Marvels of the African Great Lakes: Studying Cichlids With Dr. Frederic Schedel
Africa has long been romanticized in regards to the natural world. From crocodiles snapping at wildebeest to lions lounging on the plains, the continent is packed with charismatic animals. Often slipping under the radar is the fact that Africa’s freshwater ecosystems are also teeming with stunning wildlife.
Perhaps most fascinating are the African Great Lakes, which contain a quarter of the world’s unfrozen freshwater. The Great Lakes and their surrounding river systems are filled with an enormous diversity of cichlids, a family of fish species that rival coral reef fishes for the most colorful underwater specimens in the world. Their origins and diversity make cichlids one of the keys to understanding speciation, the process by which new species are formed.
Dr. Frederic Schedel is a postdoc at the University of Basel in Switzerland. He has researched cichlids in the Great African Lakes and its surrounding river systems, and when I spoke to him he had just gotten back from a research trip to Zambia. Frederic and I chatted for almost two hours about why cichlids are so fascinating, and what sort of threats they face.
Eva Paulus (EP): What is it that makes the Great African Lakes so interesting?
Dr. Frederic Schedel (FS): Each of the Great African Lakes (Lake Malawi, Lake Tanganyika, and Lake Victoria) are unique in terms of their individual geological histories, but also in the diversity of their species. In contrast to the ocean where fish can migrate long distances, these lakes are closed systems. That makes them perfect for studying evolution, and the impressive diversity of cichlid fishes found in these lakes makes for a great case study.
Take Lake Tanganyika, the deepest and oldest of the Great African Lakes. Although there is still some debate about the age of the lake, it’s commonly assumed that it’s about 12 million years old. Therefore, it is not surprising that the cichlids found in Lake Tanganyika are the most diverse in terms of their adaptations, for instance in their appearance, feeding strategies, or behavior.
Lake Victoria is also an interesting case: it’s the shallowest of the three lakes at around 80 m depth, and because it’s so shallow it has dried up a few times in its history. It refilled again only recently, around 15,000 years ago, but despite this it holds remarkable cichlid diversity with hundreds of species. So one key question is how such diversity has evolved in such little time. For Lake Victoria cichlids, it was recently shown that hybridization between different tribes could have triggered the onset of this adaptive radiation – the process by which organisms diversify rapidly from an ancestral species – by providing genetic variation on which selection could act upon. To sum up, if the first arrivals to the lake were already highly diverse genetically, new adaptations and species could potentially be formed faster.
Further reading: Ancient hybridization fuels rapid cichlid fish adaptive radiations
Understanding the evolutionary history of these adaptive radiations allows us to study the underlying mechanisms of evolution. I am personally especially interested in the cichlids that live in the rivers surrounding the Great African Lakes and how they contributed to the evolution of cichlids in the lakes.
EP: I heard that one of the most debated questions in this field is why they live on multiple continents. Would you say that’s the biggest mystery in the cichlid world?
FS: Cichlids are model organisms in evolutionary biology, and they are found in only a couple places around the world: mainland Africa, South America, Madagascar, Sri Lanka, India and Iran. The big question is whether the cichlid ancestors were already around when all these parts of the world were connected on the supercontinent Gondwana, or whether they dispersed across the oceans after the continents had separated. Many experts now agree that oceanic dispersal has probably taken place, and cichlids were able to cross oceans multiple times. There are some populations of cichlids that can live in brackish or marine water, and a sister group (a group of species named convict blennies) that are marine species. It is therefore plausible that the ancestor of cichlids was also marine, and that they made their way through the (at the time much narrower) Atlantic Ocean many millions of years ago.
EP: What is field work like at the great African lakes?
FS: It’s amazing! But of course it can be challenging too. In general, before you even think about booking a flight, you need to apply for a lot of different permits, which gets even more complicated if you want to cross borders in Africa. For example, depending on the country, you will need research permits, study permits, but also an export permit to bring the specimens back home. We collaborate with local scientists, which is always worthwhile and I have made a lot of good friends on my research trips. In a best-case scenario, you have all the permits, you arrive at the airport, and you meet up with your colleagues and friends who have organized a jeep already.
Depending on the project, sampling locations are quite far away from major cities or towns, which means you have to set up camp in the middle of the bush. During the field trips on riverine cichlids we camped next to rivers or waterfalls, always in remote and amazingly beautiful locations.
We usually bring fish traps, gill nets, and fishing rods. But, if possible, I prefer to catch fishes snorkeling or scuba diving. This allows me to observe the target species in their natural habitat, plus it reduces unnecessary bycatch. Of course, in some places you need to be extra careful and it is always wise to ask the locals if there are crocodiles or hippos around.
Once you’re out of the water you need to be fast in preserving the fishes – they’re euthanized as gently as possible, photographed, and tissue samples are taken. We also go to the local fishermen and see if they caught anything interesting; they often have a much better overview of what’s out there, and they have nets out at different times of the day, which gives us a good sample of the diversity as well.
EP: Cichlids are popular fishes for the aquarium trade. In the ocean, this can be a problem, for example catching fishes with cyanide. Is it similar with cichlids?
FS: They are certainly very popular aquarium fishes. As they are quite easy to breed in tanks, the ornamental fish trade thankfully doesn’t really depend on wild-caught fishes, although those individuals are always highly prized in the hobby. Therefore, it could become a problem for species or populations that are endemic to only a small area. Still, for the locals living around the African lakes it can be extra income to sell a few fishes for the aquarium trade every now and again. However, I don’t think that the ornamental fish trade is the biggest threat for the cichlids of the Great African Lakes.
EP: Is climate change an issue for cichlids?
FS: As far as we know, they do adapt to changes quite easily, but I think the major problem is that there are many species that are endemic to small areas. For example, there are species that are only found in small stretches of a river or in oases, and if a drought hits that particular area, these species will of course be highly affected and will potentially go extinct. The lakes in Africa are indirectly affected by climate change, but humans also directly impact them of course. For example, deforestation around the Lakes appears to be a huge problem as that can lead to eutrophication – the lake being filled with sediment and nutrients. This is currently the case in Lake Victoria.
Cichlids are also a big part of the total fishery catches in the Great Lakes. We have all heard of tilapia, the common name for the genus Oreochromis, which is intensively fished around Lake Malawi. But these fisheries are relatively sustainable, as long as no trawling takes place. Trawling is a huge ecological sin in a freshwater lake as it rips up the lake bed, and has already caused a collapse in fish stocks in the past.
EP: A question from a deep-sea biologist: Is there life in the deeper parts of Lake Tanganyika?
FS: There’s only oxygen down to about 200 m. It’s a tropical lake, which means there’s very little circulation of water, and oxygen doesn’t get pushed further down. But there are cichlids that live right at that oxygen border, I described one of them. They have slightly bigger eyes to adapt to a “deep-lake” habitat, which is of course much darker than the shallow coastal areas where most of the cichlid species are found.
FS: Below the oxygenated zone, there are certainly microorganisms, but what’s especially interesting for evolutionary biologists is the lake sediment – looking whether there are fish fossils, or even just otoliths or teeth preserved within the different layers might help us to reconstruct the evolutionary history of these fascinating fishes.
Eva Paulus is a recent MSc graduate in marine biology. She is interested in the ecology and evolution of deep-sea mesopelagic fishes, and is currently waiting for borders to reopen to start a PhD position in Australia. Follow her on Twitter @Deep_Sea_Dirndl.