An overview is given of our prcsent knowledge on taxonomy, phylogeny and biogcography of catfishes or Siluroidei, one of thc worlds economically important groups of liesh and brackish water fishes. In an introductory chaptcr, the taxonomic position of catfishes in the ichthyological classification is discussed; their extcrnal morphology is briefly described and synapomorphics showing their monophyly are listed. In the following chapter, cattish systematics arc prcscntcd. Thirty three families with 416 genera and 2,584 species are presently recognized, but thcsc numbers are continuously changing; it is thereforc obvious that the systematics of many taxa are still rather poorly known. For each family, data (if any) supporting their monophyly arc given together with a short description of their external morphology. The valid genera are listcd, with for each genus, the number of presently recognizcd species with a reference (if any) to a rcccnt revisionary study. Contributions to the phylogcnctic study of catfishes are scarce, and dcaling only with a few families and with a limitcd numbcr of taxa within the families. As a results only fcw data on catfish phylogeny are presently known. Therefore, also our knowledge on historical biogeography of catfisheï is seriously limitcd, until more date become available on interfamilial and intergeneric relationships.
We compare the cranial morphology of four fish species with an increasing anguilliformism in the following order: Clarias gariepinus, Clariallabes melas, Gymnallabes typus, and Channallabes apus. The main anatomicalmorphological disparities are the stepwise reduction of the skull roof along with the relative enlargement of the external jaw muscles, which occurred in each of them. Gymnallabes typus and C. apus lack a bony protection to cover the jaw muscles. The neurocranial bones of C. gariepinus, however, form a closed, broad roof, whereas the width of the neurocranium in C. melas is intermediate. Several features of the clariid heads, such as the size of the mouth and the bands of small teeth, may be regarded as adaptations for manipulating large food particles, which are even more pronounced in anguilliform clariids. The jaw musculature of G. typus is hypertrophied and attached on a higher coronoid process of the lower jaw, causing a larger adductive force. The hyomandibula interdigitates more strongly with the neurocranium and its dentition with longer teeth is posteriorly extended, closer to the lower jaw articulation. The anguilliform clariids also have their cranial muscles modified to enable a wider gape. The adductor mandibulae and the levator operculi extend more posteriorly, and the anterior attachment site of the protractor hyoidei dorsalis shifts toward the sagittal plane of the head. A phylogenetic analysis of the Clariidae, which is in progress, could check the validity of Boulenger's hypothesis that predecessors of the primitive fishes, such as Heterobranchus and most Clarias, would have evolved into progressively anguilliform clariids.
We examined patterns in fish species assemblages structure and function along environmental gradients in rivers of Gabon. Species presence-absence data from 52 sites were first analysed by canonical correspondence analysis. Results showed that the position of sites along the upstream-downstream gradient, together with elevation and water conductance were the most important predictors of local fish assemblage composition. Assemblage richness and trophic structure were further investigated using regression tree analysis. Results revealed a general increase in species richness from upstream to downstream areas and a transition from insectivorous to omnivorous, herbivorous and piscivorous species along this longitudinal gradient. There were several similarities between these previous patterns and those observed in other temperate streams suggesting a potential convergence in fish assemblage along environmental gradients in tropical and temperate riverine systems. From a conservation standpoint, these results highlight the need to evaluate all habitat types along rivers longitudinal gradient to integrate the full spectrum of species assemblages within conservation plans.
A comparative study on morphology, growth rate and reproduction of CZarias gariepinus (Burchell, 1822), Heterobranchus ZongzjWs Valenciennes, 1840, and their reciprocal hybrids (Pisces, Clariidae) Successful reciprocal hybridizations between Clarias gariepinus and Heterobranchus longifiris were performed during three experiments. The reciprocal hybrids were viable, their survival rates being similar to those found in the maternal species. Hybrid morphology was intermediate to that of the parents. No difference in external morphology was found between the reciprocal hybrids. Growth rate comparison under the same conditions and over a period of 307 days, revealed a considerably faster growth for H . IongiJilis than for C. guriepinus, and demonstrated the importance of the use of the former species in fish culture. Hybrid growth rates were similar to that of H. longifilis, although during one of the experiments female H. longifilis x male C. guriepinus hybrids displayed an even faster growth than H. longifilis.A detailed study on reproduction indicated that female C. gariepinus mature earlier (5-7 months) than female H . longifilis (12-14 months). In the reciprocal hybrids, female first sexual maturity was attained at 20-21 months. Both reciprocal hybrids and parental species displayed an equilibrated sex ratio. Macroscopical and microscopical observations revealed numerous abnormalities in gonadal development of the hybrids. GSI and fecundity in female hybrids were considerably lower than those found in the parental species. Moreover, intra-ovarian tumors occurred in 20% of the hybrids. In hybrid males, GSIs were generally higher than those found in the parental species, but the spermatozoa concentration in the semen was about 100 times less. Despite these abnormalities, viable larvae, resulting from F2 and backcross fertilizations, were obtained.
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