Signal Cloaking by Electric Fish

Journal of Experimental Biology, 2008

Animal communication systems are subject to natural selection so the imprint of selection must reside in the genome of each species. Electric fish generate electric organ discharges (EODs) from a muscle-derived electric organ (EO) and use these fields for electrolocation and communication. Weakly electric teleosts have evolved at least twice (mormyriforms, gymnotiforms) allowing a comparison of the workings of evolution in two independently evolved sensory/motor systems. We focused on the genes for two Na + channels, Nav1.4a and Nav1.4b, which are orthologs of the mammalian muscle-expressed Na + channel gene Nav1.4. Both genes are expressed in muscle in non-electric fish. Nav1.4b is expressed in muscle in electric fish, but Nav1.4a expression has been lost from muscle and gained in the evolutionarily novel EO in both groups. We hypothesized that Nav1.4a might be evolving to optimize the EOD for different sensory environments and the generation of species-specific communication signals. We obtained the sequence for Nav1.4a from non-electric, mormyriform and gymnotiform species, estimated a phylogenetic tree, and determined rates of evolution. We observed elevated rates of evolution in this gene in both groups coincident with the loss of Nav1.4a from muscle and its compartmentalization in EO. We found amino acid substitutions at sites known to be critical for channel inactivation; analyses suggest that these changes are likely to be the result of positive selection. We suggest that the diversity of EOD waveforms in both groups of electric fish is correlated with accelerations in the rate of evolution of the Nav1.4a Na + channel gene due to changes in selection pressure on the gene once it was solely expressed in the EO. . Individual variation in and androgen-modulation of the sodium current in electric organ. J. Neurosci. 15, 4023-4032. Gilbert, C. and Strausfeld, N. (1991). The functional organization of male-specific visual neurons in flies. J. Comp. Physiol. A 169, 395-411. Hanika, S. and Kramer, B. (1999). Electric organ discharges of mormyrid fish as a possible cue for predatory catfish. Naturwissenschaften 86, 286-288.

Electric sense (i.e. the ability to detect low-voltage electrical impulses from other individuals) is found in most primitive fish orders, a few derived teleost fish, some aquatic amphibians, the platypus and the echidna. Electric signals (i.e. self-generated electrical signals used for communication) are best understood in the weakly electric fish. Weakly electric fish include three freshwater teleost groups that produce dual-purpose electric signals to locate objects and communicate in the dark. Their electric organ discharges (EODs) are distinctive to species, and often to age, sex, and condition. EODs are non-propagating electrostatic fields, detectable only a few body lengths from the signaler. Electroreceptive predators may eavesdrop, and many weakly electric fish have signal adaptations that make their signals cryptic to predators. EODs are triggered by nerve impulses but their waveform shapes are regulated by steroid and peptide hormones. The electric eel is a specialized electric fish that produces a low voltage EOD for electrolocation and communication and also a high voltage discharge to stun prey and defend itself.

Fish and Fisheries

Fishes utilize electric signals for passive and active electrolocation and communication at night in murky water. To our knowledge, the mechanism of transmission of fish electric signals through water has not been addressed. In such a medium, the transmission of electrical signals can occur by movement of ions (ion conduction) under the influence of a generated electric field, or through its perturbation by nearby objects that have electric permittivity different than water. This latter property refers to the ability of a medium to resist the formation of electric fields with no involvement of ion transport. In this study, we discuss the relevance of each mechanism in the context of previous reports on transmission of signals by electric fishes in water environments.

Journal of Fish Biology, 2019

1994

Freshwater fish of the genus Apteronotus (family Gymnotidae) generate a weak, high frequency electric field (< 100 m V /em, 0.5-10 kHz) which permeates their local environment. These nocturnal fish are acutely sensitive to perturbations in their electric field caused by other electric fish, and nearby objects whose impedance is different from the surrounding water. This thesis presents high temporal and spatial resolution maps of the electric potential and field on and near Apteronotus. The fish's electric field is a complicated and highly stable function of space and time. Its characteristics, such as spectral composition, timing, and rate of attenuation, are examined in terms of physical constraints, and their possible functional roles in electroreception. Temporal jitter of the periodic field is less than 1 11sec. However, electrocyte activity is not globally synchronous along the fish's electric organ. The propagation of electrocyte activation down the fish's body produces a rotation of the electric field vector in the caudal part of the fish. This may assist the fish in identifying nonsymmetrical objects, and could also confuse electrosensory predators that try to locate Apteronotus by following its fieldlines. The propagation also results in a complex spatiotemporal pattern of the EOD potential near the fish. Visualizing the potential on the same and different fish over timescales of several months suggests that it is stable and could serve as a unique signature for individual fish. Measurements of the electric field were used to calculate the effects of simple objects on the fish's electric field. The shape of the perturbation or "electric image" on the fish's skin is relatively independent of a simple object's size, conductivity, and rostrocaudallocation, and therefore could unambiguously determine object distance. The range of electrolocation may depend on both the size of objects and their rostrocaudal location. Only objects with very large dielectric constants cause appreciable phase shifts, and these are strongly dependent on the water conductivity.

Journal of Experimental …, 2001

Sensory signals can be considered as the modulations of a specific 'carrier' for which the sensory system has specific sensitivity (Wiener, 1948). Vision is a paradigmatic example that allowed Marr (Marr, 1982) to define the main factors that determine physical image generation, considering light as the carrier of visual information. Two of these factors are the illuminating conditions and the observer's viewpoint. Just as light is necessary for vision, so the presence of a specific carrier is necessary for any other sensory system. To understand how a sensory system works, one has to characterise the carrier that holds the signals processed by that system. This paper is concerned with the signal carriers of the electrosensory system, a peculiar active sensory system that allows electric fish to communicate with each other and to explore their near environment through electric signals (Lissmann and Machin, 1958; Black-Cleworth, 1970). The carriers of these signals are the electric fields generated by activation of the electric organ that transform the body of the fish into a distributed electric source. When loaded by a surrounding impedance (the water), the electric organ discharge (EOD) generates spatio-temporal patterns of current density that flow through the skin of the emitter fish and, potentially, through the skin of nearby conspecifics (Caputi, 1999). Active electrolocation occurs when the self-generated electric field is used to image the near environment. Objects of different impedance from water interfere with the selfgenerated electric field and modulate the basal pattern of transcutaneous currents. The intensity of this field at a given point of the skin is a vector, referred to here as the selfgenerated local EOD-associated field (sLEOD). Theoretical considerations and experimental data indicate that the heterogeneous electric organs of pulse-emitting gymnotids generate sLEODs with time waveforms that are strongly dependent on their spatial coordinates along the body of the fish (

Journal of Physiology-Paris, 2014

Weakly electric fish can sense electric signals produced by other animals whether they are conspecifics, preys or predators. These signals, sensed by passive electroreception, sustain electrocommunication, mating and agonistic behavior. Weakly electric fish can also generate a weak electrical discharge with which they can actively sense the animate and inanimate objects in their surroundings. Understanding both sensory modalities depends on our knowledge of how pre-receptorial electric images are formed and how movements modify them during behavior. The inability of effectively measuring pre-receptorial fields at the level of the skin contrasts with the amount of knowledge on electric fields and the availability of computational methods for estimating them. In this work we review past work on modeling of electric organ discharge and electric images, showing the usefulness of these methods to calculate the field and providing a brief explanation of their principles. In addition, we focus on recent work demonstrating the potential of electric image modeling and what the method has to offer for experimentalists studying sensory physiology, behavior and evolution.

Communicative & integrative biology, 2008

Weakly electric fish perceive their actively generated electrical field with cutaneous electroreceptors. This active sensory system is used both for orientation and for communication. In a recent paper1 we focussed on how anatomical adaptations (pre-receptor mechanisms), biophysical constraints and behavior all contribute to active electrolocation, i.e., the fishes&#39; unique ability to determine and distinguish the electrical properties of objects based on the modulation of a self-generated carrier signal, the so-called electric organ discharge.

Brian Rasnow and James M. Bower Caltech, Division of Biology, 216-76, Pasadena, CA 91125. ... These animals detect nearby objects by sensing object-induced distortions in their electric organ discharge (EOD) electric field (reviewed in Bastian 1994; Carr 1990; Bullock and ...

The Journal of Experimental Biology, 1998