The feeding habits of a group of herbivorous rock-dwelling cichlid fishes (Cichlidae: Perciformes) from Lake Malawi,Africa

Synopsis The feeding; habits of a group of tropical herbivorous rock-dwelling cichlid fishes from Lake Malawi, Africa, are investigated using stomach content analyses. The various species fed selectively on the periphyton of the rocky shores. Blue-green alga of the genus Calothrix was the most common item ingested by the group. Diatoms (Chrysophyta) also were abundant food items. Discriminant analysis showed that dietary items were good variables to identify species. Interspecific dietary differences showed a continuum from those species feeding primarily on Calothrix to those feeding primarily on diatoms. Algal resources exhibit distinct patterns of spatial variation. Diet was correlated with foraging behavior and trophic morphology. Interspecific differences in diet could possibly facilitate ecological coexistence among various species. Such coexistence would contribute to the maintenance of the high diversity fish faunas characteristic of the Great Rift Lakes of Africa.     

A revision of the genus Labidochromis (Teleostei: Cichlidae) from Lake Malawi

The diagnosis of the genus Labidochromis is revised to include species with bicuspid outer teeth. Sixteen species, of which 13 are new, are described and illustrated and the validity of five others is discussed. The interrelationships between members of the genus and the relationship of the genus to other genera are discussed. No clearly defined sister group to the genus has been found either within or outside the Lake Malawi cichlid species flock. A key based on the coloration of known species is included.     

Description of a commercially important pelagic species of the genus Diplotaxodon (Pisces: Cichlidae) from Lake Malawi, Africa

A new species of pelagic haplochromine cichlid of the genus Diplotaxodon is described from Lake Malawi, Africa. The species, which attains a size of 14cm s. L., feeds mainly on zooplankton, shows no sexual dimorphism in size or body proportions, breeds throughout the year and lives at depths of 20 to at least 120 m. Approximately 600 tonnes of the species were caught by trawlers in 1990–1991, in the south–eastern arm of the lake alone. It appears that this species is a major component of the pelagic community, and occupies an ecological role similar to that of the clupeid Limnothrissa miodon in Lake Tanganyika.     

Midwater spawning inHaplochromis chrysonotus (Boulenger) (Teleostei: Cichlidae) in Lake Malawi

Synopsis Haplochromis chrysonotus, a semipelagic shoaling cichlid from Lake Malawi, has been observed spawning in open water up to ten metres above the substratum. It is suggested that open water spawning without the establishment of a substratum-based territory eliminates competition with other species for breeding space and that it may be a stage in the evolution of a totally pelagic mode of life.     

Morphological analyses of the neurocranium of a group of rock-dwelling cichlid fishes (Cichlidae: Perciformes) from Lake Malawi,Africa

Variation in the size and the shape of neurocrania of a group of rock-dwelling cichlid fishes (mbuna; from Lake Malawi, Africa was analysed using principal components and regression analyses on a set of 24 neurocranial measurements of 86 individuals from 11 species. The results reveal effective morphological discrimination between species. Most of the structural variation between species demonstrates positive allometry and represents size-dependent species differences. Thus, morphological differentiation in these fish may be largely the result of heterochronic changes in relative growth rates. The vomerine region, however, exhibited shape differentiation. Distinct groupings, based on vomer shape, are identified. Foraging behaviour and diet are found to be correlated with this region of the neurocrania. The correlation of the shape of the vomer and fine-scale resource partitioning suggests that this structure may be important in the evolutionary success of the group. Also, most structural shape variation in neurocranial morphology is associated with structural variation in buccal jaws.     

Feeding-site utilization in three sympatric species of Petrotilapia (Pisces, Cichlidae) from Lake Malawi

Three sympatric sibling species of Petrotilapia , with similar dietary and macrohabitat requirements, occur near Monkey Bay, Lake Malawi. Males and females of all three species fed selectively on the rocky shores. Males utilized less than 6% of the space they defended as a breeding territory for feeding and although interspecific overlap of territories was considerable, only 2.5% of the feeding space used by territorial males was shared. Males primarily utilized rich food patches which occurred within the territories of highly aggressive Pseudotropheus species, whereas females tended to feed in undefended areas. Although Petrotilapia species are not highly aggressive fishes, there was a well-defined social dominance hierarchy amongst them and it is suggested that feeding-site utilization is dependent on social rank. Inter-and intraspecific differences in feeding-site utilization probably facilitate species coexistence and thus contribute to the maintenance of the high species diversity which is a characteristic of the ichthyofauna of Lake Malawi.     

Factors in habitat partitioning in Pseudotropheus aurora (Pisces: Cichlidae), an introduced species to a species-rich community of Lake Malawi

The territorial behaviour of a small rock-dwelling cichlid, Pseudotropheus aurora , was studied at Thumbi Island West at the southern end of Lake Malawi. P. aurora is one of several species that have been introduced to an already diverse fish community. Many of the endemic and introduced species are highly territorial. The coexistence of an introduced territorial species within a speciesrich community of other territorial species is examined. Territories from which P. aurora individuals were removed were invariably re-occupied by conspecifics. Similarly, when all territories of a certain area were emptied of all territory owners of any existing species, the former P. aurora territories were re-occupied by conspecifics.
The possible roles of site specificity, competitive exclusion and behaviourally transmitted traditions regarding territories are discussed.     

A sympatric sibling species complex of Petrotilapia Trewavas from Lake Malawi analysed by enzyme electrophoresis (Pisces: Cichlidae)

The protein electromorphs of three taxa of Petrotilapia , referred to in earlier works as sibling species, were examined by starch-gel electrophoresis. The results indicated that, for the three taxa, no alternative fixed alleles occurred for any of the 25 loci (enzymes) examined. However, heterogeneous gene frequencies were indicated at seven polymorphic loci. This suggests that either the taxa are isolated 'sibling' species which recently diverged, or that they are 'incipient' species with minimal gene flow between morphs. The genetic and field data lend support to the hypothesis that sympatric splitting of morphs could be important in the explosive radiation of the cichlidae.     

Feeding habits of the clupeid Limnothrissa miodon (Boulenger), in Lake Kivu

A limnological sampling programme implemented during 1980 in Lake Kivu, Rwanda, confirmed a seasonal increase of the pelagic plankton biomass at the end of the dry season during the windy period of August/September. From plankton samples taken in the littoral and pelagic zones of the Lake it was concluded that a successful introduction of the freshwater clupeid Limnothrissa miodon Boulenger, during 1958–1960, had not obviously influenced the species composition of the Copepoda in the Lake. Examination of stomach contents of different length classes of Limnothrissa proved that the Kivu clupeid is not a strict planktonphagic species. Its diet is adapted to its life cycle and inshore/offshore migration patterns. In pelagic waters of the lake it is an exclusive plankton feeder, while in littoral waters it has more littoral bound feeding habits. Cannibalism was observed among individuals larger than 100 mm, near the margins of the lake. Echosoundings and periodic observations on stomach contents support the hypothesis that Limnothrissa feeds in the late afternoon and digests its food during the night, while a second feeding/digestion cycle starts in the early morning, at sunrise.     

Recruitment of juvenile mbuna (Pisces: Cichlidae) to experimental rock shelters in Lake Malawi,Africa

Synopsis Mbuna are species of small cichlids that inhabit the rocky shoreline of Lake Malawi. They are mouthbrooders and do not guard their young. Recruitment to the community first occurs when the female deposits free-swimming young, approximately 11 mm standard length, among the rocks. Juvenile mbuna recruit to any refuge in the rock habitat and in this study, small rock shelters were monitored daily to follow the recruitment and subsequent persistence of young. Initial mortality rate was high; less than 10% of the fish survived more than 3 weeks after deposition by the female. At the two locations studied, recruitment was greater at West Thumbi Island (11 fish per day) than at Otter Point (8 fish per day). However, fewer recruits persisted as residents at Thumbi Island because mbuna young at Thumbi were capable of excluding subsequent recruits to their shelter. Similar priority effects were not observed at Otter Point. Mbuna lack pelagic stages in the life-cycle and the species composition of local adult assemblages is an important determinant of the number and type of subsequent recruits. Priority effects may help maintain stochastic influences on recruitment success and reduce the importance of deterministic processes that are directly linked to community structure.     

Sexual selection,reproductive behavior,and speciation in the mbuna species flock of Lake Malawi (Pisces: Cichlidae)

Synopsis The cichlid species flocks of the African Great Lakes represent the most extreme case of adaptive radiation among vertebrates. Recently, attention has focused on the potential for sexual selection to drive or accelerate speciation in these fishes. Cichlids as a whole are social in nature and display complex behavior, particularly during courtship and spawning; however, the extent to which changes in species recognition cues may account for species diversity among haplochromine lineages has remained speculative. Our investigations have indicated that oral incubating haplochromines show a reduction in diversity and extent of courtship relative to substrate brooding cichlids, and apparently retain aspects of a primitive specific mate recognition system. Laboratory observations of courtship in the Malawian endemicPseudotropheus zebra suggest that organization of the spawning bout is loose, and lacking in any well defined stimulus-response chain. Interspecific comparisons of in situ courtship behavior among male mbuna, lithophilous haplochromines of Lake Malawi, revealed only one potential example of species-specific behavior, and indicated that mate choice occurred prior to the onset of intense courtship. Courtship display by male mbuna does not appear critical to species recognition and may represent an evolutionary relict. Alternatively, male courtship display may have an indirect effect on successful reproduction or may be maintained through pleiotropic effects. The mbuna display no evidence of behavioral innovation and show limited interspecific differentiation in behavioral expression. More generally, there is no evidence at present to suggest that epigamic sexual selection, acting on courtship behavior, has been a major mechanism in the diversification of the haplochromine species flocks.     

Morphological analyses of the neurocranium of a group of rock-dwelling cichlid fishes (Gichlidae: Perciformes) from Lake Malawi, Africa

Variation in the size and the shape of neurocrania of a group of rock-dwelling cichlid fishes (mbuna; from Lake Malawi, Africa was analysed using principal components and regression analyses on a set of 24 neurocranial measurements of 86 individuals from 11 species. The results reveal effective morphological discrimination between species. Most of the structural variation between species demonstrates positive allometry and represents size-dependent species differences. Thus, morphological differentiation in these fish may be largely the result of heterochronic changes in relative growth rates. The vomerine region, however, exhibited shape differentiation. Distinct groupings, based on vomer shape, are identified. Foraging behaviour and diet are found to be correlated with this region of the neurocrania. The correlation of the shape of the vomer and fine-scale resource partitioning suggests that this structure may be important in the evolutionary success of the group. Also, most structural shape variation in neurocranial morphology is associated with structural variation in buccal jaws.     

A new genus and species of Macroderoididae, and other digeneans from fishes of Lake Malawi, Africa

Malawitrema staufferi n. gen., n. sp., an unusual digenean, is described from Clarias mossambicus (type host) and Bagrus meridionalis from Lake Malawi. It has a small, pyriform body, with a spinous tegument. The ceca are relative short, not reaching to the testes. The 2 testes are symmetrical in the middle hind body. The cirrus sac is long and narrow, reaching into hind body. The genital pore is median, immediately anterior to the ventral sucker. The ovary is pretesticular, and a canalicular seminal vesicle and Laurer's canal are present. The uterus usually reaches distinctly posteriorly to testes. The eggs are small. The follicular vitellarium is in 2 small fields just anterior to testes. The ventrally subterminal excretory pore leads to a claviform vesicle. This species does not fit clearly into any known family of digeneans and is placed in the Macroderoididae as a temporary measure. Other digeneans reported from Lake Malawi include Haplorchoides cahirinus (Looss, 1896) in C. mossambicus and B. meridionalis; Astiotrema turneri Bray, van Oosterhout, Blais & Cable, 2006 in Protomelas annectens, P. cf. taeniolatus, Labeotropheus fuelleborni, Ctenopharynx (Otopharynx) pictus, and Pseudotropheus zebra; Glossidium pedatum Looss, 1899 in C. mossambicus and B. meridionalis; and an unidentifiable sanguinicolid from Bathyclarias nyasensis.     

Local adaptations in populations of rock-dwelling haplochromines (Pisces:Cichlidae) from southern Lake Victoria

The radiation of cichlid fishes in the African great lakes is often described as adaptive, because, at a superficial level, cichlid fishes seem adapted to the ecological niches they occupy. However, adaptedness has rarely been studied. We''ve investigated to what extent island populations of three species of the rock-dwelling genus Neochromis, endemic to Lake Victoria, are adapted anatomically to exploit locally abundant resources. Specifically, we asked whether different resource environments were reflected in differences in the feeding apparatus, both within species and between species. In populations of two specialized biters, the algae scrapers N. rufocaudalis and N. omnicaeruleus, the biting force of the lower jaw increased with increasing amount of items that require biting in the diet. N. greenwoodi is a less specialized biter; we found differences between two populations in the hyoid position and in the premaxilla that enhance suction feeding. These adaptations were related to the amount of items requiring suction. Comparing across three sympatric pairs of species, in each case different diets were reflected in differences in anatomy.     

The food of Bagrus docmac (Forsk.) (Pisces: Siluriformes) and its relationship with Haplochromis Hilgendorf (Pisces: Cichlidae), in Lake Victoria, East Africa

The diet of the siluriform fish Bagrus docmac (Forsk.) is described in relation to various localities in Lake Victoria, East Africa. Invertebrates which are the principal food of young fish, but generally of little significance in adults, are particularly poorly represented in fishes from Tanzanian waters and most important in those from the Kavirondo Gulf where the ontogenetic changes in diet differ from those found elsewhere in the Lake. B. docmac starts feeding piscivorously at about 10 cm standard length with the transition from a principally invertebrate to a primarily piscivorous diet occurring at 15 cm S.L. In adult B. docmac fish are found in over 95 % of individuals with food. Haplochromis is the all important food type, and Tilapia , the most valuable commercial fish in the lake, is not preyed upon at all. Bathymetric changes with respect to the invertebrate food are noted, and it is shown that piscivorous feeding occurs mainly by day and invertebrate feeding by night. Quantitative estimates of consumption of Haplochromis by B, docmac are computed from the results of a series of 24 h fishing surveys and found to exceed published data from elsewhere. The formula for the computation of a daily ration is given as Log R =2.973 Log L —3.36089 and it is calculated that the mean annual consumption of Haplochromis by B. docmac is approximately 75% of the mean standing stock of the prey fish, as compared with a commercial catch of about 5% of the mean standing stock.     

Rhopalotrema (Digenea: Didymozoidae), a new genus from Australian platycephalid fishes

Rhopalotrema n. g. is erected to accommodate two new species of didymozoid collected from platycephalid fishes from South-east Queensland, Australia and is assigned to the subfamily Didymozoinae. Rhopalotrema elusiva n. sp. is described from the fins of Platycephalus fuscus and R. pterygionastes n. sp. is described from the fins of P. indicus. The new genus is similar to Neometadidymozoon, but is distinguished from it by the structure of the testes, which are convoluted and extend from near the anterior end of the hindbody to a point only a short distance into the forebody. The two new species are distinguished from each other by their size and the size and shape of the oral sucker and pharynx.