Ethological observations on the electric organ discharge behaviour of the electric catfish, Malapterurus electricus (Pisces)

Abstract

1. Electric organ discharge (EOD) volleys emitted by the electric catfish, Malapterurus electricus, were recorded and sampled for 108 consecutive hours from the fish's natural habitat, a selected study area in the Swashi River, a tributary of the man-made Lake Kainji in Nigeria. 2. M. electricus intermittently generates different types of EOD volleys that start with a high-frequency phase, and range in duration from 2 ms to over 8 s with from 2 to over 600 pulses (EODs). 3. The catfish's EOD activity exhibited a daily activity rhythm which was correlated with local changes in light intensity. The EOD activity remained at a constant level throughout the day from 0700 h to 1800 h. Shortly after sunset, between 1800 h and 1900 h, it increased fivefold to its peak value at 1900 h. The occurrence of EOD volleys declined throughout the night with a noticeable drop to the daytime low after sunrise between 0600 h and 0700 h. During the first 6 h following sunset, the volleys were significantly longer and consisted of more EODs than the daytime volleys. It is inferred that M. electricus feeds most successfully during the early hours of the night. 4. A hitherto unknown type of Malapterurus EOD activity was discovered: during night-time, volleys consisting of more than 16 EODs were occasionally preceded by a low-frequency train of from 1 to 11 EODs. The occurrence rate of these pre-volley EODs was positively correlated with the duration of the ensuing volley (and its number of EODs). The peak pre-volley activity occurred during the first 4 h after sunset, with an average of 38% of all volleys preceded by this type of activity. The adaptive significance of the nocturnal pre-volley EODs as prey detection mechanism is discussed.