This web site describes the neural mechanisms of species recognition in the electrosensory system of weakly electric fish from Africa. Mormyrid electric fish generate complex electric signals for use in social communication; their electrosensory system is specialized for detecting these signals and recognizing subtle differences between them.
African electric fish in the family Mormyridae have become an excellent model system for the study of sensory recognition. These are animals with a complex sensory recognition task that is both ethologically-interesting, yet tractable for neuroethological analysis. The behavior relates to species recognition and to reproductive isolation. These fish produce species-typical electric organ discharges which are used for communication and for electrolocation. Species-recognition depends upon a submodality of the electric sense specialized for temporal information carried in the waveform of the electric discharge.
Electric Organs and Electroreceptors
Mormyrids are freshwater electric fish that have specialized electric organs in their tail for sending signals. They also carry specialized electroreceptors on their skin that are used for electrolocation and social communication. The electric organ is located in the caudal peduncle, the slender part of the tail from the base of the caudal fin to the posterior end of the anal and dorsal fins. Electroreceptors cover the skin surface, and may be seen in the photo above as white spots on the dark background.
The Mormyridae is a large family of fishes which is entirely endemic to Africa.
Mormyrids respond to differences EOD waveforms. To explore species recogntion behavior click here.
Mormyrids have a highly developed electrosensory system devoted to detecting and analyzing the temporal features of EODs.
What are the mormyrids related to? See Mormyrid Phylogeny and Mormyrid Sub-Families.
Where is the electric organ, and how does it work? See Electric Organ Links.
Peripheral electroreceptors are exquisitely sensitive to electric currents. Check out electroreception links
How does the neuronal system permit recognition?
Summary: Freshwater fish in the genus Brienomyrus in the family Mormyridae generate a wide range of EOD waveforms. They are species-specific and often sex-specific. The EOD waveforms are often less than a millisecond in duration yet they code for both the species and sex of the signaler. Ethological studies in the field, using playback, demonstrate that breeding males will discriminate between male and female EODs and between conspecific and heterospecific discharges. Especially important are the temporal characteristics of the discharge.
Electrophysiological studies demonstrate that one type of electroreceptor, the Knollenorgan, is specialized for detecting the weak signals from other electric fish. It has been called a "communication sensor" by some researchers. The Knollenorgan encodes the complex waveform of the EOD with a relatively simple temporal pattern of spikes. Each EOD evokes a pattern of spikes with a precisely determined time interval. Often the code is as simple as two spikes separated by several hundred microseconds.
Anatomical studies can be used to trace the pathway of the Knollenorgan receptor to the Central Nervous System. Here, the behavior one calls species-recognition depends upon a unique midbrain structure in mormyrids known as Ela (nucleus exterolateralis pars anterior). Here, temporal patterns are analyzed by a neural circuit that is composed of two cell types, a large cell and a small cell. The input to Ela generates an excitatory input onto small cells and also onto large cells. Large cells project to small cells with an inhibitory input. The axon between the collateral to the large cell and the second collateral to the small cell is very long. This serves as a delay line delays the excitation for approximately one half millisecond, thereby allowing the inhibitory input to modulate the probability of excitation of the small cell for specific durations of stimuli. This anti-coincidence model of temporal recognition of EOD wave forms appears to be a novel one among the fast time-coding pathways of electroreception and audition.
More on the phylogeny of mormyrids.
Hopkins, C. D. (1986). Behavior of Mormyridae. In Electroreception (ed. T. H. Bullock and W. F. Heiligenberg), pp. 527-576. New York: John Wiley & Sons.
Hopkins, C. D. (1988). Neuroethology of electric communication. Ann. Rev. Neurosci. 11, 497-535.
Hopkins, C. D. and Bass, A. H. (1981). Temporal coding of species recognition signals in an electric fish. Science 212, 85-87.
Mugnaini, E. and Maler, L. (1987). Cytology and immunocytochemistry of the nucleus exterolateralis anterior of the mormyrid brain: possible role of GABAergic synapses in temporal analysis. Anat. Embryol. 176, 313-336.
created by Carl D.Hopkins, Section of Neurobiology and Behavior, Cornell University
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