Fine-scale observations of the predatory behaviour of the carnivorous copepod <Emphasis Type="Italic">Paraeuchaeta norvegica</Emphasis> and the escape responses of their ichthyoplankton prey,Atlantic cod (<Emphasis Type="Italic">Gadus morhua</Emphasis>)

Paraeuchaeta norvegica (8.5 mm total length) and yolk-sac stage Atlantic cod larvae (4 mm total length) (Gadus morhua) larvae were observed in aquaria (3 l of water) using silhouette video photography. This allowed direct observations (and quantitative measurement) of predator–prey interactions between these two species in 3-dimensions. Tail beats, used by cod larvae to propel themselves through the viscous fluid environment, also generate signals detectable by mechanoreceptive copepod predators. When the prey is close enough for detection and successful capture (approximately half a body-length), the copepod launches an extremely rapid high Reynolds number attack, grabbing the larva around its midsection. While capture itself takes place in milliseconds, minutes are required to subdue and completely ingest a cod larva. The behavioural observations are used to estimate the hydrodynamic signal strength of the cod larva’s tail beats and the copepod’s perceptive field for larval fish prey. Cod larvae are more sensitive to fluid velocity than P. norvegica and also appear capable of distinguishing between the signal generated by a swimming and an attacking copepod. However, the copepod can lunge at much faster velocities than a yolk-sac cod larva can escape, leading to the larva’s capture. These observations can serve as input to the predator–prey component of ecosystem models intended to assess the impact of P. norvegica on cod larvae.     

Experimental prey species preferences of Hexaplex trunculus (Gastropoda: Muricidae) and predator–prey interactions with the Black mussel Mytilus galloprovincialis (Bivalvia: Mytilidae)

Hexaplex trunculus is one of the most widespread Mediterranean species of muricid gastropod and lives on rocky, sandy-mud and mud substrata. Although common in the Adriatic Sea, relatively little is known about its ecology especially feeding behaviour. The aim of this study was to explore the aspects of the feeding behaviour of H. trunculus using Arca noae, Modiolus barbatus and Mytilus galloprovincialis as experimental prey. Prey species preference, predator size, prey size choice, feeding rates, handling times and mode and place of attack were analysed. Typically, only M. galloprovincialis was attacked: A. noae rarely at the byssal gape and M. barbatus never. Small (40 mm) H. trunculus could not easily attack large M. galloprovincialis (65 mm) and preferred small (20 mm) and medium (35 mm) sized prey. Conversely, medium (55 mm) and large (70 mm) H. trunculus fed randomly on M. galloprovincialis of all three sizes. The feeding strategy adopted by H. trunculus individuals varied with respect to size. A tendency to drill the prey shell was recorded for small predators, whereas marginal chipping was adopted more frequently by large individuals. On average small, medium and large H. trunculus consumed 2.4±1.6 (range 0–4), 1.2±1.6 (range 0–4) and 2.0±2.1 (range 0–6) M. galloprovincialis, respectively. There was a statistically significant difference in prey-handling time with respect to the method of access adopted, predator and prey sizes. The time required to access a M. galloprovincialis individual by marginal chipping was considerably less than that required for drilling. H. trunculus consumed an average of 0.60±0.80 g M. galloprovincialis tissue dry weight over a 5-week period, that is, ~40% of its own tissue body weight. This translates to an average-sized (55 mm shell height) H. trunculus consuming ~18 M. galloprovincialis of 50 mm shell length (minimum marketable size) per year. H. trunculus showed no preference to drill either the left or right valves of M. galloprovincialis but generally attacked the posterior shell margin.     

Diet of 0-group stages of capelin (<Emphasis Type="Italic">Mallotus villosus</Emphasis>), herring (<Emphasis Type="Italic">Clupea harengus</Emphasis>) and cod (<Emphasis Type="Italic">Gadus morhua</Emphasis>) during spring and summer in the Barents Sea

Recruitment of capelin in the Barents Sea fail when juvenile herring and cod are abundant and the potential for feeding competition of wild sympatric capelin and herring larvae and small cod juveniles were investigated. The frequency of gut evacuation after capture of capelin larvae were also studied in mesocosms. Small capelin larvae (<35 mm length) fed on small prey including phytoplankton, invertebrate eggs and nauplii, bivalves, other invertebrate larvae and small copepods. Calanus copepodites were only observed in large capelin larvae (>26 mm length). Calanus copepodites were the major food sources for contemporary herring larvae (25–35 mm length) and Calanus and euphausiids were the major prey for small juvenile herring (37–60 mm length) and cod (18–40 mm length). Capelin larvae reared in mesocosms evacuated the guts shortly after capture. Capelin larvae had a smaller mouth and fed on smaller prey than herring and cod of the same length. This implies that the small capelin larvae, in contrast to sympatric small herring and cod, are not tightly linked to the food chain involving Calanus and euphausiids. Thus, exploitative competition between capelin larvae and planktivorous fish that rely on Calanus and euphausiids in the Barents Sea may be relaxed.     

Molecular prey identification in wild Octopus vulgaris paralarvae

The trophic ecology of Octopus vulgaris paralarvae collected in 2008 off the Ría de Vigo, NW Spain (42° 12.80′ N–9° 00.00′ W), was approached by both morphological and molecular methods. External digestion of prey and posterior suction of the liquefied contents by wild O. vulgaris paralarvae made the morphological identification of gut contents impossible. Thus, a PCR-based method using group-specific primers was selected to identify prey consumed by O. vulgaris paralarvae in the pelagic realm. The mitochondrial ribosomal 16S gene region was chosen for designing group-specific primers, which targeted a broad range of crustaceans and fishes but avoided the amplification of predator DNA. These primers successfully amplified DNA of prey by using a semi-nested PCR-based approach and posterior cloning. Homology search and phylogenetic analysis were then conducted with the 20 different operational taxonomic units obtained to identify the putative organisms ingested. The phylogenetic analysis clustered ingested prey into 12 families of crustaceans (11 belonging to the order Decapoda and 1 to the order Euphausiacea) and two families of fishes (Gobiidae and Carangidae). According to the Czekanowski’s Index (CI), the trophic niche breadth of O. vulgaris paralarvae is low (CI = 0.13), which means that these paralarvae are specialist predators at least during the first weeks of their life cycle. It is the first time that natural prey has been identified in O. vulgaris paralarvae collected from the wild, and such knowledge may be critical to increasing the survival of O. vulgaris hatchlings in captivity, a goal that has been actively pursued since the 1960s by aquaculture researchers.     

Temperature, salinity and prey availability shape the marine migration of Arctic char, Salvelinus alpinus, in a macrotidal estuary

The influence of salinity, temperature and prey availability on the marine migration of anadromous fishes was determined by describing the movements, habitat use and feeding behaviours of Arctic char (Salvelinus alpinus). The objectives were to determine whether char are restricted to the upper water column of the inter-/subtidal zones due to warmer temperatures. Twenty-seven char were tracked with acoustic temperature/pressure (depth) transmitters from June to September, 2008/2009, in inner Frobisher Bay, Canada. Most detections were in surface waters (0–3 m). Inter-/subtidal movements and consecutive repetitive dives (maximum 52.8 m) resulted in extreme body temperature shifts (−0.2–18.1 °C). Approximately half of intertidal and subtidal detections were between 9–13 °C and 1–3 °C, respectively. Stomach contents and deep diving suggested feeding in both inter-/subtidal zones. We suggest that char tolerate cold water at depth to capture prey in the subtidal zone, then seek warmer water to enhance feeding/digestion physiology.     

The trophic link between squid and the emperor penguin <Emphasis Type="Italic">Aptenodytes forsteri</Emphasis> at Pointe Géologie,Antarctica

Cephalopod beaks retrieved from stomachs of dead emperor penguin chicks at Pointe Géologie, Terre Adélie, provide information on taxonomic and size composition of the penguin’s squid diet, on the trophic range of the squid species preyed upon and on the fractional trophic impact of the penguin on the whole food web. Emperor penguins prey upon four squid species (Psychroteuthis glacialis, Kondakovia longimana, Gonatus antarcticus, Alluroteuthis antarcticus) and do not take squid larger than 480 mm mantle length. Larger squid live either below the penguin’s diving range or are beyond its handling capacity. Nitrogen stable isotope ratios indicate that squids cover a range of about two trophic levels (2.5–8‰ δ¹⁵N). The impact of the emperor penguin, however, concentrates on the upper part of this range, about 68% of its squid prey being >6‰ δ¹⁵N. The principal components of the emperor’s diet, fish, krill and squid, differ distinctly in average trophic level. Consequently the trophic position of the emperor penguin changes accordingly with diet composition and may differ by almost one trophic level between different emperor penguin colonies.     

Mechanics of prey selection by ephyrae of the scyphomedusa Aurelia aurita

In situ feeding patterns of ephyrae of the jellyfish Aurelia aurita (Linnaeus) revealed the importance of relatively large (>1 mm) prey in the diet of these scyphozoan predators. These studies were carried out in Narragansett Bay, Rhode Island, USA in March and April, from 1993 through 1996. Rotifers were the only small prey ingested in quantity, and then only when they were unusually abundant in the plankton. Copepod nauplii, similar in size to rotifers and equally abundant, were rarely consumed. Since copepods evince rapid escape responses, this observation suggested a role for prey escape in determining prey vulnerability, while the predominance of large prey in the diet suggested a role for prey size. Using two dimensional video observations of free-swimming ephyrae and their prey in the laboratory we tested hypotheses about the mechanisms underlying these dietary patterns, comparing mechanisms for capture of large versus small prey and for prey of equal size but differing escape behaviors. Capture efficiencies of ephyrae feeding on large prey were 4 to 12 times greater than for small prey taxa. Capture efficiencies for prey of equal size also differed significantly, indicating that other factors influence the outcome of predator–prey interactions. Most prey captures occurred while the ephyrae were swimming and creating fluid flows that entrained prey into the subumbrellar region. Even copepod nauplii were frequently drawn into the subumbrella of swimming ephyrae despite average potential escape velocities (25.7 mm s⁻¹) that exceeded mean maximum velocity of fluid flows around the ephyrae (13.1 mm s⁻¹). Large prey were more likely than small prey to contact nematocyst-bearing surfaces both before and after entrainment in flow fields. With regard to behavior, prey escape speeds were not the only predictor of prey vulnerability. Prey that continued swimming after entrainment (rotifers and brine shrimp) were captured more often than prey of equal size that ceased normal swimming (copepod nauplii and barnacle nauplii). Copepod nauplii were the prey least likely to be captured because they either “played dead” and were expelled from the subumbrella of the ephyrae before contacting a surface, or they eventually escaped at high velocity. These observations indicate that size-selective predation by ephyrae of A. aurita can be influenced by a variety of behavioral responses of the prey. Received: 9 April 1997 / Accepted: 5 September 1997     

Mechanism of predator–prey detection and behavioral responses in some anuran tadpoles

Predator–prey relationship was studied in three sympatric species of anuran tadpoles. The study design consisted of allowing predaceous Hoplobatrachus tigerinus tadpoles to devour prey tadpoles (Sphaerotheca breviceps and Bufo melanostictus) placed in a plastic tub (five tadpoles of each species, stage ~27) in 30 min. In trials without refugia, more tadpoles of Bufo fell prey compared to Sphaerotheca. In contrast, provision of refugia using hydrilla plant reversed predation risk of the two species. The swimming speed (V _max = 64.55 ± 1.45 cm/s) of Hoplobatrachus tadpoles was much higher compared to the prey species (Bufo: 3.6 ± 0.4 cm/s; Sphaerotheca: 27.6 ± 1.6 cm/s). Poor swimming ability may account for the observed vulnerability of the Bufo tadpoles to predation especially in clear waters; refugia overcame predation to some extent. On the other hand, Sphaerotheca tadpoles that swim faster than the toad tadpoles were less vulnerable in open areas; refugia actually hindered swimming and increased predation. Experiments with association choice tests show that predaceous tadpoles detect prey based on both visual and chemical cues. On the other hand, the prey tadpoles detected predator based exclusively on chemical rather than visual cues. The antipredator defense strategy of the toad tadpoles is manifested in the form of reduced movements, remaining still for longer times and, increased burst speed. The present findings also suggest that in both prey species predator detection has a genetic basis since naive tadpoles with no prior exposure to predators exhibit fright response on first encounter with them.     

Importance of rock lobster size–structure for trophic interactions: choice of soft-sediment bivalve prey

Ecologists are becoming increasingly interested in how variation in predator demographics influences prey communities. In northeastern New Zealand, the contrasting populations of previously exploited predators in highly protected marine reserves and fished areas have been used to investigate the effects of predation in soft-sediment habitats. However, these experiments have been unable to separate the role of predator size from that of density. This study provides evidence to support the model that foraging by different sizes of the rock lobster Jasus edwardsii affects soft-sediment bivalve populations in different ways. Feeding trials were conducted to investigate whether rock lobsters of different sizes vary in their choice of taxa and size of their bivalve prey. Trials with two morphologically similar species, Dosinia subrosea and Dosinia anus, indicated that lobsters of all sizes choose D. subrosea more frequently than the heavier shelled D. anus. Further results indicated that both large (>130 mm carapace length (CL)) and small (<100 mm CL) lobsters are capable of preying on a wide size range of D. subrosea (20–60 mm). However, small lobsters more frequently chose smaller shells (<30 mm) and large lobsters more frequently chose larger shells (>40 mm). Patterns in the abundance and size class distributions of these two bivalve species at protected and fished sites supported the feeding choices observed in the laboratory. These results suggest that populations of rock lobsters with large individuals inside reserves are capable of controlling the demography of bivalve populations in adjacent soft-sediment systems.     

Variation in pelagic larval growth of Atlantic billfishes: the role of prey composition and selective mortality

Atlantic blue marlin (Makaira nigricans) and sailfish (Istiophorus platypterus) larvae were collected from 10 monthly cruises (June–October 2003 and 2004) across the Straits of Florida to test (1) whether growth differed between the more productive western region near the Florida shelf, and the less productive eastern region toward the Bahamas, and (2) whether growth was related to prey consumption. Examination of larval sagittal otoliths revealed that instantaneous growth and daily growth during the first 2–3 weeks of life did not vary significantly between the two regions for either species. However, recent growth during the last two full days prior to collection was greater in the west for blue marlin larvae. Recent growth of blue marlin larvae <9 mm SL (primarily zooplanktivorous) was significantly related to prey composition (faster growth when higher proportions of Farranula copepods were consumed). Western larvae grew faster and had higher proportions of Farranula in their guts. Trends for sailfish larvae were not significant. In both species, comparison of early growth between <9 and ≥9 mm SL size groups indicated that growth trajectories diverged around 5–8 mm SL, the time when billfish larvae become capable of piscivory. Significantly faster growth of larger (older) larvae suggests that mortality was selective for fast growers and that the transition to piscivory may be a critical point in the early life of billfish.     

Evaluating the impact of caribou habitat restoration on predator and prey movement

Fragmentation of the boreal forest by linear features, including seismic lines, has destabilized predator–prey dynamics, resulting in the decline of woodland caribou (Rangifer tarandus caribou) populations. Restoration of human-altered habitat has therefore been identified as a critical management tool for achieving self-sustaining woodland caribou populations. However, only recently has testing of the response of caribou and other wildlife to restoration activities been conducted. Early work has centered around assessing changes in wildlife use of restored seismic lines. We evaluated whether restoration reduces the movement rates of predators and their associated prey, which is expected to decrease predator hunting efficiency and ultimately reduce caribou mortality. We developed a new method for using cameras to measure fine-scale movement by measuring speed as animals traveled between cameras in an array. We used our method to quantify speed of caribou, moose (Alces alces), bears (Ursus americanus), and wolves (Canis lupus) on treated (restored) and untreated seismic lines. Restoration treatments reduced travel speeds along seismic lines of wolves by 1.38 km/h, bears by 0.55 km/h, and caribou by 1.57 km/h, but did not reduce moose travel speeds. Reduced predator and caribou speeds on treated seismic lines are predicted to decrease encounter rates between predators and caribou and thus lower caribou kill rates. However, further work is needed to determine whether reduced movement rates result in reduced encounter rates with prey, and ultimately reduced caribou mortality.     

Erratum to: Prey selection by resident common bottlenose dolphins (Tursiops truncatus) in Sarasota Bay, Florida

Prey selection was investigated in wild, resident common bottlenose dolphins, Tursiops truncatus, during the summer months in Sarasota Bay, Florida, USA. Stomach content analyses of 15 dolphins with extensive sighting histories and well-documented distributions were used to determine prey use. Prey availability was assessed by purse seine surveys. We compared the relative abundances of prey available to estimates of prey use at closely matching spatial and temporal scales. G-tests determined that dolphins in this study significantly selected for prey at the species, family, and soniferous/non-soniferous prey levels (G _adj  = 753.98–1,775.93, df = 1–21, p ≤ 0.01). While comprising only 6.3% of the total available prey, soniferous fishes accounted for 51.9% of the total prey consumed. Manly’s standardized forage ratios and 95% Bonferroni confidence intervals determined significant positive selection for soniferous prey and against non-soniferous prey (β^S = 0.9461 vs. β^NS = 0.0539). Dolphins selected against Gerridae, Clupeidae, and Sparidae (β ≤ 0.0014), as well as against all the species within those families (β ≤ 0.0190). It is likely that passive listening for soniferous prey provides an ecological or energetic advantage to cetaceans utilizing this specific foraging technique.     

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     

Diet of young Atlantic bluefin tuna (<Emphasis Type="Italic">Thunnus thynnus</Emphasis>) in eastern and western Atlantic foraging grounds

Atlantic bluefin tuna (Thunnus thynnus) are highly migratory predators whose abundance, distribution, and somatic condition have changed over the past decades. Prey community composition and abundance have also varied in several foraging grounds. To better understand underlying food webs and regional energy sources, we performed stomach content and stable isotope analyses on mainly juvenile (60–150 cm curved fork length) bluefin tuna captured in foraging grounds in the western (Mid-Atlantic Bight) and eastern (Bay of Biscay) Atlantic Ocean. In the Mid-Atlantic Bight, bluefin tuna diet was mainly sand lance (Ammodytes spp., 29% prey weight), consistent with historic findings. In the Bay of Biscay, krill (Meganyctiphanes norvegica) and anchovy (Engraulis encrasicolus) made up 39% prey weight, with relative consumption of each reflecting annual changes in prey abundance. Consumption of anchovies apparently declined after the local collapse of this prey resource. In both regions, stable isotope analysis results showed that juvenile bluefin tuna fed at a lower trophic position than indicated by stomach content analysis. In the Mid-Atlantic Bight, stable isotope analyses suggested that >30% of the diet was prey from lower trophic levels that composed <10% of the prey weights based upon traditional stomach content analyses. Trophic position was similar to juvenile fish sampled in the NW Atlantic but lower than juveniles sampled in the Mediterranean Sea in previous studies. Our findings indicate that juvenile bluefin tuna targeted a relatively small range of prey species and regional foraging patterns remained consistent over time in the Mid-Atlantic Bight but changed in relation to local prey availability in the Bay of Biscay.     

Growth and grazing responses of Chrysochromulina ericina (Prymnesiophyceae): the role of irradiance, prey concentration and pH

The effect of irradiance, prey concentration and pH on the growth and grazing responses of the mixotrophic prymnesiophyte Chrysochromulina ericina under N-and P-replete conditions was studied using the pedinophyte Marsupiomonas pelliculata as prey. The two organisms were inoculated in monocultures and in mixed cultures at different predator: prey ratios at three irradiances and allowed to grow for 4–7 days. All cultures were non-axenic. Algal densities and pH were monitored throughout the experiments and growth and grazing rates were measured. An increase in growth of C. ericina cultures at irradiances of 25 and 70 μmol photons m⁻² s⁻¹ was observed after the addition of prey, while growth of C. ericina cultures at the high irradiance (150 μmol photons m⁻² s⁻¹) was unaffected by the addition of prey. However, although the growth of C. ericina increased at low irradiance (25 μmol photons m⁻² s⁻¹), it did not reach the same level as monocultures at the high irradiance (150 μmol photons m⁻² s⁻¹), suggesting that phagotrophy can only partly replace photosynthesis in C. ericina. Maximum growth rates of C. ericina at irradiances of 25 and 70 μmol photons m⁻² s⁻¹ were obtained at concentrations of > 0.15–0.3×10⁵ M. pelliculata ml⁻¹, corresponding to 50–100 μg C 1⁻¹. Ingestion of M. pelliculata cells by C. ericina did not generally follow Michaelis—Menten kinetics. Deviation from the expected saturation kinetics was especially pronounced at irradiances of 70 and 150 μmol photons m⁻² s⁻¹. At these irradiances ingestion of M. pelliculata cells by C. ericina decreased at high concentrations of M. pelliculata, indicating an increased uptake of bacterial prey in these cultures. The growth rate of C. ericina was affected in both monocultures and in mixed cultures when pH increased above 8.6, and growth stopped around pH 9. The prey alga M. pelliculata tolerated high pH better and, consequently, took over in the mixed cultures when pH exceeded 9. The ecological significance of mixotrophy in the genus Chrysochromulina is discussed. Published online: 4 July 2002     

Linking predator and prey behaviour: contrasts between Antarctic fur seals and macaroni penguins at South Georgia

Antarctic fur seals Arctocephalus gazella and macaroni penguins Eudyptes chrysolophus are the two main land-based krill Euphausia superba consumers in the northern Scotia Sea. Using a combination of concurrent at-sea (predator observations, net hauls and multi-frequency acoustics), and land-based (animal tracking and diet analysis) techniques, we examined variability in the foraging ecology of these sympatric top predators during the austral summer and autumn of 2004. Krill availability derived from acoustic surveys was low during summer, increasing in autumn. During the breeding season, krill occurred in 80% of fur seal diet samples, with fish remains in 37% of samples. Penguin diets contained the highest proportion of fish in over 20 years of routine monitoring (46% by mass; particularly the myctophid Electrona antarctica), with krill (33%) and amphipods (Themisto gaudichaudii; 21%) also occurring. When constrained by the need to return and feed their offspring both predator species foraged to the northwest of South Georgia, consistent with an area of high macrozooplankton biomass, but fur seals were apparently more successful at exploiting krill. When unconstrained by chick-rearing (during March) penguins foraged close to the Shag Rocks shelf-break, probably exploiting the high daytime biomass of fish in this area. Penguins and seals are able to respond differently to periods of reduced krill abundance (in terms of variability in diet and foraging behaviour), without detriment to the breeding success of either species. This highlights the importance of myctophid fish as an alternative trophic pathway for land-based predators in the Scotia Sea ecosystem.     

Decreased lipid storage in juvenile Atlantic cod (<Emphasis Type="Italic">Gadus morhua</Emphasis>) during settlement in cold-water eelgrass habitat

We characterized the prey field and the lipid classes/fatty acids in the flesh of age 0 juvenile cod (Gadus morhua) during their late-summer/fall arrival and settlement into eelgrass (Zostera marina) in coastal Newfoundland. Examination of available prey demonstrated a high abundance of small zooplankton (Acartia, Microsetella and Oithona sp.) with no larger Calanus sp. prey. Breakpoint analysis showed significant changes in the accumulation of relative (mg g⁻¹ wet weight) and absolute (μg fish⁻¹) amounts of lipid with standard length at the time of settlement (~60 mm standard length). Settling juvenile cod showed an alternate lipid utilization strategy where they catabolized phospholipids (PL) to a greater extent than triacylgylcerols (TAG). Polyunsaturated fatty acids (PUFA) content in cod flesh decreased as fish grew indicating that nearshore zooplankton quality was not optimal for PL formation. The dramatic reduction in cod PL was likely due to both catabolism of muscle and a lack of dietary PUFA suitable for PL synthesis. However, juvenile cod continued to grow, leading to decreased lipid stores and suggesting that cod settling into eelgrass are under intense selection pressure for growth prior to the onset of winter, possibly as a means of escaping gape-limited predation. These data contrast better-studied freshwater and estuarine systems in which lipid storage is critical for successful overwintering.     

Feeding behavior of the ctenophore <Emphasis Type="Italic">Thalassocalyce inconstans</Emphasis>: revision of anatomy of the order Thalassocalycida

Behavioral observations using a remotely operated vehicle (ROV) in the Gulf of California in March, 2003, provided insights into the vertical distribution, feeding and anatomy of the rare and delicate ctenophore Thalassocalyce inconstans. Additional archived ROV video records from the Monterey Bay Aquarium Research Institute of 288 sightings of T. inconstans and 2,437 individual observations of euphausiids in the Gulf of California and Monterey Canyon between 1989 and 2005 were examined to determine ctenophore and euphausiid prey depth distributions with respect to temperature and dissolved oxygen concentration [dO]. In the Gulf of California most ctenophores (96.9%) were above 350 m, the top of the oxygen minimum layer. In Monterey Canyon the ctenophores were more widely distributed throughout the water column, including the hypoxic zone, to depths as great as 3,500 m. Computer-aided behavioral analysis of two video records of the capture of euphausiids by T. inconstans showed that the ctenophore contracted its bell almost instantly (0.5 s), transforming its flattened, hemispherical resting shape into a closed bi-lobed globe in which seawater and prey were engulfed. Euphausiids entrapped within the globe displayed a previously undescribed escape response for krill (‘probing behavior’), in which they hovered and gently probed the inner surfaces of the globe with antennae without stimulating further contraction by the ctenophore. Such rapid bell contraction could be effected only by a peripheral sphincter muscle even though the presence of circumferential ring musculature was unknown for the Phylum Ctenophora. Thereafter, several live T. inconstans were collected by hand off Barbados and microscopic observations confirmed that assumption.     

Growth rate and retention of learned predator cues by juvenile rainbow trout: faster-growing fish forget sooner

Under conditions of spatial and/or temporal variability in predation risk, prey organisms often rely on acquired predator recognition to balance the trade-offs between energy intake and risk avoidance. The question of ‘for how long’ should prey retain this learned information is poorly understood. Here, we test the hypothesis that the growth rate experienced by prey should influence the length of the ‘memory window’. In a series of laboratory experiments, we manipulated growth rate of juvenile rainbow trout and conditioned them to recognize a novel predator cue. We subsequently tested for learned recognition either 24 h or 8 days post-conditioning. Our results suggest that trout with high versus low growth rates did not differ in their response to learned predator cues when tested 24 h post-conditioning. However, trout on a high growth rate exhibited no response to the predator cues after 8 days (i.e. did not retain the recognition of the predator odour), whereas trout on a lower growth rate retained a strong recognition of the predator. Trout that differed in their growth rate only after conditioning did not differ in their patterns of retention, demonstrating growth rate after learning does not influence retention. Trout of different initial sizes fed a similar diet (percent body mass per day) showed no difference in retention of the predator cue. Together, these data suggest that growth rate at the time of conditioning determines the ‘memory window’ of trout. The implications for threat-sensitive predator avoidance models are described.     

Induction of anti-predator responses in the green-lipped mussel Perna viridis under hypoxia

Hypoxia due to the over enrichment of waters by nutrients is becoming a global problem. In mussels, enhanced byssus thread production is an important adaptation to the presence of crustacean predators and to energetic hydrodynamic regimes. Thread production is an energy-consuming process, so this study used the green mussel Perna viridis (L.) to examine the response to predator exposure combined with hypoxia. Hypoxia is common in sheltered bays in Hong Kong, and the mussels were collected in one such bay, Lok Wo Sha (latitude/longitude: 22o18′ N/114o10′ E) in January, 2009. The predator used in the experiments was the swimming crab Thalamita danae. Oxygen concentrations used in the 48-h experiments ranged from hypoxic to normoxic (1.5 ± 0.3 mg l⁻¹, 3.0 ± 0.3 mg l⁻¹ and 6.0 ± 0.3 mg l⁻¹). Fewer byssus threads which were also shorter and thinner were produced at reduced oxygen levels, no matter if the predator was present or not; the frequency the mussels shed stalks was also lower. Mussels exposed to the predator, however, have enhanced byssus thread production at all oxygen levels when compared with the control. This has highlighted the significance of anti-predator responses for the survival of individuals even under a stressful environment in which energy supply is limited by aerobic metabolism. Interactive effects between oxygen level and predator exposure were observed for the byssus thread production (frequency of shed stalks, mean byssus thread length, cumulative byssus thread volume), with values obtained at 1.5 and 3.0 mg O₂ l⁻¹ being statistically indistinguishable for the control group without predator but not for the predator group. The lack of differences in the byssus thread production at lower oxygen levels in the absence of predator may indicate the minimum amount of byssus that is required for settlement on a substrate.