WildNothos
THE NOTHOBRANCHIUS SITE
INTRO - DISCOVERY & EARLY HISTORY - DISTRIBUTION & DISPERSAL - ECOLOGY & BIOTOPE - LIFE HISTORY & REPRODUCTION - MAINTENANCE & BREEDING - TAXONOMY & SYSTEMATICS
Past geological and geomorphological events created catchments and river basins, which play major roles in the distribution of fish species. Rivers in large parts of sub-tropical Africa exhibit seasonal cycles in water level. On the seasonally inundated floodplains temporary ponds and swamps occur that provide the necessary conditions for the survival of the annual fish species of the genus Nothobranchius.
Distribution and Dispersal of Nothobranchius Fishes
The distribution of Nothobranchius fishes across Africa reflects a combination of specialised habitat requirements, limited dispersal, and a dynamic geological and climatic history. Although the genus occupies an extensive region of sub-Saharan Africa, its species are not found everywhere within this range. Instead, they occur in a mosaic of seasonal wetlands, often separated by large areas of unsuitable habitat.
Understanding this scattered distribution requires looking beyond the present-day landscape. River systems, floodplains, climatic fluctuations, and geological changes have all influenced where these fishes occur and how populations have become isolated over time.
Geographical Distribution
Nothobranchius occurs across a broad region of northeastern, eastern, south-central, and southeastern Africa, extending from southern Chad and northern Cameroon in the west to the coastal regions of East Africa and southwards to northeastern South Africa. The genus is represented in four major African ichthyological provinces: Nilo-Sudan, Congo, Zambezi, and East Coast.
Within this extensive range, Tanzania stands out as the principal centre of species diversity. Other important areas include Kenya, the Democratic Republic of the Congo, Mozambique, Zambia, and Uganda. The distribution is far from continuous, however, and the presence of suitable habitats determines where individual species can survive.
All known Nothobranchius species inhabit seasonal freshwater environments. These include temporary pools, marshes, and seasonally inundated floodplains, particularly in savannah and woodland landscapes. Such habitats fill during the rainy season and subsequently dry out, providing the conditions to which these annual fishes are adapted. They are absent from permanent open lakes and fast-flowing rivers, and even within suitable climatic regions, the availability of appropriate soils, water regimes, and landscape features restricts their occurrence.
The geographical ranges of individual species vary considerably. Some occupy extensive areas encompassing several river systems, while others are confined to a single drainage basin, a small section of a floodplain, or even one or a few known localities. Island endemics, such as N. guentheri on Unguja (Zanzibar) and N. insularis on Mafia Island, illustrate how geographical isolation can produce particularly restricted distributions.
Currently known distribution of Nothobranchius fishes in Africa. Collecting records are organised in a grid of squares of approximately 2500 square kilometres, and the map is based on more than 2000 collecting points.
I am grateful to everyone who kindly shared collecting records, in addition to literature sources, museum database and my own collections, which have been used when plotting the map.
River Systems and the History of the Landscape
River basins and their associated floodplains provide the fundamental framework for understanding the distribution of Nothobranchius. Seasonal wetlands within a drainage system may become temporarily connected during periods of extensive flooding, allowing fish to move between habitats. In contrast, uplands and drainage divides generally lack suitable environments and act as barriers between populations.
The present-day distribution of the genus also reflects millions of years of geological and climatic change. Tectonic movements, the development of the East African Rift System, changes in river courses, and the capture or diversion of drainage systems have repeatedly altered the landscape. These processes could create temporary connections between previously separate river basins, allowing fish populations to expand into new areas. Subsequent isolation could then lead to the divergence of populations and, over time, the evolution of distinct species.
Changes in rainfall and floodplain extent have also played an important role. During wetter periods, seasonal wetlands may have extended across larger areas, creating opportunities for dispersal that no longer exist. Conversely, the fragmentation or disappearance of wetlands can isolate populations and restrict their ranges.
These historical processes help explain why closely related species may occur in neighbouring river systems while remaining geographically separated, and why some widespread species contain populations with distinct characteristics.
Dispersal and Colonisation
The dispersal of Nothobranchius is constrained by its dependence on temporary freshwater habitats. Unlike fishes that can move freely through permanent river networks, these annual fishes are restricted to shallow seasonal waters and have no known capacity for active long-distance movement across dry land.
Within a floodplain, seasonal inundation provides the most plausible mechanism for movement between habitats. During exceptional floods, water may spread across extensive low-lying areas, temporarily connecting pools and marshes that are normally isolated. Repeated flooding events over many generations can therefore facilitate gradual range expansion within a drainage system.
Dispersal between separate river basins is more difficult to explain. Geological changes, including river capture and drainage rearrangements, can create temporary aquatic connections between formerly isolated systems. Such events provide a plausible explanation for the occurrence of some species across multiple drainages.
Other explanations have been proposed, including the transport of eggs by birds or large mammals. The sudden appearance of fish in pools following rainfall has long encouraged the belief that fish somehow arrive with the rain. In reality, annual fishes survive the dry season as dormant embryos in the substrate of their temporary habitats, hatching when suitable conditions return.
Although experiments have demonstrated that eggs of some South American annual fishes can survive passage through waterfowl, there is little evidence that animal-mediated transport is an important mechanism of dispersal in Nothobranchius. The strong geographical structure of species distributions, including the persistence of distinct populations in nearby habitats, instead points to limited dispersal and a major role for historical changes in hydrological connectivity.
Distribution, Isolation and Diversity
The distribution of Nothobranchius is the product of an ongoing interaction between ecological requirements and landscape history. Suitable seasonal habitats determine where populations can persist, while flooding, changes in drainage systems, and long-term geological processes influence how populations become connected or isolated.
These processes have contributed to the remarkable diversity of the genus. The concentration of species in eastern and southeastern Africa, the existence of numerous range-restricted endemics, and the occurrence of closely related species in neighbouring drainage systems all provide clues to the evolutionary history of these fishes.
Understanding these patterns is also important for conservation. Species confined to a small number of wetlands may be particularly vulnerable to habitat loss or changes in seasonal water regimes. Their restricted distributions mean that the disappearance of even a few habitats can have consequences for the long-term survival of a species.
Explore Distribution and Dispersal in Greater Depth
The geographical patterns of Nothobranchius tell a story that extends far beyond the locations where these fishes are found today. To understand how their distributions developed, we must also consider the history of African river systems, the geological forces that reshaped them, and the mechanisms that have allowed these specialised fishes to disperse, persist, and diversify.
These subjects are explored in detail in the comprehensive reference book: Nagy & Watters - Fishes of the Genus Nothobranchius. The chapter about Distribution and Dispersal examines the geographical distribution of the genus at continental, regional, and local scales, supported by distribution maps, quantitative analyses, and detailed case studies illustrating how tectonics, river capture, and drainage reorganisation have shaped the evolution of the group.
The chapter also examines the evidence for different dispersal mechanisms, separating plausible explanations supported by biogeographical and genetic evidence from hypotheses that remain speculative.
Discover the full story of Nothobranchius distribution and dispersal in the book.
Explore the book on the Publications page.