electrosensation in electric fish

Question 1

Different types of electric sense: passive

Answer 1

passive sense (detect external electric fields):

• animate: gills, muscle, heart
• inanimate: earths geomagnetic field

25% of all fish
Use Ampullary receptors

Question 2

Different types of electric sense: active

Answer 2

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

Question 4

electric fish -

Answer 4

Send out electrical files and can detect any distortions tell them about the environment - can detect anything with a different electrical field

Question 5

Specialisations for electro-sensation:

Answer 5

• 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

Question 6

Two types of weakly electric fish:

Answer 6

Gymnotiforms and mormyriformes

• electric sense very similar but these fish evolved separately (common ancestor did not possess electric sense

Question 7

Gymnotiformes -

Answer 7

South America

E.g. knife fish - specialised tail

Almost exclusively wave type

Question 8

Generation of electric discharge - electric organ discharges (EOD):

Answer 8

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

Question 9

Mormyriformes -

Answer 9

Africa

E.g. elephant fish (specialised trunk muscles)

Almost exclusively pulse type

Question 10

Strongly electric fish electric organ discharge:

Answer 10

monopolar - best for stunning prey

Question 11

Iso voltage curves -

Answer 11

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

Question 12

Weakly electric fish electric organ discharge:

Answer 12

Bipolar or more complex waveforms

Question 13

electro-receptive sense organs:

Answer 13

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

Question 14

Ampullary receptors:

Answer 14

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

Question 15

Tuberous receptors:

Answer 15

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

Question 16

Time marker tuberous electroreceptors:

Answer 16

• 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

Question 17

Amplitude condors tuberous electroreceptors:

Answer 17

• 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)

Question 18

Receptors in mormyrid fish from Africa (elephantfish)…

Answer 18

Time coders = knollenorgane (K-receptors)

Amplitude coders = mormyromasts (D-receptors) - these are important in active object location

Question 19

Different parameters of the ‘electric image’ provide different info about an object…

Answer 19

• 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

Question 20

Distance estimation experiment:

Answer 20

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

Question 21

Proposed mechanism for depth perception in weakly electric fish =

Answer 21

Use a single stationary array of receptors without resorting to temporal or spectral measurements

Question 22

Capacitance estimation:

Answer 22

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

Question 23

Receptors in gymnotid fish from South America (knifefish)…

Answer 23

Pulse fish:
time coders = M receptors
Amplitude coders = B (burst) receptors

Wave fish:
Time coders = T (time) receptors
Amplitude coders = P (probability) receptors

Question 24

in pulse type mormyrids capacitance…

Answer 24

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

Question 25

in wave type gymnotids capacitance…

Answer 25

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

Question 26

electrocommunication:

Answer 26

an active sense like electroreception lends itself to communication, but there are two key problems when animals come together

Question 27

problem with electrocommunication 1:

Answer 27

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

Question 28

problem with electrocommunication 2:

Answer 28

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

Question 29

jamming problem…

Answer 29

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

Question 30

jamming avoidance response =

Answer 30

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

Question 31

initial hypothesis for jamming avoidance:

Answer 31

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

Question 32

individual fish can use electrocommunicaton…

Answer 32

to determine small differences in the EOD waveform of conspecifics

can probably determine - species, sex, age, individual identity