Tilapiine Species Research Articles

Introgressive hybridization levels of Tilapiine species in Lake Victoria basin, Kenya inferred from microsatellite and mitochondrial DNA genotyping based on next-generation sequencing

Despite their high abundance and species richness, tilapiines have been compromised by various factors especially overfishing, climate change, and uncontrolled fish transfers and introductions. Fish introductions have negatively impacted native tilapiine populations through competition, predation, hybridization, and introgression compromising their genetic integrity. The hybridization levels of different tilapiines in the Lake Victoria basin remain relatively understudied. The study utilized nuclear microsatellite and mitochondrial DNA (mtDNA) genetic markers to investigate hybridization signals and compare the genetic diversity of different tilapiines in Lake Victoria, Kenya, using next-generation sequencing. Low levels of hybridization from Oreochromis niloticus into other Oreochromis species were detected by Bayesian clustering analysis and principal coordinate analysis (PCoA). The results contribute to the need for conservation measures for these fish species.

Mitogenomic Features and Evolution of the Nile River Dominant Tilapiine Species (Perciformes: Cichlidae).

To better understand the diversity and evolution of cichlids, we sequenced, assembled, and annotated the complete mitochondrial genomes of three Nile tilapiine species (Coptodon zillii, Oreochromis niloticus, and Sarotherodon galilaeus) dominating the Nile River waters. Our results showed that the general mitogenomic features were conserved among the Nile tilapiine species. The genome length ranged from 16,436 to 16,631 bp and a total of 37 genes were identified (two ribosomal RNA genes (rRNAs), 22 transfer RNA genes (tRNAs), 13 protein-coding genes (PCGs), and 1 control region). The ND6 was the only CDS that presented a negative AT skew and a positive GC skew. The most extended repeat sequences were in the D-loop followed by the pseudogenes (trnSGCU). The ND5 showed relatively high substitution rates whereas ATP8 had the lowest substitution rate. The codon usage bias displayed a greater quantity of NNA and NNC at the third position and anti-bias against NNG. The phylogenetic relationship based on the complete mitogenomes and CDS was able to differentiate the three species as previously reported. This study provides new insight into the evolutionary connections between various subfamilies within cichlids while providing new molecular data that can be applied to discriminate between Nile tilapiine species and their populations.

Aspects of the biology and population structure of Oreochromis niloticus, Coptodon zillii and Oreochromis leucostictus tilapia in Lake Naivasha, Kenya

AbstractThe Nile tilapia (Oreochromis niloticus), Redbelly tilapia (Coptodon zillii) and Blue spotted tilapia (Oreochromis leucostictus) were introduced into Lake Naivasha in the 1950s to diversify and boost the lake fisheries. These species have since been exploited in the commercial gillnet fishery. The fish stocks, however, are currently facing problems of pollution, wetland degradation and intense fishing pressures. Accordingly, the present study investigated some aspects of the biology and population structure of the three tilapiine species in light of changing lake conditions. Fish samples were collected from January to December 2019 using multifilament gillnets of 2.0‐to‐6.0‐inch mesh sizes. Immediately after their capture, the fish specimens were transported to the lab for morphometric measurements and analysis of maturity. The highest and lowest mean (±SD) total length (TL) were exhibited by O. niloticus (22.9 ± 5.2 cm) and C. zillii (14.5 ± 2.1 cm), respectively. Similarly, O. niloticus exhibited a higher mean weight (268.4 ± 18.0 g) and body depth (7.6 ± 2.5 cm), compared to the other species. O. niloticus exhibited an isometric growth pattern, while C. zillii and O. leucostictus exhibited negative allometric growth. The fish species were in good condition, with mean condition factors of 1.59 ± 0.07, 1.51 ± 0.06 and 1.23 ± 0.03 for C. zillii, O. leucostictus and O. niloticus, respectively. Furthermore, the condition factors for O. leucostictus and O. niloticus varied by size class, exhibiting a decreasing trend with increased fish size. The male:female sex ratio revealed significantly more males than females for O. niloticus (2.30:1.0) and O. leucostictus (2.36:1.0), although not for C. zillii (1.15:1.0). O. niloticus (28.0–29.0 cm TL) matured at a larger size, followed by O. leucostictus (21.0–26.0 cm TL), and C. zillii (15.0–17.0 cm TL). In comparison, values of length‐weight parameters, condition factor, sex ratio and size at first maturity of the studied fishes varied in Lake Naivasha, Victoria, Albert and George. The gillnet mesh ≤3.0″ mainly targeted small‐sized immature fish, while mesh ≥4″ targeted mature fish, particularly O. niloticus. It was concluded that gillnets with ≥4.0″ mesh could safely be applied for this fishery in Lake Naivasha, whereas the use of smaller mesh should be controlled to protect the juveniles as a means of maintaining sustainable fisheries. Furthermore, the present study also provided useful information, including size at maturity that is vital for setting mesh size for managing the lake fishery.

Genome Editing Using the CRISPR-Cas9 System to Generate a Solid-Red Germline of Nile Tilapia (Oreochromis niloticus).

In recent years, there has been increasing demand for red tilapia, which are commercial strains of hybrids of different tilapiine species or red variants of highly inbred Nile tilapia. However, red tilapia phenotypes are genetically unstable and affected by environmental factors, resulting in nonuniform coloration with black or dark-red color blotches that reduce their market value. Solute carrier family 45 member 2 (SLC45A2) is a membrane transporter that mediates melanin biosynthesis and is evolutionarily conserved from fish to humans. In the present study, we describe the generation of a stable and heritable red tilapia phenotype by inducing loss-of-function mutations in the slc45a2 gene. For this purpose, we identified the slc45a2 gene in Nile tilapia and designed highly specific guide RNAs (gRNA) for its genomic sequence. Multiplex microinjection of slc45a2-specific ribonucleoproteins to Nile tilapia zygotes induced up to 97-99% albinism, including loss of melanin in the eye. Next-generation sequencing of the injected zygotes demonstrated that all injected fish carried mutant alleles with variable mutagenesis efficiencies. Sanger sequencing of the genomic target region in the slc45a2 gene from fin clips, sperm, and F1 offspring of a highly mutant male identified various genomic indels and germline transmission of the sperm-identified indels. Overall, this work demonstrates the generation of somatic and germline slc45a2 mutant alleles, which leads to complete albinism in Nile tilapia.

Application of microsatellite genotyping by amplicon sequencing for delimitation of African tilapiine species relevant for aquaculture

The introduction of new genetic lineages into a population promotes interbreeding or hybridization and can be associated with the loss of genetic adaptation. This may contribute to financial losses for fish farmers following investments into fish strains that do not perform to their expectations. Oreochromis niloticus is one of the tilapiine species that has been widely distributed in areas far from its native range mainly for aquaculture purposes. This is principally due to its fast growth rates, adaptability to new habitats in addition to attractive market prices. Oreochromis niloticus has also been previously associated with the loss of native tilapiine species such as O. variabilis and O. esculentus in Lake Victoria and other lakes in Africa. To understand the impacts of O. niloticus translocation on native fish stocks, its species borders need to be delimited. Here we use microsatellite genotyping by amplicon sequencing (SSR-GBAS) for species delimitation of O. niloticus and seven related species from different regions in East and West Africa obtained from natural and farm environments. In total, 47 markers previously developed for O. niloticus were tested for cross-species amplification resulting in thirty-eight working markers. Most species were represented by different genetic units with little admixture between them even if ocuring in the same waterbodies. Exceptions involved the farmed populations from O. niloticus and O. mossambicus, which may be indicative of hybrid introgression. Our approach did also detect lower genetic diversity on the critically endangered species O. variabilis and O. esculentus. The possibility of introgressive hybridisation between O. niloticus and some of its congenerics may affect the sustainability of its exploitation. Moreover, long-term projects to monitor the response of genetic diversity and species boundaries to anthropogenic activities are required. In this context, SSR-GBAS can be a powerful tool given its high sample throughput and higher reproducibility compared to other methods.

Molecular genetic diversity and differentiation of Nile tilapia (Oreochromis niloticus, L. 1758) in East African natural and stocked populations

BackgroundThe need for enhancing the productivity of fisheries in Africa triggered the introduction of non-native fish, causing dramatic changes to local species. In East Africa, the extensive translocation of Nile tilapia (Oreochromis niloticus) is one of the major factors in this respect. Using 40 microsatellite loci with SSR-GBS techniques, we amplified a total of 664 individuals to investigate the genetic structure of O. niloticus from East Africa in comparison to Ethiopian and Burkina Faso populations.ResultsAll three African regions were characterized by independent gene-pools, however, the Ethiopian population from Lake Tana was genetically more divergent (Fst = 2.1) than expected suggesting that it might be a different sub-species. In East Africa, the genetic structure was congruent with both geographical location and anthropogenic activities (Isolation By Distance for East Africa, R2 = 0.67 and Uganda, R2 = 0.24). O. niloticus from Lake Turkana (Kenya) was isolated, while in Uganda, despite populations being rather similar to each other, two main natural catchments were able to be defined. We show that these two groups contributed to the gene-pool of different non-native populations. Moreover, admixture and possible hybridization with other tilapiine species may have contributed to the genetic divergence found in some populations such as Lake Victoria. We detected other factors that might be affecting Nile tilapia genetic variation. For example, most of the populations have gone through a reduction in genetic diversity, which can be a consequence of bottleneck (G-W, < 0.5) caused by overfishing, genetic erosion due to fragmentation or founder effect resulting from stocking activities.ConclusionsThe anthropogenic activities particularly in the East African O. niloticus translocations, promoted artificial admixture among Nile Tilapia populations. Translocations may also have triggered hybridization with the native congenerics, which needs to be further studied. These events may contribute to outbreeding depression and hence compromising the sustainability of the species in the region.

A review on the status of some major fish species in Lake Victoria and possible conservation strategies

AbstractLake Victoria, the second largest freshwater lake in the world, supports an enormous flora and fauna biomass, with a large human population around the Lake. The lake is a source of food (fish), water for domestic use and crop and livestock production, transportation and tourism, among other uses. As a result of these varied uses, human intervention within the lake and its catchment has resulted in several ecological changes in the lake in recent years, with profound effects on its fish resources. One of the most notable changes is the reduction and risk of extinction of some fish species, including the endemic tilapiine species (Oreochromis esculentus and Oreochromis variabilis), catfishes (Xenoclarias eupogon), haplochromines and cyprinids (Labeo victorianus and Barbus altinialis) in the lake. The reduced biodiversity, as well as extinctions within the lake, has been attributed to the introduction of alien fish species (e.g. Nile perch (Lates niloticus) and Nile tilapia (Oreochromis niloticus) into the lake, habitat loss and cultural eutrophication. If left unchecked, these changes will have devastating effects on the lake's resources, as well as to people living around the lake and beyond who depend on the lake fisheries for their livelihood. Thus, there is an urgent need for management measures based on sound scientific research to be implemented in order to curb the loss of ichthyodiversity within the lake, as well as to save the livelihoods of those stakeholders who are either directly or indirectly dependent on the lake.

Effects of the acclimation to high salinity on intestinal ion and peptide transporters in two tilapia species that differ in their salinity tolerance

Tilapiine species, widely distributed across habitats with diverse water salinities, are important to aquaculture as well as a laboratory model. The effects of water salinity on two tilapia species, that differ in their salinity tolerance, was evaluated. Oreochromis niloticus reared in brackish-water, showed a significant decrease in growth and feed efficiency, whereas O. mossambicus reared in seawater did not show any significant changes. The expression and activity of Na+/K+-ATPase (NKA), V-type H+-ATPase (VHA) and carbonic anhydrase (CA), as well as expression levels of genes encoding two HCO3− and three peptide transporters (nbc1, slc26a6, slc15a1a, slc15a1b and slc15a2) were measured in three intestinal sections of these two species, grown in freshwater and brackish/sea-water. Overall, the spatial distribution along the intestine of the genes examined in this study was similar between the two species, with the exception of tcaIV. The salinity response, on the other hand, varied greatly between these species. In O. mossambicus, there was a salinity-dependent increased expression of most of the examined genes (except slc26a6 and slc15a2), while in O. niloticus the expression of most genes did not change, or even decreased (tcaIV, nbc1 and slc15a1b). This study highlighted differences in the intestinal response to salinity acclimation between closely- related species that differ in their salinity tolerance. O. mossambicus, which has a high salinity tolerance, showed expression patterns and responses similar to marine species, and differed from the low-salinity-tolerance O. niloticus, which showed a response that differed from the accepted models, that are based on marine and diadromous fishes.

Molecular characterization of sdf1 and cxcr4 in the Mozambique tilapia, Oreochromis mossambicus

Animal sexual reproduction relies on primordial germ cells (PGCs), the predecessors of the germ cell lineage, giving rise to either spermatogonia or oogonia after the completion of gonadal differentiation. There is limited information on the mechanism of PGC migration leading to the formation of the primordial gonad in Perciform fish. Oreochromis mossambicus, a tilapiine species, was investigated that is a commercially important aquaculture species in many parts of the world while in other areas it has become an invasive pest. Key components involved in PGC migration were identified, including the stromal-cell derived factor 1 (Om-sdf1a, Om-sdf1b) and the CXC receptor 4 (Om-cxcr4): both share conservation with existing model species. The spatial gene expression profiles were determined through transcript and protein analysis and displayed distinct localisation within the region of the developing gonad in larvae and within the adult gonads of certain cell populations. A recombinant Om-sdf1a was produced in Escherichia coli that activates Om-cxcr4 using a BRET-based yeast in vitro assay system, suggesting that it is structurally similar to the native Om-sdf1a and is appropriate for further structural studies. This study has improved understanding of the molecular basis of tilapia reproduction through investigation of gonad development, which may be important in the progression towards reproductive suppression methods to control tilapia populations in the wild. In addition, this research will facilitate developments in germ cell transplantation, an innovative technique that harnesses germ cell migration and allows the uptake of foreign germ cells, which differentiate to produce sperm or ova.

Intestinal transcriptome analysis revealed differential salinity adaptation between two tilapiine species

Tilapias are a group of freshwater species, which vary in their ability to adapt to high salinity water. Osmotic regulation in fish is conducted mainly in the gills, kidney, and gastrointestinal tract (GIT). The mechanisms involved in ion and water transport through the GIT is not well-characterized, with only a few described complexes. Comparing the transcriptome of the anterior and posterior intestinal sections of a freshwater and saltwater adapted fish by deep-sequencing, we examined the salinity adaptation of two tilapia species: the high salinity-tolerant Oreochromis mossambicus (Mozambique tilapia), and the less salinity-tolerant Oreochromis niloticus (Nile tilapia). This comparative analysis revealed high similarity in gene expression response to salinity change between species in the posterior intestine and large differences in the anterior intestine. Furthermore, in the anterior intestine 68 genes were saltwater up-regulated in one species and down-regulated in the other species (47 genes up-regulated in O. niloticus and down-regulated in O. mossambicus, with 21 genes showing the reverse pattern). Gene ontology (GO) analysis showed a high proportion of transporter and ion channel function among these genes. The results of this study point to a group of genes that differed in their salinity-dependent regulation pattern in the anterior intestine as potentially having a role in the differential salinity tolerance of these two closely related species.

Recent changes in fish communities of the equatorial Lake Naivasha, Kenya

Changes in fish species composition and abundance in Lake Naivasha were investigated monthly for a period of 12 months. Data were obtained both from experimental surveys as well as commercial fish landings. The structure metrics considered included species abundance and composition, length- frequency and physico-chemical attributes. Results of the study showed that the fishery is currently dominated by the recently introduced common carp,Cyprinus carpio accounting for over 95% of the total fish landings, with the naturalized fish populations performing dismally both in terms of abundance as well as health conditions. The entry of the common carp into Lake Naivasha may have completely transformed the lake’s ecosystem integrity, driving the initially abundant tilapiine species to near local extinction. The study also confirms a restricted distribution of Micropterus salmoides to rocky-bottomed habitats of Hippo Point and Crescent Lake. Conversely, the long-fin barb, Barbus paludinosus,initially confined to the north swamp and the mouth of River Malewa, currently exhibits a lake-wide distribution. The changing climatic conditions as manifested in the lake level recessions may be a real threat to the fish stocks in the lake. Human activities in the catchment areas may also have contributed to the recent ecological changes witnessed in Lake Naivasha. This study proposes a number of recommendations with a view to ensuring ecosystem integrity and consequent sustainable fisheries of Lake Naivasha. Key words: Ecosystem integrity, habitat preference, species introductions, sustainable fisheries, fish population, equatorial lake.

Mitochondrial and morphological variation of Tilapia zillii in Israel

BackgroundTilapia zillii is widespread in the East Levant inland aquatic systems as well as in artificial water reservoirs. In this study we explore the genetic and morphological variation of this widespread species, using mitochondrial control region sequences and meristic characters. We examine the hypothesis that T. zillii's population structure corresponds to the four Israeli aquatic systems.ResultsOut of seven natural water bodies, only two were found to possess genetically divergent populations of T. zillii. In addition to its presence in fish farms, the species was found in two artificial recreational ponds which were supposed to have been stocked only with other fish species. In these two artificial habitats, the haplotype frequencies diverged significantly from those of natural populations. Finally, fish from the Dead Sea springs of Ne'ot HaKikar appear to differ both genetically and morphologically from fish of the same aquatic system but not from fish of other water systems.ConclusionsOur results show that the population structure of T. zillii does not match the geography of the Israeli water-basins, with the exception of the Dead Sea and Kishon River, when considering natural populations only. The absence of a significant divergence between basins is discussed. Our results and observations suggest that the Ne'ot HaKikar Dead Sea population and those of artificial ponds could have originated from the "hitchhiking" of T. zillii, at the expense of some other cultivated tilapiine species.

Comparative time-course study on pituitary and branchial response to salinity challenge in Mozambique tilapia (Oreochromis mossambicus) and Nile tilapia (O. niloticus)

The physiological response of Mozambique and Nile tilapia transferred from fresh to brackish (15 ppt) water was compared during a one-week time course. Response in the pituitary was measured by the gene expression pattern of prolactin (PRL I), growth hormone (GH), and calcium-sensing receptor (CaSR), while the response in the gills was measured by the gene expression pattern of the prolactin receptor (PRL-R), Na(+)/K(+)/2Cl(-) cotransporter (NKCC) and Na(+)/Cl(-) cotransporter (NCC), and by activity and expression of Na(+)/K(+)-ATPase (NKA). The time-course curves of plasma osmolality levels indicate a rapid elevation 24 h after transfer, which later decreased and maintained at stable level. PRL I expression decreased in both species, but with stronger response in the Nile tilapia, while no differences were found in the slightly elevated levels of GH mRNA. The branchial response demonstrated a faster up-regulation of NKA and NKCC in the Mozambique tilapia, but similar levels after a week, while Nile tilapia had stronger and constant down-regulation of NCC. The time-course response of the measured osmoregulatory parameters indicate that 24 h after transfer is a critical time point for brackish-water adaptation. The differences in responses to saltwater challenge between Mozambique and Nile tilapia shown in this study may be associated with the differences in saltwater tolerance between these two tilapiine species.

The biology and management of Tilapia mariae (Pisces:Cichlidae) as a native and invasive species: a review

Tilapia mariae (Cichlidae) is a tropical teleost, native to West African coastal drainages in the Gulf of Guinea and naturalised in the USA, Australia and possibly Russia. This paper reviews published information on the species from its native and introduced range to improve the effective management of fisheries as well as manage current infestations, reduce further spread and promote research on control measures outside its natural range. Within its natural range, the species is abundant and contributes to the local subsistence and artisanal fisheries; however, it is not extensively utilised for aquaculture. Outside its native range, it has proliferated, particularly in disturbed aquatic ecosystems. Tilapia mariae has several characteristics that make it successful in both its natural and introduced range. These include plasticity in reproductive behaviour (particularly parental care of the brood), a variety of feeding behaviours and dietary components, tolerance of a wide range of environmental conditions and aggressive behaviour to other fish species. Although tilapiine species are considered to be amongst the world’s worst invasive alien species, surprisingly little information is available on potential impacts of T. mariae on native species and aquatic ecosystems outside its native range.

Acute salinity challenges in Mozambique and Nile tilapia: Differential responses of plasma prolactin, growth hormone and branchial expression of ion transporters

The responses of Mozambique and Nile tilapia acclimated to fresh water (FW) and brackish water (BW; 17‰) were compared following acute salinity challenges. In both species, plasma osmolality increased to above 450 mOsm by 2 h after transfer from FW to seawater (SW); these increases in osmolality were accompanied by unexpected increases in plasma prolactin (PRL). Likewise, PRL receptor gene expression in the gill also increased in both species. In Nile tilapia, hyperosmotic transfers (FW to BW and SW) resulted in increased plasma growth hormone (GH) and in branchial GH receptor gene expression, responses that were absent in Mozambique tilapia. Branchial gene expression of osmotic stress transcription factor 1 (OSTF1) increased in both species following transfer from FW to SW, whereas transfer from BW to SW induced OSTF1 expression only in the Nile tilapia. Branchial expression of Na +/Cl − cotransporter was higher in FW in both species than in BW. Branchial gene expression of Na +/K +/2Cl − cotransporter (NKCC) increased after transfer from BW to SW in Mozambique tilapia, whereas expression was reduced in the Nile tilapia following the same transfer. The difference in the SW adaptability of these species may be related to a limited capacity of Nile tilapia to up-regulate NKCC gene expression, which is likely to be an essential component in the recruitment of SW-type chloride cells. The differential responses of GH and OSTF1 may also be associated with the disparate SW adaptability of these two tilapiine species.

The Development and Validation of a Quantitative ELISA for in vivo and in vitro Synthesized Vitellogenin from Mossambicus tilapia (Oreochromis mossambicus)

The induction of vitellogenin (VTG), an egg yolk precursor found in most oviparious vertebrates, has been well established as a valuable biomarker for assessing estrogenic endocrine disruption. Oreochromis mossambicus is a tilapiine species indigenous to Southern Africa and, therefore, represents a potential African bio-indicator for the development of in vitro and in vivo screens for estrogenicity using VTG as biomarker. However, few ELISAs have been developed to quantify tilapia VTG protein levels. In the present study, commercially available VTG antiserum that shows cross reactivity with tilapia VTG was used to set up and validate a quantitative competition ELISA for tilapia VTG. This ELISA has a broad detection range between 80 ng/ml–5.4 µg/ml VTG and is able to detect both in vivo and in vitro synthesized VTG. This ELISA is highly repeatable with intra- and inter-assay variations less than 3.4% at the lowest detection limit.

Studies on the fisheries and biology of &lt;i&gt;Oreochromis urolepsis&lt;/i&gt; (Pisces: Cichilidae) in the Mtera resorvoir (Tanzania)

The fisheries and some aspects of the biology of Oreochromis urolepsis of the Mtera Reservoir (Tanzania) were examined in August 2002. It was observed that there was no proper management of the reservoir\'s fisheries. Beach seine fishing dominated the fishery. Nearly all the O. urolepsis caught by this type of gear were juveniles. The number of fish caught by the beach seines was very high, some times up to 4,000 fish per haul. The beach seines also destroy the spawning sites for the tilapias. Mean total lengths of O. urolepsis caught in gillnets of legal mesh sizes (ranging from 88 mm and 114 mm), ranged between 22.0 cm and 27 cm. Their mean weights were between 236 g and 433 g. Most of the fish caught were sexually immature. Ninety three per cent of the fish from the 88 mm gillnets were immature, while only 28% of those caught in the 114 mm mesh nets were mature. The smallest mature female had a total length of 24.0 cm. O. urolepsis is a very fecund tilapiine species. A female weighing 450 g was found to contain about 3250 eggs. It was also found that the species is a phytoplankton feeder, taking in diatoms, green and blue green algae. Tanzania Journal of Science Vol. 30 (2) 2004: pp. 33-40

DNA Barcoding of Israeli Indigenous and Introduced Cichlids

The objectives of this study were barcoding and taxonomic analysis of the five tilapiine species (Oreochromis aureus, O. niloticus, O. mossam- bicus, Sarotherodon galilaeus, and Tilapia zillii), two tilapia hybrid strains (Florida red tilapia and Philippine red tilapia), and two endemic wild cichlids (Tristramella simonis and Astatotilapia flaviijosephi) available in Israel, as well as O. urolepis hornorum. Cytochrome oxidase subunit I (COI) 619 bp sequence traces of 104 individuals were assembled, aligned, and compared (GenBank project GI 209553463). The DNA sequences of two hybrid strains were identical to those of O. hornorum and O. aureus. Absence of intra-specific variability was detected in the commercially used species, O. aureus, S. galilaeus, O. mossambicus, and O. urolepis horno- rum. Two DNA sequence variants were detected in O. niloticus originating from Ghana and Egypt. In contrast, 2-3 variants were detected in the DNA of each of the non-commercial species. Amino-acid sequences were identical in all “true tilapias” and different from the sequences in the endemic cichlids. As a whole, the protein phylogenetic tree fitted the expected conventional taxonomy as opposed to the respective DNA-based tree. Sequences FJ348047-FJ348150 were submitted to GenBank via the BOLD database (identical to FISH001-08 - FISH104-08 in this database).

Food web structure and mercury transfer in two contrasting Ugandan highland crater lakes (East Africa)

AbstractVolcanic crater lakes scattered throughout western Uganda are important local sources of water and fish. Two representative but contrasting crater lakes near the Kibale National Park were sampled in 2000; the hyper‐eutrophic Lake Saka, which is highly affected by agricultural practices, and the mesotrophic Lake Nkuruba that is still surrounded by intact forest. The food web structures in these two lakes were assessed using stable nitrogen (δ15N) and carbon (δ13C) isotope analyses, and the mercury (THg) transfer patterns were quantified. The δ15N results indicate that food webs in both lakes are abbreviated, with only one to two trophic levels from primary consumers. The Lake Saka biota had distinctively enriched δ13C values compared with those in Lake Nkuruba, which may be due to 12C‐limited phytoplankton blooms in this lake. In Lake Nkuruba, two introduced tilapiine species and the introduced guppy Poecilia reticulata fed predominantly upon invertebrates and decomposed terrestrial plant material. In Lake Saka, the introduced Nile perch Lates niloticus appeared to occupy the top trophic position, but stable isotope values of the endemic haplochromine cichlids exclude those as Nile perch prey items. THg was found to biomagnify through the food web, reaching highest concentrations in P. reticulata in Nkuruba, which tended to be higher than for L. niloticus in Saka, suggesting increased bioavailability of THg in Nkuruba. Maximum THg concentrations in fish never approached WHO recommended guidelines (200 ng g−1) designed to protect at‐risk groups.

The use of xenoantigenic antisera for the identification of tilapiine species: comparative laboratory and field studies

Summary Rapid and reliable identification of tilapiine taxa and strains is essential for selective breeding purposes and the conservation of natural genetic resources. There is evidence that antisera-mediated erythrocyte agglutination assays can fit these requirements. We evaluated the applicability of agglutination tests by studying the capacity of species characteristic antisera to recognize erythrocytes from individuals of 10 natural Ghanaian populations of Oreochromis niloticus and Sarotherodon melanotheron. The vast majority of the 218 tested individuals could be identified based on antisera-mediated erythrocyte recognition. Controls indicated the specificity of these reactions. Still, erythrocytes from 16% of all tested specimens did not respond to any antiserum (zero responders), indicating the possible existence of blood group properties in tilapias. We discuss the specificity of the antisera, the relevance of zero responses and the applicability of these tests in aquaculture and field studies.