Prey Types Research Articles

Effects of Prey Size, Microalgal Density, and Light Intensity on Survival of Pacific Red Snapper Larvae

AbstractDevelopment of Pacific Red Snapper Lutjanus peru culture has been impeded by low survival in larval culture. We studied the effects of different culture conditions on larval survival in four experiments that examined (1) different‐size prey at first feeding (nauplii of the copepods Parvocalanus crassirostris and Pseudodiaptomus euryhalinus; rotifers Proales similis; and a mix of these three prey types); (2) microalgal density (3 × 105 and 1 × 106 cells/mL); (3) the presence of adult copepods P. crassirostris in larval culture tanks compared to daily nauplius addition (Adult and Nauplius treatments); and (4) light intensity (50, 200, 500, and 1,000 lx). In experiment 1, a strong preference was observed for copepod P. crassirostris nauplii compared to other prey types. Survival at 15 d posthatch (DPH) was significantly higher in the P. crassirostris treatment (mean ± SD = 19.5 ± 10.2%) and the Mixed diet treatment (33.4 ± 9.4%). However, strange behavior of larvae was observed, followed by drastic mortality (at 15–18 DPH) when microalgal addition to larval tanks ceased. This behavior and mortality had not been observed in experiment 2 at any of the microalgal densities supplied until 20 DPH. In experiment 3, survival was evaluated at 2, 5, 7, and 9 DPH and dropped to 50% (5 DPH) in both treatments. At 9 DPH, survival was 14.8 ± 9.5% for the Adult treatment and 3.4 ± 0.9% for the Nauplius treatment. In experiment 4, a significant difference was observed for survival at 9 DPH; survival at 1,000 lx (26.5 ± 18.4%) was significantly higher than survival at 200 lx (4.9 ± 5%) and 50 lx (0.6 ± 0.6%) but was not different from survival at 500 lx (6.6 ± 3.1%). In conclusion, P. crassirostris nauplii are an adequate prey for first feeding of Pacific Red Snapper. Therefore, to improve larval survival, P. crassirostris adults should be added to the larval culture tanks. Moreover, microalgae should be present during all larval rearing, and a light intensity of 1,000 lx should be used.

Is the population increase of an invasive portunid crab (Charbydis hellerii) a concern? The effect on native prey

Biological invasions have the potential to change the local marine biodiversity through increasing predation pressure and interactions among species. Charybdis hellerii is a swimming crab native to the Indo-West Pacific, it was first collected in the Atlantic Ocean in 1987 and is currently found in high abundance in coastal areas of southeastern Brazil. We evaluated the presence of the invasive crab C. hellerii from the coast of São Paulo state, Brazil, analyzing the stomach contents and performing laboratory-based feeding experiments. In crabs with stomachs full enough to be analyzed (N = 19), mussels (30%) and brachyuran crabs (14.21%) were the most frequent prey items found. Based on these data, we experimentally tested how different densities and life stages (adults and juveniles - differentiated by carapace width - CW) of the invasive crab affected feeding patterns. Results indicated that predator density effects on the predation pressure were dependent on the prey type. Mussels were consumed highest at medium predator densities (30 crabs/m3), whereas crabs were consumed highest at high predator densities (60 crabs/m3). There was no influence of the life stage on prey consumption, suggesting that adults (CW = 57.12–67.59 mm) and juveniles (CW = 39.77–43.18 mm) exhibit similar predation. Charybdis hellerii is an important consumer of benthic native prey, having the potential to change food webs through predation pressure on bioengineers and associated fauna. As the removal of C. hellerii from the environment is difficult, the next step toward is to understand the effect of the invasive crab on the native trophic web.

Two Predators, One Prey - the Interaction Between Bacteriophage, Bacterivorous Ciliates, and Escherichia coli.

Bacterivorous ciliates and lytic bacteriophages are two major predators in aquatic environments, competing for the same type of prey. This study investigated the possible interaction of these different microorganisms and their influence on the activity of each other. Therefore, two bacterivorous ciliates, Paramecium sp. RB1 and Tetrahymena sp. RB2, were used as representative ciliates; a T4-like Escherichia coli targeting lytic bacteriophage as a model virus; and E. coli ATCC 25922 as a susceptible bacterial host and prey. The growth of the two ciliates with E. coli ATCC 25922 as prey was affected by the presence of phage particles. The grazing activity of the two ciliates resulted in more than a 99% reduction of the phage titer and bacterial cell numbers. However, viable phage particles were recovered from individual washed cells of the two ciliates after membrane filtration. Therefore, ciliates such as Paramecium sp. RB1 and Tetrahymena sp. RB2 can remove bacteriophages present in natural and artificial waters by ingesting the viral particles and eliminating bacterial host cells required for viral replication. The ingestion of phage particles may marginally contribute to the nutrient supply of the ciliates. However, the interaction of phage particles with ciliate cells may contribute to the transmission of bacteriophages in aquatic environments.

Geographical, temporal, and individual-based differences in the trophic ecology of female Cape fur seals.

Information on resource use and trophic dynamics of marine predators is important for understanding their role in ecosystem functioning and predicting population-level responses to environmental change. Where separate populations experience different local environmental conditions, geographic variability in their foraging ecology is often expected. Within populations, individuals also vary in morphology, physiology, and experience, resulting in specialization in resource use. In this context, isotopic compositions of incrementally grown tissues such as keratinous hairs offer a valuable opportunity to study long-term variation in resource and habitat use. We investigated the trophic ecology of female Cape fur seals (Arctocephalus pusillus pusillus) using carbon and nitrogen isotopic compositions of serially sampled whiskers collected at four breeding sites along the coast of South Africa. Drawing on over 900 isotopic measurements, we assessed geographic variability in isotopic niche width between colonies and the degree of individual specialization. We found slight, but clear geographic differences in isotopic ratios and isotopic niche widths, seemingly related to ecological setting, with niche widths being proportional to the area of available shelf and shelf-slope habitat surrounding the colony. We further identified periodic oscillations in isotopic ratios, which likely reflect temporal patterns in foraging distribution and prey type, linked to shifts in the availability of prey resources and their interaction with constraints on individual females throughout their breeding cycle. Finally, individual specialization indices revealed that each of the study populations contain specialist individuals that utilize only a small subset of the total population niche width. The degree of individual specialization was, however, not consistent across colonies and may reflect an interactive influence between density-dependent effects and habitat heterogeneity. Overall, this study provides important information on the trophic ecology of Cape fur seals breeding in South Africa and highlights the need to consider geographic and individual variability when assessing the foraging ecology of marine predators.

What is a mammalian omnivore? Insights into terrestrial mammalian diet diversity, body mass and evolution.

Mammalian omnivores are a broad group of species that are often treated uniformly in ecological studies. Here, we incorporate omnivorous dietary differences to investigate previously found mammalian macroevolutionary and macroecological trends. We investigate the frequency with which vertebrate prey, invertebrate prey, fibrous plant material and non-fibrous plant material co-occur in the diets of terrestrial mammals. We quantify the body size distributions and phylogenetic signal of different omnivorous diets and use multistate reversible jump Markov chain Monte Carlo methods to assess the transition rates between diets on the mammalian phylogenetic tree. We find omnivores that consume all four food types are relatively rare, as most omnivores consume only invertebrate prey and non-fibrous plants. In addition, omnivores that only consume invertebrate prey, many of which are from Rodentia, are on average smaller than omnivores that incorporate vertebrate prey. Our transition models have high rates from invertivorous omnivory to herbivory, and from vertivory to prey mixing and ultimately invertivory. We suggest prey type is an important aspect of omnivore macroevolution and macroecology, as it is correlated with body mass, evolutionary history and diet-related evolutionary transition rates. Future work should avoid lumping omnivores into one category given the ecological variety of omnivore diets and their strong evolutionary influence.

Effect of light environment on prey consumption in two species of larval stomatopods, Gonodactylaceus falcatus (Forskål, 1775) and Gonodactylellus sp. (Stomatopoda: Gonodactylidae)

Abstract While adult stomatopod crustaceans are relatively well studied, understanding of larval stomatopod ecology is lacking, largely due to difficulties studying larvae in their natural habitat. This study investigated how light environment (i.e., spectral composition) and time of day affected prey consumption in two species of larval stomatopod, Gonodactylaceus falcatus (Forskål, 1775) and Gonodactylellus sp. Individual larvae were placed with 20 Artemia nauplii prey in feeding chambers treated to produce different light environments with respect to ultraviolet (UV) light: full spectrum light UV+, full spectrum UV–, and a dark control. Chambers were lowered to a depth of 3 m for 2 hours at three times of day (noon, twilight, and night) to test 1) if larval feeding rates changed at different times of day and 2) if UV vision was involved in prey capture. We found that light was important for successful feeding, with both species eating significantly more in lighted treatments than the dark controls during daytime experiments. Gonodactylellus sp. also had a significantly higher feeding rate at twilight in the UV+ treatment than in the dark control. Both species showed decreased consumption at night compared to daytime rates, and decreased consumption in all dark controls. This study is one of the first to examine how ecological conditions affect feeding behavior in larval stomatopods. Our results suggest that light is important for larval stomatopod feeding, with differences between species in daily feeding activity periods. There was also a difference in total consumption between the two species, with the slightly larger Gonodactylaceus falcatus consuming nearly double the prey items as Gonodactylellus sp. at peak feeding times. Follow up studies should incorporate a variety of prey types to test how feeding changes based on food source and density.

Impact of exploration behavior, aptitude for pellet consumption, and the predation practice on the performance in consecutive live prey foraging tests in a piscivorous species

Within the predator–prey relationship, predator behavior is less studied. Even in natural populations, it shows great diversity, and the factors influencing this are even less known. Among these factors, the personality type of the individual, (including exploration, and neophilia) and the practice significantly influence the success of adapting to a changing environment and switching to new prey types. In the present study, we investigated the first five consecutive foraging trials on live fish prey in naïve pikeperch individuals, which previously consumed or refused pelleted food. We hypothesized that individuals which were willing to consume alternative (pelleted) food would also show higher foraging success on living prey and that the practice would influence the learning process. Our results show that the timing of prey detection is influenced by exploratory behavior, the latency of the first attack by the aptitude for consuming pellets, and both traits by the individual's practice. However, neither of the factor affects the latency and success rate of capturing the prey, suggesting that predation is an independent behavioral trait.

Life-history omnivory in the fairy shrimp Branchinecta orientalis (Branchiopoda: Anostraca)

Very little is known about the feeding of naupliar and juvenile life stages of omnivorous fairy shrimps (Crustacea: Anostraca). Here, we aim to reveal whether the fairy shrimp Branchinecta orientalis is an ontogenetic omnivore and at which age and ontogenetic stage they gain the ability to feed on zooplankton. We assess how food uptake rates change with age until reaching maturity by providing algae (pico- and nanoplanktonic unicellular algae) and zooplankton (rotifers and copepod nauplii) as food in individual experiments. We found that the fairy shrimp B. orientalis started to feed on both types of algal prey immediately after hatching. Nanoplanktonic algae likely represented the most important food source until reaching maturity. Moreover, fairy shrimps started to feed on zooplankton already when they were 7 days old. Slow-moving rotifers gradually gained importance in the fairy shrimp diet with time. Our results reveal an ontogenetic change in the prey spectrum of fairy shrimp. The systematic shift towards omnivory likely affects both phyto- and zooplankton community composition, possibly contributing to temporal changes in food web dynamics in fairy shrimp habitats, and temporary ponds, which may warrant more detailed investigations in future studies.

Ecomorphological adaptations of owl feet and talons

AbstractFeet and talons are the most specialized predatory attributes of raptor morphology. As such, the hindlimb morphology of owls should reflect their dietary specialization. Owls' diet varies widely between species with only a subset of owl species preying predominantly on small mammals. We hypothesize that different owl species have varied hindlimb morphology, adapted to their main prey types, and that only dietary specialists will be distinguishable based on talon morphology (e.g. toe length, talon curvature and thickness, etc.). We used a sample of 63 specimens from 15 owl species to obtain a variety of measurements of hindlimb morphology. We then used a discriminant function analysis to investigate the degree of variation between species and diet groups in terms of their morphological measurements. Our results indicate that talon morphology is linked to prey type in the owl species studied. Insect specialists and small mammal specialists were characterized by a low talon curvature, whereas generalists presented a pronounced talon curvature. Fish specialists presented particularly thick and robust talons, and insect specialists showed lower digit strength than other owl species. These findings could help determine the diet of other owl species, particularly endangered species for which data on diet is often scarce. This study will also be very useful in future studies of the ecology of sympatric species, resource partitioning between species, or between sexes of the same species, and more generally to the evolution of owls.

Stability Analysis of Fractional-Order Predator-Prey System with Consuming Food Resource

The cardinal element of ecology is the predator-prey relationship. The population of interacting organisms is based on many factors such as food, water, space, and protection. A key component among these factors is food. The presence of food for the organisms shapes the structure of the habitat. The present study considers a predator and two types of prey. It is assumed that one prey species utilizes the same food resource as the predator, whereas the other prey species depends on a different food resource. The existence and uniqueness of the model are studied using the Lipschitz condition. The fixed points for the fractional-order model are sorted out, and the existence of the equilibrium points is discussed. The stability analysis of the model for the biologically important fixed points is provided. These include the coexistence fixed point and the prey-free (using the same food resources as the predator does) fixed point. A fractional-order scheme is implemented to support theoretical results for the stability of equilibrium points. The time series solution of the model is presented in the form of plots. Moreover, the impact of some mathematically and biologically important parameters is presented.

Ontogenetic shifts in feeding habits of orangemouth weakfish (Cynoscion xanthulus): From estuarine benthic feeder to marine nektivore top predator

AbstractOrangemouth weakfish is a highly important commercial and game fish species in the Tropical Eastern Pacific. However, studies assessing changes in its trophic ecology relevant to size, habitat (estuarine or marine), sex, and season are non‐existent. We assessed its feeding habits and trophic position (TP) according to the aforementioned factors. Its dietary breadth was composed of 33 types of prey. The feeding habits and isotopic values (δ15N and δ13C) registered differences between sizes and habitat, indicating ontogenetic changes in habitat as well as TP, as small organisms (TP 3.1) inhabited the estuarine area, feeding on benthic and demersal organisms, whilst large individuals (TP 4.1) were found in the marine environment, preying on nektonic fauna. No differences were found according to sex or season. The isotopic niche showed that the different sizes of this species are generalist consumers with a clear separation between them and between habitats, as estuarine juvenile and preadult organisms have a larger isotopic niche than adult marine organisms, being an opportunist tertiary estuarine consumers during their early stages, and a top marine predator during the adult phase.

Is a diet of Planococcus citri nymphs and adults suitable for Chrysoperla externa for use in biological control?

Several studies have shown that the survival and reproduction of lacewings varies depending on the type of prey they consumed, especially during the first instar. In this study, we evaluated the effect of a diet consisting of Ephestia kuehniella (EK) and/or Planococcus-citri (PC) eggs offered to first instar Chrysoperla externa on subsequent development and survival. Larvae supplied only with P. citri (PC) nymphs had extended immature development, low survival, adults with lower weight and longevity, low fecundity and egg viability, and lower net reproduction rate (R0). Larvae submitted to the treatment EK+PC responded positively to the diet, showing results similar to those obtained in the control-EK treatment. Therefore, this mealybug supports suitable development and reproduction of C. externa when it is provided to second instar larvae after they fed on E. kuehniella eggs as first instars. As far as we know, this is the first report that demonstrates the effect of natural prey and alternatives on different larval stages of lacewings. This information is valuable in developing protocols for use of C. externaas a pest control agent.

The Adult Tendency of Eleven-Spotted Ladybird, Coccinella undecimpunctata for Preference of Certain Type of Aphid

Ibrahim, A.K. 2023. The Adult Tendency of Eleven-Spotted Ladybird, Coccinella undecimpunctata for Preference of Certain Type of Aphid. Arab Journal of Plant Protection, 41(4): 369-374. https://doi.org/10.22268/AJPP-41.4.369374 This study explored the extent of eleven ladybirds preference to a specific aphid species, i.e., Aphis fabae, A. nerii, Macrosiphoniella sanborni and Hyalopterus pruni and the extent of the host plant and type of prey in attracting predators. Results obtained showed a preference of adult ladybird predators for H. pruni, Macrosiphoniella sanborni, Aphis nerii, whereas Aphis fabae was the least attractive and consumed by predators. The consumption rate of apricot aphid reached 23.66% in the presence of aphids and the host plant together. The most attraction to apricot leaves in the presence of the plant only was 55.90%. The time required for ladybirds to reach the first prey was 2.15 min for the apricot aphid in the presence of aphids and plants together. Furthermore, the ladybirds reached the prey within 1.30 minutes in the presence of aphids only. The predation efficiency of the four larval instars varied according to the type of aphids. It reached 86.30% for the fourth larval instar on apricot aphids and was the highest compared with the rest of the aphid species, which ranged between 7.10 and 84%. The general average predation efficiency values on aphids were 57.45% for H. pruni, 53.13% for Macrosiphoniella sanborni, 46.05% for A. nerii and 41.16% for A. fabae. The relative efficiency values for the larval instars reached 12.25, 42.27, 60.20 and 83.50%, respectively, for the first, second, third and fourth instars. This was confirmed by the high attraction of predatory ladybirds towards H. pruni and Macrosiphoniella sanborni, compared to Aphis nerii and Aphis fabea. The proportion of aphids consumed by ladybirds in the presence of the plant host was highest for H. pruni and lowest on Aphis fabea. Keywords: Aphids, ladybugs, plant host, predation efficiency

A Filippov type predator prey system with selective harvesting of predator

In this paper, piece-wise smooth predator prey Filippov system to describe the role of refuge to prey and additional food to predators is explored. Logistic growth of prey and Holling type II predation is considered. Selective proportional harvesting of predators is included. Refuge is provided to a fraction of preys when their density is below a threshold value. This reduces the food availability for predators. To compensate this, additional food is provided to predators. The dynamics on the boundary is computed using the Filippov regularization method. The stability and existence of different equilibrium states of the independent smooth subsystems are analyzed. The role of quality of additional food with respect to prey in the characterization of the equilibrium points as regular or virtual equilibrium points for the Filippov system is investigated. It is verified that if only refuge is provided to prey with no additional food to predators then both the interior equilibrium state will never be regular simultaneously for the Filippov system. It is observed from the sliding mode dynamics that there is no escaping region. It is proved that both refuge to prey and additional food to predators enhances the sliding region on the boundary. The analysis of boundary equilibrium points, tangent points and pseudoequilibrium points on the boundary is carried out. The role of effort on harvesting on the expansion or shrinking of the limit cycle is elaborated numerically. Boundary bifurcation for the interior equilibrium state of the subsystems is demonstrated. Numerical simulation is used to elaborate the dynamics of Filippov system.

Анализ фаунистического сходства сообществ пернатых хищников некоторых местообитаний Алтайского края и республики Алтай

Birds of the orders Falconiformes, Accipitriformes, and Strigiformes are at the top of the food chain, and therefore, they are the most vulnerable to changes in the natural environment caused by human activities. The condition of populations of birds of prey can serve as a criterion for the well-being of a biotope. The current state of nesting groups of many species of birds of prey in Altai Krai is of great concern. The survival of many of them is now in question, as numerous threats are generated by lumbering, mining, farming, poaching, growing energy infrastructure, and other human activities. In this regard, there is an urgent need for a detailed study of the species composition and population numbers of birds of prey and owls in the Altai region, as well as regular monitoring of populations that still exist. In this paper, we made a comparative analysis of breeding groups of raptors of different habitats of the Russian part of the Altai region: the Ukok plateau, the Ulandryk River gorge, the Anui River valley and the Kasmalinsky pine forest. Data on the species composition are taken from both our works and literature. We carried out a comparative analysis of breeding groups of birds using the coefficients of faunistic similarity of Jaccard and Sörensen. Since we had to compare more than two breeding groups, we used Mountford cluster analysis to order the similarity coefficients, which consists in sequentially combining the most similar groups. The results of the analysis of the Jaccard and Sörensen coefficients turned out to be very similar. The greatest similarity of the nesting species composition was found between the Anui River valley and Kasmalinsky pine forest (Кj=0.63; Кs=0.77). More significant differences were found between the Ukok plateau and the Ulandryk River gorge (Кj=0.44; Кs=0.60). And the maximum differences were found between the Kasmalinsky pine forest and the Ulandryk gorge (Кj=0.17; Кs=0.29). These differences can be explained by the specifics of the physical and geographical conditions and vegetation cover of different habitats of the Altai region. The poor species composition of raptors in the Ulandryk Gorge (7 species of birds of prey and 1 species of owls) is due to the complete absence of woods in this habitat. Therefore, there are no tree-nesting species here. The gorge is inhabited only by those species of raptors that can build their nests on the rocks. There are also no species here that require special types of prey, which are not sufficient in this biotope. The richness of the species composition of raptors (18 species of birds of prey and 8 species of owls) in the Kasmalinsky pine forest is because most species are dendrophiles and find here the optimal substrate for nests – large pines. The forage base here is also optimal for most species of raptors. Only specific mountain-steppe and high-mountain species do not nest in the pine forest. According to the species composition of raptors, habitats such as the Ukok plateau and the Anui river valley are in an intermediate position between the Ulandryk gorge and the Kasmalinsky pine forest. Both have rocky outcrops suitable for nesting and trees. Therefore, in these biotopes birds that prefer nesting both on rocks and in trees can be found.

New records and a review of animal foods in the diets of the Brolga Antigone rubicunda and Australian Sarus Crane A. antigone gillae

Cranes are opportunistic omnivores, sometimes considered to be primarily vegetarian. However, there is limited knowledge in Australia of their animal prey and its importance in their diets. From field records in 2006–2021, we report novel foods including fish and crabs taken by adult Australian Sarus Cranes Antigone antigone gillae and the first records of the foods of their dependent young (crabs and grasshoppers). For Brolgas A. rubicunda, we report the first records of ducklings as prey, and of crabs, beetles and grasshoppers fed to dependent young. Both species have learnt to prey on rodents displaced or killed by sugar cane harvesting machinery in northern Queensland. A review of diet records since 1810 also reveals some overlooked foods and food-handling methods but, despite a sighting of a Brolga ingesting a whole Cane Toad Rhinella marina, there is no evidence of immunity to the toad’s toxin in either species of crane. The plant and animal components of the diet of the dependent young of both species remain almost unknown and there are also significant knowledge gaps for the animal prey of adult Australian Sarus Cranes; the vertebrate prey of adult Brolgas; and the balance of prey types for both species in the breeding and non-breeding seasons. To advance knowledge of crane diets and nutritional ecology in Australia and New Guinea, direct field techniques including faecal sampling, collection of shed feathers and camera trapping could be undertaken, supported by remote electronic technologies in relatively inaccessible breeding wetlands. Meanwhile – in anticipation of the forthcoming revision of the Handbook of Australian, New Zealand & Antarctic Birds – we urge fieldworkers, birdwatchers and others to examine their field notes for unpublished information on crane foraging and food items, especially for the dependent young of Brolgas and Australian Sarus Cranes.

Handling- or digestion-limited predators: the role of body mass and habitat complexity in predator functional response

The predator functional response quantifies the per capita feeding rate of predators as a function of prey density and is a key element of feeding interactions. Variations in its parameters are strongly associated with interaction strength and population dynamics. We examined 18 functional responses within marine whelk-bivalve systems, varying predator body size, prey species, and habitat structure. Our findings suggest that the marine whelk Rapana venosa is handling-limited, a predator type that has received less attention in previous research. We propose further categorizing handling-limited predators into 2 types: pursuit-limited (where maximum feeding rate could be influenced by habitat complexity) and ingestion-limited (where maximum feeding rate is impacted not by habitat complexity, but by predator-prey body mass ratios and prey defense strategy). We found that handling time scales negatively with predator-prey body mass ratios, but this trend exhibits layers of complexity. We propose that the transition from handling to digestion limitation with increasing predator-prey body mass ratios underlies this trend. Our study also confirms the importance of prey types, in addition to known effects of body mass ratios and habitat structure. In summary, our study reveals that simple assumptions about body masses and prey defense strategy may usefully refine estimates of feeding interactions in complex food webs.

Feeding ecology of Brevoortia aurea larvae (Clupeidae, Alosinae) from Río de la Plata estuary off Uruguay

This study analyses the feeding ecology of Brevortia aurea (Brazilian menhaden) larvae during a reproductive season in the Río de la Plata estuary (RPE), and explores its changes along larval development. Data were obtained from stomach content analyses of 205 individuals collected with a Bongo net during spring 2001. Larvae were discriminated into preflexion (PF), flexion (F) and post-flexion (PsF) stages. Feeding was described using complementary metrics like Feeding incidence (FI), the Relative Importance of different prey types consumed (IRI), the Degree of prey digestion (E) and Diet overlap (D), which were estimated and compared between stages. Also, larval morphometric relationships (body length, BL vs mouth width, MW) and between MW and maximum prey width (Wmax) were established for the range of observed larval sizes. The overall FI was 46.3% and increased with the development stage: 40.4% PF, 63.3% F, 78.6% PsF. Feeding occurred mainly during sunlit hours but no clear daily cycle could be established. Copepods and in particular Acartia tonsa were well represented among ingested preys, followed by invertebrate eggs and nauplii. Prey diversity and overlap between stages tended to decrease as development progressed. B. aurea exhibited isometric growth of MW in relation to BL. Average Wmax was 222.5 µm ± 100.9 SD, and increased non-linearly with both BL and MW. Gape size alone did not seem to be the limiting factor for prey choice (size), and we hypothesize that factors involved in the feeding mechanism other than mouth gape and linked with capture performance substantially influence the feeding ecology of this species in the RPE.

Effect of diet composition and temperature on the gastric evacuation rate of European sardine: implication for egg predation estimates

Cannibalism and intraguild predation of fish eggs are frequently observed for small and medium pelagic fish and can be an important cause of natural mortality. The European sardine Sardina pilchardus is the major predator of pelagic fish eggs in Atlantic Iberian waters. Estimates of egg mortality due to predation rely on a number of parameters that are frequently unknown for most species, such as gastric evacuation rate (GER) and feeding periodicity, and rates obtained for other fish species are generally used. These assumptions can severely affect egg mortality estimates. We conducted laboratory experiments to estimate the GER of European sardine that were fed diets composed of different proportions of fish eggs and other food types, such as Artemia franciscana nauplii, Brachionus plicatilis and microalgae, at different temperatures experienced by sardine in the wild (15, 17 and 21°C). Temperature, predator size and sex did not influence the gastric rates. Prey type significantly affected the GER of sardines, with slower evacuation when offered diets containing a higher concentration of fish eggs rather than microalgae, rotifers and Artemia nauplii. Fish eggs are more nutritionally rich compared to microalgae, rotifers and Artemia nauplii, and therefore more difficult to digest. GERs obtained here for S. pilchardus were similar to those obtained for Sardinops sagax in South Africa. This study provides important information that allows for improving estimates of daily feeding rates and egg mortality by predation, thus increasing our understanding of small pelagic fish population dynamics.

Choice of methodology and surrogate prey are decisive for the quality of protistan bacterivory rate estimates

Microeukaryote predation on bacteria is a fundamental phenomenon to understand energy and nutrient dynamics at the base of the aquatic food web. To date, the most prevalent way to estimate grazing rates is by using epifluorescence microscopy to enumerate ingestion events of fluorescently labelled tracers (FLTs) after short-term incubation experiments. However, this approach can be sensitive to the type of FLT, requires skillful preparation of the samples and is limited to small sample sizes. We tested the susceptibility of rate estimates to the choice of prey and made a side-by-side comparison between microscopy and flow cytometry when recording ingestion by a bacterivorous flagellate. Short-term uptake experiments were established using 5 types of FLTs differing in quality (living, dead or inert) and size (large or small), with Ochromonas triangulata as a model flagellate. The experiments showed that (1) each of the different prey types yielded different clearing rates, ranging from 0.5 to 3.6 nl cell-1 h-1, with the largest differences (3-fold or higher) between small prey (lower rates) and large prey (higher rates); (2) the cytometry estimate differed significantly from the microscopy estimate in 3 out of 4 experimental configurations; and (3) the precision of the cytometric analysis was greater, with >3-fold higher uncertainty associated with microscopy counting. Our results validate that flow cytometry provides a more precise bacterivory estimate, and that the choice of FLT influences the grazing rate estimate to a high extent regardless of the analytical method used.