Crabs interpret the threat of predator body size and biomass via cue concentration and diet

A global synthesis of predation on bivalves

Predation is a dominant structuring force in ecological communities. In aquatic environments, predation on bivalves has long been an important focal interaction for ecological study because bivalves have central roles as ecosystem engineers, basal components of food webs, and commercial commodities. Studies of bivalves are common, not only because of bivalves' central roles, but also due to the relative ease of studying predatory effects on this taxonomic group. To understand patterns in the interactions of bivalves and their predators we synthesised data from 52 years of peer-reviewed studies on bivalve predation. Using a systematic search, we compiled 1334 studies from 75 countries, comprising 61 bivalve families (N = 2259), dominated by Mytilidae (29% of bivalves), Veneridae (14%), Ostreidae (8%), Unionidae (7%), and Dreissenidae and Tellinidae (6% each). A total of 2036 predators were studied, with crustaceans the most studied predator group (34% of predators), followed by fishes (24%), molluscs (17%), echinoderms (10%) and birds (6%). The majority of studies (86%) were conducted in marine systems, in part driven by the high commercial value of marine bivalves. Studies in freshwater ecosystems were dominated by non-native bivalves and non-native predator species, which probably reflects the important role of biological invasions affecting freshwater biodiversity. In fact, while 81% of the studied marine bivalve species were native, only 50% of the freshwater species were native to the system. In terms of approach, most studies used predation trials, visual analysis of digested contents and exclusion experiments to assess the effects of predation. These studies reflect that many factors influence bivalve predation depending on the species studied, including (i) species traits (e.g. behaviour, morphology, defence mechanisms), (ii) other biotic interactions (e.g. presence of competitors, parasites or diseases), and (iii) environmental context (e.g. temperature, current velocity, beach exposure, habitat complexity). There is a lack of research on the effects of bivalve predation at the population and community and ecosystem levels (only 7% and 0.5% of studies respectively examined impacts at these levels). At the population level, the available studies demonstrate that predation can decrease bivalve density through consumption or the reduction of recruitment. At the community and ecosystem level, predation can trigger effects that cascade through trophic levels or effects that alter the ecological functions bivalves perform. Given the conservation and commercial importance of many bivalve species, studies of predation should be pursued in the context of global change, particularly climate change, acidification and biological invasions.

Eastern Oysters Crassostrea virginica Produce Plastic Morphological Defenses in Response to Crab Predators Despite Resource Limitation

Many prey react to predation risk by altering their phenotype to reduce their chances of being consumed but incur reductions in growth and fecundity when reacting to predators. To determine when to produce defenses, prey collect information and evaluate the costs and benefits of defense induction. Resource availability can affect prey ability and willingness to incur defense costs. When resources are scarce, defenses may suffer disproportionate decreases in energy allocation if defenses would further reduce prey access to resources or if resources are needed to maintain metabolic functions. We tested the effects of predation risk and resource availability on plastic defenses in eastern oysters Crassostrea virginica and present novel findings that oysters continued to produce defended shells in response to predators when resources were limited, even though they grew smaller, lighter shells when deprived of food in control conditions. Predation risk affected all three tested shell metrics (area, weight, and strength), but food availability did not. Although low food levels often limit expression of predator defenses, predator cues caused oysters to build shells that were larger and heavier, with a similar trend for shell strength, in treatments with both low and high food levels, suggesting that predation is an important pressure in this system. The differences between predator and control treatments were greater under conditions of low food availability, and thus, resource availability may influence interpretations of plastic responses to predators.

Disturbed flow in an aquatic environment may create a sensory refuge for aggregated prey

Predators use olfactory cues moved within water and air to locate prey. Because prey aggregations may produce more cue and be easier to detect, predation could limit aggregation size. However, disturbance in the flow may diminish the reliability of odour as a prey cue, impeding predator foraging success and efficiency. We explore how different cue concentrations (as a proxy for prey group size) affect risk to prey by fish predators in disturbed (more turbulent or mixed) and non-disturbed (less mixed) flowing water. We find that increasing odour cue concentration increases predation risk and disturbing the flow reduces predation risk. At high cue concentration fish were able to locate the cue source in both disturbed and non-disturbed flow, but at medium concentrations, predators only located the cue source more often than expected by chance in non-disturbed flow. This suggests that objects disturbing flow provide a sensory refuge allowing prey to form larger groups, but that group sizes may be limited by level of disturbance to the flow.

Does Cohistory Constrain Information Use? Evidence for Generalized Risk Assessment in Nonnative Prey

Though prey use a variety of information sources to assess predation risk, evolutionary cohistory with a predator could constrain information use, and nonnative prey might fail to recognize risk from a novel predator. Nonnative prey might instead use generalized risk assessment, relying on general alarm signals from injured conspecifics rather than cues from predators. I tested the influence of shared predator-prey history on information use, comparing responses among three native and four nonnative prey species to chemical cues from a native predator and cues from injured conspecific prey. Nonnative prey demonstrated information generalism: (1) responding stronger to alarm cues released by injured conspecific prey than to cues from predators and (2) responding similarly to alarm cues as to cues from predators consuming injured conspecific prey. By contrast, for native prey, multiple information sources were required to elicit the greatest defense. The influence of other sources of chemical information was not predicted by cohistory with the predator: only one nonnative snail responded to the predator; digestion was important for only two native species; the identity of injured prey was important for all prey; and predator and prey cues contributed additively to prey response. Information generalism, hypothesized to be costly in coevolved interactions, could facilitate invasions as a driver of or response to introduction to novel habitats.

Finding food: how marine invertebrates use chemical cues to track and select food

This review covers recent research on how marine invertebrates use chemical cues to find and select food.

You Are What you Eat: a Metabolomics Approach to Understanding Prey Responses to Diet-Dependent Chemical Cues Released by Predators

Prey responses to predator cues are graded in intensity in accordance with the degree of threat presented by the predator. In systems in which prey gather information on predators by using chemicals, prey often respond more to the odor of predators that have consumed conspecifics, as opposed to heterospecifics. We investigated the response of a prey species, the mud crab, Panopeus herbstii, to urine of blue crab, Callinectes sapidus, fed mud crabs or oysters. Behavioral analysis was combined with metabolomics to characterize bioactive deterrents in the urine of predators fed different diets. Urine from blue crabs fed oysters or mud crabs depressed mud crab foraging when presented singly, with the urine derived from a mud crab diet being more potent. The magnitude of foraging depression increased with urine concentration. When urine from blue crabs fed oysters or mud crabs was combined, response to the urine mixture was no different from that to urine derived only from a mud crab diet. Metabolomics analysis indicated diet-dependent differences were related to a set of shared spectral features that differed in concentration in the respective urines, likely consisting of aromatic compounds, amino acids, and lipids. Taken together, these results suggest mud crabs distinguish diet of, and therefore the risk imposed by, predators through detection of a suite of compounds that together represent what the predator has recently consumed.

Fear of large carnivores causes a trophic cascade

The fear large carnivores inspire, independent of their direct killing of prey, may itself cause cascading effects down food webs potentially critical for conserving ecosystem function, particularly by affecting large herbivores and mesocarnivores. However, the evidence of this has been repeatedly challenged because it remains experimentally untested. Here we show that experimentally manipulating fear itself in free-living mesocarnivore (raccoon) populations using month-long playbacks of large carnivore vocalizations caused just such cascading effects, reducing mesocarnivore foraging to the benefit of the mesocarnivore's prey, which in turn affected a competitor and prey of the mesocarnivore's prey. We further report that by experimentally restoring the fear of large carnivores in our study system, where most large carnivores have been extirpated, we succeeded in reversing this mesocarnivore's impacts. We suggest that our results reinforce the need to conserve large carnivores given the significant “ecosystem service” the fear of them provides.

Top predators may indirectly influence ecological processes through fear-induced behavioural changes in their prey. By experimentally manipulating this ‘landscape of fear', Suraci et al. show that fear of large carnivores in a mesopredator can cause cascading effects down the food web that benefit its prey.