Native species vulnerability to introduced predators: testing an inducible defense and a refuge from predation

To manage the impacts of biological invasions, it is important to determine the mechanisms responsible for the effects invasive species have on native populations. When predation by an invader is the mechanism causing declines in a native population, protecting the native species will involve elucidating the factors that affect native vulnerability. To examine those factors, this study measured how a native species responded to an introduced predator, and whether the native response could result in a refuge from predation. Predation by the green crab, Carcinus maenas, has contributed to the decline in numbers of native soft-shell clams, Mya arenaria, and efforts to eradicate crabs have proven futile. We tested how crab foraging affected clam burrowing, and how depth in the sediment affected clam survival. Clams responded to crab foraging by burrowing deeper in the sediment. Clams at shallow depths were more vulnerable to predation by crabs. Results suggest soft-shell clam burrowing is an inducible defense in response to green crab predation because burrowing deeper results in a potential refuge from predation by crabs. For restoring the native clam populations, tents could exclude crabs and protect clams, but when tents must be removed, exposing the clams to cues from foraging crabs should induce the clams to burrow deeper and decrease vulnerability. In general, by exposing potential native prey to cues from introduced predators, we can test how the natives respond, identify whether the response results in a potential refuge, and evaluate the risks to native species survival in invaded communities.     

Morphology of Coenophthalmus tridentatus first zoea (Crustacea: Portunidae: Polybiinae) hatched in the laboratory

The swimming crab Coenophthalmus tridentatus A. Milne Edwards,1879 (Decapoda, Portunidae) is an endemic species of the SouthwesternAtlantic, from southern Brazil to northern Patagonia (Argentina).Larvae of C. tridentatus from one female collected on beds ofoysters and mussels at 50 m depth in the Argentine continentalshelf (38°21'S, 57°38'W) were hatched in the laboratory.Zoea I morphology is described for the first time and comparedwith known zoeae of other portunid species.     

Foraging Decision Rules and Prey Species Preferences of Northwestern Crows (Corvus caurinus)

Categorization of similar prey types and the application of decision rules by dietary generalists can enhance the efficiency of foraging decisions and facilitate the inclusion of novel prey types in the diet. While considerable research attention has been directed toward investigation of these concepts in invertebrates, few have assessed categorization and decision rules used by generalist vertebrate predators. In this study, we experimentally investigated decision rules and prey preferences of northwestern crows (Corvus caurinus) feeding on littleneck clams (Tapes philippinarum) and whelks (Nucella lamellosa). We presented crows with three species‐size combinations: small clams (2.0–2.9 cm length) paired with large whelks (4.0–4.9 cm), small clams paired with medium whelks (3.0–3.9 cm), and large clams (4.0–4.9 cm) with large whelks. Profitability estimates based on observations of crows feeding on these prey species indicated that clams were always the more energetically profitable option; however, in prey choice trials crows consistently selected the heavier prey species, regardless of differences in profitability. These results show that crows apply a general decision rule according to which they select heavier prey items when feeding on hard‐shelled prey requiring similar handling techniques, and that while such decision rules may approximate optimal choices they may not always follow predictions based solely on prey profitability. We discuss these results in the context of behavioural flexibility of generalist predators, and predicting impacts of intertidal avian predators on prey populations.     

Responses of the saucer scallop Amusium balloti to potential predators

A high natural mortality rate has been documented for the saucer scallop Amusium balloti, an Australian scallop found on sediment bottoms at 30–60 m deep, but little is known about the causes of mortality. We studied escape responses of A. balloti to five consumers common in the bycatch of the scallop fishery as a means of identifying potential predators. The scallop showed a negligible to weak response when touched with the sea star Pentaceraster regulus and the red portunid crab Portunus rubromarginatus. The response was similar to that observed when the scallop was touched with a plastic object. In contrast, A. balloti showed a consistent and vigorous swimming response to contact with the slipper lobster Thenus orientalis, the blue swimmer crab Portunus pelagicus and the coral crab Charybdis cruciata. This was not a generalized response to crustaceans, given the scallop's weak response to the red portunid crab. This is the first report of a scallop that has a strong swimming escape response to contact with decapod crustacean predators.     

Selective crab predation on native and introduced bivalves in British Columbia

Experiments were conducted to determine whether locally abundant crab species prefer co-occurring littleneck clams, Protothaca staminea (Conrad, 1837) and Tapes philippinarum (A. Adams and Reeve, 1850), relative to a recently introduced species, the varnish clam, Nuttallia obscurata, (Reeve, 1857). Prey preference, handling time, pick-up success, profitability and consumption rates were investigated for two crab species, Dungeness crab, Cancer magister (Dana, 1852) and red rock crab, Cancer productus (Randall, 1839) crabs. Both crab species preferred varnish clams over the native species. This may be attributable to the lower handling time, higher pick-up success and increased profitability of consuming varnish clams. Handling time appeared to be a factor not only in species preference, but also in the degree of preference, with shorter handling times corresponding to stronger preference values. Both native and introduced bivalves burrow into the substratum, with the varnish clam burrowing deepest. When feeding on clams in limited substratum both crab species preferred the varnish clam. In the unlimited substratum trials Dungeness crabs preferred varnish clams (although to a lesser degree) while red rock crabs preferred littleneck clams. This was likely due to the significantly deeper burial of the varnish clam, making it less accessible. Although the morphology (i.e. thin shell, compressed shape) of the invader increases its vulnerability to predation, burial depth provides a predation refuge. These results demonstrate how interactions between native predators and the physical characteristics and behaviour of the invader can be instrumental in influencing the success of an invasive species.     

Sur une nouvelle applicaton de la theorie de P information a la description des series chronologiques planctoniques

Corrélations simples (r) et pondérées (r_p)entre S. setosa (trois stades cumulés), S. minima etS. inflata (stade 1), S. bipunctata, et les quatre paramétreshydrologiques retenus. ^aSeuil de signification 5%. ^bSeuil designification 1%     

Rodent foraging is affected by indirect, but not by direct, cues of predation risk

We used foraging trays to determine whether oldfield mice, Peromyscuspolionotus, altered foraging in response to direct cues of predationrisk (urine of native and nonnative predators) and indirectcues of predation risk (foraging microhabitat, precipitation,and moon illumination). The proportion of seeds remaining ineach tray (a measure of the giving-up density [GUD]) was usedto measure risk perceived by mice. Mice did not alter theirGUD when presented with cues of native predators (bobcats, Lynxrufus, and red foxes, Vulpes vulpes), recently introduced predators(coyotes, Canis latrans), nonnative predators (ocelots, Leoparduspardalis), a native herbivore (white-tailed deer, Odocoileusvirginianus), or a water control. Rather, GUD was related tomicrohabitat: rodents removed more seeds from foraging trayssheltered beneath vegetative cover compared with exposed traysoutside of cover. Rodents also removed more seeds during nightswith precipitation and when moon illumination was low. Our resultssuggest that P. polionotus used indirect cues rather than directcues to assess risk of vertebrate predation. Indirect cues maybe more reliable than are direct scent cues for estimating riskfrom multiple vertebrate predators that present the most riskin open environments.     

Hydrodynamic mediation of predator-prey interactions: differential patterns of prey susceptibility and predator success explained by variation in water flow

In most shallow water marine systems, fluid movements vary on scales that may influence local community dynamics both directly, through changes in the abundance of species, and indirectly, by modifying important behaviors of organisms. We examined how differences in current speed affect the outcome of predator-prey interactions for two species of marine benthic predators (knobbed whelks, Busycon carica, and blue crabs, Callinectes sapidus) foraging on two common prey species (bay scallops, Argopecten irradians, and hard clams, Mercenaria mercenaria). The predators differ in their foraging strategies and prey in their potential escape responses. Predation by blue crabs, highly mobile predators/scavengers that rely upon chemical odors transported in the water column to locate prey, could be strongly affected by changes in current speed and turbulent mixing because their foraging strategy relies on a high degree of spatial integration of prey odor plumes. Whelks, slow moving, predatory gastropods that often forage with their bodies buried in the sediment, may be less susceptible to flow-induced distortion of prey odor plumes because their sluggish movements result in a high degree of temporal integration of prey odors. Bay scallops, relatively mobile bivalves capable of rapid short-distance swimming burst, and hard clams, sedentary bivalves, have been shown to respond to varying degrees to predator odors that are dispersed in the water column. Flow regime for the predator-prey experiments was manipulated in situ using large channels. Predation by blue crabs on both juvenile hard clams and bay scallops decreased with increases in water flow (0-12 vs. 0-30 cm s⁻¹). Whelk predation on bay scallops increased with increases in water flow, whereas predation by whelks on hard clams did not differ between flow regimes. For blue crabs movement decreased at periods of high water flow. Because blue crabs locate prey through chemolocation of water-borne cues, which are diluted rapidly at higher flows, decreases in foraging may result from the inability to successfully detect prey at enhanced flows. Differences in predation by whelks could not be explained by a similar mechanism. Visual observations of foraging whelks revealed no differences in whelk behavior between the two flow regimes. The pattern of higher whelk predation on scallops at enhanced flow is likely to be related to a flow-inhibiting ability of scallops to detect predator approach. Thus, flow enhancement interferes with three of the predator-prey systems but the effect on predator success depends on whether the predator or prey is most affected.     

小峰熊蜂访花偏爱性

传粉昆虫在访花时,通常会表现出对某一类型花的偏爱性。本研究利用人工制作的大小、颜色、形态和气味不同的9种类型的花来研究小峰熊蜂Bombus hypocrita的访花偏爱性。结果表明：当增加花的大小、形态、气味等附加特征数时,小峰熊蜂的偏爱性程度与花朵附加特征数有显著相关性（P<0.01）。当花朵颜色由2种增加到4种,熊蜂对紫色花的偏爱性程度降低,但花朵颜色的种类与小峰熊蜂的访花偏爱性没有相关性（P>0.05）,花颜色的种类对熊蜂访紫色花的偏爱性影响不大。大小为5 cm的紫花被访次数（108±9次）明显高于大小为3 cm的紫花被访次数（40±4次）（P<0.01）,说明熊蜂明显喜欢访大花瓣的紫花。完全盛开的紫花被访次数（129±13次）显著高于刚绽放的花被访次数（26±3次）（P<0.01）,说明熊蜂喜欢访盛开的紫花。柠檬味的紫花被访次数（63±8次）明显低于草莓味的紫色花被访次数（88±2次）（P<0.05）,说明熊蜂喜欢访草莓味的花朵。     

Preliminary survey of a nemertean crab egg predator,Carcinonemertes, on its host crab,Callinectes arcuatus (Decapoda,Portunidae) from Golfo de Nicoya,Pacific Costa Rica

The possible presence of egg predators in brood masses of portunid crabs from Pacific Central America has not been studied yet. This survey reports the finding of a nemertean crab egg predator on the portunid crab, Callinectes arcuatus, from the Golfo de Nicoya, Pacific Costa Rica. Nemerteans were found in the egg masses of 26 out of the 74 crabs for a prevalence of 35%. The intensity (mean number of worms/ infected crab) was estimated to be 18 with a variance of 1–123 worms/infected crab. No nemerteans were observed either in the 19 Callinectes arcuatus from Golfo Dulce (southern Pacific coast) and the 10 Portunus asper from Herradura-Jaco (central Pacific coast). This nemertean is a member of the genus Carcinonemertes, which has been reported from the Caribbean coast of Panama. However, the encountered Carcinonemertes sp. is the first published finding and report from Costa Rica and Pacific Central America.     

地标对犬蝠空间记忆的影响

为了研究地标（landmarks）是否影响犬蝠（Cynopterus sphinx）的空间记忆,我们通过室内模拟试验研究犬蝠和地标在觅食过程中空间记忆形成的关系。实验组按照每天地标数分别为0、2、4、8、0的数目连续进行5天实验,对照组不设地标进行相同条件的实验。结果显示,两组犬蝠第一次取食所用的时间与实验天数之间极显著相关（Pearson Correlations: 实验组r=-0.593, P<0.01;对照组r=-0.581, P<0.01）;实验组取食成功率与实验天数之间无明显相关性（Pearson Correlations: r=0.177, P>0.05）;对照组取食成功率与实验天数之间显著相关（Pearson Correlations: r=0.445, P<0.05）。实验组与对照组犬蝠第一次取食的时间差异不显著（GLM: F_0.05,1=4.703, P>0.05）,两组间取食的成功率差异也不显著（GLM: F_0.05,1=0.849, P>0.05）。这些结果说明了随着时间增加,犬蝠对取食地的空间记忆逐渐形成,放置地标在犬蝠对取食地空间记忆形成的过程中无显著影响。     

Deleterious effect of Chattonella marina on short-necked clam (Ruditapes philippinarum); possible involvement of reactive oxygen species

In laboratory exposure experiments, Chattonella marina showeda deleterious effect on short-necked clams (Ruditapes philippinarum).Accumulated C. marina cells were observed in gill tissues ofthe clams exposed to C. marina. Immunohistochemical analysisusing antiserum against the crude glycocalyx of C. marina suggestedthat the glycocalyx was present on the gill surface. Mucus substancesprepared from gill tissue of the clams stimulated C. marinato generate increased amounts of O₂^– in a concentration-dependentmanner. These results suggest that reactive oxygen species (ROS)-mediatedgill tissue damage is one of the causative factors responsiblefor the harmful effect of C. marina on shellfish.     

Exploitation of floral signals by crab spiders (Thomisus spectabilis, Thomisidae)

Flowers exhibit characteristics through which they exploit thesensory biases of pollinating insects, and both signaler andreceiver benefit from this interaction, either through reproductiveservice or food reward. However, the preferences of pollinatorsfor certain flower traits such as color or odor might be exploitedby predators that target pollinating insects. Crab spiders,Thomisus spectabilis, position themselves on flowers to preyon pollinators such as honeybees, Apis mellifera. We gave bothhoneybees and crab spiders the choice between two randomly chosenwhite Chrysanthemum frutescens, including olfactory signalsin one experiment and excluding odor in a second experiment.When olfactory signals were included, crab spiders and honeybeesclearly preferred the same flower out of a pair. However, agreementlevel was at chance in the absence of olfactory signals. Wealso analyzed the visual flower characteristics that might influencethe decision of the animals. Neither the size of flowers (diameterof flower and diameter of reproductive flower center) nor thereflectance properties (receptor excitation values in ultraviolet,blue, and green; overall brightness) influenced the choicesof crab spiders and honeybees. Therefore, odor seems to be thefloral signal that bees use to identify high-quality flowersand that crab spiders exploit to encounter honeybees.     

Behavioral plasticity of the soft-shell clam, Mya arenaria (L.), in the presence of predators increases survival in the field

Soft-shell clams, Mya arenaria, are sessile, suspension-feeding bivalves that are preyed upon by the exotic green crab, Carcinus maenas. Clams evade crab consumers by burrowing deeper into the sediment after perceiving a threat from a nearby predator. The purpose of this study was to determine the types of signals that M. arenaria use to detect predators and the types of behaviors clams use to avoid being eaten. In a field study, clams increased their burial depth in the presence of green crab predators consuming conspecifics that were caged nearby, and also increased burial depth after artificial tactile stimulation in the laboratory assay. These results indicate that clams can use chemical and mechanical cues to detect potential predatory threats. We performed a field study to examine the difference in survivability of clams that had burrowed deeper into the sediment in response to predators vs. control clams that were burrowed less deeply. Significantly higher survival rates were observed in clams that had initially burrowed more deeply, suggesting that increasing burial depth is a valid predator avoidance strategy. Some bivalves also alter their pumping rates in the presence of predators, making them less apparent and providing more structural defense by covering soft tissue, and we measured pumping time of soft-shell clams in the presence and absence of predators, when burrowing was not an option for escape. Soft-shell clams did not alter their pumping time in the presence of green crab predators, possibly because they employ a burrowing method called “hydraulic” or “jet-propelled” burrowing, where it is necessary for the clam to pump in order to burrow. Chemical signals and tactile cues instigated behavioral changes in M. arenaria, and this change in behavior (increasing burial depth) increased clam survival in the field.     

Control of foraging behavior of individuals within an ecosystem context: the clam Macoma balthica and interactions between competition and siphon cropping

Macoma balthica (L.) is a common clam of the estuarine seafloor, belonging to an important group of invertebrates possessing the capacity to choose between the two fundamental modes of feeding available, using its siphon to inhale either suspended food particles from the water or food particles deposited on the sediment surface. Field experiments demonstrate that intraspecific competition, effects of other competing benthic invertebrates, and complex interactions between competition and partial predation (siphon cropping by fishes) modify the foraging behavior of Macoma. When protected by caging from siphon nipping by fishes, Macoma demonstrated greater siphon regeneration at lower density, indicating the importance of competition for limited resources. In the absence of siphon croppers, these same clams also exhibited more deposit feeding at the lower density either because of improved ability to deposit-feed with longer siphons or because deposited foods become more rapidly depleted than suspended foods on local spatial scales. Addition of siphon-nipping fishes caused greater reductions in siphon size of clams at lower density, presumably because the intensity of nipping per clam was greater where clam targets were fewer and because deposit feeding, which was more intense at lower densities, confers a greater risk of cropping from greater siphon extension and activity than characterize suspension feeding. Deposit feeding by Macoma was reduced in the presence of siphon croppers at both high and low density of clams, but the intensity of deposit-feeding activity at low density was substantially higher than predicted by additive effects of clam density and cropping. This suggests operation of a balancing strategy in Macoma whereby it is accepting greater risks of partial predation when rewards of greater food harvest are larger. The surprising failure to adopt a risk-averse stratery may be explained by the non-lethal nature of partial predation, which renders siphon loss an energetic penalty replacable through regeneration. The presence of a bed of suspension-feeding Rangia cuneata also altered foraging behavior of Macoma by inducing a switch to more intense deposit feeding, in response either to the documented near-bottom depletion of suspended foods or to likely enhanced biodeposition from feces and pseudofeces. The induction of greater deposit feeding by the presence of this competing suspension feeder led to greater siphon losses during exposure to croppers because Macoma was practicing more risky feeding behavior. This enhanced loss of siphon tissues to croppers in the presence of the suspension-feeding Rangia induced an interaction between the effects of siphon croppers and Rangia, such that Macoma exhibited a larger switch away from deposit feeding in the presence of siphon croppers when Rangia were also present. Clearly, the foraging decisions made by individuals can only be understood in a broad holistic context of population, community, and ecosystem processes.     

STATUS OF GIANT CLAM RESOURCES OF THE PHILIPPINES

Field surveys were conducted to determine distribution and abundanceof giant dam resources in the Philippines. All seven known speciesof giant clams were recorded with Tridacna crocea, T. maximaand T. squamosa occurring the most frequently. The larger speciesT. derasa, T. gigas, Hippopus hippopus and H. porcellanus wererelatively rare. The reduction of giant dam stocks in the Philippines is dueto the uncontrolled exploitation of this resource which is primarilygathered for shellcraft and as supplementary diet in coastalvillages. (Received 26 February 1988; accepted 6 July 1988)     

Structure of Aesthetascs in Selected Marine and Terrestrial Decapods: Chemoreceptor Morphology and Environment

We have compared the structure of the aesthetascs—thin-walledchemosensory pegs on the antennules—of Coenobita, a terrestrialhermit crab, and of various marine decapods, including the aquatichermit crab, Pagurus hirsutiusculus. In all cases, the aesthetascsare innervated by the dendrites of many bipolar neurons whosecell bodies are grouped beneath the bases of the hairs. Thedendrites have basal bodies and cilia that divide into slenderbranches, each distinguished by ovoid swellings along its lengthand containing one or more microtubules apparently continuouswith the microtubules of the cilia. The arrangement of the dendrites within the aesthetascs is distinctlydifferent in Coenobita from that in the marine animals, evenin its relative, Pagurus. There are many points of structuralconvergence between the aesthetascs of Coenobita and the thin-walledolfactory pegs of the insect antenna. These modifications mayrepresent adaptations for conservation of water in the terrestrialreceptors.     

A Non-Native Prey Mediates the Effects of a Shared Predator on an Ecosystem Service

Non-native species can alter ecosystem functions performed by native species often by displacing influential native species. However, little is known about how ecosystem functions may be modified by trait-mediated indirect effects of non-native species. Oysters and other reef-associated filter feeders enhance water quality by controlling nutrients and contaminants in many estuarine environments. However, this ecosystem service may be mitigated by predation, competition, or other species interactions, especially when such interactions involve non-native species that share little evolutionary history. We assessed trophic and other interference effects on the critical ecosystem service of water filtration in mesocosm experiments. In single-species trials, typical field densities of oysters (Crassostrea virginica) reduced water-column chlorophyll a more strongly than clams (Mercenaria mercenaria). The non-native filter-feeding reef crab Petrolisthes armatus did not draw down chlorophyll a. In multi-species treatments, oysters and clams combined additively to influence chlorophyll a drawdown. Petrolisthes did not affect net filtration when added to the bivalve-only treatments. Addition of the predatory mud crab Panopeus herbstii did not influence oyster feeding rates, but it did stop chlorophyll a drawdown by clams. However, when Petrolisthes was also added in with the clams, the clams filtered at their previously unadulterated rates, possibly because Petrolisthes drew the focus of predators or habituated the clams to crab stimuli. In sum, oysters were the most influential filter feeder, and neither predators nor competitors interfered with their net effect on water-column chlorophyll. In contrast, clams filtered less, but were more sensitive to predators as well as a facilitative buffering effect of Petrolisthes, illustrating that non-native species can indirectly affect an ecosystem service by aiding the performance of a native species.     

Advantages of social foraging in crab spiders: Groups capture more and larger prey despite the absence of a web

Among group‐living spiders, subsocial representatives in the family of crab spiders (Thomisidae) are a special case, as they build protective communal leaf nests instead of extensive communal capture webs. It could thus be inferred that antipredator benefits (e.g., enhanced protection in larger nests) rather than foraging‐related advantages (e.g., capture of more and larger prey) promote sociality in this family. Nonetheless, subsocial crab spiders do share prey, and if this behaviour does not reflect mere food scramble but has a cooperative character, crab spiders may offer insights into the evolution of social foraging applicable to many other cooperative predators that hunt without traps. Here, we performed a comparative laboratory feeding experiment on three of the four subsocial crab spider species—Australomisidia ergandros, Australomisidia socialis and Xysticus bimaculatus—to determine if crab spiders derive advantages from foraging in groups. In particular, we tested artificially composed groups of five sibling spiderlings vs. single siblings in terms of prey capture success and prey size preference. Across species, groups had higher prey capture success (measured in terms of capture rates and capture latency) and were more likely to attack large, sharable prey—dynamics leading to reduced food competition among group members in favour of living and foraging in groups. Within groups, we further compared prey extraction efficiency among the three applied social foraging tactics: producing, scrounging and feeding alone. In A. ergandros, individuals were exceptionally efficient when using the non‐cooperative scrounger tactic, which entails feeding on the prey provided by others. Thus, our multispecies comparison confirms foraging advantages in maintaining a cooperative lifestyle for crab spiders, but also demonstrates the relevance of research into exploitation of cooperative foraging in this family.