No abstract

Predation can significantly affect prey populations and communities, but predator effects can be attenuated when abiotic conditions interfere with foraging activities. In estuarine communities, turbidity can affect species richness and abundance and is changing in many areas because of coastal development. Many fish species are less efficient foragers in turbid waters, and previous research revealed that in elevated turbidity, fish are less abundant whereas crabs and shrimp are more abundant. We hypothesized that turbidity altered predatory interactions in estuaries by interfering with visually-foraging predators and prey but not with organisms relying on chemoreception. We measured the effects of turbidity on the predation rates of two model predators: a visual predator (pinfish, Lagodon rhomboides) and a chemosensory predator (blue crabs, Callinectes sapidus) in clear and turbid water (0 and ∼100 nephelometric turbidity units). Feeding assays were conducted with two prey items, mud crabs (Panopeus spp.) that rely heavily on chemoreception to detect predators, and brown shrimp (Farfantepenaus aztecus) that use both chemical and visual cues for predator detection. Because turbidity reduced pinfish foraging on both mud crabs and shrimp, the changes in predation rates are likely driven by turbidity attenuating fish foraging ability and not by affecting prey vulnerability to fish consumers. Blue crab foraging was unaffected by turbidity, and blue crabs were able to successfully consume nearly all mud crab and shrimp prey. Turbidity can influence predator–prey interactions by reducing the feeding efficiency of visual predators, providing a competitive advantage to chemosensory predators, and altering top-down control in food webs.

We investigated how changes in abiotic conditions resulting from human activities indirectly alter trophic interactions using turbidity in estuaries as a model system. Development and nutrient input are causing turbidity to increase in many coastal areas. Using an 18 yr data set from Aransas and San Antonio Bays in Texas, we found fish abundance (Sciaenops ocellatus, Pogonias cromis, Archosargus probatocephalus) to be highest in low turbidity (, 30 nephelometric turbidity units [NTU]; p , 0.01), while crab (Callinectes sapidus) abundance was highest in high turbidity (. 30 NTU; p , 0.05). In field studies, mud crabs (Panopeus spp.), an important intermediate predator on oyster reefs that are not targeted in the 18 yr data set, were more abundant on oyster reefs in St. Charles Bay, Texas, when turbidity exceeded 30 NTU ( p 5 0.03). Fish predation on tethered Panopeus herbstii was greatest when turbidity was low (, 30 NTU, p , 0.05), but predation by crabs ( p 5 0.003) and overall predation ( p 5 0.02) were greatest in high turbidity (. 30 NTU). Predation on oyster spat was not different between low-and high-turbidity sites ( p 50.64). However, oysters devoted more resources to shell growth ( p , 0.01) at a cost of less somatic growth and fecundity, a reaction known to occur in response to crab predators. Elevated turbidity can alter trophic interactions in estuaries by altering species composition and trophic interactions, leading to an increase in crab abundance, which can alter predation rates as well as growth in juvenile oysters.

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.

Eastern oysters Crassostrea virginica and hard clams Mercenaria mercenaria are key organisms for both the ecosystem services they provide and for their commercial value, but their populations have declined greatly worldwide. In an attempt to understand the interaction between bivalve physiology and environmental conditions, filter-feeding assays were carried out in a shallow estuary, the Indian River Lagoon (IRL; Florida, USA). The feeding behavior of the bivalves was studied using in situ filter-feeding devices and the biodeposition method in the 3 basins of the IRL during March and August 2015. Water characteristics (temperature, salinity, dissolved oxygen, chl a, and total, organic, and inorganic particulates) were related to possible changes in the feeding physiology of the bivalves. Oysters had higher clearance rates, filtration rates, and rejection than clams. The high rejection of inorganic matter allowed oysters to increase the organic matter ingested, leading to high absorption efficiencies. In contrast, because clam rejection was low regardless of elevated levels of inorganic matter, their absorption efficiency only increased with higher organic matter content. Both species preferred higher salinities, and the amount of organic matter in the water had a negative relationship with some feeding parameters (filtration rate for both species, and rejection for oysters). Acute environ mental change brought about by a brown tide (caused by the alga Aureoumbra lagunensis) also affected these 2 bivalve species differently, supporting the hypothesis that oysters and clams have different physiological capabilities that drive their ability to survive in dynamic estuarine ecosystems.KEY WORDS: Indian River Lagoon · Brown tide · Aureoumbra lagunensis · Seston · Clearance rate · Bivalves Resale or republication not permitted without written consent of the publisher Editorial responsibility: Romuald Lipcius,

In 2011, the Indian River Lagoon, a biodiverse estuary in eastern Florida (USA), experienced an intense microalgal bloom with disastrous ecological consequences. The bloom included a mix of microalgae with unresolved taxonomy and lasted for 7 months with a maximum concentration of 130 μg chlorophyll a L−1. In 2012, brown tide Aureoumbra lagunensis also bloomed in portions of this estuary, with reoccurrences in 2016 and 2018. To identify and understand the role of grazer pressure (top-down control) on bloom formation, we coupled DNA sequencing with bivalve feeding assays using three microalgae isolated from the 2011 bloom and maintained in culture. Feeding experiments were conducted on widely distributed bivalve species in the lagoon, including eastern oysters (Crassostrea virginica), hooked mussels (Ischadium recurvum), charru mussels (Mytella charruana), green mussels (Perna viridis), Atlantic rangia (Rangia cuneata), and hard clams (Mercenaria mercenaria), which were exposed to 3 × 104 cells mL−1 of five species of microalgae consisting of A. lagunensis and the three species clarified herein, the picocyanobacteria Crocosphaera sp. and ‘Synechococcus’ sp., and the picochlorophyte Picochlorum sp., as well as Nannochloropsis oculata used as a control. To ensure clearance rates were indicative of consumption and assimilation, the microalgae were isotopically (15N) labeled prior to feeding experiments. Clearance rates differed among bivalve and microalgal species, but enriched 15N values in bivalve tissue suggest that algal bloom species were assimilated by the bivalves. These results expand our understanding of the important ecosystem services that healthy, biodiverse filter feeder communities provide.

The subtropical mussel Mytella charruana has been reported as invasive along the southeast coast of the USA since 1986. This mussel has been found to negatively impact the keystone species in its invaded range, the eastern oyster Crassostrea virginica. To date, however, no mechanism for this negative impact has been determined. To elucidate the role of the invasive mussel on economically important oyster reefs, we compared the feeding physiology of both species in a lagoon along the east coast of Florida (USA). Three different methodologies were used: 1) in situ filter-feeding experiments using the biodeposition method to estimate feeding behavior; 2) laboratory assays to estimate the depletion of bacterial particles using a flow cytometer; and 3) stable isotope analysis in conjunction with ellipse-based metrics to investigate the niche size and overlap of these two species. The in situ filterfeeding experiments revealed that M. charruana had significantly higher clearance, filtration, rejection, organic ingestion and absorption rates, as well as higher rejection percentage and absorption efficiency, but rejected the same amount of inorganic particles. Flow cytometry data suggested that bacteria were a food source for both bivalve species. Stable isotope values confirmed that M. charruana and C. virginica filled similar functional niches in this ecosystem. These results suggest that M. charruana can outcompete native oysters, the findings also demonstrate that an invasion of M. charruana might significantly alter plankton abundance, potentially limiting food sources available to other less efficient native filter-feeders such as clams.

Turbidity is widely regarded for modulating primary production and influencing the distribution of submerged aquatic vegetation. Although less well studied, turbidity can also have significant effects on trophic interactions and food webs by modifying light penetration and scattering, influencing foraging ability of visual-hunting predators such as fishes. By interfering with visual foragers, turbidity may shift food webs towards predators that forage with other sensory modalities (e.g. chemoreception and mechanoreception), consequently altering food web structure. We analysed turbidity effects on estuarine community composition and biodiversity in the Gulf of Mexico by analysing an 18-year fisheries-independent data set and assessing communities inhabiting contemporary oyster reefs (Crassostrea virginica). In the long-term data set, elevated turbidity was associated with decreased fish species richness and diversity and higher abundances of benthic species that rely more on chemoreception for foraging and predator avoidance (e.g. crabs). High turbidity may provide a predation refuge for crabs and other benthic organisms that visually oriented fish prey upon. On oyster reefs, crabs readily consume suspension-feeding organisms including newly settled oysters and porcelain crabs (Petrolisthes armatus). Both were significantly less abundant in high turbidity. Human practices that increase turbidity may indirectly influence trophic interactions, species distributions, ecosystem function, and biodiversity.