WildNothos
THE NOTHOBRANCHIUS SITE
INTRO - DISCOVERY & EARLY HISTORY - DISTRIBUTION & BIOGEOGRAPHY - ECOLOGY & BIOTOPE - LIFE HISTORY & REPRODUCTION - MAINTENANCE & BREEDING - TAXONOMY & SYSTEMATICS
Life History and Reproduction of Nothobranchius Fishes
The biological life cycle, reproductive behaviour and egg development of the Nothobranchius species is perfectly adapted to the special conditions of the temporary natural biotopes, in which they occur. The adult fish deposit eggs into the muddy substrate of the habitat where they survive the dry season while undergoing development with intervening rest periods. Since the survival of the population must be secured during the relatively brief period of a single rainy season, the eggs must be ready to hatch as soon as the rains arrive and the habitat becomes filled with water. The development of the fish is then very rapid in order to reach sexual maturity within the shortest period of time.
Nothobranchius species are characterized by the following traits: relatively small adult size; adaptation to unstable climate and environmental conditions; produce large numbers of eggs to ensure a potentially adequate number of offspring; no parental care and protection of the offspring; and a low ability to compete, with most offspring dying before reaching reproductive age.
Rapid growth and early sexual maturation
In the nature there is, normally, plenty of food available for the newly-hatched fry. Tiny organisms, which serve as food for the fish, also survive the dry season in some way and quickly re-populate the recently formed temporary biotopes. The habitats, usually still expanding in size at the time the fry hatch, provide enough living space for the development of the newly-hatched fish. The period of growth is, however, limited and depends on the length of the rainy season and on the period when the habitat holds a sufficient volume of water. Given their seasonal nature, the habitats will be capable of hosting the adult Nothobranchius for a limited period only and, consequently, conditions dictate that the fish grow rapidly and reach an early reproductive stage. The growth rate might vary to some extent on an inter-specific level, as a result of long-term climatic conditions in a given area, but the growth of the fish is invariably rapid. When relatively slow growth does occur it is usually due to unfavourable conditions in the habitat, such as the limited availability of food, a situation that can be exacerbated by unusually dense populations. The adult lifespan of particular Nothobranchius species will depend on regional rainfall patterns but under optimal conditions most can be sexually mature in three to four weeks after hatching. At that stage the fishes will not yet have attained their full size but will continue to grow until the end of their lives. Reproduction is, however, an energetically demanding exercise, especially for the females, so growth rate may slow down after sexual maturity has been reached.
In addition, intra-population growth differences can also be observed. Some individuals within a population will grow at a relatively fast rate, providing those individuals with an advantage under exceptionally harsh conditions, when a given habitat dries up again soon after the start of the rainy season, and the most rapidly growing specimens would be the ones to establish the next generation. The author has, on a number of occasions, noted that some seasonal habitats that were on the verge of drying out completely, hosted only very young Nothobranchius specimens. However, the fish were starting to show some signs of sexual maturity and may have already begun laying the first eggs. At least, they started to breed not very long after they were collected and placed in captivity.
Intense growth rates do, however, result in shorter lifespans and those specimens that mature earlier may live for a shorter time than those that grew relatively slowly. Consequently, the slower growing specimens may inhabit the biotope for longer time and may, therefore, contribute to a greater extent to the reproductive process, if favourable conditions allow the habitat to exist for a relatively long time.
Life expectancy
The life expectancy of Nothobranchius fishes is predicted by the specifically harsh conditions of the temporary natural habitats in which they occur. The periodical drying out of the biotopes eliminates the adult fish. The time constraints of the temporary habitat pushes for rapid maturation.
Studies conducted in Tanzania after the end of the rainy season showed a significant variation in the assembly structure when habitats were visited three weeks apart. In several cases, the Nothobranchius species disappeared from the biotopes across the period of the two visits. Whereas in most cases the habitats still contained at the two visits an almost comparable amount of water, it was probably the final part of the wet period for the habitat. According to author’s opinion, the Nothobranchius species have probably disappeared from the still suitably looking habitats as a combination of loss of natural protection in the slightly shrinking habitats and, as a consequence, increasing threats by predators.
Both the periodical drying process of the habitat and the predation pressure determine a constrained life expectancy, which is restricted under natural conditions to a few months only.
Habitat use
It can often be observed in the natural biotopes that females and males inhabit different parts of the same habitat. Males typically show territorial behaviour and protect suitable spawning sites, such as areas with a thick mud base or an embayment protected by vegetation. The author often found the largest, most dominant males in those parts of the biotope where the eggs had the best protection in a deep layer of mud and the fish was protected from the predators. In these cases the females inhabited either the more open central parts of the habitat or were hiding in the dense vegetation.
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 (e.g. spots or faint cross-bar markings). These features allow the males to identify the females. In the case of several species co-inhabiting the same site, usually each species would occupy a different niche of the biotope. Observations by the author indicate that males and females of the same species tend to stay in close proximity to each other when several species are present in the same pool. For example, in an ephemeral pool near the Mbezi River, in the coastal region of Tanzania, the author found four species in the same habitat. Nothobranchius melanospilus, the largest, was abundant and inhabited the central, open part of the biotope. Only a single trio of N. ruudwildekampi could be collected, in an embayment covered by leaves. Only one pair of N. albimarginatus was found in another embayment on the opposite side of the pool. Several specimens of N. luekei, both males and females, were found at one side of the biotope only, where it was overgrown by dense thorny vegetation. Interestingly, at a second visit of the same locality three weeks later, the pool was slightly smaller and out of the four species only N. luekei was found in thorny vegetation. Incidentally, a previous collecting trip also collected there N. rubripinnis, making a total of five Nothobranchius in the habitat. This latter species occupied the same habitat niche as N. albimarginatus.
Specific habitat preference may be shown by different species that occur in the same general area but not generally in the same body of water (i.e. sympatric but not syntopic). In Uganda, N. robustus and N. ugandensis live in habitats that, in general, differ in water chemistry and ecology. N. robustus tends to favour relatively cool habitats at the edge of slow-flowing streams that most commonly have mildly acidic water, whereas N. ugandensis lives in more typical Nothobranchius pools with alkaline water and higher temperatures. The author found the two species to be syntopic in only one pool out of 20 where Nothobranchius were present.
Reproductive strategy
Adapted to the conditions of the seasonal biotopes, Nothobranchius species have developed an interesting reproductive strategy. The reproductive instinct of the annual fishes is very strong and spawning activity usually occurs daily throughout their adult lifespan. From the time they reach sexual maturity, they begin to spawn in order to ensure the survival of the population, as continuous daily reproduction is important for annual fishes that face high mortality risk given the harsh conditions of their temporary habitat. Generally speaking, the spawning activity is initiated by the female, depending on whether or not she carries mature eggs. The natural biotopes are typically turbid because of small suspended particles in the water and submerged objects can be seen through a depth of a few centimetres only. Females generally have drab grey or brown colours which are relatively difficult to spot under such conditions. However, males possess typically striking colours which help the females to find their mates. Once visual contact is made, an initial short display phase will usually occur and spawning will start within a few seconds. When the sexes are in visible distance, they approach each other and males move with particularly rapid and vigorous movements towards the females. The male shows a characteristic darting approach and attempts to drive the female towards the bottom and impress her with extended fins. The male tries to position himself above the female and, for most of the species, pushing with the lower jaw onto the dorsal surface of her head. One exception to this approach towards the female is represented by members of the N. microlepis species group. The male of this group will approach the female from below and push the typical humpback of his head against the belly of the female.
A non-receptive female would refuse by escaping and swimming away towards the surface, while a receptive female will move with the male to the substrate. The male then wraps his fins around the female, while pushing her onto, and partially into, the spawning substrate. The body of the male adopts an S-shape while the female forms her anal fin into a conical shape and presses it into the substrate. After a few seconds of trembling movement the pair stays motionless for a second and the female releases a single egg with a rapid jerk movement, while the deposited egg is simultaneously fertilized by the male. This egg laying sequence can be repeated several times.
A somewhat different spawning behaviour can be observed with the species of the subgenus Aphyobranchius, as those tend to spawn in middle levels of the water and the very small eggs sink to the substrate. Another species that may also sometimes spawn at mid-water levels is N. fuscotaeniatus.
Intraspecific competition
Males of Nothobranchius species typically engage in fights against each other, with the level of male to male aggression varying across species. The primary reason for the intraspecific competition amongst males is that dominant males have the highest mating success due to exclusive access to prime spawning sites. Fights between males include lateral displays by spreading the unpaired fins, and projections of opercular and branchiostegal membranes. Increased level of aggression includes tail beating and attempts to bite the fins of each other. In extreme cases of some species, the fight can continue until the death of the opponent.
There are major interspecific differences in the intensity, aggressiveness and the frequency of the interactions. Generally speaking, all Nothobranchius species are aggressive to a certain degree, but that not only depends on the species but often also on individual specimens. Even the less aggressive species can be aggressive to the point where the females can suffer fatal damage. In addition, wild specimens tend to be far more aggressive than tank-raised specimens of the same species.
Some of the larger species, such as N. orthonotus seems to be more aggressive than smaller species. In addition, the level of aggressiveness of the species seems also increasing by the level of red colouration. In some N. orthonotus populations, even the females can be so aggressive that they would kill each other.
The most aggressive species is Nothobranchius ocellatus, which is a predator and feed on other syntopic congeners.
Morphology
Nothobranchius species generally show little intra-specific morphological variation. Most species are 4-7 cm in total length and only a couple of species might reach 10 cm or more (Wildekamp, 2004). The general body shape of the males is usually robust, laterally compressed and deep. The greatest body depth will usually be just in front of the pelvic fin, whereas greatest body width is at the position of the pectoral fin base. Dorsal and ventral profiles of the body are convex. Males possess large and rounded dorsal and anal fins that are positioned behind the mid-length of the body. The body shape and the position of the fins are typical for fishes that are generally poor swimmers over long distances but allow them to initiate movement rapidly and swim quickly over short distances. The predatory N. ocellatus has dorsal and anal fins that are set even more posteriorly than other members of the genus, which allows faster movements when chasing prey. Another distinctive group is represented by members of the subgenus Aphyobranchius which have a larger anal fin, and a dorsal fin that is positioned more posteriorly, as an adaption to surface-dwelling behaviour.
Females of Nothobranchius species are slightly smaller than the males. Their bodies are usually less laterally compressed and more slender than those of the males. They possess a triangular anal fin with a rounded tip, and the central rays are longer and more rigid. Dorsal and anal fins are positioned more posteriorly than in males.
Phenotypic variations
Nothobranchius species are sexually highly dichromatic with males being usually colourful while females have usually grey-brown dull colouration. In some species colour polymorphism exists. One of the most typical cases is when in males of some species exhibit either red and blue, or red and yellow body colour morphs, such as with N. eggersi or N. korthausae, respectively. The two colour morphs of N. eggersi seem to appear sympatric in the same general area but not syntopically in the same habitat, whereas the colour forms of N. korthausae, including intermediary forms, might sometimes, but not always, occupy the same habitat. In some other cases, the phenotypic variations are restricted to only one component of the colour pattern, such as N. jubbi having either a red or a blue caudal fin. These phenotypic features seem to be determined by genetic factors.
Other differences in male coloration can be driven by environmental factors or dominance. Dominant males of some species might develop signals in the form of a striking colour in the fins, such as a relatively vivid submarginal band to the caudal fin. A change in environment might induce or supress the development of such an enhancement.
Dominant males typically possess the brightest coloration, suggesting that male coloration is sexually selected.
Diet and feeding habits
Nothobranchius species are generally micropedators feeding on small aquatic crustaceans, worms, insect larvae and other zooplankton. Live food is particularly important due to their high energy consumption during the fast growth period and also because of the high reproductive effort. One species, N. ocellatus has evolved as a large predator and it can always be found together with at least one or several other Nothobranchius species. In particular, it is without exception found syntopically with N. melanospilus. The members of the N. microlepis species group have specially adapted gill rakers that allow them to feed predominantly on small planktonic crustaceans. The author has also on several occasions observed them, under captive conditions, preying on other fish, such as the smaller members in the same aquarium.
Egg size
The eggs of Nothobranchius species are slightly oval in shape. Egg size is variable among species. Species with the smallest eggs size are represented by members of the subgenus Aphyobranchius, such as N. luekei and N. janpapi with egg diameter in the order of 0.6-0.7 mm, whereas the largest egg diameter is measured for N. ocellatus at around 2.5-2.6 mm. Egg diameter of most species of the genus measure typically between 1.0 and 1.5 mm. While closely related species tend to have similar egg sizes, there might be some differences. For example, the egg size of N. flagrans measures 1.5 mm wide and 1.7 mm long on average, while related N. hassoni and N. polli have eggs of 1.1-1.2 mm in diameter.
At the coastal part of Tanzania, it is common to find several Nothobranchius species occupying the same habitat. A typical assembly could be represented by N. ocellatus, N. melanospilus, N. eggersi and N. janpapi. Their egg diameter would be around 2.5-2.6 mm, 1.4-1.5 mm, 0.9-1.0 mm and 0.6-0.7 mm, respectively. Accordingly, the total volume of the eggs of N. ocellatus is 5 times more than those of N. melanospilus, 16 times more than N. eggersi and 45 times more compared to N. janpapi. The fry of Nothobranchius hatch more or less synchronously at the onset of the rainy season, and there is a considerable difference of fry size, proportional to the size of the eggs. This provides a direct advantage to the fry of the predatory N. ocellatus which can easily prey on the other species.
Embryonic development
Embryonic development of annual fishes is highly adapted to the dry phase of the temporary habitats with a prolonged phase of inactivity. The eggs are protected against the harsh conditions of the dry season by having a relatively hard outer egg shell, a chorion. The shell has a layered structure and the thickened egg wall is highly resistant against dry conditions by reducing loss of water and can even withstand physical pressure to some extent. The full development of the eggs consists of a series of different developmental and resting periods. This alteration of embryonic development represents an essential adaptation to prolonged phase of inactivity.
In order to ensure the suitable environment for the eggs to develop, the parents have to deposit the eggs in the substrate in a proper distance from the surface. This can be achieved as the female form its anal fin into a conical shape and press into the substrate. As a result of the rapid jerk movement of the spawning, as well as the vigorous fin movements, some substrate material is stirred up from the bottom. These substrate particles would cover the eggs when settling down again. Females of some species living in very dry areas have a more elongated anal fin, which allows them to place the eggs in greater depth, and thus protect them better from the heat of the direct sunshine and the drought of the upper level of the substrate.
Interestingly, not all the eggs are developing at the same pace, and this phenomenon helps to offset the unpredictability of the rainfall. Some habitats may dry out quickly again after having received some precipitations. In that case the survival of the species would not be guaranteed, as the newly hatched fry would die. However, nature is smarter than that. As the eggs develop at different rate, they are not ready to hatch at the same time. There will be some eggs resting and waiting for the next rainy period. This ensures the survival of the species under erratic rainfall conditions or during exceptionally long dry periods.
When the rainy season begins, the seasonal habitats are formed and filled again with rain water and, in consequence, by the raising level of the ground water. The temporary biotopes are then quickly inhabited by the organisms that are specially adapted to such seasonal conditions and the amazing cycle of life starts again …