Bluegills: Biology and Behavior

2: Sensory Perception

doi: https://doi.org/10.47886/9781888569933.ch2

Underwater, sound reception and hydrodynamic forces are not only difficult to distinguish, but their functions vary with level of organization. At the level of the receptor, hair cells of the lateral line and inner ear, or labyrinth, act as displacement detectors.⁵⁷ At the organ level the simple lateral line neuromasts detect velocity,⁵⁸ the movements of their cupulae being stimulated by drag forces proportional to the velocity of the water flowing past.⁵⁹ At the system level, arrays of neuromasts are detectors intermediate between velocity and acceleration, neuromasts of the lateral line canal ordinarily operating to detect acceleration.⁶⁰ The superficial neuromasts are sensitive only in still waters,⁶¹ such as those inhabited by bluegills; in running waters these organs are stimulated at all times, making them useful mainly for rheotaxis⁶² but not for detecting hydrodynamic movements.⁶³ Over short distances—that is, close to moving prey or predators—the vectors of acceleration vary in strength and direction and readily stimulate the lateral line.⁶⁴

The otoliths of the inner ear (utriculus, sacculus, and lagena) are linear acceleration detectors, their displacements occurring during displacement of the fish; the three semicircular canals and their components work as angular acceleration detectors.⁶⁵ The hydrodynamic field moves toward uniformity with distance from the source, and the absence of local disturbances leaves cupulae of the lateral line unstimulated.⁶⁶ In this situation the denser otoliths are more adapted to detect displacements of neutrally buoyant fishes.⁶⁷ Therefore, both the lateral line canals and their constituents, along with the otolith organs, have evolved to detect acceleration, the first associated with acceleration of water relative to the fish, the second with acceleration of the fish itself.⁶⁸ In addition, the lateral line senses spatial differences along its arrays; the otoliths monitor acceleration of the water as an average over the volume occupied by the fish as it swims.⁶⁹