Frans Witte

The destruction of an endemic species flock: quantitative data on the decline of the haplochromine cichlids of Lake Victoria

SynopsisThe Lake Victoria fish fauna included an endemic cichlid flock of more than 300 species. To boost fisheries, Nile perch (Lates sp.) was introduced into the lake in the 1950s. In the early 1980s an explosive increase of this predator was observed. Simultaneously, catches of haplochromines decreased. This paper describes the species composition of haplochromines in a research area in the Mwanza Gulf of Lake Victoria prior to the Nile perch upsurge. The decline of the haplochromines as a group and the decline of the number of species in various habitats in the Mwanza Gulf was monitored between 1979 and 1990. Of the 123+ species originally caught at a series of sampling stations ca. 80 had disappeared from the catches after 1986. In deepwater regions and in sub-littoral regions haplochromine catches decreased to virtually zero after the Nile perch boom. Haplochromines were still caught in the littoral regions where Nile perch densities were lower. However, a considerable decrease of species occurred in these regions too. It is expected that a remnant of the original haplochromine fauna will survive in the littoral region of the lake. Extrapolation of the data of the Mwanza Gulf to the entire lake would imply that approximately 200 of the 300+ endemic haplochromine species have already disappeared, or are threatened with extinction. Although fishing had an impact on the haplochromine stocks, the main cause of their decline was predation by Nile perch. The speed of decline differed between species and appeared to depend on their abundance and size, and on the degree of habitat overlap with Nile perch. Since the Nile perch upsurge, the food web of Lake Victoria has changed considerably and the total yield of the fishery has increased three to four times. Dramatic declines of native species have also been observed in other lakes as a result of the introduction of alien predators. However, such data concern less speciose communities and, in most cases, the actual process of extinction has not been monitored.

Biodiversity and Fishery Sustainability in the Lake Victoria Basin: An Unexpected Marriage?

Abstract Lake Victoria is Africas single most important source of inland fishery production. After it was initially fished down in the first half of the 20th century, Lake Victoria became home to a series of introduced food fishes, culminating in the eventual demographic dominance of the Nile perch, Lates niloticus. Simultaneously with the changes in fish stocks, Lake Victoria experienced dramatic changes in its ecology. The lake fishery during most of the 20th century was a multispecies fishery resting on a diverse lake ecosystem, in which native food fishes were targeted. The lake ended the century with a much more productive fishery, but one in which three species—two of them introduced—made up the majority of the catch. Although many fish stocks in Lake Victoria had declined before the expansion of the Nile perch population, a dramatic increase in the population size of Nile perch in the 1980s roughly coincided with the drastic decline or disappearance of many indigenous species. Now, two decades after the rise of Nile perch in Lake Victoria, this species has shown signs of being overfished, and some of the native species that were in retreat—or even thought extinct—are now reemerging. Data on the resurgence of the indigenous species suggest that heavy fishing of Nile perch may enhance biodiversity; this has spawned renewed interest in management options that promote both fishery sustainability and biodiversity conservation.

Species Extinction and Concomitant Ecological Changes in Lake Victoria

Lake Victorias fish fauna included a large endemic flock of 300+ haplochromine cichlid species. About two-thirds of these species have disappeared or are threatened with extinction. The main cause of this large extinction event is predation by Nile perch, an introduced predator. We describe the decline of the haplochromine species and demonstrate that the rate and sequence of their decline was determined by their relative abundance, their adult size and their habitat overlap with Nile perch. Many non-haplochromine species declined as well, but in contrast, stocks of the native pelagic cyprinid Rastrineobola argentea and the introduced Oreochromis niloticus increased. There are also indications of an increase in phytoplankton, macrophytes, prawns and benthic organisms. Many of these rapid changes in the ecosystem were probably effects of the increase of the Nile perch and the disappearance of the haplochromines. The original fish fauna included many primary and secondary consumers. Currently secondary and tertiary consumers dominate. The food web in the sub-littoral and offshore areas of the lake changed considerably due to the stock replacements.

Evolutionary origin and development of snake fangs

Many advanced snakes use fangs—specialized teeth associated with a venom gland—to introduce venom into prey or attacker. Various front- and rear-fanged groups are recognized, according to whether their fangs are positioned anterior (for example cobras and vipers) or posterior (for example grass snakes) in the upper jaw. A fundamental controversy in snake evolution is whether or not front and rear fangs share the same evolutionary and developmental origin. Resolving this controversy could identify a major evolutionary transition underlying the massive radiation of advanced snakes, and the associated developmental events. Here we examine this issue by visualizing the tooth-forming epithelium in the upper jaw of 96 snake embryos, covering eight species. We use the sonic hedgehog gene as a marker, and three-dimensionally reconstruct the development in 41 of the embryos. We show that front fangs develop from the posterior end of the upper jaw, and are strikingly similar in morphogenesis to rear fangs. This is consistent with their being homologous. In front-fanged snakes, the anterior part of the upper jaw lacks sonic hedgehog expression, and ontogenetic allometry displaces the fang from its posterior developmental origin to its adult front position—consistent with an ancestral posterior position of the front fang. In rear-fanged snakes, the fangs develop from an independent posterior dental lamina and retain their posterior position. In light of our findings, we put forward a new model for the evolution of snake fangs: a posterior subregion of the tooth-forming epithelium became developmentally uncoupled from the remaining dentition, which allowed the posterior teeth to evolve independently and in close association with the venom gland, becoming highly modified in different lineages. This developmental event could have facilitated the massive radiation of advanced snakes in the Cenozoic era, resulting in the spectacular diversity of snakes seen today.

Recovery of cichlid species in Lake Victoria: an examination of factors leading to differential extinction

234 Bottom trawl catches in the northern part of the Mwanza Gulf Fish landings of nocturnal light fishery in the Speke Gulf Discussion 236 Effects of predator decline Predictions about survival and recovery of haplochromines Potentials of comparative studies on closely related species Possible effects of the recovery of zooplanktivorous haplochromines

Transcriptome analysis of the response to chronic constant hypoxia in zebrafish hearts

Insufficient blood supply during acute infarction and chronic ischemia leads to tissue hypoxia which can significantly alter gene expression patterns in the heart. In contrast to most mammals, some teleost fishes are able to adapt to extremely low oxygen levels. We describe here that chronic constant hypoxia (CCH) leads to a smaller ventricular outflow tract, reduced lacunae within the central ventricular cavity and around the trabeculae and an increase in the number of cardiac myocyte nuclei per area in the hearts of two teleost species, zebrafish (Danio rerio) and cichlids (Haplochromis piceatus). In order to identify the molecular basis for the adaptations to CCH, we profiled the gene expression changes in the hearts of adult zebrafish. We have analyzed over 15,000 different transcripts and found 376 differentially regulated genes, of which 260 genes showed increased and 116 genes decreased expression levels. Two notch receptors (notch-2 and notch-3) as well as regulatory genes linked to cell proliferation were transcriptionally upregulated in hypoxic hearts. We observed a simultaneous increase in expression of IGF-2 and IGFbp1 and upregulation of several genes important for the protection against reactive oxygen species (ROS). We have identified here many novel genes involved in the response to CCH in the heart, which may have potential clinical implications in the future.

Phenotypic Plasticity of Anatomical Structures and Its Ecomorphological Significance

Ecomorphologists have thus far paid little attention to phenotypic plasticity of anatomical structures. In this paper we present four cases of phenotypic plasticity in African cichlids, which we suppose to be adaptive responses to environmental changes. On the basis of these cases we hypothesize that the response of a plastic anatomical structure to an environmental change may be constrained by surrounding structures. Alternatively, the surrounding structures may be affected by a plastic structure which changed as a direct response to environmental alterations. We also analyze the morphological and ecological implications of the potentials of growth retardation and accelaration on the phenotypic plasticity in organisms. The flexibility of the reaction norm of an organism may depend on these potentials of heterochrony.

An Introduction To Ecological and Taxonomic Investigations On the Haplochromine Cichlids From the Mwanza Gulf of Lake Victoria

The ecology and taxonomy of the haplochromine cichlids from the Mwanza Gulf of Lake Victoria (East Africa) have been studied by the Haplochromis Ecology Survey Team (HEST). The research area and the fishing techniques are described as basic information for following publications. Taxonomic studies resulted in the recognition of 150+ new species. The results of the ecological research on these species-including factors which influence species distribution, breeding and migration-are summarized.

Initial Results of the Ecological Survey of the Haplochromine Cichlid Fishes From the Mwanza Gulf of Lake Victoria (Tanzania): Breeding Patterns, Trophic and Species Distribution

Ecological investigations on the haplochromine Cichlidae from the Mwanza Gulf of Lake Victoria revealed that more than 140 species are living in the habitats fished with bottom trawlers (i.e. all major habitats except rocky bottoms, dense plant stands and very shallow water). These species can be classified into 9 trophic groups. The phytoplankton/detritus eaters and the zooplanktophages (both belonging to the smaller species) form the dominant group in the sublittoral waters with a mud bottom, while insect eaters dominate in shallower water, especially over sand. Most haplochromine species appear to be strongly habitat-restricted throughout life. Breeding of the bulk of the haplochromine species (phytoplankton/detritus eaters, zooplanktophages and some insectivores) in the sublittoral and littoral waters with a mud bottom appears to be seasonal. Spawning takes place at the end of the rainy season. However, some species of the littoral sand habitat breed throughout the year. A discussion is given of the factors by which the seasonal breeding pattern might be imposed. For an optimal trawl fishery recommendations based on the present data are discussed.

Species Distinction and the Biodiversity Crisis in Lake Victoria

Abstract Until the 1970s, the fish fauna of Lake Victoria in East Africa was dominated by about 500 endemic haplochromine cichlid species, which comprised about 80% of the demersal fish mass. The cichlids were extremely diverse ecologically; however, the small diversity in gross morphology and the presence of intraspecific variation made it difficult to distinguish among species. In the first half of the 1980s, the Nile perch Lates niloticus, an introduced predator, suddenly boomed and cichlids declined dramatically. During the same period eutrophication increased strongly. With the decline of Nile perch catches in the 1990s, the cichlids showed some recovery. These events have triggered many studies and debates. Disagreements about the severity and causes of the decline often stemmed from considering the cichlid flock as a single unit owing to the lack of proper taxonomic and ecological knowledge. By studying cichlid communities, trophic groups, and individual species, researchers uncovered differential ...