Morphological diversity and age dependent transformations in progeny of the large barbs (Barbus intermedius complex sensu Banister) of several morphotypes from Lake Tana (Ethiopia). Results of a long-time experiment

Results of a long-time (2005?C2011) experiment in crossing and rearing the large African barbs from Lake Tana (Ethiopia) are presented. One may conclude that progeny from homonomic crossings of large-mouthed barbs acquire diagnostic traits of their parental morphotype at different age. Some males originated from different crossings reached sexual maturity before acquiring diagnostic traits of a certain morphotype. Siblings from both homonomic and heteronomic crossings living in the same conditions considerably differed both in external and in skull characters. Differences both in external and skull characters between some siblings originated from a homonomic crossing that lived in a pond and those that lived in aquaria were equivalent to the differences between moiphotypes.     

Assessment of the role of chemoreception in the mate choice in barbs of the Barbus intermedius complex from Lake Tana, Ethiopia

The role of chemoreception in mate choice was studied in barbs of the complex Barbus intermedius from Lake Tana. It was found that chemical communication may be used in reproductive interactions of the barbs, and that males are responsible for the choice. Mature males tend to prefer ripe females and avoid both immature females and mature males, or are indifferent to them. It was shown that selectivity in mate choice by certain barb morphotypes could reduce the probability of interbreeding between different morphotypes, but not provide for reproductive isolation between them. It was hypothesized that the mate groups are finally formed as a result of direct interactions between potential partners, mediated by tactile reception or specific behavioral patterns.     

Phenetic relationships and probable ways of morphological diversification of the African large barbs (<Emphasis Type="Italic">Barbus intermedius</Emphasis> complex) in Lake Tana (Ethiopia)

29 quantitative and 18 qualitative skull characters were studied in the African large barbs (Barbus intermedius complex) from Lake Tana (Ethiopia). Based on results of principle components analysis phenetic relationships of 12 morphotypes were assessed. External features diagnostic of the morphotypes were found in individuals substantially different in the skull characters. The obtained data along with the data on morphological diversity of large barbs from Ethiopia beyond the Lake Tana basin suggest origination of the Lake Tana complex of forms from a complex of forms that existed in the upper reaches of the Blue Nile before the lake came into being, and the extant morphotypes of the Lake Tana barbs originated from three or four forms that composed the ancestral riverine local complex.     

The role of the thyroid gland hormone in ontogenesis and morphological diversification of Barbus intermedius sensu of Lake Tana in Ethiopia

The role of thyroid gland hormone in the ontogenesis of the Large African Barb Barbus intermedius sensu Banister, 1973 (Teleostei; Cyprinidae)--the supposed ancestral species which produced the species flock of the large African Barbs of the Lake Tana (Ethiopia)--has been studied. It has been shown that the thyroid hormone influences the period of many morphological processes and a change in its level causes heterochrony, reflecting on the definitive morphology of fish. Thus, it can be assumed that even a slight change in activity of the thyroid axis in the process of ontogenesis can be the cause of wide variability of the morphological features of B. intermedius and is at the basis for the fast diversification of the species flock of Big African Barbs of Lake Tana.     

Large African Barbs: Vectors of Diversification of Individuals of the Generalized Form as the Source of Phenetic Diversity of the Barbus intermedius Complex in Lake Tana,Ethiopia

Journal of Ichthyology - The paper analyzes the morphological diversification of individuals of the generalized form of the Barbus intermedius complex in comparison with the diversification of...     

Polymorphism of skull proportions in large African barbs <Emphasis Type="Italic">Barbus intermedius</Emphasis> sensu Banister (Cyprinidae) from Lakes Awasa and Langano (Rift Valley,Ethiopia)

After performing principal component analysis (PCA) of 13 characters (indices) of skulls of large African barbs (Barbus intermedius complex sensu Banister, 1973) from Lakes Awasa and Langano (Ethiopia) distributions of individuals in coordinates BL and PC1 were studied where BL is the basal skull length and PC1 is the first principal component. It is shown that in both the lakes there are groups of barbs whose individual ontogenetic trajectories are situated within separated ontogenetic channels. Separation of the channels is by far less distinct than that of the channels of generalized and specialized forms of barbs from Lake Tana and not correlated with differences in external traits. It is suggested that the revealed polymorphism is connected with food resource partitioning and might serve as a prerequisite of further divergence of sympatric forms of large African barbs.     

Evolution of MHC class II &; chain-encoding genes in the Lake Tana barbel species flock (Barbus intermedius complex)

Major histocompatibility complex (MHC) class II protein polymorphism is maintained in allelic lineages which evolve in a trans-specific manner, passing from one species to descendant species. Selection pressure on peptide binding residues should be greatest during speciation, when organisms move into new environments and their MHC molecules encounter new pathogens. The isolation ofMHC genes from teleost fishes, the most diverse group of vertebrates, has created possibilities for testing this hypothesis. The large barbels of Lake Tana have undergone an adaptive radiation within the last 5 million years, producing 14 morphotypes which inhabit different ecological niches within the lake. We studied the variability in class II beta chain-encoding genes of four of these morphotypes using polymerase chain reaction amplification and DNA sequencing. The sequences obtained were orthologous to four of the known class II genes from the common carp, from which barbels diverged approximately 32 million years ago. When subjected to phylogenetic analysis, the 48 sequences clustered into groups which represent allelic lineages. A comparison of nonsynonymous and synonymous substitutions between the peptide binding region codons and non-peptide binding region codons of these sequences revealed that they are under strong selective pressure. The nucleotide sequence data reported in this paper have been submitted to the EMBL and GenBank databases and have been assigned the accession numbers Z49869, Z49870, X93654-X93694, and X93894-X93898     

Role of thyroid hormone in the ontogeny and morphological diversification of <Emphasis Type="Italic">Barbus intermedius sensu</Emphasis> Banister, 1973 of Lake Tana in Ethiopia

The role of thyroid hormone in the ontogeny of the Large African barb Barbus intermedius sensu Banister, 1973 (Teleostei; Cyprinidae)—the presumed ancestral species for the species flock of the large African Barbs of Lake Tana (Ethiopia)—has been studied. It has been shown that the thyroid hormone influences the timing and rate of many morphogenetic processes and a change in its level causes heterochronies, affecting the definitive morphology of fish. Thus, it can be assumed that even a slight change in activity of the thyroid axis during ontogeny can be the cause of wide morphological variability in B. intermedius and may underlie explosive diversification of the species flock of Large African Barbs of Lake Tana.     

Speciation of endemic Lake Tana barbs (Cyprinidae, Ethiopia) driven by trophic resource partitioning; a molecular and ecomorphological approach

A unique species flock of large barbs (Barbus spp.) from Lake Tana is presented, from the level of fish stocks to molecules. Evidence is given for the species status of 14 morphotypes of large barbs. They distinctly differ in: (1) head and body morphometrics, (2) food preferences, (3) distribution patterns, (4) maximal body size, (5) spawning area and period, and (6) molecular genetic characters. Most types show early morphological divergence at small size. Major Histocompatibility Complex (MHC) genes, encoding cell membrane proteins involved in defence against pathogens, were found to be diagnostic for the species' genetic identity. A strong selective pressure on particular amino acid positions in the MHC protein sequence most probably arose in response to different pathogen loads from the newly invaded ecological niches after formation of Lake Tana, ca. two million years ago.Arguments for a sympatric origin of this species flock are discussed. An evolutionary scenario suggests a riverine ancestral Barbus intermedius invading Lake Tana after its formation by volcanic blocking of the Blue Nile river and its isolation by waterfalls. Specialisation for particular food types and disruptive selection on many feeding structures are hypothesized as the evolutionary drive in speciation. The causal relationship between the diversity in feeding structures and food types is explained from experiments and models. As an example, the potential food niche of three barb species is predicted from parameter values measured from a large set of feeding structures and tested against the actual gut contents. The co-occurrence of eight piscivorous barb species is unique for cyprinid fish, which lack oral jaw teeth and a stomach. The significance of this aquatic ecosystem as a multidisciplinary evolutionary laboratory and the need for a wise balance between exploitation and conservation is stressed.     

Comparative Analysis of the Structure of Differences between Some Morphotypes of Large African Barbs of Genus Barbus (Labeobarbus auctorum) from Lake Tana,Ethiopia

Journal of Ichthyology - It is shown that the phyletic relationships of the morphotypes of large African barbs Barbus (Labeobarbus auctorum) may be assessed by the results of a comparative analysis...     

Effect of large weirs on abundance and diversity of migratory Labeobarbus species in tributaries of Lake Tana,Ethiopia

Lake Tana has a remarkable fish diversity, including 17 endemic Labeobarbus species, of which nine spawn in the inflowing rivers. Three of the migratory species are threatened, namely the endangered Labeobarbus macrophtalmus and the vulnerable L. acutirostris and L. platydorsus. In July–November 2016 during the wet season netting upstream and downstream of large weirs in two rivers and in two undammed rivers showed that weirs had a severe negative effect on migratory Labeobarbus, including the three threatened species. Of eight Labeobarbus species caught below the Gelda River weir, only L. intermedius was recorded above the weir, and in the Shini River, five Labeobarbus species were caught below the weir, but none above it. Labeobarbus diversity below the Gelda River weir (H’?=?1.4) was significantly different to that above the weir (H’?=?0), whereas in the two rivers without any weirs, Labeobarbus diversity downstream (H’?=?1.39 and 1.55) was not significantly different from that upstream (H’?=?1.52 and 1.40). Therefore, specific mitigation measures, such as fishways, need to be designed from the outset of proposed weir and instream dam construction.     

Trematode metacercariae from the cranial cavity of African catfish Clarias gariepinus (Burchell, 1822) from Lake Tana,Ethiopia

Metacercariae of two species of trematodes from the fam. Diplostomidae from the cranial cavity of the African catfish Clarias gariepinus are described. The metacercaria Tylodelphys grandis n. sp. is described for the first time. Data are presented on the infection rate of catfishes in Lake Tana.     

Metacercariae of new trematode species of the genus Diplostomum (Trematoda,Diplostomidae) from fishes of Lake Tana,Ethiopia

Four new species of trematodes of the genus Diplostomum are described from metacercariae found in fishes from Lake Tana in Ethiopia: Diplostomum garrae sp. n. from the lens of Garra dembecha, D. longicollis sp. n. from the lens of Barbus humilis, D. montanum sp. n. from the lens of Barbus humilis, and D. tilapiae sp. n. from the vitreous body of Oreochromis niloticus. The metacercariae of the new species considerably differ in morphology and size from those of all known Holarctic species. It is the first African record of metacercariae of the genus Diplostomum in fish.     

Spatial and Temporal Variation of Cestode Infection and its Effects on Two Small Barbs (Barbus humilis and B. tanapelagius) in Lake Tana, Ethiopia

Pseudophyllidean cestodes as Ligula have a complex life cycle with cyclopoid copepods as first intermediate host, zooplanktivorous fish as second, and piscivorous birds as final host. We studied the effects of diet, season and habitat occupation on the prevalence of plerocercoid larvae of the tapeworm Ligula intestinalis in two closely related small barbs and the effects of the parasites on the barbs life histories in Lake Tana (Ethiopia) during 1 year. In all affected barbs L. intestinalis caused retardation in gonad development, maturation at reduced size and lower absolute fecundity. Infection rate, averaged over all habitats was significantly higher in B. tanapelagius (10%) than in B. humilis (6%). Below a threshold of 48 mm the infection rate was zero for both barbs, this coincided with a very low proportion of copepods in their diets, increasing up to 90 and 55%, respectively, for their largest size class (81–90 mm). The relatively high infection rate in B. tanapelagius is explained by its obligatory zooplanktivorous feeding behaviour, ingesting a relatively high proportion of infected cyclopoid copepods. This is in contrast with B. humilis, which is a polyphagous species, feeding both on zooplankton and benthic invertebrates. Significant seasonal effects in infection rates were observed. In both barb species infection rates were lower during the breeding season. Only for B. tanapelagius a significant negative correlation was observed between rain fall and infection rate, probably caused by an increased turbidity that decreases feeding efficiency on zooplankton. Habitat type had also a significant effect on infection rate. Barbus humilis showed a much higher infection rate in shallow clear water (10%) than in shallow turbid water (3%), whereas B. tanapelagius showed much higher infection rates in the shallow sublittoral (13%) than in the deeper pelagic (7%). Most likely, birds predate more efficiently on barbs in shallow clear waters than in shallow turbid and deep waters.     

Growth, biomass, and production of two small barbs (Barbus humilis and B. tanapelagius, Cyprinidae) and their role in the food web of Lake Tana (Ethiopia)

Growth, biomass and production of two small barbs (Barbus humilis and Barbus tanapelagius) and their role in the food web of Lake Tana were investigated. From length–frequency distribution of trawl monitoring surveys growth coefficient, Φ′ values were estimated at 3.71–4.17 for B. humilis and 3.70–4.14 for B. tanapelagius, respectively. Values for B. humilis were confirmed in pond experiments. Mean biomass of the small barbs was 13.3 kg fresh wt ha⁻¹, with B. humilis being most abundant in the littoral and sub-littoral zones, whereas B. tanapelagius was most abundant in the sub-littoral and pelagic zones. The two small barbs had a production of 53 kg fresh wt ha⁻¹ year⁻¹. Although their P/B ratios of about 4.0 were relatively high for small cyprinids, both their biomass and production were low in comparison with other small fish taxa in other tropical lakes. Of the zooplankton production only about 29% was consumed by the small barbs. However, they did not utilize calanoid copepods, which were responsible for approximately 57% of the zooplankton production and it is likely that small barb production was food limited during certain periods of the year. Piscivorous labeobarbs consumed about 56% of the small barbs production annually, but additionally, Clarias gariepinus, and many bird species were also preying on them. Therefore, limitation of Barbus production by predation during certain periods in the year cannot be excluded.     

Spawning migrations of the endemic Labeobarbus (Cyprinidae, Teleostei) species of Lake Tana, Ethiopia: status and threats

The reproductive biology of the only known intact species flock of large cyprinids, the 16 Labeobarbus species of Lake Tana (Ethiopia), has been extensively studied for the past two decades. Seven species of Labeobarbus are known to migrate >50 km upstream into tributary rivers for spawning during the rainy season (July to October), whereas eight other species are absent from these rivers and probably developed a new strategy of lacustrine spawning (macro-spatial segregation). One species (L. intermedius) probably spawns in the lake as well as in the rivers. Between the early 1990s and 2000s, the riverine spawners showed a decline of 75% in both biomass and number in both fishery independent surveys and in commercial catches. Reproductive migration makes fishes vulnerable to fisheries and other threats like habitat modifications. Lacustrine spawners are probably more resilient as they are not known to form spawning aggregations that can easily be exploited by fishermen. In addition, upstream rivers and catchments around Lake Tana are highly degraded by erosion and recently subjected to intensive habitat modification for irrigation and hydroelectric power generation. This article reviews results of field studies on the Labeobarbus spawning migration from Lake Tana to spawning rivers, giving emphasis on segregation and homing. It also summarizes existing and emerging threats which form potential causes for the decline of the migratory Labeobarbus species. Knowledge gaps on the reproductive biology are identified for further investigation.     

Development of the cranium in the large African hexaploid barb Labeobarbus (=Barbus) intermedius (Cyprinidae; Teleostei)

Developmental information on the structure and composition of the cartilaginous and bony skull in the large African barb Labeobarbus (=Barbus) intermedius (Teleostei; Cyprinidae) is provided. Sequences of cartilages and bones appearance from a large series of cleared and Alizarin red- and Alcian blue-stained laboratory-reared specimens ranging from prehatching larvae to juvenile stages are described. Comparisons of cranial development are made among cyprinids: L. intermedius, Danio rerio, Barbus barbus, and Cyprinus carpio.     

Seasonal variation in primary production of a large high altitude tropical lake (Lake Tana,Ethiopia): effects of nutrient availability and water transparency

Primary production rates, chlorophyll and phytoplankton biovolume were measured monthly from April 2003 to November 2004 in Lake Tana, a large tropical lake in the highlands of Ethiopia. The lake is characterised by low nutrient concentrations, and a low water transparency due to high silt load of the inflowing rivers during the rainy seasons (May–November) and daily resuspension of sediments in the inshore zone. The mean chlorophyll-a concentrations varied seasonally and ranged from 2.6 mg m⁻³ to 8.5 mg m⁻³ (mean: 4.5 mg m⁻³) in the offshore zone. Primary production was measured using the light–dark bottles technique. We incubated only at three depths, i.e. 0.6, 1.2 and 1.8 m. Therefore, we may have missed a substantial part of the depth production profile and probably also frequently missed P _max. Gross primary production in the openwater averaged 2.43 g O₂ m⁻² d⁻¹ and ranged between 0.03 g O₂ m⁻² d⁻¹ and 10.2 g O₂ m⁻² d⁻¹; production was significantly higher in the inshore zone. The highest production rates were observed in the post-rainy season (Oct–Nov), which coincided with a bloom of Microcystis and higher chlorophyll levels. This seasonal high production is probably caused by a relatively high nutrient availability in combination with favourable light conditions. The gross primary production rates of L. Tana are among the lowest compared with other tropical lakes. This will be partly the result of our underestimation of gross primary production by often missing P _max. Another cause is the oligotrophic nature of the lake in combination with its relatively low water transparency. The gross primary production per unit chlorophyll in the openwater zone was in the same range as in 30 other tropical lakes and reservoirs. The higher primary production in the inshore zone is probably the result of the daily water column mixing (Z _mix ≥ Z _t) in this area, enhancing nutrient recycling. A large proportion of the annual primary production is realised in one of the four seasons only. This productive post-rainy season is relatively short (2 months) and therefore efficiency of transfer of matter between the first and second trophic level of the Lake ecosystem will be poor.