Synchaeta Pectinata Research Articles

Trophi morphology and its usefulness for identification of formalin-preserved species of Synchaeta Ehrenberg, 1832 (Rotifera: Monogononta: Synchaetidae)

A study of temporal and spatial distribution of zooplankton frequently requires identification to species level. In such studies, samples are usually fixed, but according to different authors rotifer genera such as Synchaeta (Ehrenberg, 1832) can only be identified in live samples. A procedure for the identification of preserved specimens of Synchaeta is presented, permitting ecological studies without the examination of live material. Trophus morphology, investigated by light and scanning electron microscopy, was related to morphological characteristics of preserved specimens. In this way, body length was used to group formalin-preserved specimens into Synchaeta gr. tremula-oblonga sensu Ruttner-Kolisko (1974) (⩽170 μm) or Synchaeta gr. stylata-pectinata sensu Ruttner-Kolisko (1974) (⩾190 μm). Furthermore, body length, colour and shape were used to identify Synchaeta grandis (Zacharias, 1893), Synchaeta pectinata (Ehrenberg, 1832), Synchaeta kitina (Rousselet, 1902) and Synchaeta lakowitziana (Lucks, 1930). It was, however, impossible to distinguish Synchaeta tremula (Müller, 1786) from Synchaeta oblonga (Ehrenberg, 1832) by their trophi because of contradictions in the diagnostic keys and monographs regarding trophus morphology.

Phenotypic plasticity of body size at different temperatures in a planktonic rotifer: mechanisms and adaptive significance

Summary Larger body size at low temperatures is a commonly observed phenomenon in ectothermic organisms. The mechanisms that lead to this pattern and its possible adaptive significance were studied in laboratory experiments using the parthenogenetically reproducing rotifer Synchaeta pectinata. At low temperatures of 4 °C mean body volume was 46% larger than in individuals cultured at 12 °C. Egg volume was 35% larger in low vs high temperatures. Larger body size at low temperatures was caused by two mechanisms. First, when exposed to low temperatures, mothers laid larger eggs and the hatchlings of these eggs developed into larger adults (irrespective of temperature). Second, individuals cultured at low temperatures grew to a larger body size during their juvenile phase. The former mechanism had a greater influence on adult size than the latter. The production of larger eggs at low temperatures seemed to be due to a higher reproductive investment into individual offspring as it occurred independently of differences in maternal size. Life table experiments showed that offspring from small eggs (produced at high temperatures) had a significantly higher population growth rate than offspring from large eggs, when cultured at high temperatures. This was mainly due to an increase in fertility during the first days of adult life.

Seasonal variations of zooplankton abundance in the freshwater reservoir Valle de Bravo (Mexico)

Information on the density and diversity of zooplankton from drinking water reservoirs in Mexico is meagre. This is important not only from the point of view of lake management but also for providing clean drinking water for human populations. In the present work, we provide quantitative information on the seasonal variations of zooplankton and selected physico-chemical variables from Valle de Bravo, a large man-made reservoir in the State of Mexico. Based on the nutrient data, this reservoir can be regarded as mesotrophic. However, we found a high density of phytoplankton. Among Cyanophyceae, Anabaena, Microcystis, Nostoc and Oscillatoria were encountered, particularly during the warmer months. Microcystis blooms were observed from June to September. Diatoms dominated the phytoplankton during the remaining months of the year. Among zooplankton, Rotifera comprised the highest number of species. The most common species occurring throughout the year were Keratella chochlearis, Polyarthra vulgaris, Trichocerca capucina, Trichocerca similis, Asplanchna priodonta and Synchaeta pectinata. The highest abundance of K. cochlearis was observed in May at site 5 (340 ind 1−1 ). P. vulgaris occurred in high numbers throughout the year. Both Trichocerca capucina and T. similis were encountered in low numbers (mean annual average: 10 ind 1−1). Among the crustacean zooplankton, Bosmina longirostris occurred in higher numbers (up to 105 ind 1−1) regardless of the time of the year. Copepods were predominantly from the genera Mesocyclops and Megacylops; calanoids were rare. Naupliar stages were dominant during winter months (up to 670 ind 1−1). These results have been discussed from the point of view of Microcystis control in water bodies.

RESOURCE LIMITATION AND REPRODUCTIVE EFFORT IN A PLANKTONIC ROTIFER

Individuals that live in resource-limited environments are faced with an allocation problem since they generally do not have enough assimilated energy to fuel each of the processes of reproduction, somatic growth, and storage at the physiological maximum rate. Thus, they have to decide on the proportions at which they allocate energy to the various processes. In laboratory experiments and field observations, I found that the common planktonic rotifer Synchaeta pectinata increases the proportion of energy allocated to reproduction (reproductive effort, RE) as food becomes more limited. This change in the allocation scheme along a gradient of food concentrations was inferred from volumetric measurements of egg size and the size of the elliptically shaped ovary. Synchaeta pectinata used the contents of its ovary either for reproduction (increasing the size of the individually and consecutively produced eggs) or for storage (enhancing survival during starvation by resorption of this material). As egg size was relatively constant across food levels, the higher RE at low food concentrations was not due to the fact that Synchaeta channeled a larger absolute quantity of energy into reproduction, but rather that they lowered the threshold size for reproduction of the ovary and hence reproduced earlier. This pattern was also suggested by the results of the field study: when the resource concentrations declined during the study period, the ovary size of field-caught Synchaeta decreased considerably whereas the size of their eggs, collected at the same dates, stayed relatively constant. The lower reproductive threshold at low food concentrations resulted in a small ovary size after each egg deposition which implied a twofold cost. First, rotifers with small ovaries were less resistant to starvation because they could resorb only a little material from their ovaries. Second, it took longer for them to produce the next egg since a small ovary took longer to recover in size than a large ovary (for a given food concentration). The plasticity in the allocation scheme of Synchaeta pectinata can be interpreted as an adaptive strategy to variable food conditions. By lowering their reproductive threshold when food becomes limited, Synchaeta pectinata increase their chance to produce at least one offspring during their lifetime. In contrast, the higher reproductive threshold under good food conditions results in better maternal condition and, therefore, facilitates future reproduction.

Physiological responses to variable environments: storage and respiration in starving rotifers

1. Zooplankton exist in environments where food availability varies greatly over time, and success depends in part on the ability to store resources when food is abundant and to conserve them when food is scarce. This paper reports on interspecific differences in the size of stored reserves, and in respiration rate during food deprivation, of four species of planktonic rotifers.2. The size of reserves varied from 42 to 71% of initial (well‐fed) body mass. Interspecific differences in reserve size explained some of the previously observed differences in starvation time.3. The initial response of respiration rate to food deprivation was quite variable between species. Brachionus calyciflorus was the only species to conserve energy by decreasing respiration rate in response to food deprivation. In contrast, the respiration rate of starved Asplanchna priodonta increased, while that of A. silvestrii and Synchaeta pectinata did not change, during food deprivation.4. Theory predicts that temporal variation in resource level may facilitate the coexistence of competing species. This theory depends upon trade‐offs between traits that confer competitive success in different environments. Although rotifers show a trade‐off between competitive ability and maximum population growth rate, we found no evidence for trade‐offs between either of those two traits and the size of reserves.

Some observations on the diet of the backswimmer, Anisops wakefieldi (Hemiptera: Notonectidae)

In a series of laboratory experiments, A. wakefieldi was presented with a variety of prey from different systems to investigate its potential for cannibalism and for eating cladocerans, copepods and rotifers. Large (ca. 7 mm) Anisops decreased the survivorship of small (ca. 3 mm) Anisops, but ate only 0.1 individual predator-1 d-1. Ingestion rates of small Anisops on small (<0.5 mm) and large (0.5–0.9 mm) Ceriodaphnia dubia were 53 and 28 individuals predator-1 d-1, while those of large Anisops on these prey were about twice as high. Clearance rates showed that small Anisops ate small and large C. dubia with similar efficiencies, while large Anisops ate large C. dubia much more efficiently. Anisops ate C. dubia much more efficiently than the calanoid copepod Boeckella hamata. Only the large Anisops ate adult B. hamata. Large Anisops had a very low ingestion rate (0.6 individuals predator-1 d-1) on the larger congener, B. triarticulata. Small, but not large, Anisops ate the rotifer Synchaeta pectinata and small Anisops presented with a natural assemblage of rotifers ate S. pectinata and Polyarthra dolichoptera, but not four other species with a more well-developed lorica – Anuraeopsis fissa, Brachionus angularis, Keratella cochlearis and K. slacki. Ingestion rates on Synchaeta and Polyarthra were 14–20 individuals Anisops -1 d-1.

ASEXUAL DIAPAUSE INDUCED BY FOOD LIMITATION IN THE ROTIFERSYNCHAETA PECTINATA

In some clones of Synchaeta pectinata, females parthenogenetically produce two types of eggs: thin-shelled, subitaneous eggs and thicker-shelled eggs that undergo a brief diapause. The stimulus for producing diapausing eggs is food limitation, either throughout the life-span or for short intervals during early adult life. Inductive food concentrations are well above the threshold food concentration (where rm = 0). Even when severely food limited, some females produce only subitaneous eggs, and some produce a mixture of subitaneous and diapausing eggs. Production of both types of eggs may be a risk-spreading strategy: diapausing eggs provide a refuge from short periods of very low food availability, and subitaneous eggs permit some population growth as long as food availability remains above the threshold food concentration. While prolonged food limitation reduces the birth rate, the diapause response is not always associated with, and thus probably is not directly caused by, a decreased birth rate. This supposition is consistent with the fact that the presence of a toxic cyanobacterium at high food concentrations reduces the birth rate but does not induce the diapause response. The mechanism by which food limitation is assessed by the rotifer is not known. While diapause induced by food limitation is common in microorganisms, it has rarely been reported in aquatic invertebrates.

Asexual Diapause Induced by Food Limitation in the Rotifer Synchaeta pectinata

Asexual Diapause Induced by Food Limitation in the Rotifer Synchaeta pectinata

Egg size, offspring quality and food level in planktonic rotifers

1. For two species of planktonic rotifers, Brachionus calyciflorus and Synchaeta pectinata, neonates from larger eggs were more resistant to starvation than neonates from smaller eggs, so egg size was positively correlated with offspring quality.2. Theories of optimal offspring size in zooplankton predict either a decrease in offspring size with increasing food level, or maximum offspring size at intermediate food levels. In experiments using acclimated, even‐aged individuals, neither rotifer species changed its egg size as predicted by theory. B. calyciflorus egg size increased with increasing food concentration, while S. pectinata egg size did not change.3. The lack of fit to theory may have occurred because of constraints on minimum or maximum egg size, constraints on total reproductive effort, or because offspring fitness is influenced by traits in addition to neonate starvation time.

Effect of food availability on the response of planktonic rotifers to a toxic strain of the cyanobacterium Anabaena flos‐aquae

The ability of food concentration to modify the response of zooplankton to toxic cyanobacteria was examined using chronic (lifetime) toxicity tests with two rotifers, Brachionus calyciflorus and Synchaeta pectinata, and a strain of Anabaena flos‐aquae producing the alkaloid neurotoxin, anatoxin‐a. In a clone of S. pectinata, the cyanobacterium decreased both life span and lifetime fecundity to a much greater extent when the availability of food (Cryptomonas erosa) was low. Fecundity was the most sensitive parameter. In one experiment, the cyanobacterium (2.5 µg ml−1) decreased fecundity by 29% at the highest food concentration (2 × 104 cells ml−1) but by 90% at the lowest one (3 × 103 cells ml−1). In two clones of B. calyciflorus, A. flos‐aquae decreased lifespan, fecundity, and population growth rate, but there was only a tendency in one of the clones for the inhibition to be greater at lower food concentrations. The degree to which toxic cyanobacteria affect the population dynamics of susceptible zooplankton taxa in natural communities may be greatly influenced by modifying environmental factors such as food availability and temperature, especially if these factors act additively or synergistically.

Induction of diapausing amictic eggs in Synchaeta pectinata

Induction of diapausing amictic eggs in Synchaeta pectinata

Structure, development and induction of a new diapause stage in rotifers

SUMMARY1. This study shows that females of Synchaeta pectinata Ehrenberg can produce two morphologically and physiologically distinct kinds of amictic eggs that develop parthenogenetically into females. One kind, until now the only one known in monogonont rotifers, is a thin‐shelled (about 1.4 μm) subitaneous egg that develops without arrest. The other kind is a thick‐shelled (about 9 μm) egg that enters obligatory diapause after 1–3 cleavage divisions before resuming development. The thicker shell of the diapausing egg is due to an expansion of the outer sublayer of the single‐layered shell.2. The mean duration of the diapause at 19 °C (L : D 16 : 8) is about 14 days. This diapause is not broken by low temperature (5 °C), and it is not greatly extended by storage at low temperature (5 °C).3. Diapausing eggs are induced immediately after a brief starvation period, and seem to be produced at no additional energetic cost. However, a population producing a high proportion of diapausing eggs has a much reduced reproductive potential.4. Production of diapausing amictic eggs may be a strategy to increase the ability of clones to survive food limitation.5. Diapausing amictic eggs differ markedly from fertilized resting eggs produced following bisexual reproduction during favourable conditions.

Susceptibility of planktonic rotifers to a toxic strain of Anabaena flos-aquae

Reproduction in Asplanchna girodi, Brachionus calyczjlorus, Keratella cochlearis, and Synchaeta pectinata fed Cryptomonas at 19°C was inhibited by the presence of a strain of Anabaena flos-aquae (IC- 1) producing the neurotoxic alkaloid anatoxin-a. The most susceptible species, B. calycz&!orus, was suppressed at an Anabaena dry mass concentration of 0.5 pg ml-*; the others were suppressed at a concentration of 4 pg ml-‘. Reproduction of all species at 19°C was inhibited by anatoxin-a; the most sensitive species (S. pectinata) was inhibited at a concentration of 0.2 fig ml-l, and the least sensitive species (B. CalyciJorus), was inhibited at a concentration of 5 pg ml-’ but not one of 2 pg ml-‘. In contrast, A. girodi and two clones of B. calyciflorus were not inhibited by filtrates of very dense Anabaena suspensions (1 : 1 and 1 : 3 dilutions of 1 -d-old, 80 pg ml-r suspensions); this showed that the Anabaena did not release extracellular toxin, and thus that it could only inhibit rotifers that ingested it. B. calyczjlorus probably was the most susceptible to the cyanobacterium, even though it was the least sensitive to the soluble toxin, because it ingested the filaments most efficiently. Its relatively low sensitivity to the toxin may reflect an evolutionary response to its greater tendency to ingest toxic cyanobacteria. The effect of a toxic cyanobacterium on the structure of a freshwater zooplankton community should depend on the size and morphology of its colonies. Large colonies should be more readily ingested by daphniids than rotifers, and hence more likely to inhibit competitively dominant daphniids. Small, amorphous colonies or short, thin, nonmucilagecoated filaments should be ingested by, and thus inhibit, some rotifers as well as daphniids.

Jumping behavior in the oligotrich ciliates Strobilidium velox and Halteria grandinella, and its significance as a defense against rotifer predators

The jumping behavior of Strobilidium velox and Halteria grandinella was analyzed videographically. On average, undisturbed cells of these species jumped 1.7-3.6 and 8 times per minute and spent 0.8 and 1.0% of their time jumping, respectively. Both ciliate species initiated jumps after encounters with rotifer predators. S. velox jumped on contact with Asplanchna girodi, traveling a mean distance of 1.5 mm (33 body lengths) at a mean velocity of 7 mm/s (154 body lengths/s) at 17°C. H. grandinella jumped on contact or near contact with Synchaeta pectinata, traveling a mean distance of 0.37 mm (18 body lengths) at a mean velocity of 2.76 mm/s (131 body lengths/s) at 20°C. The maximum velocity recorded during these escape jumps was 16.07 mm/s for S. velox and 3.70 mm/s for H. grandinella. In S. velox, swimming velocity during jumps was not significantly correlated either with swimming velocity just before jumping (mean = 0.15 mm/s) or with distance traveled. In H. grandinella, jumping velocity and distance also were not significantly correlated. Jumping in S. velox and H. grandinella was calculated to require approximately 149% and 41 % of total metabolic rate, respectively. Jumping seemed to be an effective defense against rotifer predation. Only 3% of 93 S. velox cells contacted by A. girodi were captured, and only 12% of 92 H. grandinella cells contacted or closely approached by S. pectinata were captured; all other cells jumped away. A predation experiment showed that A. girodi was about twice as, and significantly more, likely to ingest Paramecium tetraurelia as S. velox in a mixture of equal numbers of these ciliates. The swimming velocity of S. velox during jumps is the highest one so far reported for an oligotrich, and equals the highest one reported for any ciliate (Mesodinium rubrum).

Rotifers as predators on small ciliates

Clearance rates of Synchaeta pectinata, Brachionus calyciflorus and Asplanchna girodi on Tetrahymena pyriformis (46 µm in length) at a density of 10 cells ml−1, in the presence of algal food, were 2.5 to 6.1 ml rot.−1 day−1. Clearance rates of these rotifers were, respectively, about 2, 3, and 13 times lower on Strobilidium gyrans (58 µm in length) than on T. pyriformis, indicating that the saltations of S. gyrans are an effective escape response. Clearance rates of S. pectinata were considerably lower on Colpidium striatum (81 µm) than on S. gyrans, suggesting that S. pectinata may not be able to ingest ciliates of this size. S. pectinata had a clearance rate of 19 ml rot.−1 day−1 on S. gyrans at a density of 1.2 cells ml−1, in the absence of edible algal food. Rotifers may prey extensively on ciliates in natural plankton communities, ingesting 25 to 50 individuals in the 45–60 µm size range day−1.

Suspended Clay and the Population Dynamics of Planktonic Rotifers and Cladocerans

The direct and indirect effect 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 occuring, concentrations (50—100 mg/L) of coarse, suspended clay (&lt;2—μm particle size) caused large reductions in the population growth rates (rm) of four cladoceran species (Bosmina longirostris, Ceriodaphnia dubia, Daphnia ambigua, and D. pulex). Juveniles were more susceptible than adult to suspended clay. Low concentrations (10 mg/L) of coarse clay, and high concentrations of fine clay (&lt;1 μm), did not decrease, and sometimes increased, cladoceran population growth rates. Both the inhibitory effects of high concentrations of coarse clay and the stimulatory effects of low concentrations of clay were greater at limiting food (Cryptomonas) concentrations. The population growth rates of four rotifer species (Brachionus calyciflorus, 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 rates 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 mg/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 mg/mL) was unaffected by suspended clay. Suspended clay reversed the outcome of two rotifer—cladoceran competition experiments, between K. cochlearis and D. ambigua, 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.

Prey features affecting ingestion rates by Acanthocyclops robustus (Copepoda: Cyclopoida) on zooplankton.

(1) Ingestion rates by adult female and juvenile Acanthocyclops robustus on a number of prey types were measured at a prey concentration of 100/l in experimental volumes of 300-400 ml. (2) For the adult predator, Synchaeta pectinata was most vulnerable (22.3, standard error 1.4, prey ingested per predator per day) as compared to Brachiomus calyciflorus, Brachionus diversicornis, Keratella cochlearis (two morphs), Asplanchna priodonta, Polyarthra major, Synchaeta kitina, Pompholyx sulcata, Daphnia spec., and Bosmina longirostris. For these latter prey, the lowest ingestion rate was on one morph of K. cochlearis and the highest on A. priodonta, being, respectively, 1.0, SE 0.5, and 11.3, SE 1.0, prey per predator per day. (3) With regard to the juvenile predator (mostly copepodite stages I, II and III), ingestion rates on K. cochlearis and P. sulcata were low (respectively 1.2, SE 0.7, and 0.3, SE 0.1, prey per predator per day) but quite high on S. kitina (5.7, SE 0.6). (4) In addition, the effect of increasing prey concentration on the ingestion rate (functional response) by the adult female predator was examined for B. calyciflorus, K. cochlearis, S. pectinata, S. kitina and Daphnia spec.. Increases in ingestion rate with prey density were minimal for B. calyciflorus and K. cochlearis, greater for Daphnia spec., still greater for S. pectinata and of greatest magnitude for S. kitina. (5) The reasons for these results are discussed with particular reference to prey features.

Grazing on Bacteria by Zooplankton in Australian Billabongs

Five species of rotifer (Brachionus lyratus, Conochilus dossuaris, Keratella cochlearis, Keratella quadrata and Trichotria tetractis) ingested bacteria at a prey density of 5-7 × 109 cells L.-1, and a sixth (Keratella procurva) did so at a higher bacterial concentration of 4 × 1010 cells L-1. Gastropus minor, Polyarthra vulgaris, Synchaeta pectinata and Trichocerca similis were not bacteriovores. Fluorescent-stained (DATF or acridine orange) bacteria were not suitable for determining which rotifers were potential grazers. However, bacteriovory was detected easily with safranin-red or methylene-blue stained bacteria. Clearance rates for the zooplankton community larger than 51�pm, determined with 3H-thymidine labelled bacteria, varied from &lt;0.1-71 mL L-1. H-1, but were usually &lt;5 mL L-1 h-1. Significant grazing was detected in &gt;200�m, 90-200�m and 51-90�m fractions: it was impossible to ascribe bacteriovory to discrete taxonomic groups, because zooplankton taxa were not completely separated by these mesh sizes.

The effect of Daphnia interference on a natural rotifer and ciliate community: Short‐term bottle experiments

Two bottle experiments were conducted using water from a small, eutrophic lake to assess the impact of Daphnia interference (encounter) competition on the dynamics of the rotifer and ciliate populations dominating the zooplankton community. Indirect effects of Daphnia exploitative competition were minimized by adding food resources and using short incubation periods. The introduction of Daphnia pulex (16 liter−1) to Cryptomonas‐enriched (3–3.5 × 104 cells ml−1) water for 2 d significantly suppressed numbers of the ciliate Campanella sp.; the rotifers Kellicottia bostoniensis, Keratella cochlearis, Keratella crassa, Polyarthra vulgaris, and Synchaeta pectinata; and total rotifers. The presence of Daphnia did not affect numbers of the rotifers Asplanchna girodi, Polyarthra euryptera, and Trichocerca similis. Daphnia‐induced death rates of significantly suppressed, common species were highest for K. cochlearis (0.79–1.14 d−1), intermediate for S. pectinata and Campanella (0.23–0.33 d−1), and lowest for K. crassa and P. vulgaris (0.14–0.28 d−1). The most marked effects of Daphnia on community structure were 61–77% reductions in the relative abundance of K. cochlearis and up to 100% increases in the relative abundances of K. crassa and T. similis. The observed effects of Daphnia on the rotifer species are consistent with, and largely explicable by, previously conducted laboratory experiments and behavioral observations on interference between Daphnia and these or closely related species. Interference from Daphnia can rapidly reduce the abundance and shift the species structure of rotifer and ciliate assemblages in natural communities.

Larval Mirogrex terraesanctae (Cyprinidae) of Lake Kinneret (Israel): growth rate, plankton selectivities, consumption rates and interaction with rotifers

The rotifer Synchaeta pectinata dominated gut content of first feeding Mirogrex larvae (7 mm, 10 days age) and was a selected prey of neuston-caught larvae up to 15 mm TL. A negative L-value (linear index of selection) applied to predation on nauplii and copepodites by 7 and 8 mm larvae; nevertheless, caloric intake was dominated by copepods in 8–10 mm larvae. Neuston-caught larvae 13–20 mm TL fed selectively on Cladocera, especially Bosmina, and on the rotifer Asplanchna spp.