Terry W. Snell

Resting eggs in rotifers

The biology of resting eggs of monogonont rotifers is reviewed, covering literature published since the last major review by Gilbert (1974). The topics examined include resting egg production, morphology and species specificity, hatching, and evolutionary significance.

LIFESPAN AND FECUNDITY PATTERNS IN ROTIFERS: THE COST OF REPRODUCTION

Life-history characteristics are usually viewed as adaptive. That is, changes in characteristics such as reproductive rate, duration of reproduction, and age of first reproduction usually have direct effects on an organisms fitness. However, increases in fitness gained by modifying one characteristic may lead to conflicting changes in other characteristics that decrease fitness. These conflicts are resolved and fitness maximized by tradeoffs between life-history characteristics. Reproductive pattern is the characteristic most often considered in this context (e.g., Cole, 1954. Williams, 1966; Cody, 1966; Gadgil and Bossert. 1970; Charnov and Krebs, 1973; Schaffer, 1974; Hirshfield and Tinkle, 1975; Giesel, 1976). A basic assumption in several of these studies is that of resource limitation; resources directed towards reproduction reduce those available for growth and maintenance. Thus one consequence of reproduction is presumed to be decreased survival. The existence of such a tradeoff between reproduction and future survival is a fundamental premise underlying most life-history theory. The evidence for such a tradeoff, however, is weak and indirect. Much of this evidence is summarized by Stearns (1976) and additional evidence can be found in Drosophila (Maynard-Smith, 1959), mice (Suntzeff et al., 1962), chaetognaths (MacLaren, 1966), beetles (Loschiavo, 1968), lizards (Tinkle, 1969), and plants (Harper and White, 1974). This evidence

Acute toxicity tests using rotifers: IV. Effects of cyst age, temperature, and salinity on the sensitivity of Brachionus calyciflorus

Several aspects of the response to toxicants using a standardized toxicity test with the freshwater rotifer Brachionus calyciflorus are described. Test animals are obtained by hatching cysts which produce animals of similar age and physiological condition. The acute toxicity of 28 compounds is described with 24-hr LC50s. The LC50s span five orders of magnitude, from silver at 0.008 mg.liter-1 to benzene at more than 1000 mg.liter-1. Control mortality in 84 tests averaged 2% with a standard deviation of 3%, indicating very consistent test sensitivity. Only once in 84 trials did a test fail because of excessive control mortality, yielding a failure rate of 1.2%. Cyst age from 0 to 18 months had no effect on the sensitivity of neonates to reference toxicants. Both high and low temperatures increased rotifer sensitivity to reference toxicants. Copper sensitivity was greater at 10, 25, and 30 degrees C compared with results at 20 degrees C. Likewise, sodium pentachlorophenol toxicity was greater at 10 and 30 degrees C compared with results at 20 degrees C. Survivorship curves at 25 degrees C of neonates under control conditions indicated that mortality begins at about 30 hr. This places a practical limit on toxicant exposure for the assay of 24 hr. B. calyciflorus cysts hatch at salinities up to 5 ppt and acute toxicity tests using pentachlorophenol at this salinity yielded LC50s about one-half those of standard freshwater. B. calyciflorus is preferred over Brachionus plicatilis for toxicity tests in salinities up to 5 ppt because it is consistently more sensitive.

Contribution of Water-Soluble and Insoluble Components and Their Hydrophobic/Hydrophilic Subfractions to the Reactive Oxygen Species-Generating Potential of Fine Ambient Aerosols

Relative contributions of water- and methanol-soluble compounds and their hydrophobic/hydrophilic subfractions to the ROS (reactive oxygen species)-generating potential of ambient fine aerosols (D(p) < 2.5 μm) are assessed. ROS-generating (or oxidative) potential of the particulate matter (PM) was measured by the dithiothreitol (DTT) assay. Particles were collected on quartz filters (N = 8) at an urban site near central Atlanta during January-February 2012 using a PM(2.5) high-volume sampler. Filter punches were extracted separately in both water and methanol. Hydrophobic and hydrophilic fractions were then subsequently segregated via a C-18 solid phase extraction column. The DTT assay response was significantly higher for the methanol extract, and for both extracts a substantial fraction of PM oxidative potential was associated with the hydrophobic compounds as evident from a substantial attenuation in DTT response after passing PM extracts through the C-18 column (64% for water and 83% for methanol extract; both median values). The DTT activities of water and methanol extracts were correlated with the water-soluble (R = 0.86) and water-insoluble organic carbon (R = 0.94) contents of the PM, respectively. Brown carbon (BrC), which predominantly represents the hydrophobic organic fraction (referred to as humic-like substances, HULIS), was also correlated with DTT activity in both the water (R = 0.78) and methanol extracts (R = 0.83). Oxidative potential was not correlated with any metals measured in the extracts. These findings suggest that the hydrophobic components of both water-soluble and insoluble organic aerosols substantially contribute to the oxidative properties of ambient PM. Further investigation of these hydrophobic organic compounds could help identify sources of a significant fraction of ambient aerosol toxicity.

Acute toxicity bioassays using rotifers. I. A test for brackish and marine environments with Brachionus plicatilis

Abstract A standardized 24-h acute toxicity test for the marine environment using the rotifer Brachionus plicatilis is described. Test animals are obtained by hatching cysts, thus eliminating the need for stock cultures. Since animals hatching from cysts are of similar age, genotype and physiologically condition, test variability is greatly reduced. Controlled cyst hatching is achieved by transferring to lower salinity, warmer temperature and light. After 23 h at 25°C and 15 ppt salinity, hatching begins and proceeds rapidly. By 28 h, 90% of the cysts have hatched. A protocol is outlined to collect the neonates and use them in a simple acute toxicity test to calculate a 24 h LC 50 . A reference test using sodium pentachlorophenate (NaPCP) is described as well as a range-finding test and a definitive test for unknown toxicants. The toxicity of six compounds to B. plicatilis was examined with the following decreasing sensitivities: copper > NaPCP > SDS > free NH 3 > cadmium > malathion. For 3 of the 6 compounds tested, salinity increase from 15 to 30 ppt resulted in higher sensitivity, whereas for the other 3 compounds there was no effect. In comparison with current test organisms B. plicatilis is either more, equal or less sensitive depending on the compound, confirming the species-chemical specificity of mode of action of toxicants. The repeatability of the rotifer test is 5–6 times better than that reported for Daphnia tests and twice as good as the Artemia nauplii bioassay. Like the standard brine shrimp nauplii acute test, the cyst-based rotifer test is an important advance in acute toxicity testing since it eliminates stock cultures, is rapid, sensitive, highly repeatable, easy to execute and cost effective.

Body size variation among strains of the rotifer Brachionus plicatilis

An investigation of body size variability among 13 strains of the rotifer Brachionus plicatilis was conducted under controlled laboratory conditions. Lorica lengths ranged from 123 to 292 μm and lorica widths from 114 to 199 μm. An 85% increase in lorica length was recorded as females grew from birth to adulthood. Manipulation of rotifer size by culture conditions was investigated for various salinities, diets, and temperatures. In extreme cases, diet and salinity produced a 15% and 11% change in lorica length, respectively. A factorial ANOVA of temperature × salinity × strain effects demonstrated that strain is the most important factor determining lorica size.

Acute toxicity bioassays using rotifers. II. A freshwater test with Brachionus rubens

Abstract A 24-h acute toxicity test for freshwater is described using the rotifer Brachionus rubens hatched from cysts. Hatchlings are used in a simple protocol that provides for LC50 calculation and yields highly repeatable results. Hatching is initiated by transferring cysts to warmer temperatures and light. At 25°C, hatching commences after 17 h and by 25 h, 40% of cysts have hatched. The average hatching percentage for B. rubens cysts was 53%. A reference test using sodium pentachlorophenate (NaPCP) yielded an LC50 of 0.62 mg/l, with a coefficient of variation of 9.7%. The no observed effect concentration (NOEC) for NaPCP was 0.28 mg/l. Protocols for range-finding and definitive tests of unknown toxicants are also described. Six compounds were assayed and had the following toxicity rankings: copper > NaPCP > cadmium > SDS > free NH3 > malathion. B. rubens was at least twice as sensitive as Brachionus plicatilis to all toxicants tested except malathion. The precision of the B. rubens acute toxicity test is about 3 times better than that of Daphnia. Like its marine counterpart with B. plicatilis, the B. rubens test for fresh water has a major advantage over current aquatic tests in that it eliminates culturing and maintenance of live stocks. Test animals are obtained from dormant eggs which have a shelf life of at least 1 yr. Moreover, the rotifer test proposed is fast, convenient, sensitive and repeatable, making it a useful new tool for routine assessment of aquatic toxicity of chemicals and effluents.

Effect of temperature, salinity and food level on sexual and asexual reproduction in Brachionus plicatilis (Rotifera)

The reproductive response of sexual and asexual female Brachionus plicatilis (Muller) was examined over temperatures ranging from 20° to 40°C, salinities from 5 to 40‰ S, and food levels from 0.25 to 20 μg Chlorella vulgaris dry-weight per ml. Reduced food levels, as well as temperature and salinity extremes, reduced reproduction of both sexual and asexual females, but did so differentially. Reproduction by sexual females was reduced to a greater extent at environmental extremes than asexual females. The broad, flat reproductive response curve of asexual females extended beyond the limits of the narrower, more sharply peaked curve of sexual females. Thus zones of exclusively asexual reproduction exist at environmental extremes where sexual reproduction is physiologically restricted. These results are corroborated by a comparison of the lifetime fecundity of individual sexual and asexual females over a 20°C temperature range. No differences in lifetime fecundity occurred between sexual and asexual females at 18° and 28°C. At 38°C, however, asexual female fecundity reached its highest level, while sexual female fecundity declined 15%. The appearance of sexual females in rotifer populations in the result of both inducible and repressible factors.

Estrogenic compounds affect development of harpacticoid copepod Tigriopus japonicus.

The aim of this investigation was to evaluate the impact of estrogenic compounds on the harpacticoid copepod Tigriopus japonicus after continuous exposure to environmentally relevant concentrations. Natural estrogen (17beta-estradiol), three known estrogenic compounds in vertebrates (bisphenol A, 4-nonylphenol, p-t-octylphenol), and an invertebrate molting hormone (20-hydroxyecdysone) were tested for their effects on development and reproductive characters in two successive generations of T. japonicus. Less than 24-h-old nauplii (parentals) were exposed to four sublethal concentrations of these compounds for 21 d at 25 degrees C. The first brood of nauplii (F1) produced was monitored further under the same culture conditions and exposures to test compounds. Results showed that all estrogenic compounds affected development (both in number of days to reach copepodid stage and sexual maturity) in the parental generation. Similar effects were apparent in the F1; however, fecundity, sex ratio, and survival were not significantly affected, even at concentrations as high as 10 microg/L (nominal concentration). The invertebrate molting hormone 20-hyroxyecdysone had no detectable effect on any of the endpoints tested but gave the lowest 48-h 50% lethal concentration (LC50) value. The results suggest that endocrine disruption could occur in copepods following exposure to environmentally relevant concentrations of estrogenic compounds, especially if they are exposed starting from embryonic development.

Thresholds for mictic female production in the rotifer Brachionus plicatilis (Muller)

Food concentration, free ammonia, and population density thresholds for mictic female production were characterized for the rotifer Brachionus plicatilis (Muller). Mictic female production ceased at 15.3 × 103Dunaliella cells · ml−1 (5.7 μg dry weight · ml−1) while amictic female production continued at 0.1 this food level. Free ammonia also repressed mictic and amictic female production differentially. A free ammonia threshold of 24.4 was recorded for mictic female production. Increasing population density over the range of 140–7400 females · 1−1 promoted mictic female production. The percentage of mictic daughters produced at 260 females · 1−1 was 20 times less than that at 7400 females · 1−1. The threshold population density for mictic female production was 147 females · 1−1. The rate of amictic female production strongly correlated with mictic female production. Most mictic female production occurred when the rate of amictic female production exceeded 5.2 females · day−1. A hypothesis is proposed that high rates of amictic female production trigger mixis in B. plicatilis.