Resource segregation among iliophagous fish in Lake St. Lucia,Zululand

Synopsis Sarotherodon mossambicus, Chanos chanos and 11 species of Mugilidae occur in Lake St. Lucia, Zululand. All are iliophagous and potential competitors. This investigation shows that although their diets overlap, competition is reduced by different feeding mechanisms which apparently result in the available food items being consumed in differing quantities. The diet of C. chanos consists chiefly of microfauna, that of Mugilidae microflora associated with sand grains and that of S. mossambicus, microflora associated with filamentous algae and benthic floc. Potential competition is also reduced because C. chanos reach peak numbers in summer whereas Mugilidae are more abundant in winter.     

The cyclic AMP-dependent initiation of DNA synthesis by T51B rat liver epithelioid cells.

The brief rise in the cellular cyclic AMP content which occurs late in the prereplicative phases of rat hepatocytes in vivo and T51B rat liver epitheloid cells in vitro seems to be necessary for the initiation of DNA synthesis. Thus, the extracellular calcium-deprivation in T51B rat liver cells in culture which induces a late G-1 block is rapidly reversible (cells surge into S phase within one hour) either by creating a cyclic AMP surge by the addition of calcium or 3-isobutyl-1-methyl xanthine (a cyclic 3',5'-nucleotide phosphodiesterase inhibitor) or by the exogenous addition of low concentrations of cyclic AMP itself (i.e., 10(-8)-10(-5) M). On the other hand, prevention of the calcium-induced cyclic AMP surge by imidazole (a cyclic 3',5'-nucleotide phosphodiesterase activator) blocked the initiation of DNA synthesis by the calcium-deprived T51B cells.     

The influence of zooplankton food resources on the morphology of the estuarine clupeid Gilchristella aestuarius (Gilchrist, 1914)

Synopsis The body shape of the estuarine clupeidGilchristella aestuarius from the St. Lucia system is different from that of the same species in other estuaries. The morphology ofG. aestuarius is discussed in relation to long term food availability in the St. Lucia system.     

Experimental investigations on the behaviour of the cercariae of an ectoparasitic digenean Transversotrema patialense: general activity patterns

The activity patterns of the cercariae of Transversotrema patialense have been examined by monitoring the behaviour of individual larvae. Records of three different behavioural modes, namely active, tail-first swimming, passive dropping and resting periods were made separately. The age- and temperature-dependence of the activity patterns during the spontaneously active life of the cercariae were examined and estimates made of the absolute speeds of swimming and dropping. Through the temperature range 25--32 degrees C, the spontaneously active life of cercariae declined from about 18 h to about 6 h. Within this interval, after an initial period of high activity rates, a gradual decline in activity followed. The decline was a result of a decreasing frequency of swimming events rather than any change in the durations of such events. The neural control and energetic significance of the cercarial activity patterns are discussed.     

Food and feeding ecology of piscivorous fishes at Lake St Lucia, Zululand

The food and feeding ecology of piscivorous fish in Lake St Lucia was monitored for two years. Piscivorous fishes feed predominantly on the planktivorous Gilchristella aestuarius and Thryssa vitrirostris but a wide range of prey species was recorded. Numbers of the predominant piscivores, Argyrosomus hololepidotus and Elops machnata , in an area appear to be related to the densities of their major prey, T. vitrirostris and G. aestuarius . Large piscivorous fishes are restricted to the deeper portions of the lake, whereas small piscivores such as Johnius belengerii and Terapon jarbua feed predominantly on small fishes in the littoral zone. The highly significant correlation between the composition of prey fish species in the lake and prey fish species in the diet of piscivorous fishes, indicates that piscivores are feeding in a density dependent manner. However, factors such as habitat, fish size and swimming speed of prey species are shown to be important in prey selection. Juvenile fish of species such as Sarotherodon mossambicus, Liza macrolepis and Acanthopagrus berda remain in shallow marginal areas, thus avoiding large piscivorous fishes. However by frequenting shallow areas these species become vulnerable to bird predators, especially egrets and herons.     

Molecular cloning of the rfb region of Klebsiella pneumoniae serotype O1:K20: the rfb gene cluster is responsible for synthesis of the D-galactan I O polysaccharide.

Previous chemical analyses identified two structurally distinct O polysaccharides in the lipopolysaccharide of Klebsiella pneumoniae serotype O1:K20 (C. Whitfield, J. C. Richards, M. B. Perry, B. R. Clarke, and L. L. MacLean, J. Bacteriol. 173:1420-1431, 1991). The polysaccharides were designated D-galactan I and D-galactan II; both are homopolymers of galactose. To begin investigation of the synthesis and expression of these O polysaccharides, we have cloned a 7.3-kb region of the chromosome of K. pneumoniae O1:K20, containing the his-linked rfbkpO1 (O-antigen biosynthesis) gene cluster. In Escherichia coli K-12 and Salmonella typhimurium, rfbkpO1 directed the synthesis of D-galactan I but not D-galactan II. The cloned rfbkpO1 genes did not complement a mutation affecting D-galactan II synthesis in K. pneumoniae CWK37, suggesting that another (unlinked) locus is also required for D-galactan II expression. However, plasmids carrying rfbkpO1 did complement a mutation in K. pneumoniae CWK43 which eliminated expression of both D-galactan I and D-galactan II, indicating that at least one function is common to synthesis of both polymers. Synthesis of D-galactan I was dependent on chromosomal galE and rfe genes. Hybridization experiments indicated that the rfbkpO1 sequences from different serotype O1 Klebsiella isolates showed some restriction fragment length polymorphism.     

Structural analysis of the O-antigen side chain polysaccharides in the lipopolysaccharides of Klebsiella serotypes O2(2a), O2(2a,2b), and O2(2a,2c).

The lipopolysaccharide (LPS) of Klebsiella serotype O2 is antigenically heterogeneous; some strains express multiple antigenic factors. To study this heterogeneity, we determined the structure of the O-antigen polysaccharides in isolates belonging to serotypes O2(2a), O2(2a,2b), and O2(2a,2c), by using composition analysis, methylation analysis, and both 1H and 13C nuclear magnetic resonance spectroscopy. The repeating unit structure of the 2a polysaccharide was identified as the disaccharide [----3)-beta-D-Galf-(1----3)-alpha-D-Galp-(1----] and was identical to D-galactan I, one of two O polysaccharides present in the LPS of Klebsiella pneumoniae serotype O1 (C. Whitfield, J. C. Richards, M. B. Perry, B. R. Clarke, and L. L. MacLean, J. Bacteriol. 173:1420-1431, 1991). LPS from serotype O2(2a,2b) also contained D-galactan I as the only O polysaccharide, suggesting that the 2b antigen is not an O antigen. The LPS of serotype O2(2a,2c) contained a mixture of two structurally distinct O polysaccharides and provides a second example of this phenomenon in Klebsiella spp. One polymer was identical to D-galactan I, and the other polysaccharide, the 2c antigen, was a polymer with a disaccharide repeating unit structure, [----3)-beta-D-GlcpNAc-(1----5)-beta-D-Galf-(1----]. The 2c structure does not resemble previously reported O polysaccharides from Klebsiella spp. Periodate oxidation confirmed that D-galactan I and the 2c polysaccharide are distinct glycans, rather than representing domains within a single polysaccharide chain. Monoclonal antibodies against the 2c antigen indicated that only LPS molecules with the longest O-polysaccharide chains contained the 2c epitope.     

Field observations on the lepidophagous teleost Terapon jarbua (Forsk»l)

Synopsis Foraging behaviour of the lepidophagous Indo-Pacific teleost Terapon jarbua in the Bulolo and Mtumbane estuaries of southern Africa is described. These observations suggest that scale removal from large fish and the complete ingestion of fish fry represents a modified form of predation. Prey reaction to T. jarbua shoals is also discussed.