The Response Characteristics Of Electroreception In Juvenile Siberian Sturgeon(Acipenser Baeri)

Passive electroreception of some fish is a special and ancient sensory system. It can sense both biological and non-biological sources by receiving the signals from their surrounding electric fields. It is important in preying and avoiding behaviors of these fish. Most of electroreceptors(ampullary organs) in fish are located on lateral surface of the head(opercula) and lateral/ventral of the snout. The information received by the ampullary organs is sent into the DON(Dorsal Octavolateralis Nucleus) of the hindbrain via primary afferents of the anterio-dorsal lateral line nerve, then, processed to targets of different areas in the midbrain and continue going up along the neural pathways for further analyses and decision making in the central nerve system. The capacity of perceiving the information from the electric field source and the main neural mechanism of the signal reception in fish, however, are still not clear. To approach these questions, the electroreceptive juvenile sturgeons(Acipenser baeri) was chosen for this study, and a dipole source which was designed to produce the signals as simulating the passive electro-stimulation that the fish might encounter to in the field, then, the behavioral and electrophysiological experiments were designed and performed for testing the ability and characteristics of the electroreception of the fish were studied.The results are as follows:1. The behavior characteristics of electroreception in juvenile sturgeonsThe dipole applied with sufficient current was used to simulate the bio-electric field of sturgeons’ predators in this experiment. The electrosensory responses of juvenile sturgeons to the simulated dipole electric fields were studied. The results showed that the fish avoided the dipole, and there were significant differences in avoiding thresholds among different age groups of the juvenile sturgeons. In groups of 2 months of age, the mean avoiding threshold was 457.5±32.5 μV/cm, and were 29.5±2.5 μV/cm and 10±1.0μV/cm at 3 and 7 months old respectively. The morphological observation showed that the electroreceptors were increased along with the fish development from 2 months(2234±470) to 7 months(5273±523) old. In addition, there were also differences in the behavior responses to electric fields of different frequencies for these sturgeons which avoided most obviously(avoiding distance was at 16.8±1.37 cm) at the low frequencies(0 Hz).The study results suggested that the electrosensory sensitivity was enhanced along with the increase of number of electroreceptors during the development of the juvenile sturgeons. The avoidance of behavior of juvenile sturgeons for strong electric field in this study indicated that the electrosensory system was important for their surviving.2. The electrophysiology characteristics recorded from the DON of juvenile sturgeons brain under the dipole stimulationThe activity of DON neurons of electrosensory system was recorded and their response to electric field of different intensity and distance were observed. As the results revealed, the sturgeons of different ages showed varying responses to electric fields under condition of one distance but varying intensities. The D value of the DON neurons in 2 months old fish was increased with enhancing of the stimulation, its threshold of coupling was at 13 μV/cm and the phasing degree of highest D value was at 75°. Likewise, the D value of DON neurons in 7 months old was increased as well at enhance of stimulating, but its threshold of coupling was only at 1.3 μV/cm and the phasing degree of highest D value was at 50°. In addition, the Coefficient of variation showed 3 trends(increasing, no change, decreasing) response to the enhance of stimulating. These results indicated that the physiological function of DON neurons in juvenile sturgeons were changing during development, and they coded the information from electric field at one distance but different intensities by phase coupling separately; The D value of DON neurons whose spontaneous firing rate were at low frequency(<20 Hz, Mean at 14±2 Hz) showed them increased with distance of dipole at same strength of the receiving electric field(25 μV/cm and 50 μV/cm), and it may be the result from the voltage differences received by different electroreceptors. Therefore, it indicated that the electrosensory system get the strength and distance of the electric source by getting strong enough receiving field electric field. These finds are not only new in Siberian sturgeon but also first study in passive electrosensory system.