My 2009 Fieldwork was Primarily in Gabon, West Central Africa, collaborating on an NSF-Sponsored Project Entitled “Mechanisms of Signal Diversity” to Drs. Bruce Carlson (U. Washington in St. Louis) and Carl Hopkins (Cornell University, Ithaca, NY). The work was also in collaboration with Dr. Jean Daniel Mbega, of the I.R.T institute, CENAREST, Gabon, and Dr. Matthew Arnegard (Postdoc, Peichel Lab @ Fred Hutchinson Cancer Research Center) . Our collection sites focused on two areas, near the confluence of the Louétsi and Ngounié rivers, only kilometers away from the Congo border, and along the Oogoué river, in the town of Lambarene.
Our hosts for the first part of the trip were the wonderful and generous people doing their good work at the Bongolo Hospital. Here, we focused on the collection of Paramormyrops, endemic to the Lower-Guniea basin. These fishes have explosively radiated in this region. A few of these fishes are shown on the right. As you can see, they are morphologically quite similar to each other, but several lines of evidence establish them as unique species.
Of particular interest to my research is the diversity of electric signals (EODs) exhibited among these (and other mormyrid) species! EODs among this group range in their duration, number of phases, and polarity. The signals are used for electrocommunication, and it is specifically these temporal features that the fish cue in on to determine things like sex and species. The diversity of the EODs plus the behavioral and neurophysiological data suggests to us that the electric signals are a means for reproductive isolation, similar to color morphology in cichlids. Previous collecting efforts had identified some rather interesting population biology going on in this region: we identified a single species, Paramormyrps kingsleyae, that was polymorphic for its signal type: some had biphasic EOD waveforms, whereas others had triphasic EOD waveforms. Our collections indicated that the populations exhibiting the biphasic EOD type were all from a drainage basin isolated by a large waterfall in this region, Bongolo Falls (shown left). Intriguingly, only triphasic type waveforms were found just below the waterfall, and only biphasic type waveforms were found above the waterfall. The populations were never mixed– or so we thought.
P. kingsleyae is a great fish for fieldwork: it lives in small shallow headwaters of small creeks in nearly every part of Gabon, and it gives off an electric signal, making it very easy to find. We use a small amplifier attached to an electrode to find the fishes, which remain stationary during the day, and then capture them with hoop nets. They typically hide during the day in brush, reeds and debris along these small creeks. During our analysis of old field data, this natural history became important data– there was one creek where we collected fish, Bambomo creek,that drained into the Ngounié river (below the falls), but had headwaters that came very close to creeks that drained above Bongolo Falls.
Here, we found that both signal types existed, and we even found individuals that appeared to have intermediate morphology in their electric organs: within some individuals some electrocytes were penetrating, and others were non-penetrating.
This was a very surprising finding– and we wanted to investigate this further. We focused our collecting efforts in 2009 on increasing our sampling sizes from several populations in these drainage systems. We recorded EODs, specimens, collected tissue samples for DNA genotyping, as well as electric organs from several individuals to look at expression differences between these, and other Paramormyrops that exhibit divergent electrical signals.
Following our time in Lebamba, we moved to the Oogoué river in the city of Lamberene. Here our focus was several of the ancestral Petrocephalus species that inhabit the rivers of Gabon. These fish, unlike P. kingsleyae are pelagic and travel in large groups. We had difficulty finding these in the Lebamba area, but had great successes in collecting several species in the Oogoue, particularly near a large freshwater lake in the area Lac Zile. More developments on this project can be learned from Bruce Carlson’s lab.