Rapid Morphological Changes Following Niche Shift in the Zooplanktivorous Cyprinid Rastrineobola Argentea From Lake Victoria

During the 1980s, the zooplanktivorous cyprinid dagaa (Rastrineobola argentea) ex-panded its habitat in Lake Victoria. After the eradication of the zooplanktivorous hap-lochromine cichlids from the sublittoral waters of the Mwanza Gulf (Tanzania) by intro-duced Nile perch (Lates niloticus), adult dagaa started to explore the bottom zone during daytime. This main daytime habitat for zooplankton and macrobenthic invertebrates was formerly occupied by the haplochromines. Besides extending its range of vertical distrib-ution, dagaa started to include macrobenthic invertebrates in its diet. In this paper, we present two rapid morphological adaptations of dagaa to its new niche. Samples from 1988, only one year after daytime bottom dwelling became a common habit in dagaa, showed a higher number of gill laments in comparison with samples from 1983. In contrast, the same samples showed a decrease in the number of gill rakers. The observed increase in the number of gill laments is postulated to have improved the capacity of dagaa to extract oxygen from the water, which may be crucial to survive the relatively poor oxygen conditions in the new habitat. By decreasing the number of gill rakers dagaa has probably increased its ef ciency of feeding on relatively large prey in a benthic habitat.