I have lately been studying a unique pattern of geographic variation in a species-specific electric organ discharge (EOD) signal and its morphological correlate in the mormyrid electric fish Paramormyrops kingsleaye. Our analysis includes histological analysis of electric organs and multivariate landmark analysis of 544 EOD signals collected from 12 localities across Gabon 1998-2009, with the most recent trip occurring in August, 2009.

Geographic Variation in EODs : Our results demonstrate that the majority of EOD variation in this species is explained by two factors: EOD duration and the magnitude of a very weak head-negative pre-potential, P0. Interestingly, both factors vary clinally across Gabon. EODs are longer in the East than in the West; EODs in the South have smaller P0s than those in the North.

Two signaling types of P. kingsleyae : Peak P0 magnitude is bimodally distributed for all examined EODs, corresponding to P0-present and P0-absent individuals. P0-present EODs, the ancestral EOD type for the Paramormyrops genus, is correlated with the presence of penetrating-stalked (Pa) electrocytes in the electric organ while its magnitude is correlated with the number of penetrating stalks. Most populations (10 of 12 sampled) have exclusively P0-present EOD waveforms. P0-absent EODs were associated with non-penetrating posteriorly innervated electrocytes (NPp) and occurred only in two localities, isolated from other populations by physical barriers. In one case, this was a 30-foot waterfall, the other was a costal stream separated from adjacent streams by the Atlantic Ocean.

Sympatric Signalers: In two sites along an assumed watershed boundary between populations isolated by the waterfall, both signaling phenotypes were collected in sympatry. In no other locations were these signal types found in sympatry. Three specimens collected in one of these localities had electric organs with both NPp and Pa type electrocytes, which has never been reported for a Mormyrid fish!

Why are these findings important? The absence of penetrating stalks (and P0-absent EODs) have evolved several times in Mormyrids (current estimates suggest seven of such reversions from an early Pa type!). Additionally, within the genus Paramormyrops , several independent reversions may have also taken place. We suppose that these reversions, along with our this data regarding EOD variation in Paramormyorps kingsleyae, are evidence of microevolutionary processes (population isolation and drift) that contribute to critically important macroevolutionary changes in mormyrid signal evolution, suggesting that the developmental/genetic mechanisms underlying signal evolution may in fact be very simple!