Abstract
Contemporary morphological changes in Lake Victoria cichlids Major ecological changes occurred recently in Lake Victoria. In the 1980s Nile perch, Lates niloticus, a large predator that had been introduced in 1954, boomed and the majority of the endemic cichlid species in sub-littoral and offshore waters declined or disappeared. Over the same period eutrophication and algal blooms resulted in increased water turbidity and decreased dissolved oxygen levels. Several invertebrates increased in abundance. During the 1990s, catch rates of Nile perch declined again, and some of the haplochromines started to recover in the strongly changed environment. Most recovering species changed ecologically, e.g. in diet. Morphological studies on the recovering species revealed that fishes caught in 1999-2001 differed from their conspecifics collected in 1977/78. These morphological changes, which seem to be adaptations to the new ecosystem within in a time span of only two decades, provide a rare opportunity to study aspects of rapid evolution and adaptive radiation. We have an extensive series of fish samples from the lake over a period of 30 years. Through these we can follow the patterns of morphological changes over time and compare them with the environmental changes in the lake. We will address the following questions: (1) Over what time period did the changes take place, and at what rate? (2) In relation to which environmental parameters did the morphological changes occur? (3) To what extent are the morphological changes adaptive? (4) Are the changes the result of phenotypic plasticity, genetic changes or a combination of the two? (5) Is genetic introgression through hybridisation involved? We plan to study six species at time intervals of about three years over the period 1977 till 2009. The study should reveal whether the morphological changes occurred gradually or instantly and whether they indeed concern adaptive responses to environmental changes, of which we collected data in the past. By means of stable isotope studies we will check when exactly changes in the diet of the fishes occurred. By raising fishes in the laboratory under different environmental conditions we will establish the potential contribution of phenotypic plasticity to the morphological changes. We will also investigate if there is a difference in phenotypic plasticity between old and new populations. Apart from morphological features, this will involve isohaemoglobins; we recently discovered haemoglobin plasticity in Lake Victoria haplochromines in response to chronic hypoxia. Differences in phenotypic plasticity between old and new populations may indicate the presence of genetic differences between them. With molecular techniques we will try to confirm if genetic changes are indeed present and to what extent hybridisation may have been involved.
Netherlands