WildNothos
THE NOTHOBRANCHIUS SITE
INTRO - DISCOVERY & EARLY HISTORY - DISTRIBUTION & BIOGEOGRAPHY - ECOLOGY & BIOTOPE - LIFE HISTORY & REPRODUCTION - MAINTENANCE & BREEDING - TAXONOMY & SYSTEMATICS
Ecology and Biotope of Nothobranchius Fishes
The species of the genus Nothobranchius occur mainly in river drainages of eastern and south-eastern Africa that are subject to seasonal rainfall. The temporary habitats dry out seasonally, depending on local rainfall characteristics, either annually or bi-annually. The fish deposit their eggs into the bottom substrate of the habitat. The eggs survive the dry season resting and developing in the substrate and hatch when the next rainy season arrives.
Climate and rainfall pattern
Regions hosting Nothobranchius fishes generally experience two different seasons which can be distinguished based on the distribution of the precipitation: a rainy season with abundant rainfall and a dry season of usually complete desiccation. During the rainy season the humidity is high and the temperature variation is low. In contrast, the hot summer days of the dry season are followed by cool nights, so the daily fluctuation of temperature is higher, the humidity is low. However, the intensity and the length of the seasons differ regionally.
Generally speaking, the climate across the distribution of Nothobranchius varies and the precipitation decreases from the wet tropical to the semi-arid north and south. If an area is characterized by sufficiently abundant rainfall and short dry periods, usually wooded savannah arises. To some distance to the equator, the general annual rainfall decreases and the landscape turn to grassland savannah. Getting even further from the equator, in areas where dry season can last more than ten months, no continuous cover of vegetation would grow anymore.
In Tanzania, for instance, the climate of most of the country is characterized by the alternating wet and dry seasons. The climate of the country can be roughly divided into two main parts: the coastal part near the ocean and the highland towards the interior of the continent, which is warmer and drier than the former. The eastern coastal part of Tanzania, as well as the north-western part around the Lake Victoria experience, typically, two rainy seasons per year. Generally, the longer of the two rainy seasons, the Masika, begins in March and ends in May, and the shorter rainy season, the Vuli, begins in October and ends in mid-December. Several Nothobranchius species inhabit these areas.
On another example, the north-eastern part of Kenya usually has a dry climate almost throughout the year and the annual rainfall is limited at around 150-250 mm. The scarce and unreliable rainfall is typically of bi-monsoonal pattern and so the seasonal calendar of the north-eastern part of the country is usually split into four parts. The Jilaal, the dry spell, begins in January and ends in March. Then the next season arrives with the long rains, called the Gu’, from April to June. The dry Hagai season lasts from July to September. The rest of the rainfall occurs during the short rains period of the Deyr, between October and December. The latter rainy season is reportedly somewhat more reliable than the first, but year-on-year rainfall variability is high and still years without practically any rainfall are not uncommon. Nothobranchius bojiensis is found in the north-eastern part of Kenya.
In some regions farther from the equator, the distribution of the rainfall is getting even more unpredictable and hampered with lasting periods of drought. In the area of the Nuba Mountains in Sudan, the rainy and dry seasons replace each other every six months. The rainy period lasts from May to October, while the dry season from November to April. Nothobranchius virgatus and N. nubaensis are known from the area around the foothills of the Nuba Mountains.
The rainy season is an important feature for the biology of annual fishes. When the rainy season arrives, the rivers usually overflow their banks and inundate the floodplains. This periodic flooding is essential for survival of Nothobranchius species, which occur in temporary natural habitats that dry out regularly during dry seasons.
Habitat characteristics
The typical Nothobranchius habitats are, on one hand, a variety of temporary natural biotopes, such as the floodplains of rivers and floodplains formed in shallow lakes, seasonal rivers and isolated residual pools, and seasonally drying out coastal parts of large bodies of water. A very typical natural habitat is a seasonal pond with turbid water and water lilies on the top, and at least the edge of the biotope overgrown by grass.
Another types of Nothobranchius habitats are formed by human intervention, such as rice fields or other fields created by agricultural practices, roadside drainage ditches and canals as a result of road construction activities, or even water holes excavated for human water consumption and cattle drinking places near villages.
Water balance
The length of the aquatic phase of a temporary biotope varies according to local climate and hydrographic conditions. Typically, a tropical temporary habitat contains water during and right after the monsoons. When the initial rainfall is abundant, the habitat can host small Nothobranchius fishes immediately. I collected N. neumanni from a temporary habitat where local people stated that the pool had contained water for only one week before my visit. Consequently, I collected small fish of about 1 cm in size.
The habitat will typically fill during the rainy season. At the end of the rainy season, the habitat will start losing soon water due to evaporation by the sun. If an area, however, receives extended high rainfall, the life of the water body can be extending several weeks into the dry season.
Substrate
The condition of the soil, its texture and structure, is of great importance for the reproduction of the annual fishes in Africa. A characteristic feature of the Nothobranchius habitats is a substrate of dark clay soil. The temporary habitats are generally found on low-lying floodplains, where a characteristic soil type was formed on alluvial deposits, the vertisol. Generally, it has a high content of expansive clay, which is drying relatively slowly and remains slightly wet under the surface after the water has disappeared from the temporary habitats during the dry season. The soil becomes compact and hard when dried, and develops oxygen deficient circumstances. The conditions during the drying out process provide a suitable environment for the egg development in the soil, and also protect the eggs from complete desiccation. These soils are typically alkaline.
There is another soil type of reddish colour and very widespread in tropical Africa, the latosol. Compared to vertisol, this soil contains less amounts of clay and it dries out quickly and completely. Therefore, the areas covered by latisols do not provide adequate conditions for the survival of Nothobranchius over multiple generations.
Dimension
The dimensions of the temporary habitats where Nothobranchius fish occur can vary greatly and they may be large floodplains that are under water for several square kilometres, or small waterbodies, such as puddles, pools and small ponds. Some of the largest waterbodies where I found Nothobranchius were, for instance, very large ephemeral pools formed in a river bed, as a stagnant section of a seasonal river, maybe 100 m wide and several kilometres long. The smallest collecting locations are usually represented by very small remnant pools formed on floodplains, less than half meter in length and width. Even if there is a small river that carries water on the floodplain, I could observe the phenomenon at several occasions that species belonging to the annual genus of Nothobranchius tend to stay back in water holes at the edge of a continuously restricting habitat. While that part of the habitat may totally dry out soon, which results in the death of the adult fish, it provides, however, the best spawning substrate with a broad layer of clay particles, which is essential for the survival of the deposited eggs.
Depth
Temporary habitats are usually of relatively shallow nature. Very large habitats, such as standing fragments of rivers or large floodplains might have a depth of one or two metres. However, the smallest rest pools can be very shallow. In some cases there is only a few cm water remaining in small rest pools in a drying out floodplain. If that small habitat is overgrown by dense vegetation, it facilitates the survival of the fish until the small water body literally dries up.
Flow regime
Nothobranchius fish are poor swimmers. They inhabit generally stagnant waters, such as puddles and ponds. Exceptions to this are a few species, such as N. robustus in Uganda and N. symoensi and N. rosenstocki in Zambia, which are often found in slowly flowing small streams. However, even these last mentioned species tend to inhabit the shaded and vegetated edge of the habitat, and tend to stay back at that drying out part and breed at the edge of the biotope.
Turbidity
The temporary habitats where Nothobranchius fish live are usually either slightly or strongly turbid. The source of turbidity is mainly suspended clay particles, which results of the typical fine muddy substrate of the habitat. Turbidity might influence the water quality as turbid water absorbs solar heat in the upper 5-6 cm of the water column. Furthermore, turbidity may serve for better protection, especially for the colourful males, against predation.
Water parameters
Generally speaking, water quality varies more in shallow temporary pools than in larger and deeper water bodies, subject to the influences of turbidity and shade. The animals that live in temporary pools, such as Nothobranchius fishes, have greatly adapted to these conditions. Wild Nothobranchius readily tolerate changes in water quality characteristics and, for instance, can be transferred to different quality water upon collection. Nothobranchius can be found in habitats with quite different water quality, as long as it do not reach extreme values such as lethal high temperatures or too low dissolved oxygen levels, etc.
Temperature
Temporary pools usually have a large surface area to volume ratio. The water temperature fluctuates with the elevation of the sun and temperature records reflect the time of day of measurement. Reduced water volume heats faster under the strong sun of the dry season and, in some cases, shallow waters during the latest phase of the drying out process may rise to lethal temperatures for some fish. Turbidity plays a role in heat absorption. Turbid water absorbs solar heat in the upper 5-6 cm of the water column, reducing the effect of the sun on lower layers. Many Nothobranchius habitats are often turbid and partly shaded by vegetation, both of which protect against overheating.
According to my collecting records, the average water temperature of the habitats where I found Nothobranchius species is 27.9 °C. The lowest water temperature was recorded at 21.3 °C, which was a slowly flowing stream on the highland plateau of Zambia, a locality of N. rosenstocki. The highest temperature was measured at 36.1 °C in a shallow pool in north-eastern coastal Kenya, where N. jubbi was collected.
Total dissolved solids
When the temporary habitat refills after the dry season, a rapid dilution of dissolved elements can be observed, but the large amount of rain water usually results in soft water. During the dry season the habitats shrink due to evaporation and the lowering of the level of the ground water. The total dissolved solids content of the shrinking habitat may increase due to the concentration of the dissolved elements.
Nothobranchius fishes are often found in restricted habitats and my records show an average of total dissolved solid content of 163 ppm. Lowest values of below 5 ppm were measured in a number of occasions, among others in slowly flowing streams in Zambia, where N. rosenstocki or N. symoensi were collected. While measured values above 500 ppm are very rare, I measured three exceptionally high values of 1130 ppm, 1146 ppm and 2000 ppm in habitats of N. melanospilus in Kenya and N. insularis in Tanzania, respectively.
Alkalinity
The dark clay soils that represent the substrate of almost all Nothobranchius habitats have an alkaline nature and, as a consequence, that is reflected for most of the collecting localities in the alkaline pH values measured in the water. This alkaline substrate represents a significant pH buffer that keeps alkaline conditions in the water. However, acidic water can be observed, in some cases, also on alkaline soils. Usually, a large quantity of decaying vegetation or animal dung in the water accounts for that phenomenon, and cannot be compensated by the buffering capacity of the soil.
The average pH value of my measurements for Nothobranchius habitats are at pH 7.4. The highest values of pH 8.9 to 9.0 were measured in several habitats in the coastal part of Tanzania. I recorded an exceptionally low value of pH 5.4 for a habitat of N. korthausae in Mafia Island. The soils of that island are, typically, alkaline with an average pH of 7.5 and, because of the buffering capacity of the substrate, the water of the habitats is mainly alkaline. The very acidic value resulted essentially due to the presence of a large quantity of decaying vegetation.
Dissolved oxygen content
Isolated wetlands, swamps, marshes and pools where Nothobranchius live, have typically low dissolved oxygen concentrations. In habitats with dense vegetation, the dissolved oxygen content may fluctuate diurnally, and can be at maximum in the evening when photosynthesis is about to end and fell gradually due to overnight respiration. Other part of the dissolved oxygen in water comes from the atmosphere. After dissolving at the surface, oxygen is distributed by current and turbulence. Oxygen dissolves into water at different rates depending upon variables like pressure and temperature; the colder the water, the more dissolved oxygen it can hold. For example, the dissolved oxygen content of the water at 25°C can be 8.24 mg/l as a maximum level of saturation. It was found that, in temporary habitats, the oxygen concentrations are associated with the annual or biannual flooding, but even the peak values were far below the maximum level of saturation. Values below of 5.0 mg/l are regarded as the aquatic life is put under stress.
I have measured the oxygen content in a number of localities and I found that it varies around 5 mg/l to 7 mg/l. I obtained the highest values in slowly flowing streams in Uganda, where Nothobranchius robustus was found. Lowest value I measured was just above 5.0 mg/l in a habitat in Kenya, containing decaying vegetation, where Nothobranchius microlepis was captured.
Vegetation
The temporary character of the Nothobranchius habitats and the turbidity of the water limits the growth of a rich submerged aquatic vegetations. Plants present in the temporary habitats need to withstand desiccation periods. The turbidity of the water forces the plants to grow outside of the water. Many of the habitats have rich bordering vegetation.
Nothobranchius often occupy pool margins among or under vegetation, or shallow embayments covered with leaf litter. These preferences held even when the central, open and deeper parts of the pool would have been cooler. The vegetated and leaf-littered portions of the pool are more protective against predation and also offer more spawning habitat, essential for the survival of the fertilized eggs.
Sunlight on dry exposed soil can raise soil temperature to a value that would be sufficient to endanger the eggs. Vegetation shades the soil and keeps it cooler and more suitable for development of the annual egg. As the dry season progresses, the habitats shrink and formerly protected zones at their fringe disappear. This often leads to the loss of the fish in the habitat, even if the pool would still contain some water.
Close to urban areas, the temporary pools are frequently used for agriculture, particularly to cultivate rice during the rainy season.
Accompanying fauna
The dry phase of ephemeral ponds imposes a rigorous environment that only a few species can survive. Nevertheless, many freshwater species are successful in doing that. In a typical temporary habitat, often other animals than Nothobranchius species are found as accompanying fauna. These include large water insects, tadpoles, frogs and other fishes larger than Nothobranchius. The lungfish Protopterus annectens can be often found in the same habitat. The lungfish is the only other fish species co-occurring with Nothobranchius, which can also survive the dry season. The adult fish are buried in the substrate of the desiccating habitat until the habitat is filled with water again. Other fishes are represented by non-annual species, which are usually washed into the temporary habitat by the flood.
For example, the accompanying fauna of Nothobranchius milvertzi consisted of Enteromius haasianus, Lacustricola moeruensis, Protopterus sp. and Pseudocrenilabrus cf. philander, whereas Enteromius haasianus, ‘B.’ lineomaculatus, Ctenopoma nigropannosum, Marcusenius macrolepidotus and Tilapia sparrmanii were found coexisting with N. chochamandai. The accompanying fauna of N. flagrans was represented by non-annual species, such as Enteromius multilineatus, E. paludinosus, Lacustricola moeruensis and Tilapia cf. sparrmanii.
Syntopic congeners
At many locations it happens that several Nothobranchius species can be found in the same habitat. This is especially common in the coastal region of Tanzania. The males of the different species will usually have very different coloration and body shape and it is easy to distinguish between them. The females will also show some distinguishing features, such as small differences in body or fin shape, and a pattern of darker markings on the body and fins, such as spots or faint cross-bar markings.
Predation
Some of the above-mentioned larger water insects and many of the tadpoles are carnivorous and represent predators on annual and other fishes in the isolated temporary habitats. Typically, when a habitat has large number of water insects, tadpoles or frogs, we cannot find Nothobranchius fishes there anymore. The two large predatory Nothobranchius species, N. ocellatus and N. matanduensis also shares the same pond with smaller prey species whose numbers are sustained by dense protective vegetation.
Other fishes living in the same habitat often also prey on Nothobranchius. I found in a number of cases large cichlids in the isolated pools, apparently washed in during river flooding. In these cases, Nothobranchius was either absent or present only in a very small numbers in the habitat. Small fishes vulnerable to predation occupy the shallowest marginal areas and dense stands of aquatic vegetation. As the pond dries and the protective vegetation becomes remote, refuges disappear and predation increases.