The effects of thin layers on the vertical distribution of larval Pacific herring, Clupea pallasi

Temporal and spatial heterogeneity of resources is likely to have dramatic effects on the behaviors of zooplankton and ichthyoplankton, which in turn are likely to have strong effects on ecological dynamics such as predation, growth, and mating. The objective of this study was to determine whether vertically thin layers of extreme prey concentration affect the vertical distribution of larval Pacific herring (Clupea pallasi). We employed 2-m tall experimental tanks equipped with video cameras that scanned the vertical extent of the tanks to investigate the effects of thin layers on the vertical distribution of 5- and 10-day-old herring larvae. Three treatments were established: (1) a thin layer of prey (rotifers, Brachionus plicatilis) through density (salinity) stratification, (2) homogeneous vertical distribution of both prey and density, and (3) density (salinity) stratification, but with a homogeneous distribution of prey. We found that in all treatments the majority of larval herring were at the surface, near the light, despite the absence of a peak in rotifer abundance at this depth in some instances. However, there were also clear effects of the thin layers—secondary subsurface peaks in herring abundance occurred at the mid-depths in the stratified tanks, in and around the thin layers. In addition, our results provide some evidence that thin layers specifically, rather than prey patches generally, influence the vertical distribution of larval herring, i.e., larvae may use the physical properties of thin layers to locate and distribute themselves, instead of reacting solely to the prey patches. Thus thin layers can affect directly the vertical distribution of larval herring, and perhaps indirectly their horizontal distribution, as herring larvae live in environments (e.g., estuaries) where advective transport is also often vertically heterogeneous.     

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)     

Toxicity and haemolytic activity of a newly described dinoflagellate,Heterocapsa bohainensis to the rotifer Brachionus plicatilis

The algae Heterocapsa bohaiensis is a newly described species of dinoflagellate associated with Penaeus japonicus and larvae of Eriocheir sinensis in a coastal pond of Liaodong Bay China. The rotifer Brachionus plicatilis is used as live feed for aquaculture organisms including prawns and crabs larvae. To evaluate the potential toxicity of H. bohaiensis, the effects on Brachionus plicatilis and haemolytic activity were investigated in this study. The results showed that H. bohaiensis had significant toxic effect on B. plicatilis, and this effect was dependent on the cell concentration. Significant rotifer growth suppression was observed in the ruptured cells of H. bohaiensis with ultrasonic. Relatively similar rotifer mortalities were induced both in the light and in the dark. Interestingly, haemolysis to erythrocytes was also caused in a cell density-dependent and time-dependent manner, which meant the results of haemolytic activity were consistent with the toxicity. Therefore, haemolytic toxins were considered to be involved in the toxic mechanism of H. bohaiensis against rotifers. Then, the concentrations of calcium were measured in the mastax, stomach and ovary of B. plicatilis. Obviously increased fluorescence intensity was found in the stomach, which indicated the alteration of calcium homeostasis and membrane permeability after ingesting H. bohaiensis. These results implicated haemolytic activity as a causative factor linked to the toxicity of H. bohaiensis against B. plicatilis. The results contributed to research the production and control of H. bohaiensis toxins.     

The effects of algae species and densities on the population growth of the marine rotifer, Colurella dicentra

Colurella dicentra clones isolated from bay water in the Mississippi Gulf Coast were cultured with artificial seawater. Experiments were conducted to determine the effects of six algae species (Nannochloropsis oculata, Tetraselmis chuii, Chaetoceros gracilis, Rhodomonas salina, Isochrysis galbana, and Prorocentrum micans), six C. gracilis densities, and six N. oculata densities (25,000, 50,000, 100,000, 250,000, 500,000, and 1,000,000 cells ml^− 1) on C. dicentra population growth. Algae type influenced rotifer production (p < 0.0001). C. gracilis treatment (9120 ± 3351SD) produced the highest number of rotifers followed by N. oculata (5760 ±2232SD). P. micans had the lowest number of rotifers, although not significantly different from numbers in T. chuii, R. salina, and I. galbana treatments (p > 0.05).The population growth rate (r) varied with algae species treatment. The highest values were recorded for C. gracilis treatment (0.22 to 0.26 d^− 1), followed by N. oculata (0.21 to 0.24 d^− 1), and the lowest for P. micans (− 0.19 to 0.14 d^− 1). C. gracilis and N. oculata densities had significant effects (p < 0.0001) on C. dicentra population growth. The highest rotifer production was recorded at a C. gracilis density of 100,000 cells ml^− 1, followed by 250,000 cells ml^− 1 and 50,000 cells ml^− 1. Algae densities of 500,000 cells ml^− 1 and above produced the lowest rotifer numbers. Population growth rate (r) varied with C. gracilis densities. The highest values were observed for C. gracilis concentrations of 100,000 cells ml^− 1 (0.17 to 0.19 d^− 1), and the lowest for concentrations of 500,000 cells ml^− 1 and above (− 0.19 to 0.09 d^− 1). The 100,000 cells ml^− 1N. oculata density gave the highest rotifer production followed by 50,000, 250,000, 25,000, and 500,000 cells ml^− 1. Algae densities of 1,000,000 cells ml^− 1 produced the lowest rotifer numbers. Population growth rate (r) varied with N. oculata densities, with the highest values obtained for algae densities of 100,000 cells ml^− 1 (0.35 to 0.40 d^− 1), and the lowest for concentrations of 1,000,000 cells ml^− 1 (0.05 to 0.012 d^− 1). This is the first report of C. dicentra in Mississippi Coastal waters, and perhaps the smallest marine rotifer species (93 by 49 μm) ever cultured successfully.     

Use of freeze-dried microalgae for rearing gilthead seabream, Sparus aurata L., larvae. II. Biochemical composition

The objectives of this study were to test whether freeze-dried microalgae are nutritionally adequate for rearing rotifers as food for gilthead seabream larvae. The elemental composition (C, N, H) and fatty acid composition were analysed in larvae of gilthead seabream, Sparus aurata L., rotifers Brachionus plicatilis and Brachionus rotundiformis and freeze-dried microalgae Nannochloropsis oculata. Four larval feeding treatments were tested: (A) larvae fed rotifers cultivated with freeze-dried microalgae and daily addition of freeze-dried microalgae to the larval tanks; (B) larvae fed rotifers cultivated with freeze-dried microalgae and daily addition of live microalgae to the larval tanks; (C) larvae fed rotifers cultivated with freeze-dried microalgae, without addition of microalgae to the larval tank and (D) larvae fed rotifers cultivated with live microalgae and daily addition of live microalgae to the larval tanks. No significant differences were observed between the biochemical composition of larvae with treatment A (with freeze-dried microalgae) and the composition of larvae in treatment D that were obtained with the acceptable methods for culture systems (with live microalgae).     

Presence and development of populations of the introduced brown alga Sargassum muticum in the southwest Netherlands

Of the three Brachionus species used in aquaculture, Brachionus rubens, B. calycilorus 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. calycilorus is limited, probably because inert food products are readily available as feed for freshwater larvae.The rotifer Brachionus plicatilis is used in large numbers as the first food organism in intensive cultures of marine fish and shrimp larvae. An adequate supply of these rotifers relies on mass cultures. The reproductive rate of rotifers in these cultures depends on food quality and quantity, salinity, temperature and pH of the medium. Removal of waste products from culture tanks leads to higher and more efficient production of rotifers over extended periods of time. Rotifers have to be enriched with polyunsaturated fatty acids, which are essential for proper development and survival of marine fish and shrimp larvae.The future use of preserved rotifers and their resting eggs may help to overcome unforeseen failures of live cultures and may lead to more efficient use of these organisms in raising freshwater and marine fish and shrimp larvae.     

Population growth dynamics of the rotifer Brachionus plicatilis cultured in non-limiting food condition

Population growth parameters in batch culture of Brachionus plicatilis under a continuous supply of freeze-dried microalgae powder have been determined. Two B. plicatilis strains (L- and S-types) and four microalgae species (Nannochloropsis oculata, Nannochloropsis gaditana, Nannochloris oculata and Tetraselmis suecica) have been tested, establishing the dynamics of growth at different daily food rations. Cultures showed a short lag phase, an exponential growth phase, a long post-exponential growth phase and long decline with episodic increases. In both rotifer strains, the best growth was obtained with Nannochloropsis oculata and the poorest with Nannochloris oculata.     

Population Growth of Six Iranian Brachionus Rotifer Strains in Response to Salinity and Food Type

Intrinsic rates of population increase (r) were evaluated as a measure of population dynamics of four strains of Brachionus plicatilis and two strains of B. urceolaris from Iran in response to different salinities and feeding algae. Each rotifer strain was cultured at four salinities: 5, 20, 25 and 30‰ and fed with two microalgal species: Chlorella vulgaris and Nannochloropsis oculata. Salinity of 5‰ was critical for all the examined strains, at which r was at minimum and was different from the other salinities (P < 0.05). For B. plicatilis strains, the maximum r was observed in those fed on Chlorella at salinities of 10 and 30‰ (64 ± 0.01 day⁻¹). While, in B. urceolaris, maximum r was for Nannochloropsis fed rotifers at salinity of 20‰ (0.69 ± 0.01 day⁻¹). Maximum final population density (FD) was obtained for a strain of B. urceolaris fed on Nannochloropsis at 20‰ (329.3 ± 10.9 ind.mL⁻¹). FD was relatively lower in B. plicatilis strains among all examined salinities. ANOVA showed the significant effect of salinity and rotifer strain, and algae × rotifer strain on both r and FD, and salinity × rotifer × algae on FD (P < 0.05). (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Rotifer nutrition using supplemented monoxenic cultures

The evolution of rotifer feeding/ nutritional studies is discussed together with their relevance to ecological observations. Aseptic conditions and initially synxenic cultures are regarded as a basis for nutritional work. The marine rotifer Encentrum linnhei requires the amino acid dl-tryptophan as a supplement to the food-alga Brachiomonas submarina. Observations on feeding rotifers in natural water samples, together with the morphology of their feeding mechanisms, show Encentrum to be an omnivore; a natural source of tryptophan is suggested. Vitamin B₁₂ and thiamine requirements of Encentrum and Brachionus plicatilis are examined and evidence shown for the quantitative control of the former vitamin by the rotifer's food algae. Axenic cultivation of rotifers is discussed and restricted growth of Brachionus reported under such conditions.     

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.     

Nutritional effect of freshwater Chlorella on growth of the rotifer Brachionus plicatilis

Mass production of Brachionus plicatilis is usually accomplished by feeding so-called marine Chlorella (Nannochloropsis oculata) to the rotifers in marine fish hatcheries. If the marine Chlorella are in short supply, baker's yeast is usually used as a supplementary food. Recently, a condensed suspension of freshwater Chlorella (Chlorella vulgaris, k-22) was commercially developed as another supplementary food. We have evaluated the dietary value of this freshwater Chlorella for growth of the rotifer by means of individual and batch cultures. Rotifers cultured with the freshwater Chlorella suspension under almost bacteria-free conditions, showed very suppressed growth. However if the Chlorella was supplemented with vitamin B₁₂ by adding the vitamin solution into the suspension or by culturing the Chlorella in a medium containing vitamin B₁₂, the nutritional value of freshwater Chlorella was greatly improved and almost at the same level as that of marine Chlorella. Condensed Chlorella may therefore be effective as a supplementary food if vitamin B₁₂ is supplied.     

Response of two zooptankton grazers to an ichthyotoxic estuarine dinoflagellate

The dinoflagellate Pfiesteria piscicida (gen. et sp. nov.).a toxic ‘ambush predator’, has been implicated asa causative agent of major fish kills in estuanne ecosystemsof the southeastern USA. Here we report the first experimentaltests of interactions between P.piscicida and estuarine zooplanktonpredators. specifically the rotifer Brachionus plicatilis andthe calanoid copepod Acartia tonsa. Short-term (10 day) exposureof adult B.plicatilis to P.piscicida as a food resource, aloneor in combination with the non-toxic green algae Nannochlorisand Tetraselmis. did not increase rotifer mortality relativeto animals that were given only non-toxic greens Similarly,short-term (3 day) feeding trials using adult A.tonsa indicatedthat the copepods survived equally well on either P.piscicidaor the non-toxic diatom Thalassiosira pseudonana. Copepods giventoxic dinoflagellates exhibited erratic behavior, however, relativeto animals given diatom prey. The fecundity of B.plicatiliswhen fed the toxic dinoflagellate was comparable to or higherthan that of rotifers fed only non-toxic greens We concludethat, on a short-term basis, toxic stages of P.piscicida canbe readily utilized as a nutritional resource by these commonestuarine zooplankters. More long-term effects of P.piscicidaon zooplankton, the potential for toxin bioaccumulation acrosstrophic levels, and the utility of zooplankton as biologicalcontrol agents for this toxic dinoflagellate. remain importantunanswered questions.     

Chemoreception and vertical movement in planktonic yolk-sac larvae of red sea bream Pagrus major

The capability of planktonic yolk-sac larvae of red sea bream Pagrus major in detecting food was examined in the laboratory to ascertain basic knowledge on the early life history of this marine fish. After infrequent vertical burst swimming followed by slight rising or sinking, the larvae remained motionless within thin layers of concentrated food extract (rotifer, Brachionus plicatilis). At the moment of hatching, the larvae already have receptor cells with several cilia arranged radially in their open nostrils. Thus it is likely that by means of their vertical movement they are capable of sensing the thin food patch layer. We suggest that planktonic larvae of Pagrus major are capable of detecting and remaining within food patches even before the onset of feeding. The onset of food detection in the earlier stages may be, to some extent, the more efficient strategy for larval survival and growth because this ability could contribute to a reduction in energy consumption.     

Life history response and age-specific tolerance to starvation in Brachionus plicatilis O.F. Müller (Rotifera)

To examine the life history response and age-specific tolerance to starvation in the rotifer Brachionus plicatilis O.F. Müller, we carried out two series of individual culture experiments. In the first experiment, rotifers were fed until each of the ages of 1-4 days, and were then starved during the rest of their lifetimes. The control group was fed throughout their lifespans. Rotifers stopped active reproduction just after the onset of food deprivation, and showed shorter subsequent survival times when they were starved at older ages. The finding that the larger the number of offspring produced before food deprivation, the shorter the subsequent lifetime under starvation, appeared to reflect a trade-off with the cost of reproduction. In the second experiment, newborns were starved until each of the ages of 1-5 days, and were fed thereafter. The lifespans of the rotifers starved up to the age of 3 days were not statistically different from those that were not starved. Although the starved rotifers began to reproduce once fed again, their lifetime fecundity decreased significantly from that of the non-starved group. Based on these results, it was suggested that the reproductive suppression caused by starvation would cause rotifers to have a longer lifespan to allow for future reproduction.     

Use of swimming speed and egg ratio as predictors of the status of rotifer cultures in aquaculture

This study evaluated the use of egg ratio (eggs rotifer^–1) and swimming speed (mm min^–1) as prediction criteria for production and culture quality in mass cultures of the rotifer Brachionus plicatilis. Egg ratio was determined to be a suitable predictor of rotifer growth and production in the cultures. Low egg ratios (i.e., 0–0.17 eggs rotifer^–1) indicate reduced rotifer population over time (i.e., negative net population growth rates). However, at this time egg ratio dynamics are not suitably understood to predict in advance a sudden population collapse.Swimming speed of reproductive, egg-carrying females in the exponential growth phase was 40–45 mm min^–1. During exponential growth swimming speed was independent of the food used. Lower swimming speeds were obtained in late stationary phase (10–25 mm min^–1) when yeast was used as a food source. Both environmental factors (e.g., accumulating metabolites) and changes in nutritional state of the rotifers may have affected the swimming speed, but environmental factors appear to be the most important. We believe that swimming speed has the potential of becoming an accurate predictor of culture quality in mass cultures of rotifers.     

Production and nutritional quality of the rotifer Brachionus plicatilis fed marine Chlorella sp. at different cell densities

The effect of different cell densities of marine Chlorella sp. on the growth rate, doubling time and production of the rotifer Brachionus plicatilis was investigated. A significant increase in rotifer production was achieved at a density of 50 × 10⁶ Chlorella cells ml^–1. The nutritional quality of rotifers grown at different concentrations of Chlorella is discussed.     

Steady-state rotifer growth in a two-stage, computer-controlled turbidostat

Steady-state populations of rotifers (Brachionus calyciflorus)were maintained in twostage, continuous-flow turbidostatic cultureon the green alga Chlorella pyrenoidosa. In this system, themaximum specific growth rate,µ^max of the rotifers wasmaintained by using a computer to control the concentrationof algae, as rotifer food, in the rotifer culture. As rotifersconsumed algae, the turbidity decreased until a set-point wasreached. Then fresh algal suspension (supplied from a steady-statealgal chemostat) was metered into the rotifer culture, whichwas held in the dark. Rotifer and algal populations, as wellas rotifer µ^max entered steady states. These steady-stateresults were consistent with previous data from chemostat studies,but growth transients indicated that the of the µ^maxrotifersmay be subject to selection. The system is unique in providinga means to explore population dynamics of a metazoan maintainednear its µ^max.     

Axenic culture of <Emphasis Type="Italic">Brachionus plicatilis</Emphasis> using antibiotics

The rotifer Brachionus plicatilis culture is composed of complex microcosms including bacteria, protozoans, algae, and fungi. Previous studies reported methods to establish axenic rotifer cultures, but further refinement of these techniques is needed, for molecular biological research which requires pure culture to isolate nucleic acids from rotifers only. In order to render rotifer culture axenic, we tested five antibiotics: ampicillin (Amp), chloramphenicol (Cp), kanamycin (Km), nalidixic acid (Na), and streptomycin (Sm) at 30–100 μg/ml. Except for Cp, which reduces rotifer reproduction, all other antibiotics at the tested concentrations did not affect rotifer reproduction or show any toxic effects. A rotifer disinfection method was finally established by treating the resting eggs with 0.25% (w/v) sodium hypochlorite (NaOCl) for 3 min, washing with sterilized sea water, and then exposing the neonates to an Amp, Km, Na, and Sm mixture. Using four nutrient media, we confirmed that this protocol renders the rotifer culture bacterial and fungus free. The axenic rotifer culture generated here is useful not only for genetic analysis of Brachionus plicatilis, but for studying the rotifer life cycle without bacterial influence.     

Particle size dependent feeding by the rotifer Brachionus plicatilis

Size selective feeding by Brachionus plicatilis was investigated with algae and bacteria (0.3–3.5 µm) and mono-disperse latex beads (0.3–3.0 µm) in short term feeding experiments. B. plicatilis demonstrated maximum clearance rate of particles with diameter 2µm, but particles with diameter down to 0.3 µm were also ingested. The clearance rate of bacteria was 15–55% of that obtained for optimal sized particles (2 µm), and was related to particle size. The relative reduction in retention of particles with diameter < 2 µm was more pronounced for latex beads than for natural food particles, suggesting other mechanisms than size to be important for the particle retention by the rotifer. This is emphasized by the fact that the clearance rates were much lower for latex beads than for natural food particles of comparable size. Efficient retention of bacteria was observed for rotifers in poor physiological condition, i.e. rotifers with low maximum clearance rate. This may reflect a strategy to optimize energy utilization by reducing locomotion costs and increasing energy intake.The results indicate that B. plicatilis has a low to medium ability to feed on bacteria. In natural ecosystems, its importance as a bacterial grazer is of limited importance. At high population densities, such as in live feed cultures, the rotifer may, however, efficiently remove bacteria from the culture.     

Life history characteristics of Brachionus plicatilis (rotifera) fed different algae

A detailed study of the life history of the rotifer Brachionus plicatilis was done at 20 °C, 20 ppt salinity, and 90 mg C 1^–1 food concentration. Rotifers were grown individually in culture plate wells (150 µl culture volume) and fed Isochrysis galbana Tahiti, Tetraselmis sp., Nannochloris atomus, or a l : 1 mixture (weight) of two of the algae. Observations were made every 2–8 hr and rotifers were sized and transferred to new food daily. A total of 19 different parameters were compared. Rotifers fed Isochrysis averaged 21 offspring per female, a 6.7 day reproductive period, a lifespan of 10.5 days and a mean length of 234 µm. After Isochrysis, the foods giving the highest growth, survival, and reproduction in decreasing order were Isochrysis + Nannochloris, Nannochloris, Isochrysis + Tetraselmis, Tetraselmis + Nannochloris, and Tetraselmis. Although the small volume culture system used in this study seems appropriate for studying life history of B. plicatilis, the results cannot always be directly applied to larger cultures.