THE FISHES’ TALE

By Dr Valerie Jeffries FLS – FFON’s Kenya and UK correspondent

Part 1: DARWIN’S OWN LAKE

Lake Victoria has been a highway for trade, could become a highspot for tourists, and is an evocative presence in the literature of East Africa. But why should a Dutch biologist name his book “Darwin’s Dreampond: Drama in Lake Victoria”? Tijs Goldschmidt observed, collected, studied, pondered, and theorised about the Haplochromine fish of Lake Victoria, and concluded that given the opportunity Darwin would have gone there immediately. Evolutionary biologists from all over the world study the Lake’s Haplochromine fish, (often fondly referred to as ‘Haps’), with hundreds of their species found nowhere else, and all this glorious abundance in record time. Here’s their story.

HUMANS AND HAPLOCHROMINES

Louis and Mary Leakey famously revealed to the world the fossil evidence that our species originated in East Africa, “The Cradle of Humankind”. Richard Leakey has hugely advanced the study since then, and Maeve Leakey continues research expeditions with their daughter Louise. We are fascinated by the story the scientists tease out of those dry hominin bones, depicting the extraordinary evolutionary changes since the last ancestor we shared with the apes, less than seven million years ago.

But what of Kenya’s second great story of evolution, not a handful of Hominin species, but 500 Hap species unique to Lake Victoria? No less than the fossils in the “Cradle of Humankind” the Haplochromine cichlid fish of Lake Victoria are an extraordinary demonstration of evolution.

EVOLUTION IN A RUSH

When our ancestors were discovering fire, the fish Family Cichlidae already contained some little Haplochromines that swam in East African lakes. When Darwin was writing “The Origin of Species” there was a rich abundance of them in Lake Victoria. They had diversified to eat insects, phytoplankton, zooplankton, small crustaceans, detritus, or smaller fish. Others grazed algae, or crushed snails, and the occasional scale-feeder scraped its lunch off other fish while fin-tearers stole whole limbs. For each feeding habit there were adaptations of body form to enable the food to be found and eaten. Distinctive colouring of the male Hap while breeding marks out different species to the biologist and to female Haps, who recognise their own species for courtship by his colour and pattern. What defines Haps as a group amidst all their variety is in their bones, particularly their two sets of jaw bones, the secondary pair being in their throat as ‘pharyngeal jaws’. Further back on the body is evidence of another characteristic feature: egg spots. Haps lay little yellow eggs, and when in breeding colours the male displays on his anal fin a pattern of blobs resembling little yellow eggs: his ‘egg spots’. Like other mouth-brooders Haps carry the fry in their mouth for protection, letting them out to feed but gathering them up again if threatened: (predictably, there is yet another feeding habit among Hap species: sucking the baby fish out of the mouths of brooding mothers). After courtship the female lays her eggs and then collects them up into her mouth, and among her eggs she sees near the male’s vent what looks like – little yellow eggs. Her nipping at his fin prompts the male to spawn so the sperm goes straight into the same cavity as the eggs, enclosed and safe for fertilisation to take place. These features don’t fossilise well, so it is modern molecular techniques allowing inference back in time that have enabled biologists to map the history of the Haps. But with Haps, it is not just the abundance of evolution, but also the speed. They are the fastest evolving vertebrates on Earth, and among all the African lakes teeming with Hap species it is in Lake Victoria that we see the fastest.

These unique species have all evolved in the roughly 15,000 years since the waters returned… Note, that’s in fifteen thousand years. Time for evolutionary changes that lead to speciation is generally reckoned in millions of years.

Lake Victoria is shallow, and over the few million years since its original formation as a lake, it has repeatedly disappeared. Limnological (freshwater science) evidence of dry periods and re-filling shows Lake Victoria as we know it today to be very recent. Prior to its latest refill, it was dry land: the fish were gone. There were only perhaps some swampy streams and wetlands remaining nearby, part of the network of waterways that join up the Rift Valley lakes today. Then the climate changed again and Lake Victoria re-appeared. Fish arrived and began to re-populate the lake, probably from its fringes. Estimates point to around 15,000 years ago since the lake basin finally refilled. From that time until biologists started to apply the Linnean system of classification in the 19th century, about 500 species of Haps evolved, unique to Lake Victoria. Genetic evidence shows that all these came from very few re-colonising ancestors in just a moment of geological time, and they are all related to each other more closely than to any Haps in other lakes. These unique species have all evolved in the roughly 15,000 years since the waters returned. In every part of the new lake, surface or lake bed – rocks, mud, sand – and with every new food source, a host of new species evolved , producing the Lake Victoria “Species Flock”.

Note, that’s in fifteen thousand years. Time for evolutionary changes that lead to speciation is generally reckoned in millions of years. Rapid human evolution (set out nicely in Nairobi Museum) from the last Australopithecine who shared an ancestor with the Great Apes, into you and me, took about five million years. None of the Lake Victoria Haps has had more than 15,000 years to diverge from a swamp-inhabiting ancestor who found his way into the refilling lake basin. That’s less than 0.5% as long.

Humphrey Greenwood, former Curator of the Natural History Museum in London, laboured to document and classify Haps in the 1950s, but it was a colossal task. His 134-page monograph The cichlid fishes of Lake Victoria, East Africa: the biology and evolution of a species flock (1974) was a massive advance, but not a complete catalogue. Tijs Goldschmidt‘s definitive and entertaining book “Darwin’s Dreampond” (1994) incorporates the work of Dutch biologists from the 1970s. Among them Franz Witte reported a ten minute trawl that caught 1000 Haps representing 100 species. But the dazzling diversity of the Haps presented an enormous task of classification before modern “DNA barcoding”, and Kenyan National Museum scientists are still working today to describe them all from specimens in jars.

ECOLOGICAL DISASTER

The tragedy of the 20th century was that the Species Flock was threatened through environmental changes and by the large predatory Nile perch. In the late 1950s, the Nile perch was deliberately introduced into Lake Victoria, despite the protests of some biologists, partly to develop the fishing industry for export and bring in more profit to the fishing communities beside the lake, which it did, for some. There is ample evidence against the Nile perch, but also ongoing water pollution worsened with population growth, converting Lake Victoria into a hostile environment, and Hap numbers crashed, as became apparent to the scientists, and to the local people. Until the 1070s the endemic Haps, known locally as ‘furu’, and Tilapia, their larger relative, were popular meals, and formed the bulk of fishermen’s catches, but by 1980 this had dropped below 5%. Species used to be individually recognised for their culinary possibilities and some were regarded as delicacies. After 1980 there were far fewer furu and younger Kenyans are no longer familiar with all their colours and types, since the local diet now includes Tilapia and Nile perch but very little furu.

Haps evolved in record time, but disappeared even faster through human interference. Ole Seehausen, a major recent researcher on Haps, already concluded by 1990 that the battle was lost; the Haps were gone.

Whatever the truth about the Nile perch, Lake Victoria remains polluted despite some improvements, and Haps have suffered. There was also no shortage of water hyacinth. You could watch from the Nairobi bus travelling down into Kisumu, look down to the lake, and see where the surface was blueish clear or thick green. If the water looked clear today, the wind would maybe blow the green carpet back tomorrow. Conversely, locals said you could sell ‘land’ to strangers today which will become sheer water tomorrow. Lake Victoria lived up to its history of rapid switch between lake and land, on two timescales. Recent projects to deal with the ‘noxious weed’ have had some success, but it still hinders fishing boats and shades the fish.

The extraordinarily rapid adaptive radiation of 500 Hap species that began 15,000 years ago is over. Even before the Species Flock could be fully catalogued, they disappeared. Haps evolved in record time, but disappeared even faster through human interference. Ole Seehausen, a major recent researcher on Haps, already concluded by 1990 that the battle was lost; the Haps were gone.

Or were they? . . . Watch this space!

Read: PART II: RESURGENCE , and DREAMPOND REVIVAL.

By Dr Valerie Jeffries

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