The Mormyridae (Teleostei: Osteoglossiformes) of the Upemba and Kundelungu National Parks (south-eastern DR Congo): species diversity, adaptation and speciation

This PhD has two main objectives: (1) to document the species diversity of the Mormyridae of the rivers draining the complex formed by the Upemba National Park (UNP) and the Kundelungu National Park (KNP); and (2) to study the relationships between water conductivity and the species distribution, diversity, adaptiveness and speciation patterns in the Middle Lufira (MLf).

The outline of my PhD dissertation is envisioned as follows. A general introduction (Chapter 1) which will contain a general introduction to the mormyrids, i.e., the freshwater elephant fishes, and an overview of the hydrography of the study area. It will also contain a section on the research questions and objectives. In the material and methods section, the three methods that will constitute our integrative approach when dealing with the taxonomic and evolutionary case studies selected will be presented: i.e., a morphologic, genetic, and electric signal approach (mainly the study of EODs: Electric Organ Discharge).

An annotated checklist of the Mormyridae from the UNP and KNP will be presented in Chapter 2. For that, four strategies will be adopted: (1) compiling a list of the mormyrid species currently known from the parks; (2) studying the collections from both parks mainly hosted at the RMCA and the IRSNB; (3) studying of the collections made during the 2014-2021 expeditions; and (4) collecting in the basins of the UNP and the KNP. Comparison with the relevant type specimens will be made whenever needed. This chapter will include concise (re)descriptions, identification keys, and distribution maps/data for each species, and a general discussion which will take up these different points in relation to protection issues.

A first case study, on Gnathonemus petersii (Günther, 1862), using a morphological approach only, is ongoing and will become Chapter 3. Although considered a widespread species known from the Niger Basin to the Congo Basin, it revealed to be a species-complex. This chapter will aim to: (i) clarify the status of the two current junior synonyms of G. petersii; (ii) document possible unnoticed species diversity within G. petersii as currently considered; and (iii) start to clarify the species identification of the specimens/populations from the Upper Lualaba. A second case study will aim to explore the methodological potential and problems of the integrative approach presented above to clarify the alpha-taxonomic status of Gnathonemus specimens from the Upper Lualaba (Chapter 4). Currently three morpho-species have already been identified in the basin, all different from G. petersii.

The following three chapters (Chapters 5 to 7) will focus mainly on the second main objective of the PhD which deals with the relationships between conductivity and the species distribution, diversity, adaptiveness and speciation patterns. For this, three different genera occurring in the MLf, will be studied and compared. For each of them, a similar integrative approach as in the case-study on Gnathonemus (Chapter 4) will be used. To date, five mormyrid genera, i.e., Cyphomyrus Myers, 1960, Paramormyrops Taverne, Thys van den Audenaerde & Heymer, 1977, Marcusenius Gill, 1862, Pollimyrus Taverne, 1971 and Petrocephalus Marcusen, 1854, have been found to occur in the MLf and its affluents. The former three have been selected as case-studies within the framework of our conductivity/species richness related research question(s).

Firstly, it has been observed that there are some morphological differences between specimens identified as Cyphomyrus lufirae Mukweze Mulelenu et al., 2020 from the Dikulwe River, a high conductivity (HC) left bank affluent of the MLf (778-913 μS cm-1), and those from the Luiji River, a right bank affluent of the Lufwa River, a low conductivity (LC) right bank affluent of the MLf (8-28 μS cm-1). Cyphomyruslufirae also occurs in the Kafila River, also a HC river (876-1126 μS cm-1), located further upstream on the right bank of the MLf (~880 m asl). It does not flow from the Kundelungu Plateau (KP) (~1800 m asl) like the rivers with LC, but from the more elevated part of the MLf plain (~890 m asl). Morphological differences have already been found between the C. lufirae populations from HC and those from LC rivers. Differences were found in the EODs as well (pers. obs., 2020). Therefore, Cyphomyrus has been selected as a first case study within the framework of the possible relationship between the conductivity and the morphological divergence between populations or species (Chapter 5).

Furthermore, differences were already found in caudal peduncle depth between the populations of Paramormyrops tavernei from the Dikulwe River (HC) and the Luiji River (LC). Two EOD waveforms have also been identified so far, one shared by specimens from HC (Dikulwe) and LC waters (Luiji) and a second one only found in specimens from LC waters (Luiji). These observations will form the basis for a study of this complex (Chapter 6).

The genus Marcusenius has been identified as a third potential case study (Chapter 7), because this genus has also been collected from both HC and LC tributaries. At the moment, no data are available regarding possible morphological and/or EOD differences between populations of both tributaries.

In addition to the integrative approach, the populations and species studied in Chapters 5 to 7, will also been studied using an experimental approach to try to answer three additional research questions (Chapter 8). First, is the present use of a Q10 value of 1.5 (Kramer & Swartz, 2010; Kramer & Van der Bank, 2011; Kramer & Wink, 2013; Kramer et al., 2014) or 1.6 (Hopkins, pers. comm. 2020) appropriate to study all mormyrid taxa? Secondly, are populations or species that are adapted to a certain conductivity range able to maintain or adjust their EOD signal under divergent conductivity conditions? Thirdly, is there sexual dimorphism in EOD signal related to the breeding season?

The first question is mainly a methodological one aiming to verify if the presently used Q10 values are appropriate to allow for a proper temperature adjustment to compare EODs taken at different water temperatures. The Q10 value is used to characterize the temperature dependence of biological, chemical, or physical processes. It indicates how much faster a reaction takes place if the temperature is increased by 10 °C (Zupanc et al., 2003). Indeed, the objective of this approach is to correct for observed hysteresis with decreasing and increasing temperatures (Boudinot, 1970). This will allow us, if needed, to use a temperature adjusted Q10 for mormyrids who live in an area where the water temperature varies between 16 and 29 °C. The second question should help us to shed light as to what extend populations maintain or adjust their EOD under changing conductivity conditions. Considering the importance of these EODs for orientation, prey detection, and communication with conspecifics (Hopkins, 1981; Zupanc et al., 2003), this should help us to explore the repercussions of the observed EOD maintenance or adjustment in terms of species recognition. The third question should enable us to further explore the possible occurrence of sexual dimorphism in EODs in each of the selected taxa. This will be done during the wet season which is known as the breeding period. Indeed, for some species, during the wet season sexually active males are known to produce EODs of approximately twice the duration of that of females and non-breeding males during the dry season (Bass & Hopkins, 1983; Bass et al., 1986; Kramer, 1997). Because these costly signals cannot be faked, they are honest indicators of male quality (Hopkins, 1999). It will be interesting to explore how conductivity changes might affect these EODs as well and what might be the implications at the level of species recognition.

Finally, Chapter 9 will contain a general discussion of our results and will formulate implication and perspectives by placing the results in the broader context of the mormyrid diversity of Congo basin and the African continent at large. Furthermore, our results will also be compared to what is known for the Central and South American knife fish (Gymnotiformes) about the influence of conductivity on their electrical discharges. Finally, it will situate the results within the framework of fish protection in general and of mormyrids in particular.