Midsummer heat wave effects on lacustrine plankton: Variation of assemblage structure and fatty acid composition

The effects of the 2003 European heat wave on a freshwater plankton assemblage and its fatty acid (FA) composition were investigated. Composition and FA profiles of four size categories of planktonic organisms collected in 2003 were compared to those of the colder year 2002.     

Environmental factors influencing the vertical migration of planktonic rotifers in a hypereutrophic tarn

The diel vertical migration of planktonic rotifers in a small, hypereutrophic tarn was investigated on four occasions in 1983. When the tarn was isothermal the rotifers were distributed throughout the water column. After stratification, the rotifers were confined to the top 1–2 m of oxygenated water. On all four dates the rotifers were aggregated at specific depths in the water column. On some occasions, the pattern of aggregation changed as the animals performed distinct diurnal migrations. Keratella cochlearis, K. quadrata and Polyarthra vulgaris usually followed the reverse migrations of the phytoplankton. In contrast, the movements of Anuraeopsis fissa were less pronounced and were associated with variations in the depth of the oxycline.     

Ecology of bdelloids: how to be successful

Bdelloids inhabit many different environments. The entire taxon is an order belonging to the phylum Rotifera. In spite of its wide distribution, it has a very uniform morphology, suggesting that natural selection has had almost no effect on its morphological characteristics.This paper reviews the geographical and ecological distributions of bdelloids and their ability to tolerate different conditions is discussed. Two characteristics account for the wide distribution of bdelloids, parthenogenetic reproduction and their ability to withstand unfavorable conditions through anhydrobiosis. The former is an apomictic thelytoky which may ensure genetic homogeneity within clones. However, evidence which suggests variability among and within parthenogenetic clones is discussed. Some recent experimental evidence indicates that anhydrobiosis does not affect the life history of the surviving individuals. The effects of anhydrobiosis on bdelloid populations are considered.     

Diel food utilization by woundfin,Plagopterus argentissimus,in Virgin River,Arizona

Synopsis Woundfin, Plagopterus argentissimus, fed predominantly on simuliid larvae during the day, and shifted to the larger Hydropsyche spp. at night. Ephemeropteran larvae were eaten nearly uniformly throughout the 24 hour period. Mean weight of food consumed varied from 20–40 mg per individual (0.7–2.5% body weight) throughout the 24 h period. Our estimated daily ration of about 8% body weight, at temperatures that varied between 15–25°C, is based upon our measure of mean gut contents over a 24h period, combined with intestinal evacuation rates suggested for other cyprinids in the literature. The relatively continuous feeding pattern suggests a foraging strategy that would minimize competition with other species for food during June, a period of minimum annual food abundance in Virgin River.     

Size variation in Brachionus plicatilis resting eggs

The effect of temperature and salinity on resting egg size of two Brachionus plicatilis (Rotifers) clones was investigated. Clones were selected according to their different behaviour in laying resting eggs: one clone ejects them, whereas they remain inside the females body in the other clone. The difference in resting eggs size between the two clones is noticeable, although the difference is not as great as that between female body size. An important temperature-salinity interaction on resting egg size has been observed. The general inverse relationship between size and temperature is only true at lower temperatures. At high temperatures size varies around the mean although could be greater than at intermediate temperatures. This is more evident at the intermediate salinity tested which is considered to be the closest to the optimum in our experiments. This pattern of variation suggests that mean size is bigger than expected, in relation to temperature and salinity, when these factors have values close to the extremes of their range, normally found in nature, and to which adaptative mechanisms can evolve. Size is bigger at the salinity — temperature low - low and high - high combinations which are the most commonly found in the temperate environments.     

Molecular genetics of rotifers: preliminary restriction napping of the mitochondrial genome of Brachionus plicatilis

Methods are presented to extract and purify mitochondrial DNA from the rotifer Brachionus plicatilis. The mtDNA obtained is of sufficient purity for digestion with restriction endonucleases. EcoR I restriction patterns are presented for 4 geographically separated clones. A restriction map based on digestion with 5 different restriction enzymes is included for one of these clones. Finally, use of mtDNA analysis for studies on the population structure and biogeography of rotifers is discussed.     

On choice of substrate and habitat in brachionid rotifers

Information on the distribution of 28 rotifers of the family Brachionidae from diverse waters in south and central Sweden was analyzed to reveal their relationships to substrate and habitat. Some brachionids are preferably planktic, others periphytic and/or benthic. Some non-planktic habitats are utilized more than others, but there is no evidence of a chemical attraction from any substrate. Instead, some substrates seem to be avoided, possibly depending on a poorer flora of periphytic algae. Besides substrate type, the following factors are found to be important for creating separate ecological niches in the brachionid family: temperature, oxygen content, trophic degree, chemical environment, food choice and sensitivity to predation. It is possible to delineate separate ecological niches for all brachionid rotifers, implying that Hutchinson's ideas about the plankton paradox are contradicted. Some species are specialists, other are generalists, the latter being characterized by a great morphological variation. The species are adapted in different ways to their preferential habitats, as regards foot, egg-carrying, protrusions and other lorical structures etc. Longer spines, for instance, are generally found in more transparent water, being a supposed protection against visual predators.     

Tasmania revisited: rotifer communities and habitat heterogeneity

The results of four field surveys for Rotifera in Tasmania are summarized. Most new species and records in a 1987 survey were from acid waters (pH < 4.0) of dune lakes on the west coast (42° S). Marked intra- and interhabitat differences in rotifer communities of lakes and ponds were demonstrated by cluster analysis and related to habitat heterogeneity.