I got off of the plane in Lilongwe, exhausted after my journey from Canada. I was met by a smiling man who I was to drive with for the next five hours north to the village of Kande, on the shores of Lake Malawi. We drove through the beautiful countryside and countless villages that all had their little hub of shops and restaurants nestled around the main road. We reached Kande at around 7:30; it was dark, and only the bright stars and the fireflies lit the road.
I was asleep as soon as my head hit the pillow in my cozy cabin. The next morning I was up and at it, taking a new diver out to explore the reef. It was my first dive in the lake as well. Right away fish of many different shapes and colours surround me. I know Lake Malawi is known as the most biologically diverse lake on the planet, but wow! The manifestation of this diversity astounds me.
The cichlid fishes are one of the largest animal groups on the planet. The group itself is thought to predate the continental drift events that separated Africa and South America around 140 million years ago as they are found in Africa, Latin America, Madagascar, and some parts of South East Asia. However, the African cichlids represent one of the most striking examples of adaptive radiation (rapid evolutionary divergence) in the animal kingdom.
There are over 600 identified species, and many more unidentified species that live only in Lake Malawi! The lake basin is thought to be around 8.6 million years old, yet the majority of the cichlid species have been thought to have diverged from each other much more recently than the origin of the lake. To give you an idea of that insane speed, it is thought that we diverged from our closest living relative, the chimpanzee, around 13 million years ago!
There are many hypotheses as to how this group of fish has diversified so spectacularly. The geological history of the African lakes and their rising and falling cycles have given rise to many smaller lakes that have disconnected and reconnected over evolutionary time. This has allowed for many separate, but startlingly parallel, evolutionary trajectories. There are many highly behaviourally similar species that live in different regions or lakes, yet share less genetic similarity with each other than with the other species in close range.
Like Darwin’s finches of the Galapagos Islands the Cichlids of Africa have all specialized to specific ecological niches. There are species that remove parasites from the skin of catfish, and another that eats the catfish skin itself. There are scale eaters, fin-biters, sediment sifters, and zooplankton feeders. There is a group that inhabits snail shells, another that eats snails, crabs, or flies on the surface, and yet another that flips rocks to find insect larvae hidden underneath. Some species have evolved huge eyes to enable them to see in the dim lights at depths of over 100 meters, and others have evolved lateral line systems to enable them to find creatures in the mud.
These different morphological and behavioural adaptations are associated with changes in body shape and colour, jaw structure and orientation, and teeth size and form. As I float through this underwater realm I can see these novel adaptations in action. The evolutionary history of this lake, and the resulting diversity amaze me.
It is one of the many reasons that I chose to come to Malawi, and I am looking forward to my time here amongst these crazy fish!