Taking a close look at electrosensing

The brain of the brown ghost knifefish, which uses electric fields to “see”, processes electrical signals in a way that is similar to how our brains interpret visual and auditory signals.

Indeterminate body growth and lack of gonadal decline in the brown ghost knifefish (Apteronotus leptorhynchus), an organism exhibiting negligible brain senescence

The brown ghost knifefish (Apteronotus leptorhynchus (Ellis in Eigenmann, 1912)) is the only vertebrate organism identified thus far that exhibits negligible brain senescence. The present study examines the basic growth patterns of this species, testing the hypothesis that indeterminate growth and lack of reproductive senescence correlate with negligible senescence. Analysis of length–mass relationships revealed negative allometric growth in males and isometric growth in females. Total length at first sexual maturity was 13.5 cm in males and 12.0 cm in females, whereas gonadal mass was 0.02 g in males and 0.2 g in females. Modelling of total length as a function of the number of otolith rings using attenuating growth equations revealed that lengths of up to 26.8 cm in males and 20.2 cm in females can be reached, indicating that the fish continue to grow throughout life. Gonadal mass increased significantly with age in sexually immature individuals of both sexes. In sexually mature fish, gonadal mass showed a marginal increase with age in males and no change in females. The demonstration of indeterminate growth of the fish and of the lack of gonadal regression with age has important implications for the characterization of brown ghost knifefish as a model of negligible senescence.

Echo response to chirping in the weakly electric brown ghost knifefish (Apteronotus leptorhynchus): role of frequency and amplitude modulations

Teleost fish of the order Gymnotiformes are distinguished by their ability to produce electric discharges by means of specialized organs. These electric organ discharges serve various behavioral functions, including communication. During such electric interactions, male brown ghost knifefish ( Apteronotus leptorhynchus (Ellis in Eigenmann, 1912)) generate several types of transient frequency and amplitude modulations (“chirps”) of the otherwise nearly constant discharges. Previous studies have shown that the chirps generated by one individual follow those of the other with a preferred latency of approximately 500–1000 ms. As demonstrated in the present study, signals consisting of either frequency modulations or amplitude modulations are able to trigger this echo response. Signals composed of just amplitude modulations are effective in triggering an echo response only if the reduction in amplitude is large (approximately 40%, relative to baseline of the electric organ discharge of the emitting fish). By contrast, in frequency-modulated signals, a maximum frequency increase as small as 1.2% relative to baseline frequency is sufficient to trigger an echo response. This remarkable sensitivity might be an adaptation for the detection of so-called type-2 chirps, as chirps of this type are composed of rather small frequency increases and negligible amplitude modulations. In line with this hypothesis is the observation that during electric interactions of two fish, the generation of these chirps dominates the production of any of the other five chirp types known.