Lateral Line System in Fishes ch4 2

Sensing and Electric Discharge in Fish

• Some fish have a large number of neuromasts on their heads, which might be important for sensing vortex trails in the wakes of adjacent fish in a school.
• Fish that form extremely dense schools, such as herrings, atherinids, and mullets, often lack lateral line organs along their flanks and retain canal organs only on their heads.
• These cephalic canal organs concentrate sensitivity to water motion in the head region, allowing the fish to sense the turbulence it is swimming into.
• The reduction of flank lateral line elements reduces noise from turbulence beside the fish.

Electric Fish

• Electric fish, such as the torpedo ray, electric catfish, and electric eel, can discharge enough electricity to stun prey animals and deter predators.
• Weakly electric fish, such as the knife fish and elephant fish, use electrical signals for courtship and territorial defense.
• These fish use modified muscle tissue to produce the electrical discharge, which is generated by specialized cells called electrocytes.
• Electrocytes are muscle cells that have lost the capacity to contract and are specialized for generating an ion current flow.
• When at rest, the membranes of muscle cells and nerve cells are electrically charged, with an imbalance of 84 millivolts due to sodium ion exclusion.
• When stimulated, sodium ions flow rapidly across the smooth surface, sending its potential to a positive 67 millivolts, resulting in a potential difference of 151 millivolts across the cell.

Electric Discharge in Fish

• Electric fish have stacks of electrocytes, which combine to produce high voltages.
• The South American electric eel has up to 10,000 layers of cells and can generate potentials in excess of 600 volts.
• Most electric fish are found in tropical freshwaters of Africa and South America, with few marine forms capable of generating specialized electrical discharges.

Electroreception in Other Vertebrates

• Electroreception is not restricted to a single group of aquatic vertebrates, and monotremes, such as the platypus and echidna, use electroreception to detect prey.
• Electroreception was likely an early feature of vertebrate evolution, with the brain of the lamprey responding to electric fields.
• All fish-like vertebrates of lineages that evolved before the neopterygians have electroreceptor cells, which fire when the environment around the kinocilium is negative relative to the cell.

Electroreception in Sharks and Rays

• Sharks have electroreceptors called the ampullae of Lorenzini on their heads, which detect the electrical activity that accompanies muscle contractions of their prey.
• The ampullae are sensitive to minute changes in the electric field surrounding an animal, with thresholds lower than 0.01 microvolt per centimeter.
• Sharks use their electrical sensitivity to detect prey, as all muscle activity generates electrical potential.
• The ampullae act like voltmeters, measuring differences in electrical potentials across the body surface.