitale 中，它是由钟中部的收缩驱动的。这些行为差异与核糖体 DNA 系统发育树上不同进化枝的位置有关。这一发现使我们能够确定导致巨大运动轴突和逃逸游泳出现的进化枝分支点。它们将 Rhopalonematidae 科成员的非凡双重游动行为置于进化背景中。
