Ancient Lakes

Tanganyika

There are hundreds of thousands, maybe even millions, of lakes on Earth, but only a few dozen can claim to be ‘ancient’ because they have been around for a million years or longer. This is because lakes have short lifespans (on a geological scale)! The moment a lake is created (i.e. when a hole in the ground fills up with water) it begins to die, because inflowing streams and rivers, and even the wind, are continuously depositing sediment in the lake, gradually filling it. This sediment accumulates on the bottom of the lake, which becomes increasingly shallow. Slowly, the lake changes into a swamp, and finally disappears altogether. A lake can only become ‘ancient’ therefore if it deepens faster than it fills with sediment. This is the case with lakes that are situated in ‘Rift Valleys’, for example the oldest lake in the world, Lake Baikal in Siberia (approx.. 30 million years old), and Lake Tanganyika (approx. 12 million years) and Lake Malawi (5-8 million years) in the East African Rift Valley. Rift valleys are situated in tectonically active regions. Through these tectonic processes they become wider and especially deeper over time. Rift valley lakes also accumulate sediment, but they become deeper at a faster rate. Lake Tanganyika, for example, is almost 1.5 km deep at its deepest point, but below the water column is 5-7 km of sediment!

Ancient lakes are a paradise for evolutionary biologists, because they are home to a large variety of species, many of which are found nowhere else on Earth. Such species are thus endemic to a specific lake and almost all of them also originated in that lake. Biologists can therefore investigate these species in their place of origin, which is fairly exceptional. So, ancient lakes are evolutionary laboratories just waiting to be explored by biologists!

Tanganyika

Our researchers are involved in the study of ostracods (mussel shrimps) in lakes Baikal, Tanganyika and Malawi. We are investigating the age of the species flocks that occur there. Species flocks comprise all species that presently occur in the lake and are derived from one ancestral species that colonised the lake a long time ago. Species flocks can be composed of anything from a dozen to even hundereds of species. We compared the age of the Cytherissa species flock in the older Lake Baikal, with those of the genus Cyprideis (and derived genera) in the slightly younger Lake Tanganyika. And this was our first surprise: the oldest lake (Baikal) had a younger species flock! Lake Baikal is approximately 30 million years old, but the Cytherissa species flock is ‘only’ 3-5 million years old!

A second discovery was recently made in the southern part of Lake Tanganyika. Several populations that were assigned to one species based on morphology (shape of valves, appendages, etc.) appeared, after molecular analysis, to belong to a species complex of at least four species. These are ‘cryptic’ species that can only be identified using genetic methods. The diversity of the endemic species flocks of ancient lakes could therefore be much higher than originally thought. We are now investigating if we can also find cryptic species in Lake Baikal.

Tanganyika

At the same time, we are also researching the mechanisms that have led to such incredible species diversity: what is driving this exceptional rate of speciation? Is it only the age of these lakes that allows many species to form gradually over long periods of time, or are there other processes at play? Is the main driver natural selection, like in adaptations to different types of sediment, or to food sources, or to different depths in these very deep lakes, or can we find indications that mostly sexual selection was the driver of speciation, which would mean that we are looking for a much more fundamental explanation concerning these ostracods? Our research on ancient lakes, which was initiated in 1990, has really only just begun!

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