The Oyster is Your World

Many prey species can adjust their morphology to reduce predation risk in response to predator cues. Using predator cues to enhance prey defenses may improve survival of cultivated species and enhance species restoration efforts, but assessment of such benefits across industrially relevant scales and identifying the chemical cues responsible for these morphological changes is needed. Oysters, Crassostrea virginica, develop heavier, stronger shells in response to chemical cues from predators and injured conspecifics. We investigated if oyster morphology could be manipulated on large scales to improve survival and bolster restoration. Through a series of experiments in collaboration with state agencies, NGOs, and industry, we tested the benefits and costs of this approach to improve oyster survival in the field. Our results suggested that exposing oyster spat to predator cues significantly increased their survival. Although oysters exposed to predator cues, initially developed less soft tissue, after one year, they had similar sizes and tissue masses to controls. Concurrently, we ran bioassays to identify the chemical cues predators release that stimulate oysters to grow stronger shells. We exposed oysters to eight different concentrations of urine extracted from blue crabs as well as candidate molecules for inducing defenses. Oyster shell strength increased by 20 – 100% when exposed to predator cues and followed a standard dose response curve with crab urine concentration causing shell strength to peak at ~0.19 mL urine/L seawater. Additionally, induced oysters had >50% higher survivorship than controls after being in the field after one year, although survivorship differences varied substantially depending on season and site. These findings demonstrate the utility of using predator cues to enhance the survival of target species, and highlight an opportunity to employ nontoxic methods to control pest-based mortality.

Induced defenses are well-known to increase prey survivorship in the presence of predators and carry growth costs. However, our understanding of the mechanistic underpinnings of these cost-benefit interactions and how they shift across landscapes is limited. We investigated how raising a model foundation species, oysters, under commercial hatchery conditions with cues from a common predator can alter shell microproperties, improve survival, and govern growth patterns spanning 55 sites across the Alabama coastline. Oysters exposed to predator cues developed a 4% harder and 16% thicker foliated layer in their shell which increased the crushing force shells could withstand by 52%. Oysters induced to grow defenses under these methods had 68% greater survivorship over controls after a year when used to build a new ~42 m2 reef. Developing these defenses had marked costs in shell size and soft tissue mass initially. However, growing caged oysters across the coastline with citizen scientists found that these differences were essentially nonexistent after six months in the field. Instead, oyster growth was highly dependent on local conditions, with average shell size and soft tissue mass varying 120% and 570% respectively within the region. Our findings provide insights on how minute physiological changes can structure ecological interactions.

The eastern oyster, Crassostrea virginica, is an ecologically and commercially important species in the Gulf of Mexico. A primary threat to oyster populations is predation. Oysters combat the risk of predation by increasing their shell hardness to increase survivorship. While studies have been conducted on this induced defense in controlled environments, more information is needed on how these defenses develop and persist in the field. Over the course of a year, single seed oysters will be maintained at one site on Dauphin Island, AL. This experiment consists of three treatments: no predator, predator exposure for 4 weeks in the nursery, and continuous, close predator exposure throughout their life. Periodically throughout the year, oyster physiology and shell morphology of the single seed will be assessed. Using predator cues to improve the survival of Crassostrea virginica can refine restoration and aquaculture efforts. Understanding the temporal aspect of predator induction in a less controlled environment more closely mimics naturally occurring responses and provides insight into how oysters perceive threats through time. 
 

Eastern oysters (Crassostrea virginica) are a major fishery in the south, however natural reefs are depleted and restored reefs often struggle to persist. Oysters are sessile and food selective organisms, their feeding efficiency is influenced by particle concentration and availability, which can vary across sites and season. To understand what effect changing environmental parameters like salinity and pH might have on the oyster's ability to feed, we ran in-situ filter feeding assays in Mobile Bay, AL. The filter feeding devices pump water into a head tank and through individuals chambers containing a live bivalve to allow for accurate and individual physiology assessment. Using water and biodeposit samples individual oysters’ physiological feeding rates (e.g.: clearance rate & filtration rate) are calculated via the biodeposition method using the inorganic content of the water as a tracer.

Prey experience different levels of predator exposure throughout their lifetimes. The risk allocation hypothesis suggests that organisms will adjust anti-predator responses to varying levels of perceived risk and will have the strongest anti-predator responses during brief and infrequent levels of exposure to high risk. Under constant, high risk exposure, organisms may attenuate to perceived risk and express anti-predator traits at lower levels as compared to situations where risk is variable.


Oysters (Crassostrea virginica) harden their shells in response to chemical exudates indicative of predation risk, and using oysters as a model organism, we tested how temporal variation in predation risk cues from blue crabs (Callinectes sapidus) would influence oyster morphology. We varied both risk cue concentration and temporal exposure to determine how each influenced oyster reactions to predation risk. Consistent with the risk allocation hypothesis, temporal variation in risk produced the largest increases in shell hardness. Using predator cues to harden oyster shells can improve oyster restoration and aquaculture, and varying exposure may be both logistically easier and more beneficial than maintaining constant predator exposure during nursery grow-out.