In Africa, there are actually two variations of annualism defined by the region where the species are found.
East Africa is home to the true annual species including the many forms of Nothobranchius and its more restricted sub-groups Aphyobranchius and Paranothobranchius. These species evolved to survived the extremes in weather, with eggs surviving through the dry season in a form of hibernation and hatching when the wet season causes water to once again fill rivers, ponds and pools. In some areas, the flood plains retain water in pools and ditches long enough for the fish to hatch, reach maturity and guarantee the next generation.
In certain areas of West Africa, the same can be said for a few species, mostly denoted by members of the Pronothobranchius sub-group in and around the Gambia River.
The other form of annualism is somewhat different. In West Africa many species of Killifish have evolved a mechanism which allows the eggs to enter hibernation even when the waters do not fully dry out. Members of Fundulopanchax, in particular, but also Callopanchax, exhibit these traits. Eggs of these species might well develop fully and hatch in water, but eggs are also buried along the edges of pools and streams where they undergo a partial drying – perhaps as long as a couple of months. This guarantees those species will survive an extended drought, but also continue to reproduce in a continuous way.
|NOTHOBRANCHIUS & SUB-GROUPS|