2018 Volume 35 Issue 2 Pages 85-92
Among over 2000 species in the monophyletic Nudipleura clade, fewer than 70 species have been observed to exhibit swimming behavior. Swimming behavior is produced by rhythmically flexing whole body in dorsal-ventral direction or left-right direction. These two types of behavior appear to have evolved independently several times on the phylogenetic history. Nudibranchs Tritonia and Pleurobranchaea are phylogenetically distant, but they both show dorsal-ventral swimming. There are at least two homologous neurons in the swim central pattern generator (CPG) circuits of these species. They both employ serotonergic neuromodulation of synaptic strength within the CPG circuits. The left-right swimmers distributed widely in among nudibranchs. Since Melibe and Dendronotus belong to a clade of genera that all swim with left-right body flexions, their swimming behaviors are likely homologous. However, their swim CPGs differ in both cellular composition and in the details of the neural mechanisms. Thus in Nudipleura, there are similar behaviors that have evolved independently through parallel use of homologous neurons, yet employing two distinct neural mechanisms without changing the behavioral outputs.
