Hydrodynamic stress shapes the flora and fauna that exist in wave-swept environments, alters species interactions, and can become the primary community structuring agent.Yet, hydrodynamics can be difficult to quantify because instrumentation is expensive, some methods are unreliable, and accurately measuring spatial and temporal differences can be difficult.Here, we explored the utility of barnacles as potential biological flow-indicators.Barnacles, nearly ubiquitous within estuarine environments, have demonstrated notable phenotypic plasticity in the dimensions of their feeding appendages (cirri) and genitalia in response to flow.In high flow, barnacles have shorter, stockier cirri with shorter setae; in low flow, barnacles have longer, thinner cirri with longer setae.By measuring the relative differences in cirral dimensions, comparative differences in flow among locations can be quantified.We tested our hypothesis that ivory barnacles (Amphibalanus eburneus) could be useful biological flow indicators in two experiments.First, we performed reciprocal transplants of A. eburneus from wave protected vs. wave exposed areas to assess changes in morphology over 4 weeks as well as if changes dissipated when barnacles were relocated to a different wave habitat.Then, in a second study, we transplanted barnacles into low (<5 cm/s) and high flow (>25 cm/s) environments that were largely free of waves and shielded half of the transplanted barnacles to lessen flow speed.In both experiments, barnacles had significant differences in cirral morphologies across high and low flow sites.Transplanting barnacles revealed phenotypic changes occur within two weeks and can be reversed.Further, ameliorating flow within sites did not affect barnacle morphologies in low flow but had pronounced effects in high flow environments, suggesting that flow velocity was
Abstract Mesopredator release following top predator loss may reduce biodiversity and harm foundation species. We investigated the potential for moderate environmental changes to trigger mesopredator release by disrupting the foraging ability of top predators without affecting their abundance by performing an in situ experiment designed to isolate the magnitude of mesopredator effects on oyster reefs ( Crassostrea virginica ). In estuaries, fishes occupy upper trophic levels. Most are visual foragers and become less effective predators in high turbidity. Communities were 10% more diverse, fish predation was 20% higher, and oyster recruitment four times higher in low turbidity. Crab mesopredators were 10% larger and 260% more abundant in high turbidity. Caging treatments to exclude mesopredators significantly affected communities in high but not low turbidity. Oysters had 150% stronger shells in turbid areas, a known response to crabs that was indicative of higher crab abundance. These findings indicated that increased turbidity attenuated fish foraging ability without disrupting the foraging ability of mesopredators (e.g., crabs) that forage by chemoreception. Larger and more numerous crab mesopredators significantly affected oyster reef community structure as well as the survival and growth of oysters in turbid environments. In environments where apex predators and mesopredators utilize different sensory mechanisms, sensory‐mediated mesopredator release may occur when conditions affect the foraging ability of higher order predators but not their prey.
Abstract Prey organisms reduce predation risk by altering their behavior, morphology, or life history. Avoiding or deterring predators often incurs costs, such as reductions in growth or fecundity. Prey minimize costs by limiting predator avoidance or deterrence to situations that pose significant risk of injury or death, requiring them to gather information regarding the relative threat potential predators pose. Chemical cues are often used for risk evaluation, and we investigated morphological responses of oysters ( Crassostrea virginica ) to chemical cues from injured conspecifics, from heterospecifics, and from predatory blue crabs ( Callinectes sapidus ) reared on different diets. Previous studies found newly settled oysters reacted to crab predators by growing heavier, stronger shells, but that adult oysters did not. We exposed oysters at two size classes (newly settled oyster spat and juveniles ~2.0 cm) to predation risk cue treatments including predator or injured prey exudates and to seawater controls. Since both of the size classes tested can be eaten by blue crabs, we hypothesized that both would react to crab exudates by producing heavier, stronger shells. Oyster spat grew heavier shells that required significantly more force to break, an effective measure against predatory crabs, when exposed to chemical exudates from blue crabs as compared to controls. When exposed to chemical cues from injured conspecifics or from injured clams ( Mercenaria mercenaria ), a sympatric bivalve, shell mass and force were intermediate between predator treatments and controls, indicating that oysters react to injured prey cues but not as strongly as to cues released by predators. Juvenile oysters of ~ 2.0 cm did not significantly alter their shell morphology in any of the treatments. Thus, newly settled oysters can differentiate between predatory threats and adjust their responses accordingly, with the strongest responses being to exudates released by predators, but oysters of 2.0 cm and larger do not react morphologically to predatory threats.
