Effects of algal and larval densities on development and survival of asteroid larvae

Effects of larval and algal culture density and diet composition on development and survival of temperate asteroid larvae were studied in the laboratory at Santa Cruz, California, USA, during summer and fall of 1990. Larvae of Asterina miniata were reared at two densities, 0.5 or 1.0 ml^-1, and fed one or two species of cultured phytoflagellates — Dunaliella tertiolecta alone or mixed with Rhodomonas sp. — at three concentrations of 5x10², 5x10³, and 5x10⁴ total cells ml^-1. Algal concentration strongly influenced larval development; however, larval density also had a marked effect. Development progressed further with increasing algal concentration. Larval growth and differentiation were sometimes uncoupled; i.e., growth measures were directly related to food level, while differentiation indicators were less so. At the lowest food level, growth was negative and differentiation was arrested at early precompetent stages; these larvae never formed juvenile rudiments or brachiolar attachment structures. Development times of larvae given more food ranged from 26 to 50 d and depended directly on food availability. Development time to metamorphosis at the highest food concentration was similar for siblings fed D. tertiolecta alone or mixed with Rhodomonas sp. In contrast, when food level was an order of magnitude lower, larvae fed the algal mixture metamorphosed significantly earlier than larvae fed the unialgal diet. This suggests interactive effects of food quantity and food quality. Survival was little affected by larval or food density, except at the lowest ration. Feeding experiments in well-controlled laboratory conditions are useful to predict and compare the physiological or developmental scope of response of larvae to defined environmental factors; however, results from such studies should not be extrapolated to predict rates and processes of larval development in nature.     

Feeding,growth, and survival of Engraulis mordax larvae reared in the laboratory

Methods are described for the successful rearing of northern anchovy larvae (Engraulis mordax Girard) on cultured foods. Larvae were fed successively on the unarmored dinoflagellate Gymnodinium splendens, the veliger of the gastropod Bulla gouldiana, and nauplii of the brine shrimp Artemia salina. Rearing containers ranging in capacity from 4.5 to 510 l were tested; the smaller ones were found to be most useful for laboratory experimentation. Irreversible starvation occurred when E. mordax were denied food for more than 1.5 days after yolk absorption. Growth rates of larval anchovies fed different diets were compared. Larvae fed G. splendens grew for 1 week at the same rate as animals fed wild plankton, but did not maintain this rate. Laboratory survival of E. mordax larvae on a diet of G. splendens alone, did not differ significantly when veligers supplemented the diet. However, when G. splendens and veligers were fed simultaneously to E. mordax larvae, growth rate was greatly improved, although still not matching the growth attained on a diet of wild plankton. Length (L) versus weight (W) analyses were made for all larvae at all diets. The results showed that weight could be calculated most accurately from length by the relationship log W=3.3237 log L-3.8205, regardless of diet.     

Biochemical composition and digestive enzyme activity in larvae and postlarvae ofPenaeus japonicus during herbivorous and carnivorous feeding

The growth, survival, digestive enzyme activity and biochemical composition ofPenaeus japonicus (Bate) larvae and postlarvae were measured under three feeding regimes. Larvae were reared through the protozoeal stages usingChaetoceros gracilis. From the first mysis stage, three feeding regimes were used; (A)C. gracilis plusArtemia sp. nauplii, (B)Artemia sp. nauplii alone or (C)C. gracilis alone. No significant difference was found in growth, survival, protein content or lipid content of postlarvae from the treatments receiving the single-feed type, despite the low protein (7%) and highly unsaturated fatty acid content of the alga. Growth of larvae receiving the mixed diet was significantly higher than in the other treatments. Trypsin activity was more strongly influenced than amylase activity by dietary treatment, and differences in the ratio of these enzymes between treatments suggest independent control of their secretion. Trypsin activity recorded in larvae feeding onC. gracilis was up to six time higher than in larvae feeding onArtemia sp. nauplii, apparently in response to the low protein content of the alga. Larvae receiving the mixed diet exhibited an intermediate level of trypsin activity; it is suggested that the ingestion of algae is necessary for optimal assimilation of the zooplankton component of the diet.     

Toxicity and transfer of metal oxide nanoparticles from microalgae to sea urchin larvae

Nanoparticles (NPs) contained in commercial products are released and enter into the aquatic ecosystem, posing serious possible risks to the environment and affecting the food chain. Therefore, investigating the potential toxicity of NPs on aquatic organisms has become an important issue. This study assessed the toxicity and trophic transfer of metal oxide NPs from marine microalgae (Cricosphaera elongata) to the larvae of the sea urchin Paracentrotus lividus. Larvae (24 h old) were fed on 2000 cell mL^?1 48 h of microalgae contaminated with 5 mg L^?1 of several metal oxide NPs (SiO₂, SnO₂, CeO₂, Fe₃O₄) for 15 days. Larval viability and development were monitored from the 4-arm stage to the 8-arm pluteus stage. A significant decrease in survival was observed in larvae fed with microalgae exposed to SiO₂ and CeO₂ NPs. Abnormal development, characterised by skeletal degeneration and altered rudiment growth, was observed in all larvae fed with contaminated NP algae. Our findings revealed that SiO₂ and CeO₂ NPs exerted a toxic effect in the trophic interaction analysed, by reducing sea urchin larval viability, and all metal oxide NPs induced toxicological effects. In conclusion, metal oxide NPs may enter the food chain and become bioavailable for marine organisms, affecting their development.     

Effects on larval crabs of exposure to algal toxin via ingestion of heterotrophic prey

We tested whether ingesting toxic algae by heterotrophic prey affected their nutritional value to crab larval predators, using toxic algal strains that are either ingested directly by larval crabs or rejected by them. Ingestion of toxic strains of the dinoflagellates Alexandrium andersoni and A. fundyense by the rotifer Brachionus plicatilis was confirmed. Rotifers having ingested either algal type for five days were fed to freshly hatched larvae of three crab species, with larval survival and stage durations determined. For both algal/rotifer treatments in all three crab species, larvae fed algae directly died during the first zoeal stage, while those fed rotifers that had been fed either algal strain survived to the experiment’s end (zoeal stage 3). Survival was lower, and stage duration longer, for larvae fed rotifers cultured on toxic algae when compared to those fed non-toxic algae. The role of toxic algae in the planktonic food web may be influenced by its direct or indirect ingestion by larval crabs.     

Behaviour of echinoid larvae around sharp haloclines: effects of the salinity gradient and dietary conditioning

We examined larval response to a range of sharp haloclines and determined the effect of dietary conditioning on that response in the sea urchins Echinometra lucunter and Arbacia punctulata. We reared larvae in the laboratory under a high or low concentration of either single (Isochrysis galbana) or mixed (Isochrysis galbana, Dunaliella tertiolecta, Thalassiosira weissflogii) microalgal species. For both species of sea urchins, rate of larval development was faster and age-specific larval length and width were greater in high-ration than low-ration diets. We examined the distribution of two- and four-arm larvae of E. lucunter from each diet treatment and of four-arm larvae of A. punctulata from the high-ration diets in cylinders with experimentally constructed haloclines. In three of the halocline treatments, the salinity of the bottom layer was 33‰ and that of the top layer was 21, 24 or 27‰ (21/33, 24/33 and 27/33) and in a fourth one, the salinities of the bottom and top layer were 30 and 21‰, respectively (21/30). The position of larvae in the cylinders varied with the steepness of the halocline and with dietary conditioning for both sea urchin species and all developmental stages tested. Significantly more larvae crossed the haloclines into water of 24 and 27‰ salinity than into water of 21‰ salinity. We observed an effect of diet on the position of larvae in the cylinders, and that effect varied among halocline treatments for both species. The proportion of larvae of E.lucunter that crossed the halocline was greater in low- than high-ration diets in the 24/33 and 27/33 treatments. Position of four-arm larvae in the cylinders also varied with food quality in high-ration diets: for E.lucunter in the 24/33 treatments, and for A. punctulata in the 21/30 treatments, more larvae from the single- than from the mixed-species diets were present above the halocline. Salinity in the adult habitat during most of the active reproductive period ranged from 15 to 40‰. We showed that larvae can respond to gradients in salinity, and therefore can remain within a water mass of higher salinity overlying the adult habitat. However, survival of poorly fed larvae may be increased if they are introduced into a new water mass and carried away from a nutritionally poor environment. Received: 9 July 1997 / Accepted: 12 January 1998     

Effects of monospecific and mixed-algae diets on survival, development and fatty acid composition of penaeid prawn (Penaeus spp.) larvae

Four species of microalgae (Chaetoceros muelleri, Tetraselmis suecica, Tahitian Isochrysis sp. (T-iso) and Dunaliella tertiolecta) with distinctly different fatty acid profiles were grown in continuous culture and fed to prawn larvae (Penaeus japonicus, P. semisulcatus and P. monodon) as monospecific diets. The best two diets (C. muelleri and T. suecica) were also fed as a mixed diet. Experiments were run until the larvae fed the control diet of C. muelleri metamorphosed to Mysis 1. The survival and development (i.e. performance) of the larvae were affected by algal diet, and the diets were ranked in the order of decreasing nutritional value: C. muelleri ≥ T. suecica > T-iso > D. tertiolecta. Larvae fed a mixed diet of C. muelleri and T. suecica (2:3 by dry weight) performed as well or better than those fed C. muelleri, and the performance of both these groups of larvae was better than those fed T. suecica. The lipid and carbohydrate compositions of the algae had little or no effect on the lipid and carbohydrate compositions of the larvae or their performance. However, the larvae that performed best (i.e. those fed C. muelleri) had significantly more lipid and carbohydrate than those that performed worst (i.e. those fed D. tertiolecta). Larvae fed C. muelleri or the mixed-algae diet had higher proportions of the essential fatty acids eicosapentaenoic acid [EPA, 20:5(n-3)] and arachidonic acid [ARA, 20:4(n-6)] than the larvae fed on other diets. Furthermore, the larvae fed T. suecica, which showed intermediate performance between larvae fed C. muelleri and T-iso or D. tertiolecta, also had higher proportions of EPA and ARA. Both C. muelleri and T. suecica contained EPA and ARA, but T-iso and D. tertiolecta did not, except for trace amounts of EPA in T-iso. The fatty acid ARA appears to be much more important in the diet of larval prawns than has so far been considered. The level of the essential fatty acid docosahexaenoic acid [DHA, 22:6(n-3)] in the algal diet and the larvae was not related to the performance of the larvae; only C. muelleri and T-iso contained DHA. However, the nauplii contained large proportions of DHA, suggesting that these were sufficient to meet the larval requirements for DHA during their development to Mysis 1. Mixed-algae diets could improve the performance of larvae by providing a more comprehensive range of fatty acids. Received: 22 April 1998 / Accepted: 3 December 1998     

Effect of additions of dietary triacylglycerol microspheres on growth,survival, and settlement of mussel (<Emphasis Type="Italic">Mytilus</Emphasis> sp.) larvae

This study reports the effect of additions of dietary microspheres of triacylglycerol (TAG, extracted from the diatom Chaetoceros muelleri) on larval development and settlement of Mytilus sp. The first experiment showed that mussel larvae successfully ingested TAG microspheres as soon as they acquired the ability to feed from exogenous sources. In a second experiment, larvae were fed for 28 days on diets consisting of 0, 1, 20, or 50% TAG microspheres (based on the cell concentration of a full algal ration) added as partial replacements for a ration of Isochrysis sp. (T-ISO). Lipid content and growth of larvae fed on a diet composed of 20% TAG were higher than those of the control groups, whereas survival was negatively affected. No growth or survival effect was detected with larvae fed on a diet composed of 1% TAG, whereas high mortality after 14 days was observed for larvae fed on a diet composed of 50% TAG microspheres. In a third experiment, 22-day-old larvae were fed on rations of Isochrysis sp. supplemented with 0, 1, 10, 20, 50, and 100% TAG for 2 days and allowed to settle for a 7-day period. TAG content of the larvae increased with TAG added to the diet until a saturation threshold was reached between a 20 and 50% supplementation level. Similarly, free fatty acid (FFA) content increased with TAG level in the diet and was linearly correlated with TAG content of larvae. Increased levels of FFA in larvae were attributed to digestion of TAG supplements. Settlement success and survival of larvae were not affected by diet; however, regression analysis revealed that TAG level in pre-metamorphic larvae explained 28% of survival variability among cultures.Communicated by R.J. Thompson, St. Johns     

Effect of delayed first feeding on larval performance of the spider crab Maja brachydactyla assessed by digestive enzyme activities and biometric parameters

The effect of food deprivation on larval performance of the spider crab Maja brachydactyla was studied in terms of survival, moulting capacity, size, weight and enzymatic activities. Five feeding treatments that differed in the initial age of first feeding larvae (fed from hatching, 2, 4 and 6 days post-hatching and unfed) were tested for 20 days. Newly hatched larvae kept without food supply lasted for 10 days and did not moult; with 50% survival observed at 6 days post-hatching. Larvae (zoea I stage) were only able to tolerate 2 days of food deprivation after the onset of exogenous feeding without their performance being compromised. Multivariate analyses suggest that digestive enzyme activities may be good indicators of the nutritional condition of larvae.     

Effects of temporary starvation on larvae of the sea star Asterina miniata

Patchy food distribution may force temporary starvation conditions on planktonic larvae. This potential food limitation may affect survivorship, duration of larval period, and post-metamorphic succes. In this study, larvae of the asteroid Asterina miniata were subjected to temporary food deprivation of several durations and at different stages. Developmental effects were documented by quantification of larval stage, total length, time to metamorphosis, initial juvenile radius, range of settling times, and percent survival to metamorphosis. All starved treatments were significantly affected in settling time and most in percent survival. However, larvae starved later in development demonstrated tremendous tolerance of food deprivation (e.g. the total number of settlers in the treatment starved for 28 d was not significantly different from the fed control). Survival was lower in treatments starved earlier in development than those starved later. Food is apparently required until late in larval development to facilitate metamorphosis. The range of settling times was large; for example, the continuously-fed control treatment produced juveniles from Days 58 through 136. Temporary starvation had no effect on initial juvenile radius.     

Food concentration and stocking density effects on survival and growth of laboratory-reared larvae of bay anchovy Anchoa mitchilli and lined sole Achirus lineatus

Bay anchovy (Anchoa mitchilli) eggs were stocked at densities from 0.5 to 32.0 l^-1 and larvae were fed on wild plankton (copepod nauplii) in concentrations that ranged from 50 to 5000 prey l^-1. Lined sole (Achirus lineatus) eggs were stocked at 0.5 to 16.0 l^-1 and larvae were fed wild plankton at concentrations from 50 to 1000 prey l^-1. Some larvae of each species survived at all stock and food levels to the transformation stage at 16 days after hatching. Survival rates for both species exceeded 40% when food concentration was 1000 l^-1 or higher. Growth and dry weight yields also increased significantly at the higher food concentrations. Effects of initial stocking density were not well defined, but both survival and growth decreased at the highest stocking rates. Standardized culture of bay anchovy and lined sole larvae can be based on a food concentration of 1000 copepod nauplii l^-1 to routinely produce healthy larvae.     

Energy balance of <Emphasis Type="Italic">Octopus maya</Emphasis> fed crab or an artificial diet

The present study was designed to evaluate the effect of a natural prey (the crab Callinectes sp.) and an artificial diet (pellet with squid paste and offered as a paste) on the survival and assimilation efficiency of subadult octopuses with 486 g of initial live weight. In order to reach this goal, the effects of the type of diet on energetic balance were assessed by recording ingestion rate (C), respiratory rate (R = R routine, R _rout + R apparent heat increment, R _AHI), ammonia production rate (U = U routine, U _rout + U post-prandial, U _PP) and biomass production (P) of Octopus maya during its growing process. Energy lost from faeces (H) was calculated as H=C−(U+R+P) and assimilated energy (As) as R + P. Octopuses fed an artificial diet had almost five times higher ingestion rate compared to that observed in octopuses fed crab. However, growth rate and production (P) were high in octopuses fed crab in comparison to octopuses fed artificial diet. An inverse relation between faeces (H) and type of food was observed, indicating that animals lost 77% of the ingested energy when fed artificial diet and only 5% when fed crab. A higher assimilation and production efficiency were obtained in octopuses fed crab (P/As: 61%) than in animals fed the artificial diet (P/As: −5%). The routine O : N ratio for animals in fasting was 9.1 and 2.3 for octopuses being fed crabs and the artificial diet, respectively. The post-alimentary O : N ratio was 3.6 and 2.2 for animals fed crabs and the artificial diet, respectively. This indicates that animals fed on both diets rely almost exclusively on protein. Based on energy balance data, a value of 472 kJ week⁻¹ kg⁻¹ of live octopus was estimated as the energy needed to obtain a growth rate near 9 g day⁻¹ (2.8% BW day⁻¹) for O. maya subadults. The total crab biomass needed to obtain 1 kg of fed O. maya biomass was calculated. A comparison with other different energy balance measurements made in other octopus species indicates that O. maya and Enteroctopus megalocyathus (Pérez et al. 2006) tend to be more efficient by channelling more ingested energy to biomass production (P = 69.5% of C) than O. vulgaris (P = 23% of C; Petza et al. 2006) or Paraledone charcoti (P = 4% of C; Daly and Peck 2000).     

Effects of various diets on the nutritional value of Artemia sp. as food for the prawn Penaeus monodon

The nutritional value of Artemia sp. as food for marine fish and crustacean larvae has been linked to the level of its polyunsaturated fatty acid (PUFA) content. Experiments in August 1984 were conducted to determine the effects of various artificial diets and algae on fatty acid composition of PUFA-deficient Artemia sp. (Utah GSL strain) and their resulting value as food for postlarvae of the prawn Penaeus monodon (Fabricius). Nauplii of the brine shrimp were grown on extracts of corn, copra, soybean and rice bran containing precursors (C18) to long-chain PUFA and also on algal species containing different levels of long-chain PUFA (C20). The nauplii were then used as food for P. monodon postlarvae. The results revealed that absence of C20 polyunsaturates from the feeds and their presence in the algae were reflected in the polyunsaturated fatty acid content of the tissues of Artemia sp. When fed with brine shrimp fed on algae, P. monodon displayed better postlarval survival and significantly higher growth; related to the content of polyunsaturated fatty acids in Artemia sp. A practical feeding approach in prawn hatcheries would be to grow Artemia sp. on a cheap diet such as rice bran, and then to enhance its nutritional value with a diet high in PUFA prior to harvesting, in order to improve hatchery production.     

Effects of temporary starvation on the survival,and on subsequent feeding and growth,of oyster (Crassostrea gigas) larvae

Larvae of oysters, Crassostrea gigas, were maintained without food for 1 to 8 d after fertilization, and fed daily thereafter. There was little difference in survival and growth between controls and larvae kept without food for 2 or 3 d. Survival and growth rates were depressed in larvae starved for 4 or 5 d. For larvae starved for 6 to 8 d, survival was negligible or nil; even those larvae which survived the starvation period died later in the presence of food, apparently because of impaired digestion. Therefore, food availability in the first few days after spawning appears to be of paramount importance to the successful recruitment of Pacific oysters.     

Appearance of lipid-absorption capacities in larvae of the sea bassDicentrarchus labrax during transition to the exotrophic phase

The development of intestinal lipid absorption capacities was studied in larvae of the sea bassDicentrarchus labrax from the time of mouth opening to 25 d after hatching, in individuals fed with brine shrimp,Artemia sp. (Brazil commercial strain) or artificial diets. At the time of mouth opening, before the first feed, enterocytes synthesized lipoprotein particles from endogenous lipids. Starting with the first feed, enterocytes absorbed food lipids regardless of the diet, but since these cells have a low capacity for lipoprotein synthesis, they accumulated lipids in the form of free lipid droplets. In larvae fed withArtemia sp., the rapid development of enterocytes during growth was combined with increasingly effective lipoprotein synthesis (becoming even greater than that observed in the adult), starting on Days 18 to 19. Although lipoprotein synthesis and transport were observed in larvae given artificial feed, they showed abnormalities in their intestinal mucosa. Lipid droplets formed in association with the membrane structures of specimens given egg yolk during the first days of feeding, and enterocytes degenerated. In 18 to 19 d-old larvae given artificial feed, intestinal folds disappeared and the constituent cells showed limited differentiation. In the last-mentioned individuals, the abnormalities appeared to be related to malnutrition resulting from inadequacy of the food offered, which led to total mortality of larvae, beginning on Day 20.     

Where are larvae of the vermetid gastropod <Emphasis Type="Italic">Dendropoma maximum</Emphasis> on the continuum of larval nutritional strategies?

This study evaluated whether larvae of the Indo-Pacific vermetid gastropod Dendropoma maximum are obligate planktotrophs, or whether they exhibit an intermediate feeding strategy. Experiments were conducted in Moorea, French Polynesia (149°50′W, 17°30′S), Sep–Oct 2009, to examine D. maximum larval growth and metamorphic responses to different diets and amounts of food. Dendropoma maximum larvae required particulate food to undergo metamorphosis, but were able to survive and grow in the absence of food for up to 20 days. Larvae in Low and Unfed food treatments exhibited phenotypic plasticity by growing a larger velum (the larval feeding structure) compared with those in high food. Unfed D. maximum larvae had a slower initial growth rate; however, by 11-day post-hatch fed and unfed larvae had converged on the same mean shell height (553 μm), which was only 10% larger than the initial size at hatching. Therefore, although the nutritional strategy of D. maximum larvae is best described as obligate planktotrophy, it appears to approach an intermediate feeding strategy.     

Mass culture of the rotifer Brachionus plicatilis and its evaluation as a food for larval anchovies

Growth rates of anchovy larvae, Engraulis mordax, reared for 19 days under constant environmental conditions on a diet of laboratory-cultured organisms, exceeded the growth rates of anchovies fed on a diet of wild plankton. The rotifer Brachionus plicatilis was found to be a nutritous food source when fed to the larvae in concentrations of 10 to 20/ml and in combination with the dinoflagellate Gymnodinium splendens (100/ml). Optimum conditions were determined for mass culture of the rotifer. A high food concentration was the most important parameter needed to assure a high yield of rotifers. Large volumes (464 I) of the unicellular flagellate Dunaliella sp. were cultured for feeding the rotifers. The rotifer culture technique described produces approximately 2.5×10⁶ organisms/day, providing a reliable food source for rearing studies. The lengths of B. plicatilis (without eggs) ranged between 99 and 281 , most rotifers being larger than 164 and less than 231 . Individuals weighed 0.16 g and contained 8×10^-4 cal.     

Effects of limiting access to prey on development of first zoeal stage of the brachyuran crabs Cancer magister and Hemigrapsus oregonensis

We tested the influence of limiting access to prey on larval development of the crabs Cancer magister and Hemigrapsus oregonensis by raising their Stage 1 larvae in the laboratory on different prey densities and with various periods of access to prey. Experiments were conducted in 1995 and 1996 at the Shannon Point Marine Center in Anacortes, Washington, USA. Our results show that crab larvae do not require continuous access to prey for optimal development nor do they appear to require light for prey capture. Survival and duration of Stage 1 C. magister fed continuously on only one-fourth the amount of the control density of prey and those fed at the control density for only 6 h per day were the same as for larvae fed continuously at the control density (20 ml⁻¹). Larvae with cyclic access to prey at the control density for 24 h and then starved for 72 h showed significantly lower survival and longer instar duration to Stage 2. Experiments on Stage 1 H. oregonensis which investigated a combination of prey density, period of access to prey and light/dark conditions during feeding revealed that survival decreased with decreasing prey density or with decreasing feeding period, but no differences were observed during periods of limited prey availability as a function of light or dark conditions. Stage duration was not affected by reduced prey density nor by the light/dark condition at the time of feeding, but it was prolonged when the period of access to prey was limited. The period of access to prey did not affect the weight of Day 1 Stage 2 larvae. Larvae fed high densities of prey for 4 h followed by 20 h of reduced-density diet exhibited the same survival and stage duration as controls that were continuously fed high-density prey. Our results define sub-optimal diets that can be used experimentally to determine the nutritional contributions made by naturally-occurring prey organisms during larval development in the two species. In nature, larvae may satisfy nutritional requirements through periodic encounters with dense prey patches during vertical migrations by day or night. Received: 12 August 1997 / Accepted: 5 February 1998     

Combined effects of temperature and salinity on fed and starved larvae of the mediterranean mussel Mytilus galloprovincialis and the Japanese oyster Crassostrea gigas

The combined effects of temperature, salinity and nutrition on survival and growth of larvae of the Mediterranean mussel Mytilus galloprovincialis and the Japanese oyster Crassostrea gigas were studied over a period of 7 d in the laboratory. Ripe adults, collected in spring and summer 1987 from natural populations in the Bay of Arcachon, France, were induced to spawn. Larvae of both species were cultured at four temperatures (15°, 20°, 25° and 30°C), four salinities (20, 25, 30 and 35S) per temperature, and two levels of nutrition (fed and unfed) per temperature/salinity combination. The fed larvae received a mixed algal diet of 50 cells each of Isochrysis galbana and Chaetoceros calcitrans forma pumilum per microlitre. In both bivalve species, larvae survived over a wide range of temperature and salinity, with the exception of mussel larvae, which died at 30°C. Statistical analysis indicated that nutrition had the greatest effect on larval development, explaining 64 to 75% of the variance in growth of M. galloprovincialis and 54 to 70% in growth of Crassostrea gigas. Unfed mussel larvae displayed little growth. Compared with temperature, the effect of salinity was very slight. M. galloprovincialis larvae exhibited best growth at 20°C and 35S and C. gigas at 30°C and 30S.     

Different antipredator behaviour in two anuran tadpoles: effects of predator diet

Recent investigations have indicated that animals are able to use chemical cues of predators to assess the magnitude of predation risk. One possible source of such cues is predator diet. Chemical cues may also be important in the development of antipredator behaviour, especially in animals that possess chemical alarm substances. Tadpoles of the common toad (Bufo bufo) are unpalatable to most vertebrate predators and have an alarm substance. Tadpoles of the common frog (Rana temporaria) lack both these characters. We experimentally studied how predator diet, previous experience of predators and body size affect antipredator behaviour in these two tadpole species. Late-instar larvae of the dragonfly Aeshna juncea were used as predators. The dragonfly larvae were fed a diet exclusively of insects, R. temporaria tadpoles or B. bufo tadpoles. R. temporaria tadpoles modified their behaviour according to the perceived predation risk. Depending on predator diet, the tadpoles responded with weak antipredatory behaviour (triggered by insect-fed predators) or strong behaviour (triggered by tadpole-fed predators) with distinct spatial avoidance and lowered activity level. The behaviour of B. bufo in predator diet treatments was indistinguishable from that in the control treatment. This lack of antipredator behaviour is probably related to the effective post-encounter defenses and more intense competitive regime experienced by B. bufo. The behaviour of both tadpole species was dependent on body size, but this was not related to predator treatments. Our results also indicate that antipredator behaviour is largely innate in tadpoles of both species and is not modified by a brief exposure to predators. Received: 22 August 1996 / Accepted after revision: 31 January 1997