PARASITIC CRUSTACEANS OF AFRICAN FRESHWATER FISHES FROM THE NILE AND NIGER SYSTEMS

New crustaceans parasitie on African freshwater fishes of the Nile, or former Nile, and Niger systems are described. The taxonomy of a group of closely related species of Argulus is clarified, and errors in the older literature are corrected. Additional information on distribution is presented.     

MANIPULATION OF BIOMPHALARIA GLABRATA (SAY) (GASTROPODA: PLANORBIDAE) HAEMOCYTES IN VITRO

The propensity of molluscan haemocytes to clump irreversiblyupon mixing in suspension, together with their strong adherenceto many substrates, compromises many efforts to obtain plasma-freeor treated cells for subsequent use in experimental protocols.An anti-clumping buffer developed for use with haemocytes fromthe gastropod Lymnaea stagnalis was tested for its effectivenesswith cells from Biom-phalaria glabrata. While we were unableto obtain sufficient numbers of viable haemocytes for subsequentexperimentation after centrifugation, this divalent cation-chelatingbuffer induced rounding of adherent cells monolayered on glass.Thus, cells washed in physiological buffers could subsequentlybe retrieved as monodispersed suspensions. Such haemocytes retainedsome phagocytic activity, with levels of uptake improved whencells were held in buffer with excess divalent cations beforeaddition of target particles. (Received 7 September 1992; accepted 6 January 1993)     

Tubular and glandular organs in the Gladocera, Chydoridae

Chydorid cladocerans of three closely related genera– Chydorus, Peracantha and Pleuroxus – possess a long, blindly-ending tubular organ which lies freely in the haemocoele but follows a precise course between the organs of the body, and which ultimately unites with the alimentary canal in the rectal region.
No such organ is present in Pseudochydorus globosus which, until recently, was assigned to the genus Chydorus . This species, like representatives of at least seven, and probably more, chydorid genera, has a much shorter glandular organ which, although markedly different in form, may, however, be homologous with the tubular organ. In Alona affinis what is perhaps a precursor (or regressive rudiment) of a glandular organ is present. All these are described and the only partially-elucidated situation in Alonella exigua is mentioned.
In Eurycercus lamellatus and Leydigia leydigii no trace of either a tubular or a glandular organ is present.
The function of these organs is unknown. Mention is made of their possible phyletic and taxonomic significance.     

The Influence of Calcium or Manganese on the Resistance to EDTA, Polymyxin B or Cold Shock, and the Composition of Pseudomonas aeruginosa Grown in Glucose- or Magnesium-depleted Batch Cultures

Cultures were batch grown in simple salts media in which growth was limited either by depletion of glucose and magnesium (C/Mg-dep) or by glucose alone (C-dep). Cultures were also grown in these media supplemented by calcium and/or manganese.
All cultures grown in the C-dep media were sensitive to ethylenediaminetetraacetic acid (EDTA), polymyxin and also to cold shock but were relatively resistant to ethyleneglycol-bis(2-aminoethyl ether)-N, N-tetraacetic acid (EGTA). Inclusion of calcium or manganese in the growth medium enhanced lysis by EDTA. Cultures grown in the basic C/Mg-dep media were resistant to EDTA, EGTA, polymyxin and to cold shock. Sensitivity to these agents was retained by cultures grown in C/Mg-dep media supplemented with Ca²⁺ and/or Mn²⁺. Cells grown in C/Mg-dep media with added Mn²⁺ were more sensitive to EDTA and polymyxin than those from the unsupplemented C/Mg-dep media but still resistant compared with C-dep cultures. All cultures from supplemented C/Mg-dep media were more sensitive to EGTA than those from any of the C-dep media.
Whole cells and cell walls from these various media had differing amounts of cell wall, phosphorus, amino sugar, carbohydrates, readily extractable lipid (REL), total phospholipid (PL), and especially differences in cell wall divalent metal cation content.
The differences in PL, REL and amino sugars and carbohydrate did not correlate with the response of C-dep and C/Mg-dep bacteria to EDTA, EGTA or polymyxin. The results are discussed in relation to the hypothesis that the sensitivity of Pseudomonas aeruginosa to polymyxin and EDTA is more dependent on outer membrane cation content rather than on other components, e.g. PL and lipopolysaccharide.     

PREDATION AND ITS EFFECTS ON MIGRATION AND SPECIATION IN AFRICAN FISHES: A COMMENT

Contrary to a recently expressed view very little factual evidence has been brought forward to support the idea that predatory fishes, and especially Lates and Hydrocynus , have had restrictive effects on speciation among other African freshwater fishes. A considerable body of data supports the view that the effects of these fishes do not differ from those of other predators and that predation has facilitated speciation. Further reasons why Lates and Hydrocynus cannot be considered as having effects different from those of other predators are given.
The situation in lakes containing these predators is reconsidered. There is no need to attempt to minimize their effects in Lake Tanganyika where their presence and effects are fully compatible with the existence of a rich endemic fauna. The situation in Lakes Albert and Rudolf is more satisfactorily explained by the brief duration of the existence of the present-day lakes and by their recent invasion by a fully differentiated Nilotic fauna, than by imagining that Lates and Hydrocynus have restricted speciation.
The suggestion that Lates and Hydrocynus were responsible for the inception of the habit of upstream migration for purposes of spawning is discredited, and what seem to be more probable reasons for the phenomenon are given.     

Two-egged ephippia in the chydorid Cladocera

SUMMARY. With a single reported exception, the ephippia of all specialized chydorid cladocerans hitherto described contain a single egg. Ephippia the European Chydorus ovalis and the North American species currently listed as C. cf. sphaericus have been found persistently, and so far invariably, to contain two eggs. The alternative strategies of producing 1-and 2-egged ephippia are briefly considered. Anomalous 1-egged ephippia of Daphnia (family Daphniidae) are also mentioned.