Native and Invasive Aquatic Plant Responses to Amphipod and Snail Grazing

Invasive species can negatively affect biodiversity and may be connected to plant chemical defenses. However, the extent to which small grazers affect growth and chemical defenses in native and invasive aquatic plants is not well understood. The effect of amphipod (mostly Hyalella azteca) and snail (Physella sp.) grazing on growth and phenolic content was compared in a native aquatic plant (coontail, Ceratophyllum demersum) and an invasive aquatic plant (Eurasian watermilfoil, Myriophyllum spicatum) in two laboratory experiments. In each experiment, growth of each plant was measured as changed in wet mass, and at the end of the experiments each plant was freeze-dried and ground in liquid nitrogen before running Folin-Denis assay for total phenolics. Neither amphipods nor snails had a significant effect on either coontail or milfoil growth. In both experiments (amphipods and snails) milfoil consistently grew more than coontail, and milfoil contained higher levels of phenolics than coontail in the amphipod experiment. However, phenolics were not affected by amphipod grazing itself. Phenolic results are still underway for the snail experiment. The data suggest that although invasive milfoil is more chemically defended than native coontail, grazing by small herbivores did not affect plant growth or induce phenolic production

Environmental factors may drive plant size differences between a restored and a natural marsh in the Housatonic River Estuary

Spartina alterniflora (smooth cordgrass) provides habitat for many species and prevents erosion along coastlines globally and is a common restoration target. However, restored and natural marshes often have different traits due to genetic or environmental differences. Previous data has shown that S. alterniflora from a restored marsh in Stratford, Connecticut are consistently smaller than those from a naturally occurring marsh in Milford, Connecticut. We aimed to determine whether environmental differences such as nutrients play a role in observed size differences by performing two greenhouse experiments in which we grew S. alterniflora shoots from each location 1) in identical conditions and 2) under nutrient enriched and unenriched conditions. Height and diameter were measured weekly for four weeks in each experiment. When grown in identical conditions, Milford plants were taller with larger diameters than those from Stratford throughout the experiment, but growth rates were similar in plants from the two sites. Although not statistically significant, nutrient enrichment seemed to increase growth rates in plants from Stratford more than plants from Milford, with no difference in growth in unenriched plants from the two sites. These results suggest that environmental factors do play a role in the observed Spartina alterniflora height differences between sites

Idiosyncratic Responses of Seagrass Phenolic Production Following Sea Urchin Grazing

While chemical defenses can determine plant persistence in terrestrial ecosystems and some marine macroalgae, their role in determining seagrass persistence in areas of intense grazing is unknown. As a first step toward determining if concentrations of feeding deterrents in seagrasses increase following herbivore attacks, we conducted 4 experiments using a common macrograzer (sea urchin Lytechinus variegatus) and 2 phylogenetically divergent seagrass species (Thalassia testudinum and Halodule wrightii). Macrograzer impacts on production of phenolic acids and condensed tannins varied somewhat idiosyncratically with season, urchin density, and distance from urchin damage. In general, phenolic concentrations were higher in both turtlegrass and shoalgrass in summer than in fall. Grazing led to increased condensed tannin concentrations in T. testudinum but had few effects on turtlegrass phenolic acid concentrations. Turtlegrass p-hydroxybenzoic acid concentrations increased locally in the grazed area and in areas \u3e2 cm above grazing. Similarly, condensed tannins in grazed H. wrightii leaves increased with urchin density in summer but decreased in fall while having few predictable effects on phenolic acid concentrations. Shoalgrass gallic acid concentrations increased in the lowest portions of grazed leaves. In choice feeding experiments in which urchins were offered agar food made with nighand low-phenolic seagrass tissue, high phenolic concentrations did not reduce urchin feeding, suggesting that the increases in phenolic concentrations we saw in turtlegrass and shoalgrass did not protect these plants against future sea urchin herbivory but rather some other factor, such as mesograzer feeding or pathogenic infection

Seagrass Deterrence to Mesograzer Herbivory: Evidence from Mesocosm Experiments and Feeding Preference Trials

Two laboratory experiments documented the effects of mesograzers (i.e. the gastropod Crepidula ustulatulina and the isopod Paracerceis caudata) on phenolic acid and condensed tannin production in 2 regionally abundant seagrasses—Thalassia testudinum (turtlegrass) and Halodule wrightii (shoalgrass). Subsequent paired choice experiments tested the hypothesis that phenolic acids and condensed tannins produced by these seagrasses deter mesograzer feeding. At the scale of the shoot, grazing by gastropods and isopods led to ~40 to 50% decreases in concentrations of some phenolic acids and ~20% decreases in condensed tannins in turtlegrass leaves. At a more refined spatial scale, concentrations of 2 of these compounds increased by 25 to 85% in areas near tissues damaged by C. ustulatulina and P. caudata in turtlegrass. In contrast, isopod feeding increased the concentrations of some shoalgrass phenolic acids by ~30 to 50%, while gastropod grazing led to ~25 to 50% higher concentrations of condensed tannins in shoalgrass leaves, suggesting that grazer identity and seagrass species play important roles in seagrass deterrent production. Amphipods (Batea catharinensis) consistently preferred agar food made from seagrass leaves with low phenolic concentrations in choice feeding experiments, indicating that phenolics can act as feeding deterrents to these mesograzers

Potential Role of Phenolics in Eurasian Watermilfoil (Myriophyllum spicatum) Invasion Success

Although the detrimental effects of invasive aquatic plants like Eurasian watermilfoil (Myriophyllum spicatum) are well documented, the factors leading to successful aquatic plant invasions are poorly understood. High levels of chemical feeding deterrents in invasive plant species may be at least partially responsible when invasive plants overgrow and dominate the invaded community. To investigate the role of phenolics in Eurasian watermilfoil invasions, whole M. spicatum (invasive) and Ceratophyllum demersum (native) plants were collected from Osbourndale Pond in Derby, Connecticut during September 2016 and frozen at -80°C until phenolic analysis. Colorimetric assays were used to measure total phenolic and condensed tannin content of the two plant species. A choice feeding experiment was conducted to determine if phenolics produced by M. spicatum affected feeding by amphipods, the dominate herbivore at our study site. Artificial diets were prepared by incorporating palatable fish food into an agar matrix and pouring the agar food over window screen. The control diet contained only a palatable food, while the gallic acid diet contained the palatable food plus gallic acid at a concentration common in M. spicatum. Amphipods were simultaneously offered the control food and the gallic acid treated food. The number of squares that were cleared of food after 48 hours was recorded. Data analysis is currently underway, but results from this study can shed light on the role of phenolics in aquatic plant invasion success

Use of Shallow Estuarine Habitats by Nekton in the Mobile-Tensaw River Delta, Alabama

We compared nekton density, composition, and biomass in fall 2009 and spring 2010 among three major habitat types (marsh, SAV=submerged aquatic vegetation dominated by Vallisneria americana, SNB=shallow nonvegetated bottom) commonly found throughout the Mobile-Tensaw River Delta (MTD) using 1-m2drop samplers. In the sampling design, habitat selection was based on vegetation composition. Sample locations (TR=Tensaw River, CB=Chocolatta Bay, and BC=Below Causeway) were selected based on their degree of tidal connectivity with the wider estuary (BC \u3e TR \u3e CB). Nekton distributional patterns varied among both locations and habitat types. Species richness was greater at BC than CB. The young of most estuarine-dependant fishery species (e.g., white shrimp, blue crab, gulf menhaden) were more abundant, and had more biomass, at BC and TR than CB. Estuarine residents (e.g., riverine grass shrimp, rainwater killifish) dominated the nekton in CB. Within locations, mean densities and biomass of abundant species were concentrated in vegetated (marsh, SAV) habitat types, and most species associated with vegetation structure were more abundant in SAV than marsh. Tidally unrestricted areas of the MTD may provide an important nursery for fishery species such as white shrimp, blue crab, gulf menhaden, and southern flounder. Additional studies will be needed to determine if these fishery species represent strong conduits for cross ecosystem transfer of energy and nutrients between the MTD and northern Gulf of Mexico

El herbivorismo de Sarpa salpa en los tramos someros de los lechos de Posidonia oceanica

Here, we examined the temporal and small–scale spatial variability of grazing by the herbivorous fish Sarpa salpa on shallow beds of the temperate seagrass Posidonia oceanica. Herbivory intensity expressed as the percent of leaf area taken by fish bites was higher in September 2006 than in February 2007, and at 0.5 m than at 1.5 m during both sampling times. All S. salpa feeding at the shallow locations studied were juveniles, with bite sizes ranging from 0.03 to 0.62 cm2. Juveniles feeding at 1.5 m were larger in February 2007 than in September 2006, as evidenced by significant differences in mean bite size per shoot. However, the larger juveniles feeding at 1.5 m in February 2007 did not appear to feed as frequently as the comparatively smaller juveniles feeding at the same depth in September 2006, as suggested by significant differences in number of bites per shoot. The number of bites per shoot was also lower at 1.5 m than at 0.5 m in February 2007, although mean bite size did not differ significantly between the two depths at that sampling time. In general S. salpa juveniles did not select a particular range of leaf ages when feeding in the study locations, although the juveniles feeding at 1.5 m in September 2006 appeared to select mid–aged leaves. Fish did not show a preference for more epiphytized leaves. These results show that grazing activity by S. salpa juveniles in shallow reaches of P. oceanica meadows may vary temporally and across small changes in depth, which in turn may affect the overall intensity of herbivory on the seagrass.En el presente artículo analizamos la variabilidad temporal y espacial en pequeña escala de la actividad de alimentación del pez herbívoro Sarpa salpa en los lechos someros de la pradera submarina de clima templado Posidonia oceanica. La intensidad del herbivorismo expresada como el porcentaje de superficie foliar mordida por el pez fue superior en septiembre de 2006 que en febrero de 2007, y a una profundidad de 0,5 m que de 1,5 m durante los dos períodos de muestreo. Todos los individuos de S. salpa que se alimentaban en las zonas someras estudiadas eran juveniles y el tamaño de mordedura se situaba entre 0,03 y 0,62 cm2. Los juveniles que se alimentaban a 1,5 m de profundidad fueron más grandes en febrero de 2007 que en septiembre de 2006, tal como ponen de relieve las diferencias significativas existentes en el tamaño medio de mordedura por haz. No obstante, los juveniles más grandes que se alimentaban a 1,5 m de profundidad en febrero de 2007 no parecían alimentarse con tanta frecuencia como los juveniles comparativamente más pequeños, que lo hacían a la misma profundidad en septiembre de 2006, tal como sugieren las diferencias significativas halladas en el número de mordeduras por haz. Asimismo, el número de mordeduras por haz fue inferior a 1,5 m de profundidad que a 0,5 m en febrero de 2007, si bien el tamaño medio de las mordeduras no difería en medida significativa entre las dos profundidades en aquel período de muestreo. En general, los juveniles de S. salpa no elegían un intervalo de edad concreto de las hojas a la hora de alimentarse en los lugares del estudio, a pesar de que los juveniles que se alimentaban a 1,5 m de profundidad en septiembre de 2006 parecían elegir hojas de edad mediana. Los peces no mostraron preferencia por las hojas con mayor cobertura de epífitos. Estos resultados muestran que la actividad de alimentación de los juveniles de S. salpa en los tramos someros de las praderas de P. oceanica puede variar con el tiempo y con pequeños cambios de profundidad, lo que a su vez puede afectar a la intensidad general del herbivorismo sobre las praderas submarinas

Sarpa Salpa Herbivory on Shallow Reaches of Posidonia Oceanica Beds

Sarpa salpa herbivory on shallow reaches of Posidonia oceanica beds.— Here, we examined the temporal and small–scale spatial variability of grazing by the herbivorous fish Sarpa salpa on shallow beds of the temperate seagrass Posidonia oceanica. Herbivory intensity expressed as the percent of leaf area taken by fish bites was higher in September 2006 than in February 2007, and at 0.5 m than at 1.5 m during both sampling times. All S. salpa feeding at the shallow locations studied were juveniles, with bite sizes ranging from 0.03 to 0.62 cm2. Juveniles feeding at 1.5 m were larger in February 2007 than in September 2006, as evidenced by significant differences in mean bite size per shoot. However, the larger juveniles feeding at 1.5 m in February 2007 did not appear to feed as frequently as the comparatively smaller juveniles feeding at the same depth in September 2006, as suggested by significant differences in number of bites per shoot. The number of bites per shoot was also lower at 1.5 m than at 0.5 m in February 2007, although mean bite size did not differ significantly between the two depths at that sampling time. In general S. salpa juveniles did not select a particular range of leaf ages when feeding in the study locations, although the juveniles feeding at 1.5 m in September 2006 appeared to select mid–aged leaves. Fish did not show a preference for more epiphytized leaves. These results show that grazing activity by S. salpa juveniles in shallow reaches of P. oceanica meadows may vary temporally and across small changes in depth, which in turn may affect the overall intensity of herbivory on the seagrass

High Phenolic Content Fails to Deter Mesograzer Consumption of Myriophyllum spicatum (Eurasian watermilfoil) in New England

Eurasian watermilfoil (Myriophyllum spicatum) is often considered one of the most aggressive macrophyte invaders in freshwater habitats throughout the United States. However, conditions leading to successful milfoil invasions are not well understood. This study sought to illuminate the role of 4 herbivores in determining milfoil invasion success via either enemy release or biotic resistance. We determined feeding preferences of three herbivores native to the northeastern United States and measured milfoil phenolic content, which may act as an herbivore feeding deterrent. We found that phenolic content in milfoil was two times higher than in two of the most abundant native macrophytes at our study sites, consistent with enemy release. However, laboratory feeding experiments demonstrated that milfoil phenolics did not deter amphipod (Hyalella azteca), snail (Physella sp.), or weevil (Euhrychiopsis lecontei) herbivory, contrary to the predictions of enemy release. Amphipods and snails from habitats invaded by milfoil consumed similar quantities of both milfoil and the low-phenolic native plant Elodea canadensis. In contrast, weevils consumed milfoil but not E. canadensis. Amphipods collected from milfoil-free habitats also readily consumed milfoil, and they consumed two to three times more milfoil than E. canadensis in choice feeding trials. These results suggest that high phenolic levels do not prevent native herbivores from consuming invasive milfoil. Instead, native generalist grazers like amphipods and snails may limit milfoil proliferation and provide a measure of biotic resistance

Seagrass–Pathogen Interactions: ‘Pseudo-Induction’ of Turtlegrass Phenolics Near Wasting Disease Lesions

Marine protists of the genus Labyrinthula cause the seagrass wasting disease, which is associated with regional die-offs of eelgrass Zostera marina and also infects turtlegrass Thalassia testudinum . The ability of seagrasses to resist pathogen attack is determined by multiple factors, which are poorly understood. One factor hypothesized to influence seagrass disease resistance is the presence of (poly)phenolic natural products such as caffeic acid, which inhibits the growth of L. zosterae in in vitro laboratory bioassays. This hypothesis has been supported by reports of pathogen-induced phenolic accumulations in eelgrass Z. marina. To test the response of T. testudinum to inoculation with Labyrinthula sp., we conducted a series of culture experiments wherein plants were inoculated with Labyrinthula sp. isolated from turtlegrass beds in Perdido Bay, Florida (USA). Concentrations of phenolic acids and condensed tannins were quantified in diseased leaves as well as those treated with 5 mM salicylic acid, a signaling molecule associated with pathogen-induced responses in plants. In infection experiments, increases in the concentrations of several phenolic acids, but not condensed tannins, were observed in tissues above, but not below, microbial lesions. Salicylic acid (SA) treatments did not induce any phenolic compound, either when applied alone or in concert with the pathogen. The induction of phenolic acids above, but not below, infection sites suggests that T. testudinum leaves did not respond to the pathogen specifically. Instead, the pattern is consistent with the predictions of the sink/source model of plant defense, which predicts increased phenolic contents in cases where wounds disrupt plant resource allocation and cause a local overabundance of carbonbased resources. Thus, we suggest that the emergence of Labyrinthula sp. lesions on turtlegrass blades causes a ‘pseudo-induction’ of specific phenolics as carbon resources over-accumulate in tissues located above wound sites