Freshwater crab origins—Laying Gondwana to rest

We review the available data on the phylogeny, palaeontology and divergence time estimation of primary freshwater crabs in relation to a hypothesized Gondwanan origin of these brachyurans, as postulated by some workers in recent decades. Known phylogenetic relationships within the Old World freshwater crabs do not correspond to the successive fragmentation of the Gondwana continent. This is strong evidence against an ancestral Gondwanan distribution of Afrotropical Potamonautidae and Asian-Australian Gecarcinucidae. The fossil record of freshwater crabs (no older than the Oligocene) and heterotreme brachyurans also postdate the initial break up of Gondwana. Molecular-clock based time estimates for the most common recent ancestor of freshwater crab families differ profoundly, depending on the method of calibration used, and whether freshwater or marine brachyuran fossils are used as calibration points. As such, molecular clock estimates calibrated on freshwater crab fossils favour a post-Gondwanan evolution of freshwater crabs whereas calibration based on marine brachyuran fossils date their last common ancestor before the fragmentation of Gondwana.     

Effects of livestock exclusion on density,survival and biomass of the perennial sagebrush grass Hymenachne pernambucense (Poaceae) from a temperate fluvial wetland

In Argentina, the intensification of soybean production has displaced a substantial proportion of cattle ranching to fluvial wetlands such as those in the Delta of the Paraná River. Cattle grazing affects structure and dynamics of native forage plants but there is little information on this impact in populations from fluvial wetlands. This study addresses the effect of cattle ranching on density, survival, mean life-span and aerial biomass of Hymenachne pernambucense (Poaceae), an important forage species in the region. The study was carried out monthly for one year in permanents plots subject to continuous grazing and plots excluded from grazing in the Middle Delta of the Paraná River. In plots excluded from grazing, tillers showed significantly higher population density and survival, and a two-fold increase in mean life-span, while continuous grazing decreased survival of cohorts. The largest contribution to tiller density in ungrazed and grazed populations was made by spring and summer cohorts, respectively. Total and green biomass were significantly higher in the ungrazed population, with highest differences in late spring-early summer. Cattle grazing affected the relationship between tiller density and green biomass suggesting that cattle prefer sprouts because they are more palatable and nutritious than older tissue.     

Strategies of reproduction, dispersion, and competition in river plants: A review

The present paper reviews the literature on reproduction, survial, dispersion and competition in aquatic plants of mostly European rivers. The specificity of the strategies of colonization in aquatic plants in comparison with terrestrial plants is noted. The importance of the knowledge of these life history traits in the control of the harmful effects caused by aquatic plants and for ecosystem management is also considered. The difficulty of applying current ecological theories concerning adaptative strategies to aquatic plants is discussed, focusing on the great interest of studying the species traits of aquatic macrophytes for testing such theories and interpreting recolonization patterns of disturbed areas. Knowledge of strategies of reproduction, dispersion and competition among aquatic plants remains very fragmentary, particularly from a quantitative view-point, and further studies are required both for theoretical and practical applications.     

The effect of lake fertilisation on the rotifers of Seathwaite Tarn,an acidified lake in the English Lake District

Seathwaite Tarn, in the English Lake District, was used as an experimental site to test the feasibility of increasing the pH of an acidified waterbody by adding a phosphorus-based fertiliser solution. The impact of this experiment on the planktonic rotifer community was assessed by monitoring for two years before treatment (1990–1991), and for a further two years once treatment had begun (1992–1993). The pH of the water rose by 0.5 pH units over the period of fertilisation, and the lake became very eutrophic. This had little effect on the species composition of the rotifer assemblage, which continued to reflect the acid, oligotrophic nature of the untreated lake. However, there were marked changes in the levels of abundance of most planktonic species. Population densities of Keratella serrulata and Trichocerca longiseta increased in early spring, compared to pre-treatment levels, apparently due to the rises in chlorophyll-a concentrations. In contrast, Polyarthra dolichoptera became very scarce after fertilisation began. The treatment seemed to have little effect on the population dynamics of the dominant openwater rotifer, Polyarthra remata. In 1993, very high densities of Bosmina coregoni (up to 250 × 10³ ind M^–3 ) developed in response to the eutrophication programme. This seemed to suppress the rotifer community in the summer months.     

Feeding ecology of three Astyanax species (Characidae, Tetragonopterinae) from a floodplain lake of Mogi-Guaçú River, Paraná River Basin, Brazil

Synopsis The feeding ecology of three characids (A. fasciatus, A. bimaculatus and A. schubarti) was studied monthly during 1988 in Lake Inferñao, a major floodplain lake of the Mogi-Guaçú River in the State of São Paulo. River flooding directly influenced the diet of the omnivorous A. bimaculatus and A. fasciatus which responded to maximum inundation (March) by consuming predominantly allochthonous insects. In contrast, A. schubarti was less influenced by the river flood cycle on qualitative changes in diet and relied basically on aquatic macrophytes and periphytic algae. The importance of zooplankton in the diet of the three species was low, and may be attributed to its low density in the water column (< 1 ind l^–1). Ontogenetic diet changes were evident for the three species. For A. fasciatus and A. bimaculatus the importance of zooplankton was high at the early stages, decreasing with size. A. schubarti at younger stages consumed diversified items which gradually decreased quantitatively up to the size class of 65 mm; from this size onwards, the diet became restricted to the consumption of periphytic algae and macrophytes.     

TOXIC CONCENTRATION OF FREE AMMONIA TO BRACHIONUS CALYCIFLORUS PALLAS,A ROTIFER PEST SPECIES FOUND IN HIGH RATE ALGAL PONDS (HRAP's)

SUMMARY

Laboratory tests on Brachionus calyciflorus Pallas indicate that at low concentrations of free ammonia (< 7 mg l ^?1) pH levels up to 9,85 could be tolerated for 96 h without apparent ill effect. Free ammonia concentrations > 14,4 mg l ^?1 (as N) could not be tolerated by this species. It is recommended that ammonium hydroxide be added to HRAP's in order to elevate the concentration of free ammonia above this level.     

A NOTE ON THE EFFECTS OF DRASTIC CHANGES IN TOTAL DISSOLVED SOLIDS ON THE AQUATIC PULMONATE SNAIL BULINUS (PHYSOPSIS) AFRICANUS KRAUSS

SUMMARY

THE transfer of a sample of Bulinus (Physopsis) africanus from water in their natural habitat containing 490 ppm TDS to a laboratory culture medium of 126 ppm TDS was found to be accompanied by a marked increase in the variability of the rate of oxygen consumption among individuals, and resulted in a 46% mortality. Transfer through the same gradient, but through two acclimatory stages of intermediate dissolved solid content, eliminated both these effects, suggesting an ability to acclimate and therefore to cope with the more gradual changes which may be expected to occur under field conditions.     

Competition between mobile species using patchy resources: an example from a freshwater,symbiotic assemblage

Summary Three species of freshwater mites that are symbiotic with mussels in St Mark's River, north Florida, have consistently high rates of colonization to and occupy high proportions of mussels. The mites Unionicola poundsi and U. serrata are territorial and have limited numbers/host, whereas the non-territorial U. abnormipes has highly variable numbers/host. U. abnormipes and U. serrata are most common in Villosa villosa and Uniomerus declivis respectively, patterns that can not be explained by host species preferences, whereas U. poundsi is equally abundant in both host species. Field experiments showed that both U. poundsi and U. serrata were limited most by intraspecific competition between adult mites, presumably for access to food and oviposition sites. Additionally, U. serrata did not remain within small hosts, most of which were V. villosa. In contrast, numbers of U. abnormipes were limited by both other mite species although the nature of the interactions differed. U. serrata may prey on U. abnormipes when they co-occur, whereas U. poundsi probably only excludes U. abnormipes from certain areas within hosts. Hence, U. abnormipes occurs mostly in V. villosa because most of these mussels do not contain U. serrata, but even so its numbers are still depressed by U. poundsi. The results were consistent with the general expectation of Holmes and Price (1986) that parasite assemblages where species have high colonization levels should be organized primarily by biotic interactions. However, specific outcomes of competition between mites were consistent with the more general model of Levins (1979) for competition between species using variable resources. Failure of other models to apply to Unionicola pinpointed at least five key biological characters that may form a better basis of comparison than taxonomic or habitat-based contrasts.     

The genus Synchaeta (rotifers) in a north-western Mediterranean coastal lagoon (Etang de Thau, France): taxonomical and ecological remarks

During a survey of a north-western Mediterranean coastal lagoon, (the Etang de Thau, and at a nearby marine station, 1994–1998), six species of the genus Synchaetawere identified. The systematics of this illoricate genus is difficult and identification is virtually impossible in preserved material, although some indication may be obtained from an examination of the trophi after treatment with sodium hypochlorite. In this study, taxonomic characters (mastax, body morphology and egg size) were obtained from living material prior to preservation. We add some ecological observations: distribution of species (neither temperature nor salinity were found to be determining factors), co-existence (up to five species can co-occur) and consideration of trophic links (competition with tintinnids, copepod nauplii and cirripedes).