Responses of the black sea urchin Tetrapygus niger to its sea-star predators Heliaster helianthus and Meyenaster gelatinosus under field conditions

We ran field experiments to examine the responses of the black sea urchin Tetrapygus niger to predatory sea stars. Trials involving simulated attacks (one or several arms of a sea star being placed on top of half the urchin) showed that the urchin differentiated between the predatory sea stars, Heliaster helianthus and Meyenaster gelatinosus, and a non-predatory sea star, Stichaster striatus, and showed almost no response to a sea star mimic. We further compared the responses of the urchin to different threat levels presented by the two predatory sea stars. The highest threat level was a simulated attack, then mere contact, and subsequently sea stars being placed at different distances from the urchin. All urchins responded to simulated attacks and contact with both sea stars. The proportion responding decreased with distance and more rapidly in trials with H. helianthus (0% at a distance of 30 cm) than with M. gelatinosus (33% at a distance of 50 cm). At each of the threat levels where there was a response to both sea stars, the urchins responded more rapidly to M. gelatinosus than to H. helianthus. In a third experiment where a predatory sea star was added to a circular area (1-m diameter) in which either 4-8 or 11-19 undisturbed urchins were present, the urchins fled the area more rapidly when the added sea star was M. gelatinosus, but the rate of fleeing did not vary with density, as might occur if there was communication among urchins using alarm signals. Our observations suggest that M. gelatinosus presents a stronger predatory threat than H. helianthus. This corresponds to field observations showing that the urchins are more frequently consumed by M. gelatinosus. These are the first field experiments demonstrating distance chemodetection by a marine invertebrate under back-and-forth water flow from wave activity.     

Brown shrimp (Crangon crangon, L.) functional response to density of different sized juvenile bivalves Macoma balthica (L.)

Variability in infaunal bivalve abundance in the Wadden Sea is largely determined by recruitment variability. Post-settlement, but pre-recruitment bivalve mortality is high and related to the occurrence of their most abundant predator, the brown shrimp Crangon crangon. To investigate if the mortality patterns of newly settled bivalves can be explained by the foraging behavior of brown shrimp, we carried out experiments on shrimp functional response to three size classes of juveniles of the Baltic Tellin Macoma balthica. The functional response curves for all three prey sizes (0.62 mm, 0.73 mm, and 0.85 mm) were the hyperbolic Holling's type II. The attack rate was highest for the smallest prey size (a = 0.31, medium and large prey a = 0.22); the handling time was longest for the largest prey size (T_h = 29 s, small and medium prey T_h = 15 s). Thus, a large body size is advantageous for the bivalves over the whole density range. Knowledge of individual foraging behavior is needed to model predation mortality of bivalves. The consumption rates in the experiment were theoretically high enough to account for M. balthica mortality in the field.     

Mantle defensive response of marine pulmonate Trimusculusperuvianus

The marine pulmonate Trimusculusperuvianus (Sowerby, 1835) which is found in caves or narrow crevices along the Chilean coast was studied to determine a possible chemical defense against the intertidal key predator Heliasterhelianthus. T.peruvianus releases a white secretion through the extended mantle animal covers, to contact with the tube feet of starfish. This behaviour significantly decreases predation of gastropod and it has not been described previously for members of the Trimusculidae family.     

Rearing trials of Halla parthenopeia under laboratory conditions (Polychaeta: Oenonidae)

A small scale attempt to maintain and rear the worm Halla parthenopeia in laboratory conditions was conducted. Five bivalve species (Paphia undulata, Cerastoderma glaucum, Venerupis pullastra, Ruditapes decussata, and Gafrarium pectinatum) were used to investigate preferred food item, feeding rate and growth of the worm. Hallaparthenopeia are capable of using different types of clams, although they grew better with Paphia undulata and C.glaucum as food items. The highest average daily predation rates in case of P. undulata as a prey were 1.73, 2.13 and 2.57 individuals per predator per day for small, medium and large groups of H. parthenopeia, respectively. The daily predation rate on C. glaucum was low with an average: 0.50, 0.63 and 0.73 individuals per predator per day for the small, medium and large worm groups, respectively. The daily growth rate of H. parthenopiea increased when it was fed P. undulata (average: 0.083, 0.071 and 0.038 g/day for small, medium and large worm groups respectively), compared to an average of 0.044, 0.034 and 0.020 g/day for small, medium and large worm groups, respectively, when worms were fed with C. glaucum. The biochemical composition of three different sizes of the worm was also determined. Protein was the highest biochemical constituent with an average of 51% of the dry weight, followed by lipids with an average of 25.88% of the dry weight; meanwhile carbohydrate was present at an average of 20.72%. Our findings indicate that growth of H. parthenopeia can be improved when fed with a suitable prey item and suggest that it is feasible to successfully culture this protein-rich worm in captivity.     

Type and nature of cues used by Nucella lapillus to evaluate predation risk

The ability of prey to detect and adequately respond to predation risk influences immediate survival and overall fitness. Chemical cues are commonly used by prey to evaluate risk, and the purpose of this study was to elicit the nature of cues used by prey hunted by generalist predators. Nucella lapillus are common, predatory, intertidal snails that evaluate predatory risk using chemical cues. Using Nucella and a suite of its potential predators as a model system, we explored how (1) predator type, (2) predator diet, and (3) injured conspecifics and heterospecifics influence Nucella behavior. Using laboratory flumes, we determined that Nucella responded only to the invasive green crab (Carcinus maenas), the predator it most frequently encounters. Nucella did not respond to rock crabs (Cancer irroratus) or Jonah crabs (Cancer borealis), which are sympatric predators but do not frequently encounter Nucella because these crabs are primarily subtidal. Predator diet did not affect Nucella responses to risk, although starved predator response was not significantly different from controls. Since green crabs are generalist predators, diet cues do not reflect predation risk, and thus altering behavior as a function of predator diet would not likely benefit Nucella. Nucella did, however, react to injured conspecifics, a strategy that may allow them to recognize threats when predators are difficult to detect. Nucella did not react to injured heterospecifics including mussels (Mytilus edulis) and herbivorous snails Littorina littorea, suggesting that they are responding to chemical cues unique to their species. The nature of cues used by Nucella allows them to minimize costs associated with predator avoidance.     

Reciprocity in predator–prey interactions: exposure to defended prey and predation risk affects intermediate predator life history and morphology

A vast body of literature exists documenting the morphological, behavioural and life history changes that predators induce in prey. However, little attention has been paid to how these induced changes feed back and affect the predators’ life history and morphology. Larvae of the phantom midge Chaoborus flavicans are intermediate predators in a food web with Daphnia pulex as the basal resource and planktivorous fish as the top predator. C. flavicans prey on D. pulex and are themselves prey for fish; as D. pulex induce morphological defences in the presence of C. flavicans this is an ideal system in which to evaluate the effects of defended prey and top predators on an intermediate consumer. We assessed the impact on C. flavicans life history and morphology of foraging on defended prey while also being exposed to the non-lethal presence of a top fish predator. We tested the basic hypothesis that the effects of defended prey will depend on the presence or absence of top predator predation risk. Feeding rate was significantly reduced and time to pupation was significantly increased by defended morph prey. Gut size, development time, fecundity, egg size and reproductive effort respond to fish chemical cues directly or significantly alter the relationship between a trait and body size. We found no significant interactions between prey morph and the non-lethal presence of a top predator, suggesting that the effects of these two biological factors were additive or singularly independent. Overall it appears that C. flavicans is able to substantially modify several aspects of its biology, and while some changes appear mere consequences of resource limitation others appear facultative in nature.     

Piscivory by the mummichog (Fundulus heteroclitus): Evidence from the laboratory and salt marshes

While piscivory is common in many fishes, there are few accounts for the fundulid, Fundulus heteroclitus (mummichog). We suspected that this species might be involved in several forms of piscivory including predation, cannibalism and scavenging. To evaluate these possibilities we conducted several laboratory experiments and field observations in its primary habitat, salt marshes. We found that digestion of larvae and small juveniles of F. heteroclitus was fast (< 1 h) and this makes detection of small fish prey difficult for any form of piscivory. In addition, laboratory experiments (one-on-one encounters, feeding on dead prey) indicated that both cannibalism and scavenging of conspecific prey were possible. Field observations (n = 2449 stomachs from fish 12-106 mm TL over four years at two salt marshes) also suggested that piscivory occurred frequently (4.3-24.7% of stomachs examined) based on the detection of numerous fish hard parts, especially pharyngeal jaws. These structures allowed us to determine that most prey were F. heteroclitus and to back-calculate the size of the prey and thus the piscivore/prey size relationship. As a result, it appears that both cannibalism (0.2-9.1% of stomachs) and scavenging (0.5-9.9%) are common feeding modes of F. heteroclitus in salt marshes and, subsequently, modes of energy transfer for salt marshes in general. We suggest that future studies of fish feeding consider that cannibalism and scavenging may be frequent possibilities when fish remains are detected in stomach contents.     

Interaction strength between a predator and dangerous prey: Sinistrofulgur predation on Mercenaria

The lack of direct empirical evidence of predator evolution in response to prey adaptation is a fundamental weakness of the arms race analogy of predator-prey coevolution. I examined the interaction between the predatory busyconine whelk Sinistrofulgur sinistrum and its bivalve prey Mercenaria mercenaria to evaluate whether reciprocal adaptation was likely in this predator-prey system. Thick-lipped whelks use their shell lip to chip open the shell of their prey, often resulting in breakage to their own shell. Thus, hard-shelled prey, such as Mercenaria, may be considered dangerous because they are able to inflict damage to the predator as a consequence of the interaction. The strength of interaction between whelks and their bivalve prey was viewed by regressing predator performance (the incidence of shell breakage in encounters with prey) on prey phenotype (a function of size). Interaction with Mercenaria of varying sizes has strong and predictable consequences (r²=0.946; p=0.028) for Sinistrofulgur. Predators that select large, thick bivalve prey increase the likelihood that their shell lip will be broken in the process of attempting to open their prey. Ecological consequences of feeding-induced breakage may include reduced growth rate, reproductive success, and survivorship. These results suggest that natural selection should favor predator phenotypes that reduce feeding-induced breakage when interactions with damage-inducing prey occur.     

Modeling changes in predator functional response to prey across spatial scales

Extrapolation of predator functional responses from laboratory observations to the field is often necessary to predict predation rates and predator-prey dynamics at spatial and temporal scales that are difficult to observe directly. We use a spatially explicit individual-based model to explore mechanisms behind changes in functional responses when the scale of observation is increased. Model parameters were estimated from a predator-prey system consisting of the predator Delphastus catalinae (Coleoptera: Coccinellidae) and Bemisia tabaci biotype B (Hemiptera: Aleyrodidae) on tomato plants. The model explicitly incorporates prey and predator distributions within single plants, the search behavior of predators within plants, and the functional response to prey at the smallest scale of interaction (within leaflets) observed in the laboratory. Validation revealed that the model is useful in scaling up from laboratory observations to predation in whole tomato plants of varying sizes. Comparing predicted predation at the leaflet scale, as observed in laboratory experiments, with predicted predation on whole plants revealed that the predator functional response switches from type II within leaflets to type III within whole plants. We found that the magnitude of predation rates and the type of functional response at the whole plant scale are modulated by (1) the degree of alignment between predator and prey distributions and (2) predator foraging behavior, particularly the effect of area-concentrated search within plants when prey population density is relatively low. The experimental and modeling techniques we present could be applied to other systems in which active predators prey upon sessile or slow-moving species.     

Effect of the degree of autotomy on feeding, growth, and reproductive capacity in the multi-armed sea star Heliaster helianthus

The term autotomy refers to the process by which some species lose limbs or parts of limbs in response to adverse biotic or abiotic conditions, as for example, predation or abnormally high temperatures. The multi-armed sea star Heliaster helianthus is a key predator of the intertidal and the shallow rocky subtidal communities of north-central Chile. Natural populations of this sea star have been found with up to 60% of the individuals showing some degree of autotomy. The present study evaluated the effects of autotomy on feeding rate and growth of juvenile and adult H. helianthus after experimentally induced autotomy of 17% and 33% of their arms, as well as on the energy content of the pyloric caeca and gonads of adults during the reproductive period. Experimental juvenile sea stars were maintained and fed in the laboratory over a period of five months and adult sea stars for one month, Intact individuals were maintained as parallel controls. The results showed that juveniles undergoing 33% autotomy decreased their feeding rates, and as a consequence showed lowered net individual growth. In contrast, adults with 17% and 33% autotomy showed marked reductions in feeding. The results showed that autotomized adults had between five and seven times lower contents of carbohydrates, lipids, and proteins (and thus energetic content) in their pyloric caeca and gonads. The loss of the arms not only decreased the capacity for feeding in sea stars, but also allocated energy away from growth and reproduction into the process of regeneration of arms. This suggests that autotomy reduces the fitness of H. helianthus. Growth was reduced in the juveniles, while adults became limited in their ability to store energy which then limited their reproductive potential. Finally, based on the important effect of autotomy on reducing the feeding capacity of H. helianthus, the role of this sea star as a predator in the environment may be strongly affected.     

Predator odor recognition and antipredatory response in fish: does the prey know the predator diel rhythm?

We studied in a laboratory experiment using stream tanks if two percid prey fish, the perch (Perca fluviatilis) and the ruffe (Gymnocephalus cernuus), can recognize and respond to increased predation risk using odors of two piscivores, the pike (Esox lucius) and the burbot (Lota lota). Burbot is night-active most of the year but pike hunts predominantly visually whenever there is enough light. Perch is a common day-active prey of pike and dark-active ruffe that of burbot. We predicted that besides recognizing the predator odors, the prey species would respond more strongly to odors of the predator which share the same activity pattern. Both perch and ruffe clearly responded to both predator fish odors. They decreased movements and erected the spiny dorsal fins. Fin erection showed clearly the black warning ornamentation in the fin and thus erected fin may function besides as mechanical defense also as warning ornament for an approaching predator. No rapid escape movements were generally observed. Both perch and ruffe responded more strongly to pike odor than to burbot. There were no clear differences in response between day and night. In conclusion, we were able to verify clear predator odor recognition by both prey fish. Both perch and ruffe responded to both predator odors and it seemed that pike forms a stronger threat for both prey species. Despite of diel activity differences both perch and ruffe used the same antipredatory strategies, but the day-active perch seemed to have a more flexible antipredatory behavior by responding more strongly to burbot threat during the night when burbot is active.     

Danger Comes from All Fronts: Predator-Dependent Escape Tactics of Túngara Frogs

The escape response of an organism is generally its last line of defense against a predator. Because the effectiveness of an escape varies with the approach behaviour of the predator, it should be advantageous for prey to alter their escape trajectories depending on the mode of predator attack. To test this hypothesis we examined the escape responses of a single prey species, the ground-dwelling túngara frog (Engystomops pustulosus), to disparate predators approaching from different spatial planes: a terrestrial predator (snake) and an aerial predator (bat). Túngara frogs showed consistently distinct escape responses when attacked by terrestrial versus aerial predators. The frogs fled away from the snake models (Median: 131°). In stark contrast, the frogs moved toward the bat models (Median: 27°); effectively undercutting the bat’s flight path. Our results reveal that prey escape trajectories reflect the specificity of their predators’ attacks. This study emphasizes the flexibility of strategies performed by prey to outcompete predators with diverse modes of attack.     

Prey productivity effects on the impact of predators of the mussel, Mytilus californianus (Conrad)

The relationship of increasing prey productivity, a measure incorporating prey settlement and body growth, to changes in the relative impact of two predator groups, birds and the sea star, Pisaster ochraceus (Brandt) on a competitively dominant mussel, Mytilus californianus were examined. The purpose of this experiment was twofold, 1) to determine if the separate effects of each predator group on prey abundance increased as prey productivity increased and 2) to determine if the relative impacts of the two predator groups diverged as prey productivity increased. In this experiment, the separate impact of each predator group increased with increasing prey productivity. However, the relative impact of each predator group did not diverge with increasing prey productivity. Unlike previous studies that suggested with increasing prey productivity the relative effect of two predator groups should diverge, this experiment suggested that communities can have more redundant predator groups than originally thought. The results of an analysis using a proportional hazards model suggested that despite increasing prey productivity, birds and the sea star were equal in their ability to curb population increases by M. californianus. These results highlight the need to carefully consider what type of species to species comparisons to make when attempting to discern the relative roles of different predator groups in a community.     

Behavioural mechanisms of sea stars (Asterias vulgaris Verrill and Leptasterias polaris Müller) and crabs (Cancer irroratus Say and Hyas araneus Linnaeus) preying on juvenile sea scallops (Placopecten magellanicus (Gmelin)), and procedural effects of scallop tethering

We compared predation rates and behaviours of sea stars (Asterias vulgaris and Leptasterias polaris) and crabs (Cancer irroratus and Hyas araneus) preying on juvenile sea scallops (Placopecten magellanicus, 25-35 mm shell height) in laboratory. These predatory species co-occur with sea scallops on the sea bed of the Gulf of St. Lawrence, Canada, and limit scallop survival in seeding operations. We also examined, under controlled conditions, the effect of tethering scallops on predator-prey interactions. Predation rates, time budgets and encounter behaviours observed for A. vulgaris and C. irroratus preying on free (untethered) scallops were comparable to previous studies. C. irroratus were more effective predators as they consumed 3.1 scallops predator^− 1 day^− 1, although they spent only 0.9% of their time searching for prey. A. vulgaris consumed 0.9 scallops predator^− 1 day^− 1 and spent 7.6% of their time searching. Sea stars L. polaris had a lower predation rate (0.02 scallop predator^− 1 day^− 1) than A. vulgaris. The frequent avoidance behaviour of L. polaris and its low ability to capture scallops support the notion that scallops are not a main component of this sea star's diet. Crabs H. araneus had similar predation rates (1.3 scallops predator^− 1 day^− 1) and behaviours to C. irroratus, although the probability of consumption upon capture was affected by relatively high numbers of rejections and post-capture escapes of scallops. As expected, the tethering procedure increased predation rate of L. polaris (about 19 times higher), but surprisingly did not significantly affect that of A. vulgaris. Examination of behaviours indicated that A. vulgaris offered tethered scallops tended to have a higher probability of capture, but spent less time searching for prey (possibly because satiation was reached) than A. vulgaris offered free scallops. Predation rates and behaviours of both crab species were not affected by tethering, since encounter rate was the primary determinant of crab-scallop interactions. Identification and quantification of behaviours underlying the predation process allowed us to mathematically model predator-related mortality for the four predator species.     

Influence of cruising and ambush predators on 3-dimensional habitat use in age 0 juvenile Atlantic cod Gadus morhua

Predators and prey often co-exist at high densities within the same habitat, yet the behavioural and spatial dynamics underlying this co-existence are not well known. To better understand small-scale, predator-prey co-occurrence, the spatial patterns and behaviour of age 0 juvenile cod Gadus morhua 75-88 mm SL and two of their known predators, age 2+ cod and short-horn sculpin Myoxocephalus scorpinus, were examined in two habitats (i.e., sand and eelgrass) using three-dimensional video analysis. Both habitat and predator type interacted to result in unique spatial patterns of prey. Spatial overlap between predators and prey was highest in open habitat in the presence of the cruising predator but lowest in the presence of sculpin in the same habitat. In eelgrass, age 0 cod avoided predators primarily along the vertical axis (i.e., distance off bottom). Age 0 cod stayed above eelgrass in the presence of sculpin but lowered themselves into the eelgrass while in the presence of predator cod. Anti-predator behaviour (i.e., predator-prey distance, prey cohesion and freezing) was significantly reduced over eelgrass compared to sand, suggesting eelgrass has lower ‘inherent risk’ than open habitats. However, predator consumption was similar across all treatments, suggesting that, 1) complex habitat also impairs the visual cues needed for anti-predator behaviour (e.g., schooling) and assessing the location of predators, and 2) predators change their behaviour with habitat to enhance their opportunities for finding and capturing prey.     

Prey selection and the impact of the starfish Marthasterias glacialis (L.) and other predators on the mussel Choromytilus Meridionalis (Krauss)

The diet of the starfish, Marthasterias glacialis (L.), consists of a variety of mollusc species, as well as ascidians and barnacles. Starfish densities are maximal where mussels, Choromytilus meridionalis (Krauss), are abundant and in such areas mussels form the bulk of the diet. Laboratory feeding experiments indicate that Marthasterias glacialis select mussels of particular sizes and that the length of prey taken is an increasing function of predator arm length. The time taken to consume each mussel is determined by the ratio of shell length to starfish size. The number of mussels consumed per day increases only slightly with starfish size, but because the prey taken increase in size, energy consumption is maintained at a relatively consistent 1% of predator body energy per day. Using prey selection and feeding rate data for different sized starfish, predictive three dimensional predation surfaces are developed for a natural starfish population feeding on either one or two cohort Choromytilus meridionalis populations. The models indicate that predatory effort should be concentrated on the smallest mussels when a single adult cohort is present, but on recruiting mussels just above the minimum prey size limit where two cohorts are present. Other major predators of mussels, the rock lobster, Jasus lalandii (Milne Edwards), and the whelk, Natica tecta Anton, appear to select similar size-ranges of prey to starfish, despite their differing body forms and feeding methods. Since the juveniles of all three predators can only take small mussels, predator recruitment may well depend upon the successful settlement of strong mussel cohorts. Evidence for such entrainment of predator cohorts to settlements of mussels is presented.     

Do anuran larvae respond behaviourally to chemical cues from an invasive crayfish predator? A community-wide study

Antipredator behaviour is an important fitness component in most animals. A co-evolutionary history between predator and prey is important for prey to respond adaptively to predation threats. When non-native predator species invade new areas, native prey may not recognise them or may lack effective antipredator defences. However, responses to novel predators can be facilitated by chemical cues from the predators’ diet. The red swamp crayfish Procambarus clarkii is a widespread invasive predator in the Southwest of the Iberian Peninsula, where it preys upon native anuran tadpoles. In a laboratory experiment we studied behavioural antipredator defences (alterations in activity level and spatial avoidance of predator) of nine anurans in response to P. clarkii chemical cues, and compared them with the defences towards a native predator, the larval dragonfly Aeshna sp. To investigate how chemical cues from consumed conspecifics shape the responses, we raised tadpoles with either a tadpole-fed or starved crayfish, or dragonfly larva, or in the absence of a predator. Five species significantly altered their behaviour in the presence of crayfish, and this was largely mediated by chemical cues from consumed conspecifics. In the presence of dragonflies, most species exhibited behavioural defences and often these did not require the presence of cues from predation events. Responding to cues from consumed conspecifics seems to be a critical factor in facilitating certain behavioural responses to novel exotic predators. This finding can be useful for predicting antipredator responses to invasive predators and help directing conservation efforts to the species at highest risk.     

Bt maize fed-prey mediated effect on fitness and digestive physiology of the ground predator Poecilus cupreus L. (Coleoptera: Carabidae)

We investigated the effects of a Bt maize hybrid on fitness and digestive physiology of the ground-dwelling predator Poecilus cupreus L., as compared with the near-isogenic hybrid. A tritrophic assay revealed that there was a great decline in the detection of Cry1Ab toxin through the trophic chain, the concentration of the toxin being 945, 349 and 37 ng g⁻¹ of fresh weight in Bt maize leaves, Spodoptera littoralis (Boisduval) larvae and P. cupreus larvae, respectively. Moreover, the toxin was only detected in 8% of the P. cupreus adults collected from fields growing Bt maize. Developmental time of both larvae and pupae of P. cupreus was not adversely affected by the Cry1Ab toxin via fed-prey. To elucidate potential detrimental effects due to a reduction in the quality of the prey, we assessed the digestive proteolytic activities of P. cupreus adults from a laboratory culture and insects collected in commercial Bt and non-Bt maize fields. Field-collected P. cupreus adults had higher proteolytic activities than those reared in the laboratory, whereas no significant differences were found between P. cupreus adults reared on Bt and non-Bt maize fed-S. littoralis or between P. cupreus adults collected in commercial Bt and non-Bt maize fields.     

Trophodynamics of two interacting species of estuarine mysids, Praunus flexuosus and Neomysis integer, and their predation on the calanoid copepod Eurytemora affinis

Community structure is shaped by external factors (i.e., habitat, temperature, and food) frequently modified by interactions among its members. This study focusses on trophic interactions between two sympatric mysids Praunus flexuosus and Neomysis integer of the Elbe Estuary, northern Germany. Based on an experimental approach, intraguild predation was evaluated. Predation rate of P. flexuosus on N. integer was positively related to predator size and temperature. Predation rate was significantly correlated with prey size, juvenile N. integer released just from the mysid marsupium being most vulnerable. However, adult P. flexuosus were able to gain more energy in terms of body carbon by catching larger N. integer, whereas immature P. flexuosus assimilated more energy by capturing large numbers of the small-sized N. integer. In contrast to N. integer, P. flexuosus showed an efficient escape behaviour that prevented all stages of N. integer from preying on any size class of P. flexuosus. When Eurytemora affinis was offered as prey, both N. integer and P. flexuosus increased predation rates with predator size and temperature. In mixed prey (N. integer and E. affinis) experiments at 10 °C, predation rates of adult P. flexuosus on N. integer released just from the marsupium declined from 17±8 to 6±4 N. integer mysid⁻¹ day⁻¹. We conclude that intraguild predation exists between the two species but is one sided with small N. integer being strongly suppressed. This heavy predation pressure is modified by the addition of alternative food resources, in this case, E. affinis.     

Predator versus prey: on aerial hunting and escape strategies in birds

Predator and prey attack-escape performance is likely to bethe outcome of an evolutionary arms race. Predatory birds aretypically larger than their prey, suggesting different flightperformances. We analyze three idealized attack-escape situationsbetween predatory and prey birds: climbing flight escape, horizontalspeeding, and turning and escape by diving. Generally a smallerbird will outclimb a larger predator and hence outclimbing shouldbe a common escape strategy. However, some predators such asthe Eleonora's falcon (Falco elenorae) has a very high rateof climb for its size. Prey species with an equal or highercapacity to climb fast, such as the swift Apus apus, usuallyadopt climbing escape when attacked by Eleonora's falcons.To analyze the outcome of the turning gambit between predatorand prey we use a Howland diagram, where the relative lineartop speeds and minimum turning radii of prey and predator definethe escape and danger zones. Applied to the Eleonora's falconand some potential prey species, this analysis indicates thatthe falcon usually wins against the example prey species; thatis, the prey will be captured. Level maneuvering hunting isthe most common strategy seen in Eleonora's falcons. To avoidcapture via use of this strategy by a predator, the prey shouldbe able to initiate tight turns at high linear speed, whichis facilitated by a low wing loading (weight per unit of wingarea). High diving speed is favored by large size. If close enough to safe cover, a prey might still opt for a verticaldive to escape in spite of lower terminal diving speed thanthat of the predator. On the basis of aerodynamic considerationswe discuss escape flight strategies in birds in relation tomorphological adaptations.