Ontogenetic development of electric‐organ discharges in a mormyrid fish, the bulldog Marcusenius macrolepidotus (South African form)

Abstract

The emergence and development of the electric-organ discharge (EOD) in larvae and juvenile bulldog Marcusenius macrolepidotus was investigated. Larvae hatched 4-5 days after spawning, and the first EODs were recorded on days 9 and 10 at a standard length (L S ) of c. 6Á5 mm. The larval EOD waveform was virtually monopolar, with a strong head-positive phase followed by a weak head-negative phase of long duration. A small separate potential preceded the EOD by c. 1Á6 ms (believed to represent postsynaptic potential from electrocyte stalks). In contrast to previous reports on Pollimyrus adspersus with its distinct larval and adult EODs, in M. macrolepidotus there was a gradual transformation of the larval into the adult EOD waveform. The transformation started at an L S of c. 17 mm (at an age of c. 40 days), first indications being a decrease in duration of the head-negative phase, and an increase of its peak amplitude relative to that of the head-positive phase. Still later, the weak postpotential of the adult EOD emerged on the rising edge of the head-negative phase. The transformation was nearly completed at an L S of c. 30 mm (at an age of c. 60 days). Evolutionary and behavioural consequences of this alternative path of EOD ontogeny are discussed.

Allopatric differentiation in theMarcusenius macrolepidotusspecies complex in southern and eastern Africa: the resurrection ofM. pongolensisandM. angolensis, and the description of two new species (Mormyridae, Teleostei)

“…The larvae of the three Pollimyrus species and of M. pongolensis all generate head-positive larval EODs of long duration and almost identical waveforms, but only those of the three Pollimyrus species that have been studied are functionally replaced by a distinct adult discharge of short duration, reversed polarity, and following each larval EOD at an interval of about 0.7 ms during a short transition period after which the larval electric organ degenerates (Westby and Kirschbaum 1978;Baier et al 2006, Werneyer and. The monopolar larval EOD of M. pongolensis, however, is gradually transformed into the bipolar adult EOD (Werneyer and Kramer, 2006), with no ''electric'' evidence for two distinct, subsequently active electric organs.…”

Larval electroreceptors in the epidermis of mormyrid fish: II. The promormyromast

“…However, during the transformation of the larval discharge into the adult discharge in M. r. proboscirostris (Schugardt and Kirschbaum, unpubl. results) and also in Marcusenius macrolepidotus (Werneyer and Kramer, 2006), the change in discharge waveform is less visible than in P. isidori (Westby and Kirschbaum, 1978). Nevertheless, the degeneration of the LTOs and the transformation of the promormyromast still takes place as in P. isidori.…”

Signal diversification is associated with corollary discharge evolution in weakly electric fish

“…A previous 490 study demonstrated that EOD duration changes substantially with growth in C. numenius, and 491 this correlates with ontogenetic changes in EO anatomy (Paul et al, 2015). For many mormyrid 492 species, EOD waveform varies with ontogeny, sex, relative dominance, and season (Bass, 1986;493 Hopkins, 1999;Carlson et al, 2000;Werneyer and Kramer, 2006). The existence of individual 494 variation in corollary discharge delays raises the question whether the same mechanism is used 495 for individual variation of corollary discharge as for species differences.…”