A study of the electric organ discharges and sounds
produced by selected southern African Synodontis
A study of electric organ discharges (EODs) and sounds was done on hybrids
(Synodontis zambezensis x S. nigromaculatus) and pure species of S. zambezensis
and S. nigromaculatus respectively. Fish were placed between a pair of carbon rod
electrodes that was connected to a differential amplifier with a variable gain to record
EODs (Kramer and Van der Bank, 2000). A hydrophone was used for the underwater
sounds and a microphone for the sounds outside the water. No EODs were produced
by the species and no distinct sound differences were found outside the water or
underwater during 24-hour studies.
Electrosensory and electromotor systems are used for orientation and communication
over short ranges. Two groups of weakly electric fish are known (the gymnotiform fish
of South America and mormyriform fish of Africa). In addition, Lissmann and Machin
(1963) recorded the first weak pulse-like electric discharges from Clarias in Africa.
Kramer and Van der Bank (2000) used species-specificity of electric organ discharges
(EODs), genetics and morphology to described a new species (Petrocephalus
wesselsi, Mormyridae). Hagedorn et al. (1990) presented evidence that synodontid
catfish species (S. nigrita , Valenciennes, 1840; S. obesus, Boulenger, 1898 and S.
schoutedeni, David, 1936) can produce weak electric discharges independent of
sounds. It indicated that these signals might be species-specific in Synodontis.
Selected southern African synodontid species and S. schoutedeni as control were
studied in an attempt to verify this.
5.3 MATERIALS AND METHODS
EODs of the Synodontis fishes were recorded in the field immediately after capture,
using a 37-1 plastic aquarium filled with river water where the fish was collected. Fish
were placed between a pair of carbon rod electrodes that was connected to a
differential amplifier with a variable gain (up to x10; 0,2 Hz…100Hz; filter slopes, -3 dB
per octave). A fish finder (with electrodes connected to electronic sound apparatus to
enhance sounds and headphones) and recorder were used to record EODs in a 24-
hour study. A hydrophone and modified oscilloscope (John Bell, RAU technical
division) was used for the electric and sound recordings. The electrodes were placed
inside the aquarium containing the different species. A 24-hour study was done in
order to cover all the activities. Three species (S. nigromaculatus, S. zambezensis and
a hybrid) were analysed for sounds made outside the water. The fishes were held by
their dorsal fins outside the water and the sounds produced recorded.
5.4 RESULTS AND DISCUSSION
No EOD results were obtained for any of the Synodontis species analysed. Kramer
(pers. com, 2003) attempted to find EODs for various other Group II species in the
Caprivi without any success. Analysis of the fish sounds (out of the water) between the
hybrid (S. nigromaculatus x S. zambezensis), S. nigromaculatus and S. zambezensis
showed no distinct differences. The hybrid had a frequency range of 660-2860 Hz with
a dominant frequency of 2140 Hz. Synodontis nigromaculatus had an intermediate
frequency range of 400 – 2840 Hz with a dominant frequency of 1700 Hz and S.
zambezensis had a frequency range of 620 – 2040 Hz (dominant frequency = 1520
Hz). The duration of each pulse of the call was also not significantly different (Mason,
pers. com.) between the species studied; the hybrid had an intermediate duration of
0.139 sec, S. nigromaculatus 0.144 sec and S. zambezensis 0.102 sec. The calls
emitted by the fish are probably distress calls produced to cause a potential predator to
realise it. These are stress calls and probably not being used for mating or conspecific
recognition. The 24-hour study was necessary to monitor any sounds that might be
made during night time when these fishes are more active. No underwater sounds
could be detected within the time duration.
I would like to acknowledge Mervyn Mason of the Communication and Behaviour
Research Group from the Department of Zoology at the University of the Witwatersrand
for his assistance with regards to the analysis of the fish sounds produced out of the
Hagedorn, M., Vomble, M., Finger, T.E., 1990. Synodontide Catfish: a new group of
weakly electric fish. Brain Behav. Evol. 35, 268 - 277.
Kramer, B., Van der Bank, F.H., 2000. The southern churchill, Petrocephalus wesseli, a
new species of mormyrid from south Africa defined by electric organ discharges,
genetics and morphology. Env. Biol. of Fishes 59, 393 - 413.
Lissmann, H., Machin, K.E., 1963. Electric receptors in non-electric fish (Clarias). Na.
199, 88 - 89.