@article {672, title = {Review of the southern African slender stonebashers, genus Heteromormyrus Steindachner 1866 (Teleostei: Mormyridae), with description of six new species}, journal = {Journal of Fish Biology}, volume = {108}, year = {2025}, month = {Sep-17-2025}, abstract = {

Recent molecular studies have advanced our knowledge of the taxonomic diversity and generic placement of the slender stonebashers, previously placed in the genus Hippopotamyrus, in southern Africa. These fishes were recently transferred to the genus Heteromormyrus whose range encompasses the Kwanza, Kunene, Okavango, Zambezi, Pungwe and Buzi River systems in southern Africa, as well as the southern tributaries of the Congo River system. The present study builds on previous research that identified at least eight candidate species within the Heteromormyrus ansorgii species complex by providing formal descriptions for six new species and redescriptions of Heteromormyrus pauciradiatus and H. ansorgii s.s. The Kwanza River system is peculiar because it currently has five known species in this genus, some of which are co-distributed, whereas the other river systems have only one or two species in this genus. Two of the new species, Heteromormyrus dolichorhynchus sp. nov. and Heteromormyrus angusticaudata sp. nov., are endemic to the Kwanza River system. Heteromormyrus xanekweorum sp. nov. is confined to the Okavango River system, Heteromormyrus chilembwei sp. nov. occurs in the Ruo River (lower Zambezi River system), Heteromormyrus tangwenai sp. nov. is endemic to the Pungwe River system and Heteromormyrus ndauorum sp. nov. is endemic to the Buzi River system. Species in this genus exhibit high morphological similarity, but they can be separated by a combination of characters, including scale counts, dorsal- and anal-fin ray counts, vertebral counts, caudal peduncle depth, position of nostrils, head shape and variation in colour pattern. Taxonomic diversity within this genus is likely to be higher than currently known, and future studies, particularly in the Kwanza and upper Zambezi rivers, are anticipated to uncover additional new species.

}, keywords = {Diversity, freshwater, mormyrids, southern Africa, Taxonomy}, issn = {0022-1112, 1095-8649}, doi = {10.1111/jfb.70191}, url = {https://doi.org/10.1111/jfb.70191}, author = {Mutizwa, Tadiwa I. and Kadye, Wilbert T. and Bragan{\c c}a, Pedro H. N. and Chakona, Albert} } @article {670, title = {Gene expression correlates and mechanistic insights into electric organ discharge duration changes in mormyrid electric fish}, journal = {Journal of Experimental Biology}, volume = {228}, year = {2025}, month = {Jun-01-2025}, abstract = {

Electric organ discharge (EOD) duration in African weakly electric fish (Mormyridae) is the most variable waveform component between species and the basis for distinguishing species-specific signals. EOD duration is thought to be influenced by morphological and physiological features of electrocytes (the cells that comprise the electric organ), but the mechanistic details are poorly understood. It has long been known that EOD duration is modulated by androgen hormones, affording an opportunity to identify gene expression correlates of EOD duration differences. We induced EOD elongation in the mormyrid Brienomyrus brachyistius by administering 17\α-methyltestosterone (17\αMT) to three treatment groups: control (no 17\αMT exposure), T1day and T8day (samples taken 1 and 8 days after a single exposure to 17\αMT, respectively). We then performed RNAseq, differential gene expression and functional enrichment analysis to detect gene expression changes during EOD duration change. Our analyses indicate 44 genes whose expression changed in tandem with EOD elongation and include genes responsible for actin filaments and microtubules, extracellular matrix organization and membrane lipid metabolism. Additionally, we found expression changes in one Na+ channel \β-subunit, and five voltage-gated K+ channels. Together, these genes point toward specific cellular processes that contribute to morphological and physiological changes that contribute to EOD duration changes.

}, keywords = {Androgen hormones, electrocyte, K+ channel, Na+ channel, Testosterone}, issn = {0022-0949, 1477-9145}, doi = {10.1242/jeb.249548}, url = {https://doi.org/10.1242/jeb.249548}, author = {Losilla, Mauricio and Gallant, Jason R.} } @article {669, title = {High-frequency electrical tuning and linear filter properties of Knollenorgan electroreceptors of mormyrid electric fish}, journal = {The Journal of Physiology}, year = {2025}, month = {Jun-15-2025}, pages = {1-22}, chapter = {1}, abstract = {

Electrical tuning allows auditory, vestibular, and electrosensory receptor cells to filter sensory signals and selectively transmit specific stimulus frequencies. In auditory hair cells, electrical tuning results from membrane potential resonance produced by voltage-gated Ca and K(Ca) channels, with variable kinetics that generate different tuning properties. Such resonance has been observed only up to \∼1 kHz, however. Additionally, in most species that employ electrical tuning, hearing is constrained to this relatively low-frequency range, raising the question of whether electrical tuning can extend to higher frequencies. Here we investigated this possibility by studying tuning and transduction properties of Knollenorgans, a class of tuberous electroreceptors of mormyrid electric fish. These organs, which generate spike-like receptor potentials, detect species-specific electric organ discharges (EODs). To test whether fish with brief EODs had correspondingly high-frequency electrical tuning, we recorded tuning curves from Knollenorgans of three species, Brevimyrus niger, Gnathonemus petersii, and Pollimyrus adspersus, which have EODs with spectral components exceeding 5 kHz. All species had receptors tuned to a range of frequencies tiling the species-specific EOD spectrum, with best frequencies extending beyond 10 kHz in P. adspersus. We also computed the impulse response of each Knollenorgan by reverse-correlating spikes elicited by white noise stimuli. After incorporation of a spike threshold non-linearity, convolving the impulse response with arbitrary stimulus waveforms successfully predicted spike patterns experimentally evoked by these inputs. These analyses demonstrate that differential electrical tuning properties of Knollenorgans produce distinct, well-timed spike responses that reliably encode time-varying electrical signals at frequencies up to 20 kHz. imageKey points Knollenorgans, among the tuberous electroreceptors of mormyrid electric fish, are modified hair cells that transduce electrical signals into spike-like receptor potentials. Knollenorgans in three species of mormyrids are tuned to frequencies matched to the frequencies present in the species-typical electric organ discharges, suiting them for electric communication. The frequency of highest sensitivity of Knollenorgans can extend well beyond 10 kHz, far exceeding the limit for electrical tuning mechanisms estimated from mechanosensitive hair cells. The timing and probability of spiking by Knollenorgans are accurately predicted by a model composed of linear filtering followed by non-linear rectification and spike thresholding. Differential filtering by different Knollenorgans produces distinct outputs to the same input, with high-tuned receptors effectively transmitting well-timed spikes, on a microsecond time scale, in response to electrical stimuli up to 20 kHz.

}, issn = {0022-3751, 1469-7793}, doi = {10.1113/jp288299}, url = {https://doi.org/10.1113/jp288299}, author = {Raman, Indira M. and Hopkins, Carl D.} } @article {668, title = {Length-weight relationships and condition factors of Mormyridae species in the Niger River: implications for conservation and management}, journal = {Limnology and Freshwater Biology }, volume = {2025}, year = {2025}, month = {Jan-01-2025}, pages = {215-221}, abstract = {

The length-weight relationship and condition factor of the ten most abundant Mormyridae species in the Niger River at Niamey were examined from August to November 2023. The species assessed were\ Mormyrus rume,\ Campylomormyrus tamandua,\ Pollimyrus isidori,\ Brienomyrus niger,\ Hyperopisus bebe, Mormyrus macrophthalmus,\ Mormyrops anguilloides,\ Brevimyrus niger,\ Mormyrops oudoti,\ Cyphomyrus Psittacus, and\ Marcusenius cyprinoides. Analysis of their length-weight relationships revealed \“b\” values ranging from 2.08 to 2.87, suggesting negative allometric growth across all species. Condition factor (k) values varied from 1.22 \± 0.31 for\ H. bebe\ to 10.48 \± 2.83 for\ M. cyprinoides, with no significant differences in average condition factors among the species. These findings highlight the potential vulnerability of Mormyridae species to environmental changes and anthropogenic pressures, emphasizing the need for targeted conservation measures. The results underscore the importance of sustainable fisheries management practices to preserve biodiversity and ensure the long-term productivity of the Niger River ecosystem. The observed negative allometric growth patterns indicate that environmental conditions may be suboptimal for these species, potentially due to factors such as habitat degradation, water quality issues, or overfishing. Addressing these challenges through habitat restoration and regulatory measures could enhance the resilience of fish populations and support sustainable fisheries.

}, keywords = {condition factor, length-weight analysis, Mormyridae, sustainable fisheries management, the Niger River}, issn = {2658-3518}, doi = {10.31951/2658-3518-2025-a-2-215}, url = {https://doi.org/10.31951/2658-3518-2025-a-2-215}, author = {Souley, S.M.N. and Harouna, M. and Ado, M.I. and Youssoufa, I.} }