Implications of a fluctuating fish predator guild on behavior, distribution, and abundance of a shared prey species: the grass shrimp Palaemonetes pugio

In some systems, the identity of a prey species' dominant predator(s) may not be constant over time. In cases in which a prey species exhibits different responses to various predator species, such changes in predator identity may have population-wide consequences. Our goals were to determine (1) whether mortality of and refuge use by the grass shrimp, Palaemonetes pugio, were predator-specific, and (2) how effects of prey size and habitat interacted with predator type. Striped bass (Morone saxatilis) exerted twice as much predation pressure as mummichog (Fundulus heteroclitus), although not equally as great on large (female) and small (male) shrimp. Mummichog, which fed preferentially on large shrimp, forced a partitioning of habitat between the two shrimp size classes. In contrast, large and small shrimp occupied similar habitats when subjected to striped bass, which fed on both size classes equally. Refuge use of grass shrimp depended on predator type. In the presence of mummichog, which occupied shallower depths in the water column than striped bass, shrimp stayed deep and close to structural habitat. Striped bass, which were deeper, caused shrimp to move high in the water column away from structural habitat. When both predators were present, shrimp distribution was similar to that when only striped bass were present, striped bass predation rate was enhanced, and overall mortality was higher than with either predator alone. Results suggest that at times when mummichogs are the dominant predators, large (female) shrimp experience higher predation than small (male) shrimp and are physically separated from their potential mates. When striped bass are more abundant, male and female shrimp may share a similar, shallow, less structure-oriented distribution and be subjected to higher mortality. When both predators are present, mortality rates may be higher still. This predator-, size-, and habitat-specificity of grass shrimp behavior suggests significant population and distribution consequences of fluctuating predator guilds and fluctuating cover of structural habitats in the field.     

Prey bacteria shape the community structure of their predators

Although predator–prey interactions among higher organisms have been studied extensively, only few examples are known for microbes other than protists and viruses. Among the bacteria, the most studied obligate predators are the Bdellovibrio and like organisms (BALOs) that prey on many other bacteria. In the macroscopical world, both predator and prey influence the population size of the other''s community, and may have a role in selection. However, selective pressures among prey and predatory bacteria have been rarely investigated. In this study, Bacteriovorax, a predator within the group of BALOs, in environmental waters were fed two prey bacteria, Vibrio vulnificus and Vibrio parahaemolyticus. The two prey species yielded distinct Bacteriovorax populations, evidence that selective pressures shaped the predator community and diversity. The results of laboratory experiments confirmed the differential predation of Bacteriovorax phylotypes on the two bacteria species. Not only did Bacteriovorax Cluster IX exhibit the versatility to be the exclusive efficient predator on Vibrio vulnificus, thereby, behaving as a specialist, but was also able to prey with similar efficiency on Vibrio parahaemolyticus, indicative of a generalist. Therefore, we proposed a designation of versatilist for this predator. This initiative should provide a basis for further efforts to characterize the predatory patterns of bacterial predators. The results of this study have revealed impacts of the prey on Bacteriovorax predation and in structuring the predator community, and advanced understanding of predation behavior in the microbial world.     

Predator-prey interaction between hatchery-reared Japanese flounder juvenile, Paralichthysolivaceus, and sandy shore crab, Matutalunaris: daily rhythms, anti-predator conditioning and starvation

Predator-prey interaction between sandy shore crab, Matuta lunaris (Forskål, 1775), and juvenile Japanese flounder, Paralichthys olivaceus (Temminck et Schlegel), was investigated under controlled laboratory conditions. Possibility of training and conditioning hatchery-reared flounder to avoid predators was also examined. Crabs took over 75% of their daily ration at night when they were given access to prey 24 h a day. Large (64.8±5.4 g)- and medium (30.68±3.33 g)-sized crabs ate ca. 5.5±1.45 and 3.9±1.99 individuals of flounder (TL=4.96±0.23 cm) a day, respectively. When flounder juveniles that have experienced predation pressure by crabs encountered predators again, they exhibited better survival compared to the naive fish. Flounder juveniles were also conditioned either using small and, thus, benign predators, or large crabs over fence. The conditioned fish with either method were better able to avoid capture by crabs than naive fish, revealing that learning process should play an important role in their predator avoidance. Anti-predator performance was also compared between starved and fed flounder juveniles. Fed fish were rarely eaten by predators after 3 h of exposure, whereas starved fish continued to be eaten. Our results suggest that stock-enhancement program of Japanese flounder can be improved by applying proper feeding protocol and conditioning to avoid predators prior to release. Present research supports the idea that behavioural and ecological consideration for the target species is indispensable for the success of stock enhancement.     

Effect of an invasive crab upon a marine fishery: green crab, Carcinus maenas, predation upon a venerid clam, Katelysia scalarina, in Tasmania (Australia)

Nonindigenous species are increasingly recognized as altering marine and estuarine communities, causing significant changes in abundance and distribution of native species. Such effects are of particular concern to coastal fisheries. We experimentally determined the effect of the nonindigenous European green crab, Carcinus maenas, upon the stepped venerid clam, Katelysia scalarina, the basis for a fledgling clam fishery in Tasmania, Australia. First, we observed a trend of decreased juvenile (<13-mm shell length or SL) abundance of K. scalarina at sites with C. maenas relative to those without this invasive predator. Additionally, relative predation intensity on these juveniles was significantly higher in invaded areas. To better understand the dynamics of predation by this invader, we conducted a number of manipulative experiments. In cage experiments testing per capita predation rates, we found that: (1) of the various sizes of C. maenas, large C. maenas were the most significant predators; (2) the smallest size class of K. scalarina tested (6-12-mm SL) was preferred by C. maenas; (3) C. maenas had much higher predation rates than any native predator tested; and (4) while the native shore crab, Paragrapsus gaimardii, was found to have a constant predation rate over an eightfold range of densities of juvenile K. scalarina (16-128 individuals·m⁻²), C. maenas significantly increased its per capita predation with increasing prey density. Notably, in open field plots at a site where C. maenas was abundant, predation was constant over the range of tested prey densities. We predict, therefore, that the invasion of C. maenas will have significant negative consequences for the Tasmanian K. scalarina fishery.     

The effect of prey size and prey density on the functional response,survival, growth and development of a predatory pentatomid bug,Podisus maculiventris say

Two experiments on the nymphal predation of Podisus maculiventris were conducted using Spodoptera litura larvae as prey. First experiment: The predator nymphs divided into three groups were reared individually from second instar to adult in a small vessel. Each nymph in the groups 1, 2 and 3 was allowed to attack the serially growing larvae (these were supplied at the rate of one per day) from 3-, 5- and 7-day old after hatching, respectively. The first prey used for the group 1 was so small that it was not only insufficient to satiate the predator but also was difficult to be searched out. But these disadvantages were soon recuperated due to the rapid growth of the prey and all nymphs could survive to adults. The survival rate of third and fourth instar nymphs in the group 3 was severely affected by vigorous counterattack of older prey larvae. Second experiment: The predator nymphs were individually reared either in a small vessel or in a large one at various rates of food supply (the prey larvae of 7-day old were used). The functional response curves obtained for each instar of the predator took a saturation type within a certain range of the prey density. The saturation level specific to each instar was generally higher for the predator reared in the large vessel than in the small one. The functional response of fourth and fifth instar nymphs was accelerated at a high prey density, viz. 16 larvae per vessel. Even at the low rate of food supply, viz. one larva per day per predator, the predator nymphs could survive to adults, but the size of resultant adults were abnormally small.     

Predatory behavior and preference of a successful invader, the mud crab Dyspanopeus sayi (Panopeidae), on its bivalve prey

Predator-prey relationships between the panopeid crab, Dyspanopeus sayi, and the mytilid, Musculista senhousia, were investigated. Through laboratory experiments, prey-handling behavior, prey size selection, predator foraging behavior and preferences for two types of prey (M. senhousia and the Manila clam Ruditapes philippinarum) were assessed. Handling time differed significantly with respect to the three prey sizes offered (small: 15.0-20.0 mm shell length, SL; medium: 20.1-25.0 mm SL; and large: 25.1-30.0 mm SL); mud crabs were more efficient in predating medium-small than large prey. Although differences in prey profitability were not evident, D. sayi exhibited a marked reluctance to feed on larger-sized prey whilst smaller, more easily predated mussels were available. Size selection may be the result of a mechanical process in which encountered prey are attacked but rejected if they remain unbroken after a certain number of opening attempts. D. sayi exhibited inverse density-dependent foraging. A significant higher mortality of prey was evident at low prey density. Thus, at low predator density, the D. sayi-M. senhousia interaction was a destabilizing type II functional response. Interference responses affected the magnitude of predation intensity by D. sayi on M. senhousia, since as the density of foraging crabs increased, their foraging success fell. At high density (4 crabs tank⁻¹), crabs engaged in a high amount of agonistic activity when encountering a conspecific specimen, greatly diminished prey mortality. Finally, presenting two types of prey, Manila clam juveniles were poorly predated by mud crabs, which focused their predation mostly on M. senhousia. It is hypothesized that, when more accessible prey is available, mud crabs will have a minimal predatory impact on commercial R. philippinarum juvenile stocks.     

Comparative Study of Functional Response of Common Hemipteran Bugs of East Calcutta Wetlands,India

The common and abundant hemipteran water bugs Anisops bouvieri, Diplonychus rusticus, D. annulatus, of the wetlands of East Kolkata are known predators of a wide range of aquatic insects including the mosquito larvae. In the laboratory their predation were assessed in respect to short term and long term periods using the larvae of Culex quinquefasciatus to reveal their possible role in regulating the dipteran population in nature. The attack rate (a) and handling time (T_h ) of these predators varied with respect to the prey size. For the backswimmers A. bouvieri the values for a and T_h for the small prey were 5.47 L and 18.72 min respectively, while in case of the belostomatid bugs, the values for the same were 5.37 L and 8.64 min (for D. rusticus), 5.81 L and 20.16 min (for D. annulatus). The predation rate varied with prey and predator densities for both the prey sizes. It was revealed that on an average A. bouvieri can kill and consume 10–82 and 6–44, D. rusticus 10–118 and 10–84 and D. annulatus 10–70 and 10–138 small and large sized prey per day, respectively. However the mutual interference (m) values of the three predators varied with the prey size and ranged between 0.053–0.326 for A. bouvieri, 0.0381–0.066 for D. rusticus and 0.0556–0.115 for D. annulatus, respectively. In the long term experiments A. bouvieri killed between 6–119 small preys and 3–31 large preys, D. rusticus killed 50–94 small preys and 50–96 large preys and D. annulatum were found to kill between 14–74 small prey and 50–131 large prey per day, respectively. The clearance rates were found to be proportional to the predator density as well to the prey size and density, and differed between the predator species significantly. These data are supportive of qualifying the water bugs, A. bouvieri, D. rusticus, and D. annulatus as potential biological resources in regulating the population of mosquito larvae in the wet‐lands. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Notonecta exhibit threat-sensitive,predator-induced dispersal

Dispersal is a central process determining community structure in heterogeneous landscapes, and species interactions within habitats may be a major determinant of dispersal. Although the effects of species interactions on dispersal within habitats have been well studied, how species interactions affect the movement of individuals between habitats in a landscape has received less attention. We conducted two experiments to assess the extent to which predation risk affects dispersal from an aquatic habitat by a flight-capable semi-aquatic insect (Notonecta undulata). Exposure to non-lethal (caged) fish fed conspecifics increased dispersal rates in N. undulata. Moreover, dispersal rate was positively correlated with the level of risk imposed by the fish; the greater the number of notonectids consumed by the caged fish, the greater the dispersal rate from the habitat. These results suggest that risk within a habitat can affect dispersal among habitats in a landscape and thus affect community structure on a much greater scale than the direct effect of predation itself.     

Temporary pools are not `enemy-free'

Temporary pools are traditionally considered as refuges where the conspicuous anostracans are protected from predation. While this is true for the size-selective predation by fish, there is compelling evidence that invertebrate predation is an important biotic stress regulating temporary pool communities. In rock pools in southeastern Botswana, we studied the impact of some suspected invertebrate predators on populations of the freshwater anostracan Branchipodopsis wolfi by means of observations and manipulative experiments. In a survey of 45 pools, the relationship between B. wolfi natural population sizes and the abundance of suspected predators were never negative for turbellarians and mosquito larvae. When dragonfly larvae, notonectids or tadpoles were present, the anostracan populations were generally non-existent or very small. In enclosure experiments with turbellarians, there was a significant effect of predation within one hour of the start; the average daily predation rate was about 1/4 anostracan per turbellarian. Anostracans from a pool with few turbellarians were slightly less vulnerable than those from a turbellarian-rich pool. Furthermore, there was an indication of males being predated on more than females. With dragonfly larvae and notonectids, the predation effect was marked with all six anostracans in an experiment eaten in less than one day by a single predator (predation rate: about one anostracan every 2 h per predator). In a behavioral study, both sexes of B. wolfi avoided swimming above sediment that held more turbellarians than the open patches; there was no evidence for chemical communication with respect to this behavior.     

Effects of substrate on interactions between juvenile sea scallops (Placopecten magellanicus Gmelin) and predatory sea stars (Asterias vulgaris Verrill) and rock crabs (Cancer irroratus Say)

We investigated the effect of substrate (glass bottom, sand, granule, pebble) on predation of juvenile sea scallops (Placopecten magellanicus) by sea stars (Asterias vulgaris) and rock crabs (Cancer irroratus) at two prey sizes (11-15 mm and 24-28 mm shell height), and two prey densities (10 and 30 scallops per aquarium) in laboratory experiments. Specifically, we quantified predation rate and underlying behaviours (proportion of time a predator spent searching for and handling prey, encounter rate between predators and prey, and various outcomes of encounters). We detected a significant gradual effect of particle size of natural substrates on sea star predation: specifically, predation rate on and encounter rate with small scallops tended to decrease with increasing particle size (being highest for sand, intermediate for granule, and lowest for pebble). Substrate type did not significantly affect predation rates or behaviours of sea stars preying on large scallops or of rock crabs preying on either scallop size classes. Other factors, such as prey size and density, were important in the scallop-sea star and scallop-rock crab systems. For example, predation rate by sea stars and crabs and certain sea star behaviours (e.g. probability of consuming scallops upon capture) were significantly higher with small scallops than with large scallops. As well, in interactions between small scallops and sea stars, predation rate and encounter rate increased with prey density, and the proportion of time sea stars spent searching was higher at low prey density than high prey density. Thus, substrate type may be a minor factor determining predation risk of seeded scallops during enhancement operations; prey size and prey density may play a more important role. However, substrate type still needs to be considered when choosing a site for scallop enhancement, as it may affect other scallop behaviours (such as movement).     

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.     

Diel variation in predator abundance, predation risk and prey distribution in shallow-water estuarine habitats

Predation by visual predators is often affected by light conditions and may therefore exhibit strong diel variation. The dominant predators on grass shrimp, Palaemonetes pugio, are finfish predators that are thought to locate their prey by visual cues. We examined the response of grass shrimp to diel variation in predation risk in the nearshore shallow waters of the Chesapeake Bay. We used diel shoreline seines to assess the relative abundance of predators. We assessed the relative risk of predation with shrimp tethered at refuge (30 cm) and nonrefuge (60 cm) depths. To measure grass shrimp response to predation risk, we used dipnets to monitor habitat use. Four predominantly visual predators dominated the shoreline seine catches, Fundulus heteroclitus, Micropogonias undulatus, Morone americana and Morone saxatilis. Total predator abundance had a diel component, with dramatic nighttime decreases in total abundance, whereas guild composition and relative abundance remained unchanged. Relative predation risk for tethered shrimp exhibited significant time by habitat interaction. During the day, depth negatively affected survivorship of tethered shrimp while at night overall survivorship increased and there was no effect of depth. Shrimp habitats use reflected diel predation risks. Abundances in the near shore were highest during the day with decreased abundances at night. Together, the seine and tethering data highlight the importance for a refuge (e.g., shallow water) from predation during the daytime and a relaxation of predation pressure at night.     

Hyperendemicity of Haplorchis taichui Infection among Riparian People in Saravane and Champasak Province,Lao PDR

In this study, we found that Haplorchis taichui, a heterophyid intestinal fluke, is highly prevalent, with heavy worm loads, among riparian people in Saravane and Champasak province, Lao PDR. Fecal specimens were collected from 1,460 people (717 men and 743 women) in 12 riparian (Mekong river) districts and were examined by the Kato-Katz fecal smear technique. The overall helminth egg positive rate was 78.8% and 66.4% in Saravane and Champasak province, respectively. The positive rate for small trematode eggs (STE), which included H. taichui and other heterophyids, Opisthorchis viverrini, and lecithodendriids, was 69.9% and 46.3% in Saravane and Champasak province, respectively. To obtain adult flukes, 30 STE-positive people were treated with 40 mg/kg praziquantel and then purged. Whole diarrheic stools were collected 4-5 times for each person and searched for fluke specimens using a stereomicroscope. Mixed infections with various species of trematodes (H. taichui, Haplorchis pumilio, O. viverrini, Prosthodendrium molenkampi, Centrocestus formosanus, and Echinochasmus japonicus) and a species of cestode (Taenia saginata) were found. However, the worm load was exceptionally high for H. taichui compared with other trematode species, with an average of 21,565 and 12,079 specimens per infected person in Saravane and Champasak province, respectively, followed by H. pumilio (41.9 and 22.5, respectively) and O. viverrini (9.4 and 1.5, respectively). These results show that diverse species of intestinal and liver flukes are prevalent among riparian people in Saravane and Champasak province, Lao PDR, with H. taichui being the exceptionally dominant species.     

Habitat effects on second-order predation of the seed predator Harpalus rufipes and implications for weed seedbank management

Seed predators provide a valuable ecosystem service to farmers by reducing densities of weed seeds, and, in turn, densities of weed seedlings they must manage. The predominant invertebrate weed seed predator in Maine, USA, agroecosystems is the carabid beetle Harpalus rufipes DeGeer. Pitfall trapping has shown that H. rufipes prefers sites with vegetative cover to fallow sites, preference speculated to be driven by predator avoidance behavior. To test this hypothesis, ‘second-order predation assays’ were developed, in which live H. rufipes prey were presented to second-order predators. Field experiments were conducted to determine foremost if H. rufipes was subject to second-order predation, and secondly, whether (a) vegetative cover affords H. rufipes protection from second-order predators, and (b) high rates of second-order predation correspond with decreased invertebrate seed predation rates. Two 72-h experiments were conducted (mid August and September 2012) at crop and non-crop sites across a 28 ha diversified farm in Stillwater, ME, USA.Second-order predation was 2.8% per day. Based on images from motion-sensing cameras, H. rufipes’ predators included birds and small mammals. Neither a relationship between second-order predation and vegetative treatment, nor an empirical relationship between second-order predation and invertebrate seed predation were detected. However, a simulation model predicted that 2.8% per day second-order predation could increase the number of seeds entering the seedbank by more than 17% annually. Additionally, complex habitats supported higher rates of second-order predation than did simple habitats.     

Predator-driven fragmentation of fiddler crab droves into selfish miniherds of biased composition

An explanation for animal groups is the selfish herd, characterized by aggregation as each member tries to shield itself from a predator by moving into a tight gap between other members. We test the hypotheses that: (1) droves, the large feeding groups of fiddler crabs, are selfish herds; (2) the miniherds that form when droves fragment on approach of a large predator are selfish herds; (3) selfish herds form when refugia are unlikely to be reached before an approaching predator arrives; and (4) the composition of selfish miniherds is biased toward individuals most vulnerable to predation. The study was conducted in South Carolina (USA) by videotaping the movements of sand fiddler crabs Uca pugilator when approached by a human predator. In both droves and miniherds, interindividual distance decreases with predator approach, consistent with behavior in a selfish herd. However, two other expectations for selfish herds—herd cohesion and sacrificing distance from the predator in order to get closer to other herders—are only met in miniherds. Crabs farther from refugia are more likely to form and remain in miniherds, indicating that selfish herding is only favored when refugia cannot be quickly reached. The composition of the smallest miniherds, consisting of 2-18 crabs, is biased toward females and small males. These individuals may be more vulnerable to predation because they lack the enlarged claw of large males that deters some predators. The small miniherds are relatively homogeneous with respect to the size and sex of their members, which may enhance cohesion and effectiveness as selfish herds. Miniherds will be effective selfish groups when predator attack has a significant vertical component and when the strike distance is large relative to both the size of the prey and the distance between group members. Droves are not selfish herds but permit crabs to flee feeding grounds as members of selfish miniherds.     

Predator-prey interactions from in situ time-lapse observations of a sublittoral mussel bed in the Gulf of Trieste (Northern Adriatic)

Hexaplex trunculus (Linnaeus, 1758) is one of the most abundant and widespread muricid gastropods in the Northern Adriatic Sea, but relatively little is known about the feeding ecology of this predator. We examined the activity of H. trunculus on a sublittoral mussel bed at 24 m depth through in situ time-lapse observations and bulk samples. The camera photographed a 0.25 m² section of the mussel bed at 6-min intervals for ~ 23 h. Photos were examined frame-by-frame for gastropod movement and activities, especially interactions between H. trunculus and Mytilus galloprovincialis (Lamarck, 1819). Our survey indicates high activity-levels of H. trunculus on the sea floor: all gastropods made minor movements, most made major movements, and most left the field of view during the study-interval. On average, individuals remained stationary for only 7.3 h. Two predation attempts on Mytilus involving conspecific competition were documented, and one Hexaplex was consuming a mussel at the onset of the deployment. Additionally, 487 M. galloprovincialis from four diver-taken 0.25 m² quadrates were measured and examined for traces of marginal chipping and drilling predation. Mytilus from surface samples ranged from 11.1 mm to 95.5 mm in length, and one of the four samples had a significantly different average shell length from the others. 114 H. trunculus were collected and measured. Hexaplex ranged from 22.1 mm to 86.1 mm and the mean shell length did not differ among samples, though they were overwhelmingly medium and large. Predation frequency (the ratio of successfully preyed upon bivalves to the total number of bivalves sampled) is high at the studied site (> 55%), and large gastropods preferred a chipping mode of predation to drilling, supporting earlier laboratory studies showing a preference for M. galloprovincialis and this predation strategy. Prey effectiveness (the ratio of failed predatory attacks to total predatory attacks) is also high (63.8%), and no evidence of a size refuge was found. Feeding in H. trunculus is highly facultative, calling for caution when using drill holes to estimate predation intensities; whenever possible, traces of multiple predation modes should be considered.     

Optimal foraging versus shared doom effects: interactive influence of mussel size and epibiosis on predator preference

In the western Baltic Sea, the highly competitive blue mussel Mytilus edulis tends to monopolize shallow water hard substrata. In many habitats, mussel dominance is mainly controlled by the generalist predator Carcinus maenas. These predator-prey interactions seem to be affected by mussel size (relative to crab size) and mussel epibionts.There is a clear relationship between prey size and predator size as suggested by the optimal foraging theory: Each crab size class preferentially preys on a certain mussel size class. Preferred prey size increases with crab size.Epibionts on Mytilus, however, influence this simple pattern of feeding preferences by crabs. When offered similarly sized mussels, crabs prefer Balanus-fouled mussels over clean mussels. There is, however, a hierarchy of factors: the influence of attractive epibiotic barnacles is weaker than the factor ‘mussel size’. Testing small mussels against large mussels, presence or absence of epibiotic barnacles does not significantly alter preferences caused by mussel size. Balanus enhanced crab predation on mussels in two ways: Additional food gain and, probably more important, improvement in handling of the prey. The latter effect is illustrated by the fact that artificial barnacle mimics increased crab predation on mussels to the same extent as do live barnacles.We conclude that crab predation preferences follows the optimal foraging model when prey belong to different size classes, whereas within size classes crab preferences is controlled by epibionts.     

Predation on eggs and larvae ofHeliothis virescens [Lep.: Noctuidae] by an adult predator complex in cage studies on cotton

Predation by a predator complex consisting of adults of the lygaeidGeocoris punctipes (Say), the nabidsNabis roseipennis Reuter and/orTropiconabis capsiformis Germar and the coccinelids,Hippodamia convergens Guerin-Meneville andColeomegilla maculata (DeGeer) onHeliothis virescens (F.) eggs, 1st-instar larvae or both on late pinhead square and early bloom stage cotton was evaluated in field cages in Mississippi. Prey densities of 4 (11, 512/ha) or 8 per cage and predator: prey (P:p) ratios of 1∶1, 2∶1, 3∶1, 4∶1 and 5∶1 were used. Prey were labeled with phosphorus-32 or carbon-14 to determine predator ingestion and effectiveness. Average percent egg predation as a function of P:p ratio ranged from 2.1 to 12.1 for a 48 h period. The average percent predation on larvae as a function of P:p ratio ranged from 5.3 to 22.0. The hemiptera fed more than the coleoptera on larvae, andG. punctipes was the best egg predator. For the range of predator densities used, the average area of discovery for the predator complex was 6.98 × 10⁻³ m/day and 2.34 × 10⁻² m/day when exposed to eggs and larvae, respectively. Publication N^o 5936-Mississippi Agricultural and Forestry Experiment Station, Mississippi State, MS 39762.     

Animal Models for Echinostoma malayanum Infection: Worm Recovery and Some Pathology

Echinostomes are intestinal trematodes that infect a wide range of vertebrate hosts, including humans, in their adult stage and also parasitize numerous invertebrate and cold-blooded vertebrate hosts in their larval stages. The purpose of this study was to compare Echinostoma malayanum parasite growth, including worm recovery, body size of adult worms, eggs per worm, eggs per gram of feces, and pathological changes in the small intestine of experimental animals. In this study, 6-8-week-old male hamsters, rats, mice, and gerbils were infected with echinostome metacercariae and then sacrificed at day 60 post-infection. The small intestine and feces of each infected animal were collected and then processed for analysis. The results showed that worm recovery, eggs per worm, and eggs per gram of feces from all infected hamsters were higher compared with infected rats and mice. However, in infected gerbils, no parasites were observed in the small intestine, and there were no parasite eggs in the feces. The volume of eggs per gram of feces and eggs per worm were related to parasite size. The results of histopathological changes in the small intestine of infected groups showed abnormal villi and goblet cells, as evidenced by short villi and an increase in the number and size of goblet cells compared with the normal control group.     

Trading off safety against food: state dependent habitat choice and foraging in crucian carp

The influence of hunger level and predation risk on habitat choice and foraging in crucian carp, Carassius carassius, were studied in a laboratory experiment. Experiments were carried out in aquaria with or without a predator (pike, Esox lucius). Habitat use and foraging activity of three-fish foraging groups of either fed or hungry crucian carp were studied. Fish were allowed to choose between an open (risky) habitat with Tubifex worms and a habitat with dense vegetation (safe) without food. Habitat use was significantly affected by both risk of predation and hunger level. Crucian carp spent less time in the open habitat when there was a predator present and they also spent less time there when fed than when hungry. Furthermore, there was a significant interaction between risk of predation and hunger level, indicating a state-dependent trade-off between food acquisition and predator avoidance.