Mixed Algal Diet Research Articles

Evaluation of suitable micro‐algal diets for intensive culture of the calanoid copepod Acartia bilobata

Copepods are alternative live feed in marine fish hatcheries, and the quality and type of micro-algal diets influence their culture productivity. Three algal species of different groups including prymnesiophyte: Isochrysis galbana (Iso), the diatom: Chaetoceros muelleri (Cht) and eustigmatophyte: Nannochloropsis occulata (Nan) were fed to the calanoid copepod Acartia bilobata. Selected algae were offered as mono-algal diets: T1-Iso, T2-Cht and T3-Nan or as mixed algal diets: T4-Nan + Cht, T5-Nan + Iso, T6-Iso + Cht and T7-Iso + Cht + Nan. Reproductive parameters, egg production, hatching success, nauplii and copepodite survival, development and first maturity, were closely monitored. Results illustrated that I. galbana was the most suitable diet for this copepod species, supporting higher average relative egg production (5631.62 ± 133.49 egg L−1 day−1), hatching success (96.32 ± 1.40%) and superior adult survival (44.44 ± 3.20%). Different algal diets influenced the nauplii and copepodite survival rate. Maximum survival to adults (56.67 ± 3.78%) and shortened development time (5.75 ± 0.12 days) and time to first maturity (4.50 ± 0.12 days) were observed when fed with I. galbana. The groups fed with C. muelleri exhibited lower survival (6.67 ± 0.67%) during development into adults, and the group fed with N. occulata alone or with C. muelleri did not survived till the adults. Amongst seven algal diets tested, I. galbana was found ideal for maximizing culture productivity, and hence recommended for intensive culture of A. bilobata.

Application of enriched Cyclops abyssorum divergens with mixed algal diet compared to Artemia franciscana for improving larval growth and body composition of Dicentrarchus labrax

Larval rearing techniques with suitable nursery feed should be prioritized for better survival and growth. We evaluate the potential use of Cyclops abyssorum divergens as a natural live food candidate instead of the costly traditional Artemia franciscana in the feeding strategy for Dicentrarchus labrax larvae. C. abyssorum divergens and A. franciscana were enriched with mixed valuable algae strains (Isochrysis galbana, Tetraselmis chuii, Chlorella marina and Nannochloropsis oculata) that were used in equal biomass proportions (1:1:1:1) on dry weight basis. Amino acids and fatty acid profiles were found to be highly significantly different in enriched C. abyssorum divergens (EC) compared to enriched A. franciscana (EA). The findings also revealed compensated deficiency in some amino acids such as histidine, isoleucine, and methionine in EC and also increased levels of total essential amino acids (EAA) in EC by 8.2 % more than that of EA. Further metabolizing of palmitic acid (C16:0) and bio-conversion of α- linolenic acid, ALA (C18:3 n-3) to high accumulation of DHA fatty acid lead to higher PUFAs contents in EC and lower levels in EA. Specific growth rates (SGR) and survival percentages (S %) of D. labrax were higher in larvae fed with EC compared to the lower values in EA. Moreover, analyzing final carcass composition revealed increased levels of DHA fatty acid attaining DHA/EPA ratio in fish-fed EC of 2.95 % (>1) compared to 0.90 % (<1) in larvae-fed EA. The elevated levels of the amino acids in the larvae-fed EC were a yardstick to the surged growth and survival rate of the examined species. The present study concluded that C. abyssorum divergens is the best promising candidate for Sea bass larvae-culture.

Influence of Different Algal Diets on Larval Growth Rates in the Marine Serpulidae Polychaete Worm Spirobranchus kraussii

Abstract Effects of food availability on larval growth and survival of Spirobranchus kraussii were studied by feeding larvae different algal diets. Newly hatched larvae of S. kraussii were fed four different marine microalgae species, singly and in various mixtures. The best growth was observed when fed C. vulgaris, N. oculata as a single species and mixed-algal diet during day 15 after fertilization. Mortality was low for larvae (max. 5%); survival rate more than 95%. These results suggest that S. kraussii larvae have the capacity to feed using alternative sources of energy, and food size and quality can affect their growth and sustainability.

Food quantity–quality co‐limitation: Interactive effects of dietary carbon and essential lipid supply on population growth of a freshwater rotifer

Abstract Food quantity and quality are highly variable in natural systems. Therefore, their interplay and the associated effects on consumer population growth are important for predator–prey interactions and community dynamics. Experiments in which consumers were exposed to elemental nutrient limitations along food quantity gradients suggest that food quality effects on consumer performance are relevant only at high food quantities. However, elemental nutrients act differently on physiological processes than biochemical nutrients. So far, the interactive effects of food quantity and biochemical compounds on consumer performance have been insufficiently studied. We studied interactive effects of food quantity and biochemical food quality on population growth, including fecundity and survival, of the freshwater rotifer Brachionus calyciflorus. We hypothesised that these life history traits are differently affected by the availability of biochemical nutrients and that food quality effects gain importance with increasing food quantity. In a first experiment, we established food quantity and quality gradients by providing rotifers with different concentrations of a low‐quality food, the sterol‐free cyanobacterium Synechococcus elongatus, supplemented with increasing amounts of cholesterol. In a second experiment, food quantity and quality gradients were established by providing different proportions of two prey species differing in biochemical food quality, i.e. S. elongatus and the lipid‐rich alga Nannochloropsis limnetica, at different total food concentrations. We found that the effects of cholesterol supplementation on population growth increased with increasing food quantity. This interactive effect on population growth was mainly due to food quality effects on fecundity, as effects on survival remained constant along the food quantity gradient. In contrast, when feeding on the mixed algal diet, the food quality effect associated with increasing the proportion of the high‐quality alga did not change along the food quantity gradient. The data on survival and fecundity demonstrate the missing interactive effect of food quantity and quality on population growth, as both traits were oppositely affected. Survival was affected by food quality primarily at low food quantity, whereas food quality effects on fecundity were stronger at high food quantity. Our results highlight the significance of essential biochemicals in mediating the interactive effects of food quantity and quality on population growth. The interplay between food quantity and biochemical food quality limitation seems to influence resource allocation patterns in order to optimise survival or reproduction, which may strongly affect population dynamics in variable environments. As opposed to exploring the function of a single nutrient via supplementation, using algae mixtures allowed us to assess food quality effects on consumer performance in a more natural context by taking potential interactive effects of multiple co‐limiting nutrients into account.

Captive breeding of the caramote prawn (Penaeus kerathurus) for restocking purpose: effect of single- and mixed-algal diets on larval growth and survival

Attempts to breed crustacean species of commercial interest to promote the conservation, restoration and enhancement of the stocks are increasing. Optimization of diet for larval growth is a crucial step for successful actions. Here we focus on the caramote prawn (Penaeus kerathurus), a primary target of small-scale fishery because of its high commercial value in the North Mediterranean region and particularly in Italy. Three species of microalgae (Isochrysis galbana, Nannochloropsis oculata, and Chaetoceros calcitrans) with different fatty acid contents were used to feed caramote prawn larvae as monospecific diets. Further, C. calcitrans in combination either with I. galbana or N. oculata were tested as mixed diets. Measurements of developmental performance were taken until the larvae metamorphosed to Mysis 1, while survival rate was estimated at day 10 of benthonic post-larval stage. Larvae fed on mixed diets performed as well or slightly better than those fed on single-C. calcitrans and -N. oculata diets both in terms of development and survival, while larvae fed with I. galbana diet performed worse overall. Diets that performed best (C. calcitrans and I. galbana mix) are known to have a higher proportion of the essential fatty acids eicosapentaenoic acid (EPA, 20:5n-3) and docosahexaenoic acid (DHA, 22:6n-3), respectively. We suggest that mixed-algal diets could improve the performance of larvae by providing a wider availability of fatty acids.

Grazing and performance of the copepod Pseudodiaptomus poplesia on a Chinese strain of Aureococcus anophagefferens

Brown tides have recurred in estuary areas globally, but trophic interactions between the causative species Aureococcus anophagefferens and planktonic copepods remain poorly understood. In this study, we investigated performance (ingestion, growth, development and reproduction) of the planktonic copepod, Pseudodiaptomus poplesia, offered either mono-algal or mixed-algal diets containing a Chinese strain of A. anophagefferens. A typical Michaelis-Menten pattern existed between ingestion rate and food level when copepod fed on the monoalgal diet of this species. Nauplii exhibited the highest maximum ingestion rate (Imax) than copepodids and adult females. In addition, Imax value was higher in nauplii feeding on A. anophagefferens than on Skeletonema costatum. When fed mixtures of A. anophagefferens and S. costatum, P. poplesia selected against A. anophagefferens cells, but less strongly at the naupliar stage. Nauplii did not undergo metamorphosis and died at late naupliar stages feeding on A. anophagefferens alone, similar to those under starvation. Furthermore, the presence of A. anophagefferens greatly reduced the reproduction rate of females in mixtures but did not influence the growth rate of copepodids. These results suggest that P. poplesia nauplii may exert grazing pressure on A. anophagefferens population during a brown tide, which, however, may not be persistent because of copepod population decline.

Rotifers enriched with a mixed algal diet promote survival, growth and development of barramundi larvae, Lates calcarifer (Bloch)

Fulfilling the nutritional requirements of marine fish larvae, including barramundi (Lates calcarifer), remains a bottleneck for optimal commercial hatchery production. This study investigated the effect of mixed algal diets for the enrichment of rotifers on the survival, growth, development and stress resistance of barramundi larvae. Four groups of 2–13 days-post-hatch (dph) barramundi larvae were fed rotifers enriched with either Nannochloropsis oculata, Chlorella vulgaris, or a blend of 50:50 or 75:25 of N. oculata to C. vulgaris (based on dry weight, DW), respectively. The highly unsaturated fatty acids (HUFAs) docosahexaenoic acid (DHA), eicosapentaenoic acid (EPA) and arachidonic acid (ARA) were used as qualitative proxies of the feed. The barramundi larvae fed with the 50:50 diet outperformed all other treatments, with significantly greater larval length (P<0.05), eye diameter (P<0.05) and body depth (P<0.05) at 10 dph, a trend conserved over the whole growth period. The barramundi larvae fed the 50:50 diet also achieved the highest flexion percentage at 10 dph (P<0.05) and scored the highest survival past the stress test (85%; P<0.05). The best performing treatment had the second highest level of DHA which suggests that while DHA is an important fatty acid for barramundi larvae it is likely its interaction with other EFA and monounsaturated fatty acid also plays a critical role in their development. Overall, blending microalgae species can enhance the balance of dietary fatty acid for barramundi larvae which in turn can lead to better growth, faster development and higher stress resistance when compared to monospecific microalgal diets.

Bioavailability and Effects of Ingested Metals on Aquatic Organisms

Laboratory toxicity studies and a caged bivalve field study complemented by computer modeling were conducted to investigate the significance of effects on aquatic organisms due to dietary exposure to metals. The lab studies were performed with saltwater and freshwater organisms. The saltwater studies provided partial support for previous research that showed dietary effects could occur when the food source, a mono-algal culture, was loaded with Ag, Cu, Ni or Zn at dissolved metal concentrations below than the current ambient water quality criteria (WQC). However, in contrast to the earlier studies, continuous 7-day exposures to Ag rather than a 4-hour pulse exposure were required to elicit a response by Acartia tonsa. The effects concentrations were also much higher than in the earlier studies and slightly higher than the proposed chronic water quality criteria (WQC) for Ag. The three other metals were only tested under a continuous-exposure regime and yielded similar test results to Ag, though the dissolved concentrations in which the algae were cultured were below the current chronic saltwater WQC. In the freshwater studies performed with Ceriodaphnia dubia exposed to dietary Cu and Ag, reproductive impairment was not consistent among repeat tests. Variations in reproductive vitality of the C. dubia might have contributed to the variability in adverse responses by the organisms. Questions remain regarding the applicability of the saltwater and freshwater results to field conditions, where mitigating factors such as a more nutritional mixed-algal diet or increased metal complexation capacity could decrease the amount of the bioavailable form of the metals, thereby decreasing exposure and effects. This title belongs to WERF Research Report Series ISBN: 9781843397656 (eBook)

Does allelopathy affect co-culturing Haslea ostrearia with other microalgae relevant to aquaculture?

Haslea ostrearia is a marine diatom known to produce marennine, a water-soluble blue-green pigment responsible for the greening of oysters in ponds along the French Atlantic coast. This phenomenon occurs seasonally when H. ostrearia blooms in oyster ponds, and it increases the economic value of cultured oysters. From an ecological perspective, H. ostrearia blooms are accompanied by a decrease in the abundance of other microalgae, suggesting that this diatom produces allelochemicals. Recent studies showed that purified marennine has other biological activities, for instance antioxidant, antibacterial, and antiviral activities, which could be used in aquaculture to promote this pigment as a natural antipathogen agent. One important issue regarding the possible use of H. ostrearia in aquaculture as a mixed algal diet, however, is the importance of marennine allelopathy. In this study, we investigated the allelopathic effect of H. ostrearia on the growth of five microalgal species relevant to aquaculture: Chaetoceros calcitrans, Skeletonema costatum, Phaeodactylum tricornutum, Tetraselmis suecica, and Tisochrysis lutea. Allelopathic tests were realized by co-culturing these microalgae with H. ostrearia in batch and in semi-continuous mode, based on initial biovolume ratios. Our findings showed that inhibition of the growth of microalgae due to the presence of H. ostrearia and marennine was species dependent. Skeletonema costatum, C. calcitrans, and T. lutea were significantly more sensitive, whereas T. suecica and P. tricornutum appeared to be more resistant. Growth irradiance significantly influenced the allelopathic effect against the sensitive species S. costatum, and the H. ostrearia production of marennine increases with irradiance. Data presented in this study partly support the hypothesis that marennine released into the culture medium possibly acts as an allelochemical compound, thus explaining the dominance of H. ostrearia and the loss of sensitive algae in oyster ponds, but also that some species are insensitive, which allows co-culturing and use in a mixed algal diet in aquaculture.

Laboratory conditioning modifies properties of gills mitochondria from the Pacific oyster Crassostrea gigas

Although laboratory experiments allow greater control of environmental conditions than field studies, they have several drawbacks. To analyze physiological responses to forcing environmental variables, experimental conditions should mimic natural conditions as closely as possible. For filter-feeding organisms in particular, diet quality and quantity is one of the environmental parameters that can differ markedly between experimental and field conditions. In the hatchery, Pacific oysters, Crassostrea gigas, commonly show good physiological performance and growth on a mixed algal diet of Tisochrysis lutea, formerly Isochrysis aff. galbana clone Tahiti (T-Iso), and Chaetoceros calcitrans, presumably as it provides a good supply of essential polyunsaturated fatty acids (PUFA) as 20:4n-6, 20:5n-3 and 22:6n-3. The present study tests whether the fluctuating biotic and abiotic conditions in the field modify the structure and function of oyster mitochondria. One group of oysters was maintained in the intertidal zone, and the other group was fed the mixed diet in a nearby experimental hatchery under salinity and temperature conditions equivalent to those in the field. After 4 weeks of conditioning, the functional capacities and membrane lipid composition of gill mitochondria were measured. For essential polyunsaturated fatty acids, only the proportion of 20:5n-3 differed between field and laboratory oysters, and confirmed the capacity of the mixed diet T-Iso + C. gracilis, to provide the essential PUFA. Nevertheless, proportions of other FA (e.g., 22:5n-6 and non-methylene-interrupted FA) differed markedly between laboratory and field-conditioned oysters. Mitochondrial oxygen uptake, cytochrome c oxidase activity, content of cardiolipin and concentration of cytochrome b were significantly lower in laboratory-conditioned than in field-conditioned oysters. These results indicate that laboratory conditioning, although allowing similar growth and gonad maturation, only partially mimics conditions that allow C. gigas to maintain mitochondrial function. Although our experimental design cannot ascertain what difference between experimental laboratory and field conditions led to changes in membrane composition and mitochondrial function, differences in nutritional quality (other than known essential PUFA) and abiotic factors (e.g., oxygen availability, emersion or daily temperature fluctuations) had a major impact on mitochondrial properties in oysters.

The effects of mixed algal diets on population growth, egg productivity and nutritional profiles in cyclopoid copepods (Thermocyclops hyalinus and Mesocyclops aspericornis)

This experiment was carried out to evaluate the effect of mixed algal diet on dietary profiles of copepods. The microalgae like Spirulina platensis, Chlorella vulgaris and Azolla pinnata were mass cultured in artificial medium for a period of 20days, cells are harvested, dried, powdered and used as feed. The small freshwater cyclopoids Thermocyclops hyalinus were compared to Mesocyclops aspericornis a species of copepod genera commonly preferred by most of the fish larvae. Both species are easily maintained in culture, when fed with mixed algal diets of equal ratios (1:1:1). Biochemical composition, egg production ratios, growth performance and fatty acids profile of the two different species were analyzed after an experimental period of 15days, all the nutritional values were found to be high and statistically variable. On the basis of biochemical composition, egg production ratio, growth performance, amino acids and fatty acids profile it is found that M. aspericornis was the suitable candidate for larval fish diets.

A note on the laboratory culture of the benthic foraminifer Cornuloculina balkwilli (MacFadyen)

BackgroundGenetic studies of the Foraminifera provide valuable insights into marine speciation and biogeography, yet the discovery of vitally needed new genetic markers for this important group is being severely limited by an extreme lack of genetic data. The establishment of a laboratory culture from a single, asexually reproducing foraminifer, will be essential to provide enough pooled genetic material from these unicellular organisms, to facilitate full genome sequencing and genetic marker discovery, using next-generation sequencing techniques.FindingsThe aim of this study was to develop a simple and inexpensive method of culturing benthic foraminifera, via asexual reproduction, in a controlled laboratory environment. Individual specimens of the benthic foraminfer Cornuloculina balkwilli (MacFadyen) were placed in 7 cm plastic beakers, containing 50 ml natural seawater, filtered to 0.2 μm, and kept at 23°C, with a 12-hour light/dark cycle, and fed weekly on a mixed algal diet of Dunaliella tertiolecta and Phaeodactylum tricornutum. Asexually derived cultures were successfully established from 4 specimens of Cornuloculina balkwilli, originally added to the culture vessels as immature specimens. Many thousands of individuals were present after 6 months.ConclusionsThe method presented here demonstrates that only basic laboratory equipment is required to establish and maintain a thriving culture of the benthic foraminfer, C. balkwilli, from a single asexually reproducing specimen, providing an excellent source of genetic material for use in next generation sequencing. The method is easily reproducible and will greatly aid in the discovery of critically needed new genetic markers in the Foraminifera. It also highlights C. balkwilli as a good candidate species for use in the field of environmental micropaleontology.

Reproductive performance, growth and development time of a tropical harpacticoid copepod, Nitocra affinis californica Lang, 1965 fed with different microalgal diets

This study examines the growth, reproductive performance and survival of a copepod Nitocra affinis californica Lang, 1965 fed with three microalgal food items singly (a diatom, C. calcitrans ; a green algae, N. oculata ; flagellated green algae, T. tetrathele) and in combinations (50% C.calcitrans +50% N. oculata; 50% C. calcitrans+50% T. tetrathele; 50% N. oculata+50% T.tetrathele; 50% C. calcitrans+25% N. oculata+25% T. tetrathele). The experiments were performed under controlled laboratory condition (30ppt salinity, temperature of 30°C, light intensity of 25μmolm−2s−1 and 12h light:12h dark cycle). Feeding densities were maintained at 106 cells ml−1 throughout the study. The highest (p<0.05) egg production was achieved by copepods fed with C. calcitrans and mixed algal diet (50% C. calcitrans+25% N. oculata+25% T. tetrathele) with mean values of 133.6±3.9 and 128.2±2.0 eggs female−1, respectively. Similarly, the highest (p<0.05) offspring production (132.0±3.8 offsprings female−1), survival from nauplii to adult (98.8±0.2) and maximum specific growth rate (K=0.43±0.0) was achieved with N. affinis californica fed C. calcitrans. The shortest (p<0.05) embryonic development time, time interval between egg sac, development time from nauplii to copepodid and copepodid to adult resulted from feeding with T. tetrathele. The longevity of the female N. affinis was found to be longest (p<0.05) when fed with mixed algal diet (42.2±0.7days) and combination of C. calcitrans+T. tetrathele (41.4±0.7days). The present study revealed that amongst the microalgae offered, C. calcitrans was the best food item as indicated by the highest reproductive capacity (eggs female−1 and offsprings female−1) of the N. affinis californica. Significantly higher nutritional values of C. calcitrans compared to other microalgae were probably responsible for the best reproductive and growth performance of N. affinis.

Changes in enzymatic activity during early ­development of bay scallops Argopecten irradians and sea scallops Placopecten magellanicus

Poor growth and survival of scallops is often reported during early developmental stages. Stage- and species-specific dietary requirements have been identified, but the mechanisms responsible for these differences remain largely unknown. Dietary success depends on food bio- chemical properties, digestive and/or assimilative capacity, as well as the animal's specific nutri- tional demands, which can vary over development and between species. The present study exam- ines developmental changes in the activities of key digestive enzymes in larval and postlarval bay scallops Argopecten irradians and sea scallops Placopecten magellanicus raised on a mixed algal diet until ~4 to 5 mm in shell height (SH) and sampled at intervals encompassing major transitions in feeding organ development. Colorimetric assays measured general protease, lipase (esterase) and carbohydrase (α-amylase, cellulase and laminarinase) activities. The most pronounced changes in enzymatic activity occurred prior to scallops attaining ~1.2 mm SH in both scallop species. The esterase:protease ratio exhibited similar ontogenetic patterns in both scallop species, decreasing sharply between larval and immediate postlarval stages before increasing to an intermediate level and stabilizing around 1.2 mm SH. In contrast, mean carbohydrase activities measured over the experimental duration differed between species, with bay scallops exhibiting higher specific activi- ties of α-amylase and cellulase but much lower activities of laminarinase than sea scallops. These findings have implications for understanding scallop utilization of the food supply in nature, as well as developing targeted diets that could enhance their growth and survival in culture.

Reproductive performance, survival and development of nauplii and copepodites, sex ratio and adult life expectancy of the harpacticoid copepod, Euterpina acutifrons, fed different microalgal diets

A series of laboratory experiments were conducted on the harpacticoid copepod, Euterpina acutifrons, to assess the influence of 10 different microalgal diets (four monoalgal and six mixed algal diets) on several parameters related to its productivity in culture. The four monoalgal diets were the Tahitian strain of Isochrysis sp. (T-Iso), Pavlova salina (Pav), Tetraselmis chuii (Tet) and the diatom Chaetoceros muelleri (Chaet), the five binary diets were T-Iso+Tet, Pav+Tet, T-Iso+Pav, Tet+Chaet and Pav+Chaet, while the tri-algal diet was T-Iso+Tet+Pav. All diets were fed to copepods at 1500 μ gC L−1 and in the case of binary or trialgal diets, carbon concentration was divided equally between the two or three algae offered. Among monoalgal diets, the diatom Chaet was excellent for E. acutifrons. Out of the 10 diets tested, the binary diet Tet+Chaet, which contained the diatom Chaet, was the best for naupliar production of single pair E. acutifrons (19.5±1.7 nauplii female−1 day−1), significantly (P 0.05). Similarly, in the group naupliar production trial (50 adult E. acutifrons per replicate), Tet+Chaet produced a significantly higher number of nauplii (405.8±18.6 nauplii day−1) than the other treatments (P<0.05). Tet+Chaet further supported the highest naupliar survival (82.0±2.8%) and copepodite survival (89.0±2.8%), while the mono-algal diet Chaet produced the second highest naupliar (76.7±2.6%) and copepodite survival (83.5±2.6%). In contrast, Pav produced the lowest overall survival at the naupliar stage (30.0±2.9%), significantly lower than all other treatments (P<0.05). While development from newly hatched nauplii to copepodites was not significantly affected by diets, mean development time from nauplius to adult was significantly different among treatments. Mean development time from hatching (naupliar I stage; NI) to the adult stage was the fastest with Tet+Chaet and Chaet (6.8±0.0 days for both treatments), which was significantly faster than that of Pav, T-Iso Pav+Tet and T-Iso+Pav+Te treatment (P<0.05). E. acutifrons sex ratio was significantly affected by diets, and always skewed in favour of males. Feeding on Pav resulted in the lowest proportion of females (23.7±1.2%), significantly lower than for six of the other treatments (P<0.05). Adult females had longer average life expectancy than males for all treatments, and were the longest when fed Tet+Chaet (9.5±0.4 days), which was more than twice as long as the shortest lifespan recorded for the Pav treatment (4.2±0.6 days) (P<0.05). In summary, among 10 diets tested, the binary diet Tet+Chaet appeared to support the highest culture productivity of E. acutifrons while the diatom Chaet also performed well as a monoalgal diet.

Potential for the hatchery production of spat of the green mussel Perna viridis Linnaeus (1758)

The Asian green mussel Perna viridis Linnaeus (1758) was spawned in hatchery and the larvae were successfully reared to setting. The larvae grew through the straight hinge, umbo, eyespot and pediveliger stages in the pelagic phase before metamorphosing into the plantigrade stage and setting as spat on a suitable substratum. Isochrysis galbana was used as a standard algal food until settlement of spat after which a mixed algal diet of I. galbana and Chaetoceros calcitrans was provided. Spat fall began on 21st day and continued up to the 35th day on a variety of substrates. The shell length–height relationship in the larvae is given by Y = −0.397 + 1.149 X, r = 0.987. Growth of the larvae is linear. Prospects for large scale hatchery production of green mussel spat are discussed.

Effects of different dietary microalgae on survival, growth, settlement and fatty acid composition of blue mussel ( Mytilus galloprovincialis) larvae

The diatom Chaetoceros calcitrans is a major component of many bivalve hatcheries, yet it is expensive and notoriously difficult to culture on a commercial scale. In an attempt to reduce dependence on the diatom C. calcitrans, mussel larvae ( Mytilus galloprovincialis) were subjected to feeding experiments which altered levels of the diatom under controlled hatchery conditions. Growth, survival and settlement success of mussel larvae were determined in response to five mixed algal diets in which the relative contributions of C. calcitrans was varied over the experimental period (30 days). Fatty acid profiles of the larvae and algal diets were also assessed. The exclusion of C. calcitrans from the diet had no significant differences on larval growth and only minor differences in total fatty acid content were found between treatments. Fatty acid analysis revealed that larval survival was strongly influenced by the proportions of dietary docosahexaenoic acid (DHA), while settlement was positively correlated with higher ratios of the n-3 long-chained polyunsaturated fatty acid (n-3 LC-PUFA) (namely, DHA and eicosapentaenoic acid, EPA), to the n-6 LC-PUFA (arachidonic acid, ARA). Despite similar relative and absolute n-3 LC-PUFA levels in the larvae under different dietary treatments, the larvae receiving high levels of C. calcitrans performed significantly better in terms of survival and settlement success. These results indicate that the (DHA + EPA)/ARA ratio is a key factor in determining larval performance, rather than the total amount of these fatty acids.

Evaluation of dietary microalgae for culture of the calanoid copepod Pseudodiaptomus pelagicus

A series of five experiments were conducted to evaluate the effects of various dietary microalgae on survival, reproduction, sex ratio, fecundity, time to first maturation, brood interval, brood size, total nauplii production, and population development of the calanoid copepod Pseudodiaptomus pelagicus. Three experiments utilized 10 reproductive pairs of adults cultured in 1 L beakers. In the first experiment, the microalga Isochrysis galbana (T-iso) was fed at five densities to determine the optimal density for production. In the second experiment, five microalgae, T-iso, Thalassiosira weissfloggi (Tw), Chaetoceros gracilis (Ch), Rhodomonas lens (Rh), and Tetraselmis suecica (Tet), were each fed at a carbon equivalency of 80,000 cells/mL of T-iso. In the third experiment, the same five microalgae were fed along with four mixed microalgae diets, consisting of T-iso/Tw and T-iso/Ch at two ratios, 50:50 and 75:25, all were fed at a carbon equivalency of 160,000 cells/mL of T-iso. The fourth experiment cultured individual pairs in 30 mL beakers to determine the effects of dietary microalgae on nauplii production, brood size, and brood interval. The fifth experiment cultured 100 early stage nauplii (N1–N3) until the population reached sexual maturity. The optimal density of T-iso was 160,000 cells/mL. At 80,000 cells/mL, Tw produced a significantly greater number of copepods than the other microalgal diets. In the 160,000 cells/mL experiment, total production with T-iso/Tw at 50:50 ratio was the highest and was significantly higher than all monoalgal diets except Rh. In general, the total number of copepods produced was greater with mixed algal diets than those attained with the monoalgal diets. Results of the nauplii growout experiment showed that the survival of nauplii to adult was highest with Rh and lowest with T-iso. Time to first maturity was significantly affected by dietary microalgae with T-iso being the slowest. The percentage of ovigerous females was significantly different among dietary microalgae, with T-iso and Ch exhibiting the lowest percentage. P. pelagicus can be successfully cultured with various monoalgal diets but results suggest that a mixed algae ration of T-iso and Tw will produce the greatest population growth.

Influence of food supply on the δ13C signature of mollusc shells: implications for palaeoenvironmental reconstitutions

Compared to oxygen isotopes, the carbon isotope composition of biogenic carbonates is less commonly used as proxy for palaeoenvironmental reconstructions because shell δ13C is derived from both dissolved inorganic (seawater) and organic carbon sources (food), and interactions between these two pools make it difficult to unambiguously identify any independent effect of either. The main purpose of this study was to demonstrate any direct impact of variable food supply on bivalve shell δ13C signatures, using low/high rations of a 13C-light mixed algal diet fed to 14-month-old (adult) cultured Japanese Crassostrea gigas under otherwise essentially identical in vitro conditions during 3 summer months (May, June and July 2003, seawater temperature means at 16, 18 and 20 °C respectively) in experimental tanks at the Argenton laboratory along the Brittany Atlantic coast of France. At a daily ration of 12% (versus 4%) oyster dry weight, the newly grown part of the shells (hinge region) showed significantly lower δ13C values, by 3.5‰ (high ration: mean of −5.8 ± 1.1‰, n = 10; low ration: mean of −2.3 ± 0.7‰, n = 6; ANOVA Scheffe’s test, p < 0.0001). This can be explained by an enhanced metabolic activity at higher food supply, raising 13C-depleted respiratory CO2 in the extrapallial cavity. Based on these δ13C values and data extracted from the literature, and assuming no carbon isotope fractionation between food and shell, the proportion of shell metabolic carbon would be 26 ± 7 and 5 ± 5% for the high- and low-ration C. gigas shells respectively; with carbon isotope fractionation (arguably more realistic), the corresponding values would be 69 ± 14 and 24 ± 9%. Both groups of cultured shells exhibited lower δ13C values than did wild oysters from Marennes-Ol eron Bay in the study region, which is not inconsistent with an independent influence of diet type. Although there was no significant difference between the two food regimes in terms of δ18O shell values (means of 0.1 ± 0.3 and 0.4 ± 0.2‰ at high and low rations respectively, non-significant Scheffe’s test), a positive δ13C vs. δ18O relationship recorded at high rations supports the interpretation of a progressive temperature-mediated rise in metabolic activity fuelled by higher food supply (in this case reflecting increased energy investment in reproduction), in terms not only of δ13C (metabolic signal) but also of δ18O (seawater temperature signal). Overall, whole-shell δ18O trends faithfully recorded summer/winter variations in seawater temperature experienced by the 17-month-old cultured oysters.

Mitigation of adverse effects of rising CO2 on a planktonic herbivore by mixed algal diets

AbstractPutative future increase in atmospheric CO2 is expected to adversely affect herbivore growth due to decrease in contents of key nutrients such as nitrogen and phosphorus (P) relative to carbon in primary producers including plant and algal species. However, as many herbivores are polyphagous and as the response of primary producers to elevated CO2 is highly species‐specific, effects of elevated CO2 on herbivore growth may differ between feeding conditions with monospecific and multiproducer diets. To examine this possibility, we performed CO2 manipulation experiments under a P‐limited condition with a planktonic herbivore, Daphnia, and three algal species, Scenedesmus obliquus (green algae), Cyclotella sp. (diatoms) and Synechococcus sp. (cyanobacteria). Semibatch cultures with single algal species (monocultures) and multiple algal species (mixed cultures) were grown at ambient (360 ppm) and high CO2 levels (2000 ppm) that were within the natural range in lakes. Both in the mono‐ and mixed cultures, algal steady state abundance increased but algal P : C and N : C ratios decreased when they were grown at high CO2. As expected, Daphnia fed monospecific algae cultured at high CO2 had decreased growth rates despite increased algal abundance. However, when fed mixed algae cultured at high CO2, especially consisting of diatoms and cyanobacteria or the three algal species, Daphnia maintained high growth rates despite lowered P and N contents relative to C in the algal diets. These results imply that algal diets composed of multiple species can mitigate the adverse effects of elevated CO2 on herbivore performance, although the magnitude of this mitigation depends on the composition of algal species involved in the diets.