Brachionus

Abstract: Rotifera is a Phylum of primary freshwater Metazoa containing two major groups: the heterogonic Monogononta and the exclusively parthenogenetic Bdelloidea. Monogononta contains 1,570 species-level taxa, of which a majority (1,488) are free-living fresh or inland water taxa. Bdelloidea contains 461 “species,” only one of which is marine, but with many limnoterrestrial representatives or animals of unknown ecology. Actual numbers may be much higher, considering the occurrence of cryptic speciation in Monogononta and the unsatisfactory nature of taxonomic knowledge. Rotifers, mostly monogononts, occur in all types of water bodies, worldwide. They are particularly diverse in the littoral zone of stagnant waterbodies with soft, slightly acidic water and under oligo- to mesotrophic conditions. The rotifer record is highest in the Northern hemisphere, which may be due to the concentration of studies in those regions. Diversity is highest in the (sub)tropics; hotspots are northeast North America, tropical South America, Southeast Asia, Australia, and Lake Baikal, endemicity is low in Africa (including Madagascar), Europe, the Indian subcontinent, and Antarctica. Although the lack of fossil evidence and of molecular phylogenetic studies are major hindrances, contrasting hypotheses on the origin and evolutionary history of Brachionus, Macrochaetus, and Trichocerca are presented.

Abstract: The direct and indirect effects of suspended clay on the population dynamics of several species of planktonic rotifers and cladocerans were investigated using long-term laboratory experiments. Life table and population growth experiments showed that high, but naturally occurring, concentrations (50-100 mg/L) of coarse, suspended clay (<2-jm particle size) caused large reductions in the population growth rates (rm) of four cladoceran species (Bosinina longirostris, Ceriodaphnia dubia, Daphnia ambigua, and D. pulex). Ju- veniles were more susceptible than adults to suspended clay. Low concentrations (10 mg/L) of coarse clay, and high concentrations of fine clay (< 1 Ajm), did not decrease, and sometimes increased, cladoceran population growth rates. Both the inhibitory effects of high concen- trations of coarse clay and the stimulatory effects of low concentrations of clay were greater at limiting food (Cr pptomnonas) concentrations. The population growth rates of four rotifer species (Brachionus calvciflorus, Keratella cochlearis, Polyarthra vulgaris, and Synchaeta pectinata) were not affected by high concentrations of coarse or fine clay, even at very low food levels. The population growth rate of the rotifer K. crassa was decreased by coarse clay. The threshold food concentration (the food concentration, measured as dry mass, at which population growth rate equals zero) of the cladoceran D. ambigua was increased from 0.15 to 0.40 jg/mL in the presence of 50 mg/L coarse clay, indicating a reduced ability to compete in exploitative contests. The threshold food concentration of the rotifer K. cochlearis (0.05 jg/mL) was unaffected by suspended clay. Suspended clay reversed the outcome of two rotifer-cladoceran competition experiments, between K. cochlearis and D. arnbigua, and between K. cochlearis and C. dubia. In the absence of clay, the cladocerans dominated; in the presence of clay, the rotifer dominated. The presence of suspended sediments in natural ecosystems, such as turbid lakes and reservoirs, should favor rotifers over cladocerans and thus influence the structure of zooplankton communities.

Abstract: Of the three Brachionus species used in aquaculture, Brachionus rubens, B. calyciflorus and B. plicatilis, the latter is most widely used in raising marine fish and shrimp larvae due to its tolerance to the marine environment. In freshwater aquaculture the use of B. rubens and B. calyciforus is limited, probably because inert food products are readily available as feed for freshwater larvae.