Plants and animals that inhabit the intertidal zone of wave—swept shores are generally small relative to terrestrial or subtidal organisms. Various biological mechanisms have been proposed to account for this observation (competition, size—specific predation, food—limitation, etc.). However, these biological mechanisms are constrained to operate within the mechanical limitations imposed by the physical environment, and these limitations have never been thoroughly explored. We investigated the possibility that the observed limits to size in wave—swept organisms are due solely or in part to mechanical, rather than biological, factors. The total force imposed on an organism by breaking waves and postbreaking flows is due to both the water's velocity and its acceleration. The force due to velocity (a combined effect of drag and lift) increases in strict proportion to the organism's structural strength as the organism increases in size, and therefore cannot act as a mechanical limit to size. In contrast, the force due to the water's acceleration increases faster than the organism's structural strength as the organism grows, and thus constitutes a potential mechanical limit to its size. We incorporated this fact into a model that predicts the probability that an organism will be destroyed (by breakage or dislodgement) as a function of five parameters that can be measured empirically: (1) the organism's size, (2) the organism's structural strength, (3) the maximum water acceleration in each wave, (4) the maximum water velocity at the time of maximum acceleration in each wave, and (5) the probability of encountering waves with given flow parameters. The model was tested using a variety of organisms. For each, parameters 1—4 were measured or calculated; the probability of destruction, and the size—specific increment in this probability, were then predicted. For the limpets Collisella pelta and Notoacmaea scutum, the urchin Strongylocentrotus purpuratus, the mussel Mytilus californianus (when solitary), and the hydrocoral Millepora complanata, both the probability of destruction and the size—specific increase in the risk of destruction were determined to be substantial. It is conjectured that the size of individuals of these species may be limited as a result of mechanical factors, though the case of M. complanata is complicated by the possibility that breakage may act as a dispersal mechanism. In other cases (the snails Thais canaliculata, T. emarginata, and Littorina scutulata; the barnacle Semibalanus cariosus), the size—specific increment in the risk of destruction is small and the size limits imposed on these organisms are conjectured to be due to biological factors. Our model also provides an approach to examining many potential effects of environmental stress caused by flowing water. For example, these methods may be applied to studies of: (1) life—history parameters (e.g., size at first reproduction, age at first reproduction, timing of reproductive cycles, length of possible reproductive lifetime), (2) the effects of gregarious settlement on the flow encountered, (3) the physical basis for patterns of disturbance, (4) the optimum (as opposed to the maximum) size of organisms, and (5) the energetic cost of maintaining a skeleton with an appropriate safety factor. A definitive answer regarding the possibility of mechanical limits to size depends both upon an accurate measurement of the probability of encountering a wave of specific flow parameters and upon factors that are external to the model considered (e.g., life—history parameters). Further, due to their ability to move with the flow, organisms that are sufficiently flexible can escape the size limits imposed on more rigid organisms. Thus, some macroalgae attain large sizes (2—3 m in maximum dimension). The precise role of these factors awaits further research.

Mechanical Limits to Size in Wave-Swept Organisms

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.

/pdf/fear-of-large-carnivores-causes-a-trophic-cascade-2g03dd8gek.pdf

Fear of large carnivores causes a trophic cascade.

Inducible defenses are found in both terrestrial and aquatic organisms and appear to be a common defensive strategy of both sessile and mobile species. Here we report the presence of previously undescribed inducible defenses in marine mussels, a dominant member of temperate rocky intertidal communities. In the Gulf of Maine, estuarine shorelines with low water flow are characterized by high crab predation on the common blue mussel, Mytilus edulis, compared to nearby shore- lines with high water flow. Mussels at these high predation sites had thicker shells, relatively more shell mass, and were more tightly attached to the substrate than mussels at low predation sites. Field and laboratory experiments were used to test whether the between- site differences in these traits were the result of phenotypic plasticity. Mussels outplanted to high predation sites produced more byssal threads and attached more firmly to the substrate than did mussels outplanted to low predation sites. In the laboratory, thicker shells were induced by waterborne cues from both the common predator, Carcinus maenus, and damaged conspecifics, and these thicker shells were mechanically stronger than thinner, noninduced ones. Our results contribute to growing evidence that inducible defenses are a pervasive feature of intertidal assemblages. In addition to the bivalve example presented here, gastropods, barnacles, bryozoans, and seaweeds are also known to have inducible defenses. Phenotypic plasticity for defensive traits has likely evolved repeatedly in intertidal organisms and appears to play a major role in the dynamics of rocky intertidal communities.

Crab predation, waterborne cues, and inducible defenses in the blue mussel, mytilus edulis

A survey of studies examining crab and lobster predation on hard-shelled molluscs indicated that most predators prefer small-sized prey when offered a range of sizes I suggest that this pattern of preference for small-sized prey can be explained by considering the mechanical cost of predation If predation costs are assumed to be non-renewable, then the probability of exhausting them can be included as an additional parameter in the classical diet breadth model The importance of predation costs in determining prey size selection can be extended to other predators that use mechanical means to attack their prey

https://www.int-res.com/articles/meps/87/m087p239.pdf

Why do decapod crustaceans prefer small-sized molluscan prey?

We examined the predator avoidance behav- iors of two common freshwater snails, Physella virgata and Planorbella trivolvis, to the crayfish Procambarus simulans. In response to crayfish predation, the snails crawled above the waterline for several hours, then re- turned to the water. A significant size-dependent rela- tionship existed between crawlout (vertical migration above the waterline) and vulnerability to predation. All observed size classes of P. virgata, and small P. trivolvis, were vulnerable and crawled out in response to crayfish predation. Large, invulnerable P. trivolvis did not display any overt avoidance behavior, but relied instead on strong shell architecture for defense. We suggest that, in these species, crawling above the waterline reduces the proba- bility of an encounter between vulnerable thin-shelled snails and crayfish. This behavior is an adaptive response to predation.

http://www.biosurvey.ou.edu/oliver/pdf/Alexander1991b.PredationRiskandAvoidanceBehavior.pdf

Predation Risk and Avoidance Behavior in Two Freshwater Snails

The feeding behavior of rocky intertidal crabs in the tropical and temperate eastern Pacific was studied in relation to specific mechanical properties of the shells of their limpet prey. A series of laboratory experiments, involving direct observations, records of shell remains, and measurements of the forces generated by a feeding crab, showed that by far the most common feeding technique was to pry the margin of the limpet shell away from the substratum. The pattern of deformation in models of limpet shells subject to a similar prying force indicated (1) that the greatest stress on the shell was at the point of force application at the shell margin and (2) that the thickness of the shell margin contributed more to shell strength than did thickness in more apical regions of the shell. Measurements of the strength of real shells provided further support for this latter conclusion.In addition, the strength of foot attachment, which sets the maximum prying force that the shell can experience, closely parall...

Richard B. Lowell

Papers

Crab predation on limpets: predator behavior and defensive features of the shell morphology of the prey.