Silva A, Perrone R, Macadar O. (2007) Environmental, seasonal, and social modulations of basal activity in a weakly electric fish. Physiology & Behavior. 90: 525-536.

Brachyhypopomous pinnicaudatus, a species of freshwater, weakly electric, EOD fish from the Gymnotiformes in America were studied for their pulse-type wave form. The effects of water temperature and day-night cycle were observed and analyzed by comparing differences in the rate of EOD. Their signal is usually biphasic. Analyzing temperature in this species is important because most fish of this type have been studied in tropical waters, where temperature is relatively constant; this study focuses on the temperate climate, where seasons significantly affect the water temperature. EOD rate has been found to be temperature-dependent in other species studied. Additionally, studies have found it to be effected by the night phase of the day-night cycle and by breeding fish in activities of courtship and other social situations. The significance of this study (seeing as so many others have been done on EOD pulse fish) is that not much research has been performed on species that live in temperate zones.

In the study, 63 adult fish were kept in tanks that mimicked their natural habitat in temperature, light cycles, and in optimal female to male ratios (depending on the breeding time). They were collected using a “fish detector” and could be clearly distinguished by sex during the breeding phase because males display a long caudal filament upon sexual maturation (males are also significantly longer). The temperature variation was kept between 8 and 33 degrees Celsius, and the light cycle stayed within 10-14 hours of light, and 10-14 hours of dark. Recordings of the fish EOD rates were taken using electrodes attached to a non-conducting net that surrounded the fish and measurements were recorded when the fish was not making significant movements. The temperature sensitivity was measured in response to a gradual change (1 degree per 10 min) and to an acute change (instant 10 degree change). Q₁₀ (up/down) was calculated for gradual temperature changes and for acute change (Q₁₀ step): it is the ratio between EOD rates before and after the temperature change which allows comparison of rate of change over time. The fish we also video-taped during the night-day experimental setup and an infrared-sensitive camera (and infrared light) were used during night cycles. Two fixed electrodes recorded EOD rates and fed the information into high input impedance amplifiers. From this data mean EOD intervals were calculated to compare variability over the cycles. The experimental groups of fish were non-breeding isolated fish, male-female dyads (during breeding and non-breeding), non-breeding isolated males, and breeding isolated males. EOD rate was also tested before and after acclimation to the acute change in water temperature. Results could not be analyzed as a normal distribution so significance was tested using the Wilcoxon matched pair test and the Mann Whitney U test. Water temperature and day-night cycle effects were tested independently, meaning temperature was tested only during the light phases and day-night cycle was only tested under constant temperature.

Results showed that EOD rate increased in the dark phase and that nocturnal basal rates were significantly higher than diurnal basal rates. Mean EOD interval decrease immediately after “sunset” in both breeding and non-breeding fish. Non-breeding males showed higher temperature sensitivity before acclimation (biphasic to monophasic signal upon change in temperature), but after acclimation sensitivity decreased (biphasic at either temperature). This decrease in sensitivity was also found to be an accurate indicator of sexual maturation. Males in sexual interaction were found to exhibit a significant decrease in mean EOD intervals after sunset, but when put into isolation; the same male fish had a less significant decrease in intervals. Overall, results show that both temperature and day-night cycles effect the EOD rates of the Brachyhypopomous pinnicaudatus species. They also indicate that these effects are affected by social situations and the reproductive stages. Those that performed the study predict that these dynamic changes in rate indicate a plastic biological mechanism in the fish that allows them to adapt and react to various situations. Future studies were encouraged in looking at the role of pacemaker or pre-pacemaker cells and steroid hormones in these mechanisms.

The relevance of this study to BSCI338W is that the EOD form of communication is one which can provide a large amount of information due to the variations in presence of certain environmental/social conditions. Weakly electric fish, such as Brachyhypopomous pinnicaudatus, need to continue to be studied on the molecular/neurological level to understand the mechanisms behind the variations of EOD pulse rate – perhaps this would also help us understand the reasons behind the variations in rate and therefore the information that is transmitted during these interactions (or non-interactions). Finally, the fact that EOD fish use electrical signals is yet another example of how many mediums exist for communication, allowing the amount of diversity we see in animals.