lakes in the region dried out, as happened to Lake Victoria about 15,000 years ago. In recent history only Lake Kyoga was easily accessible to Victoria cichlids, as further downstream movement by the Victoria Nile (to Lake Albert) is prevented by a series of waterfalls, notably Murchison. In contrast, the Owen Falls (now flooded by a dam) between Victoria and Kyoga were essentially a series of rapids that did not effectively block fish movements between the two lakes. The Victoria haplochromines are distinctly sexually dimorphic (males relatively brightly colored; females dull), and their ecology is extremely diverse, falling into at least 16 groups, including detritivores, zooplanktivores, insectivores, prawn-eaters, molluscivores and piscivores. As a result of predation by the introduced Nile perch, eutrophication and other changes to the ecosystem, it is estimated that at least 200 species (about 40 percent) of Lake Victoria haplochromines have become extinct, including more than 100 undescribed species. Initially it was feared that this number was even higher, by some estimates 65 percent of the total species, but several species that were feared extinct have been rediscovered after the Nile perch started to decline in the 1990s. Several of the remaining species are seriously threatened and additional extinctions are possible. Some species have survived in nearby small satellite lakes, have survived in refugias among rocks or papyrus sedges (protecting them from the Nile perch), or have adapted to the human-induced changes in the lake itself. Such adaptions include a larger gill area (adaption for oxygen-poor water), changes in the feeding apparatus, changes to the eyes (giving them a better sight in turbid water) and smaller head/larger caudal peduncle (allowing faster swimming). The piscivorous (affected by both predation and competition from Nile perch), molluscivorous and insectivorous haplochromines were particularly hard hit with many extinctions. Others have become extinct in their pure form, but survive as hybrids between close relatives (especially among the detritivores). The zooplanktivores have been least affected and in the late 1990s had reached densities similar to, or above, the densities before the drastic declines, although consisting of fewer species and often switching their diet towards macroinvertebrates. Some of the threatened Lake Victoria cichlid species have captive "insurance" populations in zoos, public aquaria and among private aquarists, and a few species are extinct in the wild (only survive in captivity). Before the mass extinction that has occurred among the lake's cichlids in the last 50