Causes and Consequences of Individual Phenotypic Differences in Brachyuran Crabs With A Focus on Behavior

Individual variation defines almost every morphological, physiological, and behavioral aspect of populations and is a fundamental component of many ecosystem processes. Recent work indicates that accounting for these individual differences can enhance our ability to predict community responses to environmental disturbances which is becoming increasingly important in an era of extraordinary global change. However, our understanding of how different individual characteristics are connected to each other and governed by the environment remains limited. This study sought to evaluate the relationship between individual behavior, physiological condition, and local habitat for Brachyuran crabs as well as the subsequent strength of their predator-prey interactions within oyster reefs communities. Here, I examined the effects of parasitic infection, diet, habitat quality, season, and conspecific density on crab behavioral traits and physiology. I also evaluated how consistent individual behavioral differences, i.e. personalities, interact with predator type and habitat quality to influence individual mortality and movement patterns. These relationships are essential for calculating population dynamics across multiple spatial scales. My research found that crab energy stores were strongly dependent upon diet and spawning season. In turn, individual crab activity level and reproductive effort were tied to these energy stores and the local environment. Individuals in structurally degraded oyster reefs would generally exhibit lower activity levels and decreased reproductive output in comparison to crabs which inhabited healthy, structurally complex reefs. Individual activity was further decreased by reductions in conspecific density, which correlated to habitat quality, and by parasitic infection. Additionally, individual behavior influenced predation risk with bold crabs predominantly consumed by active hunters and shy crabs preferentially selected by ambush predators. Personality also interacted with habitat quality as crabs on low quality reefs rapidly left the region, starting with the boldest individuals; whereas high quality reefs had greater levels of predation, particularly among bold crabs. These findings demonstrate that individual phenotypic variation mediates divergent community interactions across habitat quality and provides several mechanisms through which spatially structured populations may develop

Drift Macroalgal Distribution In Northern Gulf of Mexico Seagrass Meadows

Drift macroalgae, often found in clumps or mats adjacent to or within seagrass beds, can increase the value of seagrass beds as habitat for nekton via added food resources and structural complexity. But, as algal biomass increases, it can also decrease light availability, inhibit faunal movements, smother benthic communities, and contribute to hypoxia, all of which can reduce nekton abundance. We quantified the abundance and distribution of drift macroalgae within seagrass meadows dominated by turtle grass Thalassia testudinum across the northern Gulf of Mexico and compared seagrass characteristics to macroalgal biomass and distribution. Drift macroalgae were most abundant in areas with higher seagrass shoot densities and intermediate canopy heights. We did not find significant relationships between algal biomass and point measures of salinity, temperature, or depth. The macroalgal genera Laurencia and Gracilaria were present across the study region, Agardhiella and Digenia were collected in the western Gulf of Mexico, and Acanthophora was collected in the eastern Gulf of Mexico. Our survey revealed drift algae to be abundant and widespread throughout seagrass meadows in the northern Gulf of Mexico, which likely influences the habitat value of seagrass ecosystems

The Influence of Diet Composition on Fitness of the Blue Crab, Callinectes sapidus.

The physiological condition and fecundity of an organism is frequently controlled by diet. As changes in environmental conditions often cause organisms to alter their foraging behavior, a comprehensive understanding of how diet influences the fitness of an individual is central to predicting the effect of environmental change on population dynamics. We experimentally manipulated the diet of the economically and ecologically important blue crab, Callinectes sapidus, to approximate the effects of a dietary shift from primarily animal to plant tissue, a phenomenon commonly documented in crabs. Crabs whose diet consisted exclusively of animal tissue had markedly lower mortality and consumed substantially more food than crabs whose diet consisted exclusively of seaweed. The quantity of food consumed had a significant positive influence on reproductive effort and long-term energy stores. Additionally, seaweed diets produced a three-fold decrease in hepatopancreas lipid content and a simultaneous two-fold increase in crab aggression when compared to an animal diet. Our results reveal that the consumption of animal tissue substantially enhanced C. sapidus fitness, and suggest that a dietary shift to plant tissue may reduce crab population growth by decreasing fecundity as well as increasing mortality. This study has implications for C. sapidus fisheries

Effect of diet on egg size.

<p>Mean ± SE egg volume (μm³) of crabs fed either ribbed mussels (<u><i>Geukensia demissa</i></u>), fish (<i>Fundulus heteroclitus</i>), or seaweed (<i>Ulva lactuca</i>) at one of two portion sizes (large or small) for ~2.5 months (n = 10).</p

Effect of diet on tissue investment towards the hepatopancreas and reproduction.

<p>Mean ± SE <b>A)</b> Hepatosomatic Index (HSI) and <b>B)</b> gonadosomatic Index (GSI) of crabs fed either ribbed mussels (<u><i>Geukensia demissa</i></u>), fish (<i>Fundulus heteroclitus</i>), or seaweed (<i>Ulva lactuca</i>) at one of two portion sizes (large or small) for ~2.5 months (n = 10). For reference purposes, starved crabs were expressed in the figures as separate columns (n = 4). Starved crabs were not used in the statistical analyses.</p

A) Analysis of percent lipid composition of the hepatopancreas.

<p>The effect of food type (ribbed mussels, <u><i>Geukensia demissa</i></u>; mummichogs, <i>Fundulus heteroclitus</i>; seaweed, <i>Ulva lactuca</i>) and portion size offered (large, small) on the mean ± SE percent lipid composition of the crab hepatopancreas (n = 10). Starved crabs (n = 4) were represented in the figure to serve as a visual reference and were not included in the statistical analysis. Lower case letters denote statistical differences (<i>p</i> < 0.001, 2-way ANOVA, Tukey test). <b>B)</b> The relationship of percent lipid composition of the hepatopancreas for individual crabs and their corresponding hepatosomatic index (HSI, n = 60).</p

Analysis of crab behavior.

<p>Mean ± SE aggression probability of crabs as a function of <b>A)</b> diet (n = 10), <b>B)</b> time since last feeding (24 or 48 h; n = 1168 and 1163 respectively), and <b>C)</b> daily temperature (°C) within holding containers for crabs fed within 24 or 48 h (n = 53–159). Replicates depend on the number of surviving crabs and times exposed to the same temperature (max = 3 same temperature).</p

Barnacles as biological flow indicators

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 between wave protected and 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 (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 the primary driver of barnacle morphology in our experiment. These results highlight the utility of barnacles as cheap, accessible, and biologically relevant indicators of flow that can be useful for relative comparisons of flow differences among sites