Differentiation in electrical pulse waveforms in a pair of sibling Dwarf Stonebashers, Pollimyrus castelnaui and P. marianne : possible mechanisms and functions (Mormyridae, Teleostei)

Abstract

The recently discovered snoutfish Pollimyrus marianne is virtually indistinguishable from its vicariant sibling species P. castelnaui by general appearance, but well differentiated in the waveform of its communication signal, an electric organ discharge (EOD) of the pulse type. We here describe the differentiation in EOD waveform in detail, and also differences between the sexes in P. castelnaui . In an attempt to better understand how such strong differentiation is possible in an evolutionarily short time, we mathematically modelled the three successive electrical excitations at the level of the electrocytes that constitute an EOD by three Gaussians. By varying amplitudes, duration and relative delay times, all tri- and pentaphasic EOD waveforms observed in the two species are generated. We suggest that in speciation only slight quantitative modifications in the anatomy and physiology of the electrocytes were necessary to generate spectacular differentiation. Using a spontaneous choice paradigm in untrained fish, four out of four P. marianne , and two out of eight P. castelnaui showed a preference for conspecific rather than heterospecific playback EODs. The results suggest that EOD waveform differentiation was involved when their common ancestor differentiated into a pair of allopatric sibling species of Dwarf Stonebashers.