Influence of electrical stimulation and deprivation on the electric organ discharge behaviour and metabolism of neuronal gangliosides of the tapirfish (Gnathonemus petersi, Mormyridae, Teleostei)

The influence of electrical stimulation ("attack"-frequency of 40 Hz, 2 V, 2 days) and of social and electrical deprivation on the metabolism of gangliosides of various brain structures and the electric organ of the weakly electric tapirfish (Gnathonemus petersi, Mormyridae) was investigated. After stimulation the daily average discharges of the electric organ increased from 9.4 to 11.1 Hz, whereas after deprivation they decreased to 7.9 Hz as compared with controls. There were significant and structure specific differences in some ganglioside-fractions (GM1, GD3, GD1a, GD1b and GP1) in concentration and in specific radioactive NeuAc-labelling between stimulated and deprived animals respectively, compared with controls.     

Sex differences in the waveform of the pulse-type electric fish,Pollimyrus isidori (Mormyridae)

Summary This paper reports the finding of a sexual dimorphism in the electric organ discharge of the weakly electric fishPollimyms isidori. The difference in the head-positive components of the waveform (Fig. 1) was found in 95% of adults. Juvenile fish always possessed female type discharges. Fourier analysis (Fig. 3) revealed significant sex differences in the power spectra of the discharges, indicating that specific phase information is perhaps unnecessary for sexual identification. The results are discussed with reference to our current knowledge of coding of electrocommunication signals in the electrosensory system.Abbreviation EOD electric organ discharge     

On the electromotor neurons of both electric organs of Pollimyrus isidori (Mormyridae, Teleostei)

The electromotor neurons (EMNs) of the adult electric organ of Pollimyrus isidori and the "giant neurons" (GNs) found in the more rostral part of the spinal cord were investigated both with light and electron microscopical methods. The two kinds of neuron are more or less similar in size, about 30 microns, and are found in the more dorsal part of the spinal cord. The EMNs show somatosomatic gap junctions, as do the GNs. In addition, chemical synapses could be found between nerve fibres and both types of neurons. Histochemical investigations show that the EMNs and the GNs are both AChE positive. On the basis of the arguments presented in the discussion, we believe that the "giant neurons" are the electromotor neurons of the larval electric organ of Pollimyrus isidori.     

Reproduction and development of the weakly electric fish,Pollimyrus isidori (Mormyridae,Teleostei) in captivity

Synopsis No clear sexual dimorphism occurs in Pollimyrus isidori. Females usually grew slightly larger than males. The anal-fin reflex, however, makes it easy to discriminate between males and females. Spawning took place during the first six hours of the dark phase, in the territory of the male. During each spawning act 2–4 eggs were laid. The male put the eggs into a well-hidden nest previously constructed from plant material. The eggs, free embryos, and larvae of several spawnings were guarded by the male for several weeks. The females laid on average 120 eggs per fractional spawning. The eggs were 2 mm in diameter, not adhesive and very yolky. There was a relationship between pH and viability (% embryos hatched). Hatching occurred on the third day after spawning. The respiratory network of segmental origin in the median fin fold was well developed in the free embryos. Transition to exogenous feeding occurred on day 14. The larval period ended when the fish were 15 mm long and 40–50 days old. First gonadal recrudescence occurred at a total length of 6 cm (about 200 days old). The environmental factors decreasing conductivity and pH, increasing water level, and imitation of rain led to gonadal recrudescence, but pH did not act as a cue. Recrudescence was triggered by a decrease of conductivity, but absolute values or ionic composition of the water were not important. Maturation was completed after about fifty days. Mature and spawning fish no longer required any variation of these environmental factors to maintain mature gonads. A steady and considerable increase in conductivity led to gonadal regression. The fish did not show postbreeding refractoriness.     

Temperature sensitivity of the electric organ discharge waveform in Gymnotus carapo

At the southern boundary of gymnotiform distribution in America. water temperature changes seasonally, and may be an environmental cue for the onset of breeding. In this study, we aim to describe the role of temperature upon electric organ discharge waveform in Gymnotus carapo, order Gymnotiformes, family Gymnotidae, and to analyze its interactions with the effects of steroid hormones. The effects of water temperature within its natural range were explored using different protocols. All fish tested had temperature-sensitive electric organ discharge waveforms: the amplitude of the last head-negative component (V4) decreased as temperature increased. Rate increases elicited by electrical stimulation had similar but smaller effect on waveform. Temperature sensitivity is a peripheral phenomenon that depends on the conductivity of the aquatic media. We found hormonal-dependent changes in the electric organ discharge waveform not previously described in this species. The amplitude and duration of V4 increased after testosterone administration. Both testosterone treatment and acclimation by sustained temperature at 27-28 degrees C (environmental simulation of breeding conditions) induced a decrease in temperature sensitivity. As in the related species Brachyhypopomus pinnicaudatus, our data strongly suggest interactions between temperature sensitivity of the electric organ discharge waveform and sexual maturity that might be crucial for reproduction.     

Dim light vision – Morphological and functional adaptations of the eye of the mormyrid fish, Gnathonemus petersii

The African weakly electric fish Gnathonemus petersii is well known for its electrosensory capabilities. These animals can detect and distinguish objects through active electrolocation in complete darkness. Because of their nocturnal lifestyle, a low contribution of vision for orientation and object detection has been expected. However, as we show in this review, the retina of G. petersii is highly specialized with hundreds of rods and tens of cones grouped together in bundles in a complex way, ensheathed by a tapetum lucidum. The structure of the bundles goes beyond what would be expected if only photon catch was supposed to be increased. During daytime, the structure of these “macro-receptors” changes dramatically depending on retinomotor movements. During the day, the rods and cones are located in different compartments of the bundle, separated by a narrow canal in the form of a “bottle neck”. Investigations on cell structure and neurochemistry in the retina indicate a general organization that is simpler in terms of bipolar and ganglion cell diversity than in tetrachromatic species such as goldfish, yet similar in terms of neurochemical differentiation of amacrine cells. In both respects, the inner retina of the elephantnose fish bears the greatest similarity to catfish and some deep-sea fish retinae. Neuronal circuits and bundle structure give hints of possible adaptations for contrast and/or movement detection. Behavioral experiments suggest that, in contrast to the vision specialists Lepomis gibbosus, pattern detection of G. petersii is not affected by higher spatial frequencies. A pattern of low spatial frequencies, however, was equally well detected by G. petersii and L. gibbosus. Optomotor response experiments indicate that motion vision is important for Gnathonemus, narrowing down the search for the functional specialization of the Gnathonemus retina and providing a starting point for work on multisensory integration in these fish.     

Sex recognition and neuronal coding of electric organ discharge waveform in the pulse-type weakly electric fish, Hypopomus occidentalis

1. Hypopomus occidentalis, a weakly electric gymnotiform fish with a pulse-type discharge, has a sexually dimorphic electric organ discharge (Hagedorn 1983). The electric organ discharges (EODs) of males in the breeding season are longer in duration and have a lower peak-power frequency than the EODs of females. We tested reproductively mature fish in the field by presenting electronically generated stimuli in which the only cue for sex recognition was the waveshape of individual EOD-like pulses in a train. We found that gravid females could readily discriminate male-like from female-like EOD waveshapes, and we conclude that this feature of the electric signal is sufficient for sex recognition. 2. To understand the possible neural bases for discrimination of male and female EODs by H . occidentalis, we conducted a neurophysiological examination of both peripheral and central neurons. Our studies show that there are sets of neurons in this species which can discriminate male or female EODs by coding either temporal or spectral features of the EOD. 3. Temporal encoding of stimulus duration was observed in evoked field potential recordings from the magnocellular nucleus of the midbrain torus semicircularis. This nucleus indirectly receives pulse marker electroreceptor information. The field potentials suggest that comparison is possible between pulse marker activity on opposite sides of the body. 4. From standard frequency-threshold curves, spectral encoding of stimulus peak-power frequency was measured in burst duration coder electroreceptor afferents. In both male and female fish, the best frequencies of the narrow-band population of electroreceptors were lower than the peak-power frequency of the EOD. Based on this observation, and the presence of a population of wide-band receptors which can serve as a frequency-independent amplitude reference, a slope-detection model of frequency discrimination is advanced. 5. Spectral discrimination of EOD peak-power frequency was also shown to be possible in a more natural situation similar to that present during behavioral discrimination. As the fish's EOD mimic slowly scanned through and temporally coincided with the neighbor's EOD mimic, peak spike rate in burst duration coder afferents was measured. Spike rate at the moment of coincidence changed predictably as a function of the neighbor's EOD peak-power frequency. 6. Single-unit threshold measurements were made on afferents from peripheral burst duration coder receptors in the amplitude-coding pathway, and midbrain giant cells in the time-coding pathway.(ABSTRACT TRUNCATED AT 400 WORDS)     

Adaptive radiation in the Congo River: An ecological speciation scenario for African weakly electric fish (Teleostei; Mormyridae; Campylomormyrus)

The ultimate aim of this study is to better understand the diversification of African weakly electric fish in the Congo River. Based on a robust phylogenetic hypothesis we examined the radiation within the mormyrid genus Campylomormyrus. Morphological traits relevant for the divergence between the identified species were detected. Among them, the variation in the shape of the trunk-like elongated snout suggested the presence of different trophic specializations. Furthermore, the chosen model taxon, the genus Campylomormyrus, exhibits a wide diversity of electric organ discharge (EOD) waveforms. A comparison of EOD waveform types and phylogenetic relationships showed major differences in EOD between closely related species. This indicates that the EOD might function as a reproductive isolation mechanism. In conclusion, we provide a plausible scenario of an adaptive radiation triggered by sexual selection and assortative mating based on EOD characteristics, but caused by a divergent selection on the feeding apparatus. These findings point towards an adaptive radiation of at least some Campylomormyrus species occurring in the Lower Congo River.     

Phylogenetic relationships of mormyrid electric fishes (Mormyridae; Teleostei) inferred from cytochrome b sequences

The nasute termite genus Nasutitermes is widely distributed over all tropical regions. The phylogenetic relationships among 17 Nasutitermes species from the Pacific tropics were inferred from sequences of mitochondrial cytochrome oxidase II and 16S ribosomal RNA genes. Several methods of analysis yielded phylogenetic trees showing almost the same topology and in good agreement with reconstructions based on morphological or behavioral characters. Neotropical and Australian species came out as separate, apical clades. Asian species split between an apical branch, appearing as sister group to the neotropical clade, and basal taxa. New Guinean species were spread among several clades, suggesting a derivation from multiple origins. A well-supported clade includes the neotropical, Australian, and New Guinean species, with the southeast Asian N. takasagoensis and N. matangensis. It excludes the Asian species N. regularis, N. parvonasutus, and N. longinasus, which might deserve to be removed from Nasutitermes, as well as the long-legged Asian genera Hospitalitermes and Longipeditermes. A Gondwanan origin is proposed for the former clade, although an Old World origin of Nasutitermes followed by dispersal to Australia and South America cannot be excluded.     

Ontogeny of the electric organ discharge and the electric organ in the weakly electric pulse fish Brachyhypopomus pinnicaudatus (Hypopomidae, Gymnotiformes)

I recorded the electric organ discharges (EODs) of 331 immature Brachyhypopomus pinnicaudatus 6–88 mm long. Larvae produced head-positive pulses 1.3 ms long at 7 mm (6 days) and added a second, small head-negative phase at 12 mm. Both phases shortened duration and increased amplitude during growth. Relative to the whole EOD, the negative phase increased duration until 22 mm and amplitude until 37 mm. Fish above 37 mm produced a “symmetric” EOD like that of adult females. I stained cleared fish with Sudan black, or fluorescently labeled serial sections with anti-desmin (electric organ) or anti-myosin (muscle). From day 6 onward, a single electric organ was found at the ventral margin of the hypaxial muscle. Electrocytes were initially cylindrical, overlapping, and stalk-less, but later shortened along the rostrocaudal axis, separated into rows, and formed caudal stalks. This differentiation started in the posterior electric organ in 12-mm fish and was complete in the anterior region of fish with “symmetric” EODs. The lack of a distinct “larval” electric organ in this pulse-type species weakens the hypothesis that all gymnotiforms develop both a temporary (larval) and a permanent (adult) electric organ. Accepted: 1 March 1997     

Patterns of reproductive morphology in the genus Gobiodon (Teleostei: Gobiidae)

The reproductive system of gobiid fishes (family Gobiidae), especially among hermaphroditic goby species, is morphologically diverse. Two hermaphroditic species in the genus Gobiodon, G. okinawae and G. oculolineatus, have several modifications of the gonoduct and gonad that are associated with secretion production and storage. In this study, an examination of six additional Gobiodon species, G. citrinus, G. fulvus, G. histrio, G. micropus, G. quinquestrigatus and G. rivulatus, revealed similar reproductive modifications. Among these six Gobiodon species, a number of features were found to be shared amongst each other and with G. okinawae and G. oculolineatus. All individuals had either an ovariform gonad or an ovotestis; no individuals had a purely testicular gonad. The gonadal lobes extended caudally past, rather than terminating at, their union with the gonoduct. Accessory secretory structures associated with the reproductive complex, termed accessory gonoduct structures, or AGdS, always originated from the gonoduct. The ovariform gonad was comprised entirely of ovarian tissue, while the ovotestis was divided into three morphologically distinct regions. Only one of the ovotestis regions was strictly gametogenic, consisting of both early stage oocytes and sperm-filled seminiferous lobules. The second region of the ovotestis was made up of stromal tissue surrounding some compressed lumina and a small number of early-stage oocytes. The third region was highly lobulated and acted as a storage region for eosinophilic secretions. Anteriorly, the stromal region of each of the two ovotestis lobes disappeared and the gametogenic and secretory storage regions of the ovotestis separated into two discrete lobes. In all of the examined Gobiodon species, all individuals having an ovotestis also had AGdS. However, AGdS presence among individuals having an ovariform gonad varied in a species-specific manner, with the AGdS being fully differentiated and well-developed in a number of species, and either in a very early stage of development, or absent, in others. The distribution of these AGdS states among Gobiodon species corresponds with that of several other morphological features that have been proposed by Harold et al. (Bull Mar Sci 82:119–136, 2008) as phylogenetically informative for intra-generic clade identification. Reproductive characters may prove informative in the development of hypotheses of relationships among gobiid fishes.     

Plasticity of the electric organ discharge waveform of the electric fish Brachyhypopomus pinnicaudatus I. Quantification of day-night changes

The electric organ discharge of the gymnotiform fish Brachyhypopomus pinnicaudatus is a biphasic waveform. The female's electric organ discharge is nearly symmetric but males produce a longer second phase than first phase. In this study, infrared-sensitive video cameras monitored the position of unrestrained fish, facilitating precise measurement of electric organ discharge duration and amplitude every 2 h for 24 h. Males (n=27) increased electric organ discharge duration by 37 ± 12% and amplitude by 24 ± 9% at night and decreased it during the day. In contrast, females (n=8) exhibited only minor electric organ discharge variation over time. Most of a male's increase occurred rapidly within the first 2–3 h of darkness. Electric organ discharge values gradually diminished during the second half of the dark period and into the next morning. Modulation of the second phase of the biphasic electric organ discharge produced most of the duration change in males, but both phases changed amplitude by similar amounts. Turning the lights off at mid-day triggered an immediate increase in electric organ discharge, suggesting modification of existing ion channels in the electric organ, rather than altered genomic expression. Exaggeration of electric organ discharge sex differences implies a social function. Daily reduction of duration and amplitude may reduce predation risk or energy expenditure. Accepted: 12 September 1998     

Differentiation of morphology, genetics and electric signals in a region of sympatry between sister species of African electric fish (Mormyridae)

Mormyrid fishes produce and sense weak electric organ discharges (EODs) for object detection and communication, and they have been increasingly recognized as useful model organisms for studying signal evolution and speciation. EOD waveform variation can provide important clues to sympatric species boundaries between otherwise similar or morphologically cryptic forms. Endemic to the watersheds of Gabon (Central Africa), Ivindomyrus marchei and Ivindomyrus opdenboschi are morphologically similar to one another. Using morphometric, electrophysiological and molecular characters [cytochrome b sequences and amplified fragment length polymorphism (AFLP) genotypes], we investigated to what extent these nominal mormyrid species have diverged into biological species. Our sampling covered the known distribution of each species with a focus on the Ivindo River, where the two taxa co-occur. An overall pattern of congruence among datasets suggests that I. opdenboschi and I. marchei are mostly distinct. Electric signal analysis showed that EODs of I. opdenboschi tend to have a smaller initial head-positive peak than those of I. marchei, and they often possess a small third waveform peak that is typically absent in EODs of I. marchei. Analysis of sympatric I. opdenboschi and I. marchei populations revealed slight, but significant, genetic partitioning between populations based on AFLP data (F(ST) approximately 0.04). Taken separately, however, none of the characters we evaluated allowed us to discriminate two completely distinct or monophyletic groups. Lack of robust separation on the basis of any single character set may be a consequence of incomplete lineage sorting due to recent ancestry and/or introgressive hybridization. Incongruence between genetic datasets in one individual, which exhibited a mitochondrial haplotype characteristic of I. marchei but nevertheless fell within a genetic cluster of I. opdenboschi based on AFLP genotypes, suggests that a low level of recent hybridization may also be contributing to patterns of character variation in sympatry. Nevertheless, despite less than perfect separability based on any one dataset and inconclusive evidence for complete reproductive isolation between them in the Ivindo River, we find sufficient evidence to support the existence of two distinctive species, I. opdenboschi and I. marchei, even if not 'biological species' in the Mayrian sense.     

Androgen correlates of socially induced changes in the electric organ discharge waveform of a mormyrid fish

Weakly electric fish from the family Mormyridae produce pulsatile electric organ discharges (EODs) for use in communication. For many species, male EODs are seasonally longer in duration than those of females, and among males, there are also individual differences in EOD duration. While EOD elongation can be induced by the administration of exogenous androgens, androgen levels have never before been assessed under natural or seminatural conditions. By simulating the conditions occurring during the breeding season in the laboratory, we provide evidence of a sex difference in EOD duration as well as document levels of circulating androgens in males. In this study, we analyzed the nature of social influences on male EOD duration and plasma androgen levels in Brienomyrus brachyistius. Individual males, first housed with a single female and then placed into social groups consisting of three males and three females, showed status-dependent changes in EOD duration. Top-ranking males experienced a relatively large increase in EOD duration. Second-ranking males experienced a more modest increase, and low-ranking males experienced a decrease in EOD duration. These changes were paralleled by differences in circulating levels of plasma 11-ketotestosterone (11-KT), but not testosterone, suggesting that the changes in EOD duration may have been mediated by changes in plasma 11-KT levels. Thus, it appears that EOD duration is an accurate indicator of male status, which is under social and hormonal control.