electrosensation in electric fish Flashcards
Different types of electric sense: passive
passive sense (detect external electric fields):
- animate: gills, muscle, heart
- inanimate: earths geomagnetic field
25% of all fish
Use Ampullary receptors
Different types of electric sense: active
Active sense (detect perturbation in electric field emitted by the fish itself):
- animate: predators, prey
- inanimate: anything with electrical conductivity different to the water
All weekly electric fish and some strongly electric fish
Rquires tuberous receptors and an electric organs
Can detect:
- location
- Conductance
- Capacitance
- Distance
Generate a somatotropin map of the electrosensitive body surface
electric fish -
Send out electrical files and can detect any distortions tell them about the environment - can detect anything with a different electrical field
Specialisations for electro-sensation:
- ability to detect electric fields in surrounding medium (specialised sensory structures, brain regions and body shape)
- ability to produce and emit electric field (specialised electric organ controlled by a specialised motor nucleus in the brain stem)
- modified behaviours related to use of electric signals for prey detection and communication
Two types of weakly electric fish:
Gymnotiforms and mormyriformes
- electric sense very similar but these fish evolved separately (common ancestor did not possess electric sense
Gymnotiformes -
South America
E.g. knife fish - specialised tail
Almost exclusively wave type
Generation of electric discharge - electric organ discharges (EOD):
Electric organs usually consist of modified muscle cells (electrocytes) - excitation contraction coupling is disabled =no contraction
Flattened muscle electrocyte cells are stacked to form electroplaques - surrounded by an insulating connective sheath (so electrocurrent doesn’t escape into environment)
Motor neurones innervate one side of the muscle fibre = produces action potential causes electrocytes to become excited allowing current to flow through into environment
Mormyriformes -
Africa
E.g. elephant fish (specialised trunk muscles)
Almost exclusively pulse type
Strongly electric fish electric organ discharge:
monopolar - best for stunning prey
Iso voltage curves -
Form a map of voltage (governed by shape of fish)
Current flows (at right angles to voltage) from one end of fish to the other end
Weakly electric fish electric organ discharge:
Bipolar or more complex waveforms
electro-receptive sense organs:
Two types: Ampullary and tuberous
- All electroreceptors are hair cells (similar to mechanoreceptors)
- they form part of the octavo-lateral sensory system - which includes the receptors for hearing, eqbm, gravity/rotation and water currents
Ampullary receptors:
Have a jelly-filled opening (canal) through the epidermis to the outside
- Low resistance, high sensitivity to low frequencies
Found in many fish - weakly electric fish, rays and sharks
Very sensitive to weak electric field gradients
Sensory neurones are spontaneously active
Respond only to low frequency signals
Tuberous receptors:
Canal loosely ‘plugged’ with epidermal cells…
- 100x less sensitive
- only respond to high frequencies
- only found in electric fish
Respond to the discharged of electric organs
Two types: time markers and amplitude condors - these receptors together permit…
- detection of EODs generated by conspecifics and thus electrocommunication
- the presence of objects in the environment and thus electrolocation
Time marker tuberous electroreceptors:
- High sensitivity
- Fixed latency (fire at precise time)
- detect the timing of the fish’ own EOD or that of a conspecific
- fire a single action potential per EOD
Amplitude condors tuberous electroreceptors:
- Low overall sensitivity but very high sensitivity to tiny changes in the amplitude of a fish’ own EOD
- bursts of spikes in which the latency of the first spike signals amplitude (higher amplitude = shorter latency)
Receptors in mormyrid fish from Africa (elephantfish)…
Time coders = knollenorgane (K-receptors)
Amplitude coders = mormyromasts (D-receptors) - these are important in active object location
Different parameters of the ‘electric image’ provide different info about an object…
- the location of receptors detecting a distortion indicates the location of the object relative to the body
- the sign of the distortion indicates conductance
- changes in waveform or timing indicate capacitance
Distance estimation experiment:
Electrodes placed near body of fish to see what different objects ‘look like’ to the fish’ sensory receptors at different distances
Conductors - have increased EOD amplitude
Insulators - have decreased EOD amplitude
Shorter distance = small image, Higher contrast = larger signal
Therefore can work out distance despair of size/shape of object
Proposed mechanism for depth perception in weakly electric fish =
Use a single stationary array of receptors without resorting to temporal or spectral measurements
Capacitance estimation:
Capacitance = electrical property of material that store charge
Living organism = high capacitance
Inanimate objects = low capacitance
= capacitance is useful in distinguishing other organisms from environment
Receptors in gymnotid fish from South America (knifefish)…
Pulse fish:
time coders = M receptors
Amplitude coders = B (burst) receptors
Wave fish:
Time coders = T (time) receptors
Amplitude coders = P (probability) receptors
in pulse type mormyrids capacitance…
modifies the shape, but not the timing of EOD pulses
- receptors detect waveform distortion - responses of A and B cells are compared to allow fish to distinguish between resistive and capacitance objects
in wave type gymnotids capacitance…
modifies the timing (phase) of the field relative to the emission
- T type receptors detect timing distortions, by comparing distortions at receptors all over the body the fish can localise capacitive objects
electrocommunication:
an active sense like electroreception lends itself to communication, but there are two key problems when animals come together
problem with electrocommunication 1:
how to maintain the sensitivity of receptors when the fish is itself producing a powerful EOD?
- corollary (about same time as) discharge inhibition of knollenorgans in mormyrid pulse fish
problem with electrocommunication 2:
how to avoid interference from nearby conspecific? - affects communication and electrolocation
- mormyrid pulse type fish = use sparse signals (less interference)
- gymnotid wave type fish = individuals have characteristic frequencies + use jamming avoidance response
jamming problem…
occurs in gymnotid wave type fish
- summation of similar frequency of two fish’ EODs distorts detected EOD by creating a lower frequency ‘beating’ pattern = this leads to jamming of the ability to decipher the EOD
jamming avoidance response =
both fish shift the frequency of their EODs = fish with high-frequency shifts higher, fish with lower frequency shifts lower
the ‘beating’ interference does not occur when the two EOD frequencies differ markedly
initial hypothesis for jamming avoidance:
fish could monitor its own centrally generated EOD pacemaker pattern (‘efference copy’) and compares it to the conspecific pattern detected on the body surface
- wrong because fish detects its own and conspecific pattern together on the body surface, and computes the frequency difference
individual fish can use electrocommunicaton…
to determine small differences in the EOD waveform of conspecifics
can probably determine - species, sex, age, individual identity
