The Temperature–Size Rule in Lecane inermis (Rotifera) is adaptive and driven by nuclei size adjustment to temperature and oxygen combinations

The evolutionary implications of the Temperature–Size Rule (TSR) in ectotherms is debatable; it is uncertain whether size decrease with temperature increase is an adaptation or a non-adaptive by-product of some temperature-dependent processes. We tested whether (i) the size of the rotifer Lecane inermis affects fecundity in a way that depends on the combination of low or high temperature and oxygen content and (ii) the proximate mechanism underlying TSR in this species is associated with nuclei size adjustment (a proxy of cell size).Small-type and large-type rotifers were obtained by culturing at different temperatures prior to the experiment and then exposed to combinations of two temperature and two oxygen conditions. Fecundity was estimated and used as a measure of fitness. Nuclei and body sizes were measured to examine the response to both environmental factors tested.The results show the following for L. inermis. (i) Body size affects fecundity in response to both temperature and oxygen, supporting a hypothesis regarding the contribution of oxygen in TSR. (ii) Large individuals are generally more fecund than small ones; however, under a combination of high temperature and poor oxygen conditions, small individuals are more fecund than large ones, in accordance with a hypothesis of the adaptive significance of TSR. (iii) The body size response to temperature is realised by nuclei size adjustment. (iv) Nuclei size changes in response to temperature and oxygen conditions, in agreement with hypotheses on the cellular mechanism underlying TSR and on a contribution of oxygen availability in TSR. These results serve as empirical evidence for the adaptive significance of TSR and validation of the cellular mechanism for the observed response.     

The temperature–size rule in a rotifer is determined by the mother and at the egg stage

In most ectotherms, compared with development at low temperatures, development at high temperatures results in the acceleration of maturation, which in turn results in a smaller size (temperature–size rule, TSR). It is not known at which developmental stages this thermal response is determined. We exposed different life stages of the rotifer Lecane inermis to 15, 20, or 25 °C to determine whether the TSR in the F1 generation is governed by the thermal conditions experienced by the mothers (F0 generation) during their development, during egg production, or during the development of the eggs or hatchlings. We found that the adult size was affected by the thermal conditions experienced by the mothers and embryos, but not by the conditions during post-hatching growth. We suggest that the thermal plasticity producing the TSR in rotifers may reflect the joint impacts of a maternal effect and a direct effect of the environment during egg development. The two-point control of the TSR resembles the thermal determination of other biological phenomena, similar to the thermally determined sex determination in ectotherms. Our results contribute not only to better understanding the proximate mechanisms of TSR, but also to comprehending the general biological mechanisms of response to temperature, which is one of the most important ecological factors.     

Reproductive investment of several rotifer species

A comparative study was carried out on the seasonal variation in egg and body volumes of four species of rotifers, Brachionus calyciflorus, Brachionus angularis, Keratella quadrata and Anuraeopsis fissa, from ponds near Sevilla and in Doñana National Park. Temperature and food concentration significantly affected egg volume in all four species; clutch size significantly affected egg volume in A. fissa, B. calyciflorus and B. angularis, but not in K. quadrata. As temperature increased, egg and body volumes declined in A. fissa and B. angularis but increased in K. quadrata and B. calyciflorus due, perhaps, to complex interactions between temperature, food level and clutch size.All four rotifer species responded similarly to changes in food concentration: below a certain food level, egg and body volumes were small but increased to a maximal size, which differed in the four species, as the food concentration was raised. At higher food levels, there was a reduction in egg and body volumes. The food levels for maximum egg and body volumes are interpreted as optimal ones; below these, food limitation reduces the size of the reproducing adult and, consequently, the size of eggs. Above this optimal level, higher food levels accelerated the rotifer life cycles, resulting in adults maturing at smaller sizes with a larger number of smaller eggs.     

Effect of two microalgae concentrations on body size and egg size of the rotifer <Emphasis Type="Italic">Brachionus calyciflorus</Emphasis>

The rotifer, Brachionus calyciflorus, was grown with two algae species (Chlorella sp. and Scenedesmus obliquus) at different concentrations (0.1, 1 and 10 × 10⁶ cells ml⁻¹). The body size (lorica biovolume) of individual rotifer and their egg size were measured when the populations were roughly in the exponential phase of population growth. The body size of the rotifers differed significantly (P < 0.05) among the two algae species used, however this effect was not observed for egg size. The body size of rotifers fed on higher densities of Chlorella sp. (10 × 10⁶ cells ml⁻¹) was significantly larger than for those fed on lower and medium densities (0.1 and 1 × 10⁶ cells ml⁻¹). Body size and egg size of rotifers fed with different amounts of Scenedesmus did not differ significantly. The egg size was significantly larger at higher food level of Chlorella. A significantly positive correlation was observed between the adult rotifer body size and their egg size.     

Utilization of cyanobacteria by Brachionus calyciflorus: Anabaena flos-aquae (NRC-44-1) as a sole or complementary food source

The rotifer Brachionus calyciflorus can utilize the cyanobacterium Anabaena flos-aquae as either a sole or supplementary food source in laboratory culture. Positive population growth rates accompany food densities of 10 or 100 µg dry weight ml^–1, but slightly negative rates are found at a lower density (1.0 µg ml^–1). These results are consistent for rotifers feeding on two strains of A. flos-aquae, UTEX-1444 and NRC-44-1, with slightly enhanced survivorship and reproduction with the latter food. A 1:1 mixture (by dry weight) of Euglena gracilis and A. flos-aquae (NRC-44-1) produces survivorship comparable to that of control rotifer cohorts fed E. gracilis alone, but elicits significantly greater fecundity and population growth rates than found with the control food suspension at the same biomass density.     

流溪河水库敞水区轮虫多样性与群落的动态特征

流溪河水库是位于北回归线上的大型峡谷型供水水库,于2013—2014年对该水库敞水区轮虫群落及其环境因子进行观测,分析敞水区轮虫多样性与群落动态特征。流溪河水库介于贫营养到中营养水平之间,丰水期(5月—9月)的总磷、透明度均高于枯水期(10月—4月),两年的总磷的平均值为0.019mg/L,透明度为2.55m。两年共检出轮虫40种,单月检出物种数波动范围为8—19种,月均检出轮虫13种。螺形龟甲轮虫、真翅多肢轮虫、沟痕泡轮虫和胶鞘轮虫是主要优势轮虫。2013、2014年轮虫年均总丰度分别为68.9、66.9个/L,两年物种丰富度在丰水期大于枯水期,但枯水期轮虫群落Simpson多样性指数均大于丰水期。采用Bray-Curtis距离测度群落之间的相异性,计算表明两年的枯水期轮虫群落之间的平均距离均大于丰水期,即丰水期时轮虫群落相似性较高,枯水期时轮虫群落变异较大。丰水期的水温和水质参数的变化相对稳定,种类的优势度更为明显,导致丰水期群落之间更为相似。主成分分析表明,无柄轮属、胶鞘轮属、多肢轮属、异尾轮属、泡轮属、晶囊轮属和皱甲轮属的主要种类的全年分布有较大的差异,而其余多数轮虫种类全年分布差异较小,反映了我国南亚热带地区水温全年变幅小,可维持较多的全年性种类共存。群落的方差解析与RDA分析表明,相对于生物与化学变量,物理环境变量主导了流溪河水库敞水区轮虫群落的变异,就单个变量而言,水温和食物是影响流溪河水库敞水区轮虫群落结构的重要因素,降雨则是影响轮虫群落结构变异的宏观因素。绝大多数轮虫为滤食性的,处于食物链底端,个体小、生活史短,轮虫群落在对环境因子变化的响应上与浮游植物具有相似性。     

Rotifers as food in aquaculture

The rotifer Brachionus plicatilis (O.F. Muller) can be mass cultivated in large quantities and is an important live feed in aquaculture. This rotifer is commonly offered to larvae during the first 7–30 days of exogenous feeding. Variation in prey density affects larval fish feeding rates, rations, activity, evacuation time, growth rates and growth efficiencies. B. plicatilis can be supplied at the food concentrations required for meeting larval metabolic demands and yielding high survival rates. Live food may enhance the digestive processes of larval predators. A large range of genetically distinct B. plicatilis strains with a wide range of body size permit larval rearing of many fish species. Larvae are first fed on a small strain of rotifers, and as larvae increase in size, a larger strain of rotifers is introduced. Rotifers are regarded as living food capsules for transferring nutrients to fish larvae. These nutrients include highly unsaturated fatty acids (mainly 20: 5 n–3 and 22: 6 n–3) essential for survival of marine fish larvae. In addition, rotifers treated with antibiotics may promote higher survival rates. The possibility of preserving live rotifers at low temperatures or through their resting eggs has been investigated.     

Species composition,distribution and seasonal dynamics of Rotifera in a Rift Valley lake in Ethiopia (Lake Awasa)

The species composition of rotifers in Lake Awasa was studied and 40 species recorded. Seven species appeared in large numbers in the plankton seasonally. Brachionus and Keratella species made up more than 50% of the rotifer community by numbers during the observation period (1983–1987). The standing stock numbers of rotifer species are low with a maximum of 50 individuals L^–1, and some possible reasons for this observation are discussed. Most rotifer species are distributed randomly in the lake and show 3-fold fluctuations between consecutive days.The seasonal dynamics of most rotifer species are correlated with mixing periods in the lake, and the amplitude of seasonal fluctuation may be as high as 50-fold. Clear-cut seasonal succession of rotifer species was observed during the study period, but no consistent seasonal pattern for individual species was obvious. Also, observations indicate that rotifer biomass is partly sustained by availability of carbon through the bacterial pathway and that competitive exclusion for food by nauplii and ciliates probably keeps rotifer abundance low.     

Effect of Food Quantity and Quality on Population Growth Rate and Digestive Activity in the Euryhaline Rotifer Brachionus plicatilis Müller

We investigated the nutritional effects of both food quantity and quality on Brachionus plicatilis. Decomposition of particulate and dissolved organic matter by rotifer digestive enzymes play a crucial role in rotifer nutrition. Among other enzymes, rotifers produce phosphatases, non‐specific enzymes that allow for the release of orthophosphate from a variety of organic phosphorus compounds. Phosphatase saturation was measured in B. plicatilis homogenates using the spectrofluorimetric method. We examined population growth rate, reproduction and phosphatase activity in the homogenate of rotifers (PA_RH) fed by nutrient‐replete algal food supplied at different quantities. Population growth rate, number of eggs per individual and PA_RH were affected by food quantity. Growth rate and number of eggs per individual significantly increased in rotifers fed by food supplied at the highest quantity. The highest population growth rate was reached by rotifers fed by nutrient‐replete food, while it did not significantly differ between rotifers fed on nitrogen (N)‐depleted and phosphorus (P)‐depleted food. The number of eggs per individual was more affected by N than P supply. PA_RH and rotifer RNA content were not influenced by different food quality. The results indicate that B. plicatilis is not able to regulate its digestive apparatus in terms of efficiently getting access to essential nutrients when scarce, but do this when nutrient‐replete food is available in different quantity. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Why get big in the cold? Towards a solution to a life-history puzzle

The temperature–size rule (TSR), which states that body size increases at lower developmental temperatures, appears to be a near-universal law for ectotherms. Although recent studies seem to suggest that the TSR might be adaptive, the underlying developmental mechanisms are thus far largely unknown. Here, we investigate temperature effects on life-history traits, behaviour and physiology in the copper butterfly Lycaena tityrus in order to disentangle the mechanistic basis for the above rule. In L. tityrus the larger body size produced at a lower temperature was proximately due to a greater increase in mass, which was caused by both behavioural and physiological mechanisms: a much-increased food intake and a higher efficiency in converting ingested food into body matter. These mechanisms, combined with temperature-induced changes at the cellular level, may provide general explanations for the TSR. Body fat and protein content increased in butterflies reared at the higher temperature, indicating favourable growth conditions. As predicted from protandry theory, males showed reduced development times, caused by higher growth rates compared to females. The latter was itself related to a higher daily food consumption, while the total food consumption (due to the females’ longer developmental period) and assimilation was higher in females and may underly the sexual body size dimorphism.     

Microgeographic variation in body size and development time in the waterstrider,Limnoporus notabilis

Summary Body size and development time were compared among four population of Limnoporus notabilis (Heteroptera: Gerridae), in the southwestern corner of mainland British Columbia, Canada. Comparisons based on both field-caught and lab-reared animals indicated significant, heritable variability in body size among these four populations. Although body size and development time tended to be positively correlated, the variability in development time among populations was significant only for females. Comparisons of food availability at two of the study sites indicated that the observed patterns of variability in body size and development time could not be the result of genetic or phenotypic adaptations to food supply. These patterns also do not appear to reflect direct adaptation to season length. Both body size and development time were, however, positively associated with environmental heterogeneity, as indicated by habitat stability and continuity. In seasonal habitats, this positive association is complicated by selection for rapid development, leading to populations of large L. notabilis with relatively short development times. The hypothesis is thus advanced that microgeographic variability in body size and development time in L. notabilis reflects the interplay between selective adaptation to environmental heterogeneity, as proposed by Roff (1977, 1978) and dingle et al. (1980), and selection for rapid development in seasonal habitats.     

Characterization of the bacterial flora of mass cultivated Brachionus plicatilis

Bacterial density and composition in association of mass cultivated rotifers (Brachionus plicatilis, SINTEF-strain) was investigated, during experimental conditions identical to the procedures used for preparing rotifers as live food for marine cold water fish larvae. These procedures include cultivation, enrichment with squid meal and acclimation to low temperature by storage of the rotifer culture at 6 °C. Large variations were observed in the number of rotifer associated (1.8–7.6 · 10³ colony forming units per rotifer^–1) and free-living (0.6–25 10⁷ cells·ml^–1) bacteria. An increase of 50–150% in the bacterial number was normally observed after feeding the rotifer with squid meal, but after three days of acclimation at 6 °C, the bacterial numbers decreased to the initial level.After enrichment of the cultures with squid meal, the similarity in the composition of the bacterial flora between the rotifers and water was reduced. However, acclimation of the culture at 6 °C resulted in better agreement of the rotifer associated flora and that in water. Enrichment of the cultures induced a shift in the bacterial composition from Cytophaga/Flavobacterium dominance to Pseudomonas/Alcaligenes dominance. The bacterial flora of the rotifer cultures are dominated by presumably opportunistic species after enrichment, which may have detrimental effects when rotifers are fed as live food to marine fish larvae.     

Monoxenic cultivation of the rotifer Brachionus calyciflorus in a defined medium

Summary Techniques are described for the initiation and maintenance of axenic cultures of Euglena gracilis strain Z and monoxenic cultures of Brachionus calyciflorus variety pala with the Euglena, using in both the same defined, buffered medium. The medium, which is inorganic—except for the citrate chelating agent, the buffer, and vitamins B₁ and B₁₂ — has been used for the axenic cultuvation of the Euglena for more than 13 months. The monoxenic Brachionus cultures, established by inoculating rotifers into Euglena cultures, have been maintained for more than 8 months. Contamination tests on the rotifer cultures were performed frequently in three different test media.Mictic females, males, and resting eggs of Brachionus were observed in the monoxenic cultures, and considerable variation in the length of the posterolateral spines was noted.The compatibility of a rotifer to a defined medium which sustains the axenic culture of its food organism is a feature of this system which is convenient, useful, and unique to date in synxenic rotifer culture work.Supported by National Science Foundation Grant No. GB 7717.     

Regulation of rotifer densities by crustacean zooplankton in an oligotrophic montane lake in British Columbia

Summary To examine the relative demographic effects of predation and competition for food in rotifers during spring and summer in an oligotrophic lake, predator and competitor densities and food supplies were experimentally altered inside large enclosures. Abundances of rotifer species were positively correlated with experimental densities of fourth instar Chaoborus trivittatus larvae, a major crustacean predator in 1976. Experimental alteration of the densities of Daphnia rosea and Diaptomus leptopus and D. kenai in 1978 produced highly significant increases in rotifer biomass only under Daphnia removal, but not under copepod removal. Inorganic fertilizer additions to enclosures in 1978 and 1979 revealed minimal rotifer increases unless pulsed additions were large or Daphnia were also excluded. Large demographic reponses of rotifers to low fertilizer loadings in the absence of Daphnia confirmed the pre-eminence of competitive food limitation in producing rotifer scarcity in summer.     

Maternal age and spine development in the rotifer Brachionus calyciflorus: increase of spine length with birth orders1

1. Maternal effects have long been known to influence phenotypic plasticity in rotifers. Females in Brachionus calyciflorus and several other species produce long‐spined offspring when the predatory rotifer Asplanchna is present; B. calyciflorus also develops short spines when food concentrations are low. These spines protect against predation and decrease food threshold concentrations. 2. Some strains of B. calyciflorus develop long spines even in the absence of Asplanchna and other environmental stimuli. We demonstrate in this study that spine length in such cases is dependent on the age of the mother. 3. In strains from Florida and Georgia, offspring spine length increased significantly with birth order, sometimes to lengths formerly observed only in the presence of Asplanchna. Significant variation in this trait was found among and within clones of a strain. Offspring body size also increased with maternal age. This is the first time maternal age has been shown to affect rotifer morphology. 4. These birth‐order effects may have important ecological implications and explain phenotypic plasticity and polymorphism in body size and spine length in populations when predators are absent and food concentrations are high. They may be a bet‐hedging mechanism to assure adaptation to rapid changes in predation pressure or food conditions.     

Effects of an Ostracod (Cypris pubera) on the Rotifer Keratella tropica: Predation and Reduced Spine Development

This study investigates two different effects of an ostracod on a rotifer. The rotifer Keratella tropica and the ostracod Cypris pubera were cultured on the alga Cryptomonas erosa. Adult ostracods (2.2 mm body length) significantly reduced the population growth rate (r day^–1) of K. tropica from 0.42 to 0.13. Individuals of this size ingested live rotifers and produced fecal pellets with rotifer loricas. Smaller ostracods (both 0.59 and 1.61 mm body lengths) did not affect K. tropica 's population growth rate. Surprisingly, C. pubera significantly inhibited spine development in K. tropica. Rotifers cultured with juvenile, non‐predatory ostracods had similar lorica lengths but right and left posterior spines that were 10 and 30% shorter, respectively. The spine reduction induced by the ostracod kairomone is in striking contrast to the spine elongation induced in this rotifer by kairomones from copepods, cladocerans and Asplanchna. In shallow ecosystems, large ostracods that swim in the plankton may be important predators of rotifers. In addition, the presence of ostracods in plankton communities may be one of many factors affecting rotifer spine development. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Life cycle patterns of rotifers in Lake Peipsi

Life cycle strategies of rotifers in Lake Peipsi (Estonia) were examined. Bisexual (mictic) reproduction was detected in 26 species. Life cycle patterns were determined for 17 species. All three basic life cycle patterns were represented, with some evidence of intraspecific variability. The midcycle pattern, with mixis occurring near the population maximum, prevailed. Extended mixis was observed in littoral populations of Polyarthra luminosa and P. remata. Most species probably are monocyclic. Polyarthra dolichoptera and Synchaeta oblonga may be dicyclic. Mictic periods were not detected in Conochilus hippocrepis and C. unicornis, which probably are acyclic in L. Peipsi. Mictic ratios and egg ratios were calculated for some of the dominant species. The highest amictic egg ratios were observed slightly before or at the population maxima. High mictic ratios (1.0) indicated very intense bisexual reproduction in populations of A. fissa, P. dolichoptera and S. verrucosa. Definite periods of bisexual reproduction could not be distinguished in the rotifer community. During May–October, the spread of mictic reproduction merely reflects the general seasonal distribution of rotifers. Their life cycle strategies represent specific adaptations to unpredictability of their habitat. The results of the study confirm that mixis is an anticipatory event, and not a response to environment deterioration, or an ending of a rotifer population cycle.     

Space and time distribution of zooplankton in a Mediterranean lagoon (Etang de Berre)

In the Berre lagoon, a large brackish and swallow area near Marseille, the environmental factors (temperature, salinity, oxygen, suspended particulate matter and chlorophyll) generally display strong space and time variations. The rotifer Brachionus plicatilis and the copepod Acartia tonsa constitute the bulk of the zooplankton population during all the year. Their space and time distributions were studied in 23 stations distributed all over the lagoon, during four seasonal cruises (February, June, October, November), at surface and bottom layers. There is no marked difference in the horizontal and vertical distribution of the two species, (except in November when rotifers were prevailing in surface and copepods at depth) and in their time occurence. When the four series of data are pooled, correlation analysis show that A.tonsa is positively correlated with temperature, salinity and seston and negatively to oxygen and chlorophyll. B. plicatilis is positively correlated with temperature and seston, but also with chlorophyll, while salinity has a negative effect. The specific eggs number of both species is chlorophyll dependent. Considering seasonal cruises separately, some differences appear in the sense or the significance of these different correlations. The respective distribution of the two species is only partly dependent on the variation of the environmental factors: most of the variance remains unexplained, as indicated by the result of a stepwise multiple regression analysis using the most significant factors (temperature, salinity and oxygen explain 33 to 42% of the variance in Acartia, while temperature and salinity explain 27 to 28% of the variance in Brachionus). Thus, internal behavioral factors could also play a role in the distribution of organisms, particularly in some cases of aggregations of organisms observed during this study. As the two species occupied the same space habitat most of the year, they are potentially in competition for food. A way to optimize the food utilization could be the time separation of their feeding activity, nocturnal in Acartia and diurnal in Brachionus. Another way could be selective feeding upon food particles depending on their size (Brachionus being able to use finer particles than Acartia) or their quality (Brachionus being more herbivorous than Acartia) as demonstrated in some grazing experiments carried out in parallel.     

Ecology of rotifers of Surinsar,a subtropical,fresh water lake in Jamme (J & K), India

17 species of rotifers have been recorded from Lake Surinsar, Jammu (J & K), India of which some are either exclusively limnetic (Brachionus angularis, Hexarthra sp., Filinia opliensis), or littoral (Brachionus patulus, M. ventralis, Trichotoria sp., Platyias quadricornis, Lecane (Monostyla) decipiens, L (M), bulla and Lecane sp.) and others (Keratella tropica, Anuraeopsis fissa, Brachionus quadridentatus, B. calyciflorus, Trichocera sp., T. similis, and Polyarthra sp.) seem to be wandering species. Seasonal maxima for both littoral and limnetic zones are reported. Most population maxima are contributed mainly by one or at best two species.Physico-chemical factors like temperature, pH, dissolved oxygen, free carbon dioxide, calcium, magnesium, and total alkalinity have been studied and their infuence on these rotifer species are discussed. On their thermal responses, the available rotifer species have been classified as warm stenothermal, cold stenothermal or eurythermal. The importance of Mytilina ventralis as a biological indicator for dissolved oxygen in this lake has been pointed out.