Natural selection arising from resource competition and environmental heterogeneity can drive adaptive radiation. Ecological opportunity facilitates this process, resulting in rapid divergence of ecological traits in many celebrated radiations. In other cases, sexual selection is thought to fuel divergence in mating signals ahead of ecological divergence. Comparing divergence rates between naturally and sexually selected traits can offer insights into processes underlying species radiations, but to date such comparisons have been largely qualitative. Here, we quantitatively compare divergence rates for four traits in African mormyrid fishes, which use an electrical communication system with few extrinsic constraints on divergence. We demonstrate rapid signal evolution in the Paramormyrops species flock compared to divergence in morphology, size, and trophic ecology. This disparity in the tempo of trait evolution suggests that sexual selection is an important early driver of species radiation in these mormyrids. We also found slight divergence in ecological traits among closely related species, consistent with a supporting role for natural selection in Paramormyrops diversification. Our results highlight the potential for sexual selection to drive explosive signal divergence when innovations in communication open new opportunities in signal space, suggesting that opportunity can catalyze species radiations through sexual selection, as well as natural selection.

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Electroreception, the capacity to detect external underwater electric fields with specialised receptors, is a phylogenetically widespread sensory modality in fishes and amphibians. In passive electroreception, a capacity possessed by c. 16% of fish species, an animal uses low‐frequency‐tuned ampullary electroreceptors to detect microvolt‐range bioelectric fields from prey, without the need to generate its own electric field. In active electroreception (electrolocation), which occurs only in the teleost lineages Mormyroidea and Gymnotiformes, an animal senses its surroundings by generating a weak (< 1 V) electric‐organ discharge (EOD) and detecting distortions in the EOD‐associated field using high‐frequency‐tuned tuberous electroreceptors. Tuberous electroreceptors also detect the EODs of neighbouring fishes, facilitating electrocommunication. Several other groups of elasmobranchs and teleosts generate weak (< 10 V) or strong (> 50 V) EODs that facilitate communication or predation, but not electrolocation. Approximately 1.5% of fish species possess electric organs. This review has two aims. First, to synthesise our knowledge of the functional biology and phylogenetic distribution of electroreception and electrogenesis in fishes, with a focus on freshwater taxa and with emphasis on the proximate (morphological, physiological and genetic) bases of EOD and electroreceptor diversity. Second, to describe the diversity, biogeography, ecology and electric signal diversity of the mormyroids and gymnotiforms and to explore the ultimate (evolutionary) bases of signal and receptor diversity in their convergent electrogenic–electrosensory systems. Four sets of potential drivers or moderators of signal diversity are discussed. First, selective forces of an abiotic (environmental) nature for optimal electrolocation and communication performance of the EOD. Second, selective forces of a biotic nature targeting the communication function of the EOD, including sexual selection, reproductive interference from syntopic heterospecifics and selection from eavesdropping predators. Third, non‐adaptive drift and, finally, phylogenetic inertia, which may arise from stabilising selection for optimal signal‐receptor matching.

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A comparative study was undertaken of Amazonian fish diversity and density (abundance and biomass) in nutrient poor (blackwater) and richer (whitewater -varzea) habitats in the vicinity of Teff, Brazil. The whitewater sampling sites, in the floodplain of the Rios SolimSes and Japura, had high turbidity (Secchi disc 0.03-0.7 m), a conductivity of 64-110 \lS cm" 1 at 25°C and a pH of 6.6-6.9. The blackwater sites Lagos Amana, lama and Tefe had a low turbidity (Secchi disc 2.0-2.2 m), a conductivity of 7-11 JlS cm" 1 at 25°C and a pH of 5.3-6.0. The fish communities of open water, floating meadow and forest margin were sampled. Both whitewater and blackwater sites held high diversity fish communities with many species in common. Whitewater habitats were more diverse yielding 108 species, compared with only 68 from blackwater. Fish density within floating meadow was estimated during high water April and May 1994, and low water October 1994 and March 1995. During the high water season fish biomasses in blackwater lakes (31.1 gm" 2 ) were significantly greater than those of either whitewater lakes (13.4 gm" 2 ) or whitewater river channels (3.45 g m" 2 ). At low water, in October, whitewater channels were generally found to hold the highest fish biomass (204 g m~2) although in March a blackwater site yielded the highest fish biomass recorded (285.9 g m" 2 ). The amount of floating meadow habitat within Whitewaters is however much greater than that in blackwaters. It is suggested that blackwaters may offer a stable habitat resulting in a high standing crop. In comparison, variation in conditions such as dissolved oxygen concentration in the whitewater may limit standing crop, but still allow periods of high productivity. Evidence was found for migration of fish from varzea lakes during periods of low oxygen availability. The high diversity and biomass offish caught in both whitewater and blackwater lakes indicates that water acidity and conductivity are poor predictors of fish diversity and density in tropical floodplain lakes.