By Dr Valerie Jeffries FLS
If you missed Part I, it’s here.
Franz Witte back in Leiden could hardly believe what he was hearing on the crackly phone line from Tanzania: was his research student Ole really claiming there were dozens of Haplochromine (Haps) fish in his net ? Ole Seehausen, (who’s now a Professor at Berne), had returned to Lake Victoria in 1991 to document the Haps’ last gasp after sad decades of predation and habitat degradation. The wonderful news, which Witte could hardly hear and which no-one expected, was that the Haplochromines were coming back How ?!
The Nile perch had been overfished and was itself becoming scarce, and eventually forced to turn its attention from the disappearing Haps (its favourite prey) to small shrimps, and to cannibalism. Aspects of pollution from human sources had been addressed. The water hyacinth, which clouds the water and lowers its oxygen concentration, was being vigorously cleared away by angry fishermen because it immobilises their boats. Tentatively, Haps began to re-appear.
Franz Witte wrote that the changes had happened “over a time span of only two decades”, almost before our eyes.
But it was not the Species Flock lost 30 years ago. It’s a new, smaller assembly including types not previously known. The 21st century is seeing fresh innovation in these remarkably adaptable little fish.
Several species show shifts in feeding habits, in their response to predators, and in courtship, and some individuals, intriguingly, belong to no recognisable species at all. They had changed within a few decades, demonstrating that they are still fast-evolvers, that we can now track in our lifetimes. Franz Witte wrote that the changes had happened “over a time span of only two decades”, almost before our eyes.
NATURAL SELECTION EXPRESS
Water quality is a key aspect of the Haps’ environment, so we’d predict that increasingly polluted conditions would have promoted natural selection for features increasing a fish’s ability to acquire oxygen.
Sure enough, a species well characterised before it nearly vanished in the 1980s, Haplochromis pyrrhocephalus, was found upon its reappearance in 2008 to possess 64% more gill surface area than before.
As well as gills, sight and breeding colour have been influenced, by interaction of environmental change and mate choice.
The colour range of light penetrating water is affected by turbidity, which reduces blue relative to red. Suspended sediment, algae, or pollutants, plus shade from a cloak of water hyacinth, reduce light penetration, making male colours less visible to the females. Trials in 2010 compared female Haps from clear and cloudy regions, and found that those from the murky water showed reduced preference for males of their own species. Equally, the breeding colours of male Haps living in cloudy water showed less difference between species than the difference distinguishing them in clear water habitats. This further blursthe species signal to females. So, barriers to hybridisation had crumbled.
It seems sexual selection has broken down, if females are less able to perceive distinctive colours and patterns, and so throw caution to the winds and mate with any male rather than none. With hybridisation thus being more likely, it’s not surprising that DNA analysis found that among a group of closely similar species, those now living in turbid water showed a general reduction of overall genetic difference between them. This suggests the approach of speciation (the process of one species separating into two) and would account for the new types that matched no known species. How far hybridisation contributes to forming new species and how far environmental changes are the sole spur is a matter of lively debate, but it’s increasingly recognised that some animals have bred across species lines, with a novel and viable outcome, as plants more often do.
Modern fish have a wider range of colour vision than mammals, having probably always used sight since they began over 400 million years ago, whereas mammals began with less need for seeing colour in their nocturnal lives. Primates experienced selection for improved colour vision as diurnal animals much later, maybe to see ripe fruits from afar. We have not caught up. The cones of human retinas are of three types, as we have genes for just three types of opsin protein: sensitive to red, green, or blue light.
Haps have seven opsin genes, so we’d guess they were seeing a wider range of wavelengths than we can, and we’d predict that their extra wavelengths would match their needs. In fact scientists have used the sophisticated equipment of medical ophthalmology to examine the eyes of these fish and measure the wider spectrum they can perceive. Sure enough, surface swimming species do see beyond our blue, and bottom dwellers have a range stretched beyond our red. Generally the deeper the habitat, the redder they can see.
Hap males of the Pundamilia group range from more blue in surface swimmers to more red in deeper murkier habitats, their colours in tune with the ambient wavelengths reaching their eyes.
Photos by E. Schraml
As we’d expect, Natural Selection fine-tunes their sight because of its importance for feeding, escaping predators, and especially breeding colours which play a key role in speciation. “Speciation through sensory drive in cichlid fish” was the title used by Ole Seehausen and his co-authors for their paper which received the accolade of adorning the front cover of Nature magazine in 2008.
The following year Haps were back in the limelight, with several American studies looking for new mutations in the opsin genes themselves, mutations that had been selected for and become the norm since the population crash. They found them: in turbid waters. The total possible spectrum of wavelengths the little fish could see had been widened even further.
DREAMPOND REVIVAL
A new Species Flock may yet appear.
After Darwin’s Galapagos finches, Haps have contributed most to an understanding of how new species are formed. The actual mechanism of speciation, that is the origin of species, is a question which Charles Darwin in fact avoided as was only sensible at the time with so little means of answering. It has generated an industry of theory and research, and we have seen that Haps are ideal subjects. They had long thrilled field biologists and collectors with their number and unique variety in Lake Victoria, and now we can go further as data is analysed with more sophisticated techniques. The wonderful Haplochromines are now changing again, offering unrivalled opportunities to examine the processes of hybridisation and speciation.
A new Species Flock may yet appear.
By Dr Valerie Jeffries FLS
A KENYAN POSTSCRIPT
Haps research has produced leading articles in “Nature” more than once in recent years, and in several other prestigious journals. Kenyan scientists have played their part in the big European and American research groups: important Hap publications in 2011 and since have involved museum scientists in the Ichthyology Department at Nairobi National Museum.
The Museum has a preserved Coelocanth, a rare example of a species that seems to have evolved hardly at all in the 425 million years since its relatives lived. It was known only from fossils until a live one was caught off South Africa in 1938, to general astonishment. Richard Dawkins (who was born in Kenya), echoing “The Lamb” by William Blake, wrote in 2006, “As William Blake might have written to a coelacanth: Did he who made the haplochromids make thee?”
Besides their world-famous fossils of Human ancestors, and the Big Five mammals, Kenyans can be proud of the spectacular nursery of new species below the surface of the Lake, the richest on Earth.
Acknowledgements
Many thanks are due to author and photographer Erwin Schraml for generous permission to use his photographs. Erwin edits the website http://www.worldfish.de and produces a magazine, Eggspots, all about cichlids.
FFON. Written by nature detectives. 