No description available.

Mating behaviours and reproductive decisions are of key importance to evolutionary fitness. However, it is often difficult to measure and compare the fitness of different individuals, especially in long-lived species, and for behaviours that are subject to multiple selective forces. A striking exception though is the field of sex-allocation research, which is a triumphant success of evolutionary theory (Charnov 1982). The reason for this success is that, following Fisher (1958), the fitness consequences of different sex ratios can be readily predicted and tested analytically. These predictions are very amenable to empirical testing and provide a platform to measure the precision of adaptation, which is crucial for understanding the nature of evolutionary change.

Reproductive success is heavily influenced by life-history traits; a series of energy investment trade-offs that organisms must optimise according to their environmental conditions. These include considerations such as how many offspring and when to reproduce? The consequences of multiple trade-offs can be extremely complex, making research difficult. However, there are notable exceptions. Simple clutch size theory enabled great strides in assessing trade-offs in resource allocation, though it quickly becomes more complicated when considering investment in current versus future reproduction. Arguably, even greater success has come from consideration of investment in a particular sex. Sex allocation theory provides simple models that can be empirically tested, and has provided some of the strongest evidence for natural selection and evolution. Much of this work has focused on certain parasitoids due to their extraordinary sex ratios and the finite resources available to offspring in a host. Whilst clutch size and sex allocation theory have provided many answers, there are still questions regarding the impact of other life-history traits. In this thesis I have used the gregarious parasitoid wasp Nasonia vitripennis in laboratory experiments to assess some of these traits. I have focused on the impact of larval competition, inbreeding, host condition and host feeding on longevity, fecundity, sex allocation and mating success. By manipulating host quality through host-feeding, I was able to vary the level of resources available to offspring. Simultaneously, by manipulating the matedstatus and number of females ovipositing on a host, I was able to vary the number and sex ratio of offspring competing for resources. My research has provided an insight into how larval competition and host-feeding impact on optimal clutch size and sex allocation. Furthermore, I have attempted to assess the extent to which body size, which is commonly associated with reproductive success, can be used to predict fitness. The appendix includes work using molecular data to understand the mating behaviour and population structure of N. vitripennis in the wild, enabling models based on assumptions of laboratory-based behaviour to be applied to wild populations.

No description available.

No description available.

I examined variability in the shell form of 25 Littorina obtusata populations from Massachusetts to northern Maine. I chose this scale because the history of one of L. obtusata's principal predators, the crab Carcinus maenas, has changed dramatically in the past 100 years. Before 1900, Carcinus did not occur north of Cape Cod, Massachusetts, but by 1950 it had reached Canadian border. Moreover, in summer water temperatures during average 6-8&\sp\circ&C colder at northern locations. Shell thickness and mass increased and body mass decreased with increasing latitude. to test whether these patterns reflect plasticity in response to predators, snails from two northern and two southern populations were raised in the laboratory with and without Carcinus effluent. Snails raised with Carcinus produced thicker shells than conspecifics raised without Carcinus. This response was accompanied by reduced body size and body growth. Another experiment examined whether geographic differences in water temperature induce changes in shell form. Snails reciprocally transplanted between a northern (Maine) and southern (Massachusetts) exhibited substantial plasticity in shell form. Southern snails transplanted north produced significantly thinner, lighter, shells than snails raised at their native shore, while northern snails transplanted south produced thicker, heavier shells than snails raised at the native shore. In addition, snails producing thicker, heavier shells exhibited reduced body mass and growth. Although patterns in final phenotypes exhibited cogradient variation, growth in both shell thickness and mass exhibited countergradient gradient variation. Most examples of countergradient variation are associated with temperature differences suggesting that differences in water temperature are responsible for this pattern. A third experiment involved reciprocally transplanting snails between the same two sites (temperature effect) and raising them with and without Carcinus effluent. In general, Carcinus effluent and warmer water temperatures induced thicker, heavier shells and reduced body mass and growth. Overall, it appears that predator effluent and water temperature have similar effects on shell form. My work on phenotypic plasticity encourages a more pluralistic view of phenotypic variation. Moreover, my results suggest that phenotypic plasticity is a ubiquitous strategy in adapting to different environments and that its evolution may be driven by life history trade-offs.

Following larval development in coastal waters, the postlarval stage, the megalopa, of the blue crab enters estuaries and colonizes juvenile/adult habitats. I examined the dynamics of the immigration of blue crab megalopae in the York River, Virginia. Plankton samples were collected nightly during flood tide in the York River Jul-Nov 1987, 1988 and 1989 to quantify variation in abundance of blue crab megalopae during the recruitment season. The abundance of megalopae was highly episodic, and significantly correlated with wind stress to the west (1987, 1988 and 1989), wind stress to the north (1989), and observed tidal range at Gloucester Point (1987 and 1988). These factors accounted for 22-56% of the variation in megalopal abundance. Megalopal abundance was not correlated with current speed, water temperature, salinity, or Chesapeake Bay subtidal volume. While in the estuary, megalopae vertically migrated in response to light and tide. Blue crab megalopae were more abundant during flood than ebb, indicating that megalopae selectively occupy flood waters. Ascent of megalopae into the water column appeared to be light limited. at night, megalopae were concentrated at the surface; during day, they were concentrated near the bottom in deep water, but apparently did not ascend from the bottom in shallow water. Thus, megalopae utilize flood currents for transport up the estuary while reducing the risk of predation to visual predators. Spatial patterns of abundance of megalopal and juvenile blue crabs were not consistent across habitats (plankton, grassbeds, or artificial settlement substrates) or time (days, months). Densities of megalopae and first-stage juveniles in grass beds correlated with megalopal abundance in the plankton; settlement on artificial substrates did not. Total juvenile abundance exhibited lower spatial and temporal variability in grass beds than that of megalopae or first-stage juveniles, suggesting that high variability in settlement is tempered by post-settlement processes. In laboratory studies, sand shrimp, Crangon septemspinosa, consumed &>&99% of available megalopae, while grass shrimp, Palaemonetes pugio, consumed 37% of megalopae in the first trial and 5% in the second trial. Predation by grass shrimp was not related to the rate of metamorphosis of megalopae, but may have been related to water temperature.

Woody and herbaceous vegetation were sampled in 23 tidal swamps along a tidal freshwater tributary of lower Chesapeake Bay. Four vegetative categories were ordinated with Detrended Correspondence Analysis (DECORANA). Species distribution patterns of each strata were compared with respect to edaphic factors, a wetness index, and mean water table depth. Woody species are restricted to hummocks (topographic highs). Hummocks drain as quickly as the tide drops and so are partially inundated for only short periods each day. Although low in canopy diversity, tidal swamps are floristically rich in herbaceous and woody understory species, ranking them among the most speciose in temperate North America. Canopy composition is related to the wetness of a site as determined by the percent of the forest floor covered by hollows (low inter-hummock depressions) and by mean water table depth. Fraxinus spp. and Nyssa biflora dominated swamps are best developed in wetter sites, which contain higher calcium (Ca) and organic matter (Om) levels and where the mean water table depth is about &-&17 cm. In contrast, Acer rubrum-Liquidambar styraciflua-Nyssa biflora dominated swamps occur at less wet sites where mean water table depth is deeper than 20 cm. Although DECORANA separated canopy and herbaceous strata similarly, the woody subcanopy (shrubs and small understory tree species) did not separate into the same two communities. to determine whether this pattern might be indicative of forests in general, distributional data of canopy and subcanopy species were also compared using similarly collected data from a southern Appalachian forest. Sapling (juvenile canopy species) distribution patterns were also compared in both systems. Separate ordinations were performed on canopy, sapling, and subcanopy species. Canopy trees and saplings showed a similar pattern of distribution, suggesting that resource requirements of saplings and canopy-statured adults are similar. In contrast, the subcanopy species of neither ecosystem showed any discernable distributional relationship to the canopy or sapling layers, suggesting that subcanopy life-forms may partition different resources than canopy species in temperate forests. If so, the common practise of combining sapling and subcanopy species in structural analyses may hinder our understanding of subcanopy structural patterns in forests.

A model of zinc accumulation by the American oyster, Crassostrea virginica, is developed by relating in-situ zinc body burden to time-integration of uptake. Short-term uptake rates are estimated in laboratory by introducing &\sp{lcub}65{rcub}&Zn to oysters of various weights in aquaria with salinities of 18&\perthous& and 12&\perthous&. Uptake of &\sp{lcub}65{rcub}&Zn by an oyster: (1) varies as a power function of the body weight (soft tissue dry weight) of the oyster (&dy\over{lcub}dt{rcub}& = kW&\sp{lcub}\beta{rcub}&), (2) is inversely related to the salinity of ambient water, and (3) increases linearly with ambient concentration. Zinc body burdens of oysters of various weights from oyster beds with different salinity regimes of the James River and of the Rappahannock River are measured. When the zinc body burden of oysters is fitted to a power function of body weight (y = aW&\sp{lcub}\rm b{rcub}&), the values of power, b, are 1.33, 1.30, and 1.06 for salinities of 13, 15, and 20&\perthous&, respectively, in the James River and 1.16 for a nominal salinity of 18&\perthous& in the Rappahannock River. The values of b agree with the values of &\beta& derived from the &\sp{lcub}65{rcub}&Zn uptake experiments; b = &\beta& + 1. The model is calibrated using data for Horsehead Shoals and Nansemond Ridge, two sites in the James River having average salinities roughly the same as those used in &\sp{lcub}65{rcub}&Zn laboratory studies. The model is verified by the use of data for Wreck Shoal, a mid-salinity sampling site of the James River, and the pooled data for the Rappahannock River sampling sites. The weight-specific zinc concentration of an oyster increases continuously, but rate of the increase is reduced as the oyster grows larger. Both uptake parameters, k and &\beta& vary with salinity. It is suggested that the body weight effects, and their variation with salinity, should be incorporated in the design of monitoring programs for trace metals as well as in experimental studies.

Human development has eroded Chesapeake Bay's health, resulting in an increase in the extent and severity of hypoxia (≤2 mg O2 l-1). The Bay's hypoxic zones have an adverse affect on community function and secondary production of macrobenthos. The production of macrobenthos is important as these fauna link energy transfer from primary consumers to epibenthic predators and demersal fish, and serve as the foremost pathway that carbon is recycled out of the sediment. Additionally, bioturbation, an essential macrobenthic function that causes the displacement and mixing of sediment particles, increases the quality of marine sediments. In the marine environment bioturbation is primarily mediated by macrofauna which are susceptible to perturbations in their surrounding environment due to their sedentary life history traits. The effect of hypoxia on macrobenthic production was assessed in Chesapeake Bay and three of its tributaries (Potomac, Rappahannock, and York rivers) from 1996 to 2004. Each year, 25 random samples were collected from each system and macrobenthic production estimated using Edgar's allometric equation. Efforts were then focused on the Rappahannock River, a sub-estuary of Chesapeake Bay known to experience seasonal hypoxia, to assess changes in macrobenthic production and function. During the spring, summer, fall, and following spring of 2007 and 2008, samples were collected each season in each year, and DO concentrations were measured continuously at two sites in 2007 and two in 2008. A benthic observing system (Wormcam) was also deployed in 2009 from early spring to late fall to assess the impact of hypoxia on bioturbation. Wormcam transmitted a time series of in situ images and water quality data in near real-time. Results from the previous projects was used to develop a continuous-time, biomass-based model, including phytoplankton, zooplankton, and macrobenthic state variables. The primary focus aimed at predicting the effect of hypoxia on macrobenthic biomass. Z', a sigmoid relationship between macrobenthic biomass and DO concentration, was derived from macrobenthic data collected from the 2007 and 2008 field experiments. Annual fluctuations in macrobenthic production were significantly correlated with DO. Hypoxia led to a 90% reduction in daily macrobenthic production relative to normoxia, and production at hypoxic sites was composed primarily of smaller, disturbance-related annelids. The reduced production resulted in an annual biomass loss of approximately 7320 to 13,200 metric tons C, which equated to a 6 to 12% annual displacement of the Bay's total macrobenthic productivity due to hypoxia. Macrobenthic production differed across seasons, and sediment reworking rates were significantly higher during normoxia, indicating a change in the functional role of the macrobenthic community. Hypoxia was found to significantly reduce bioturbation through reductions in burrow lengths, burrow rates, and burrowing depth. Although infaunal activity was greatly reduced during hypoxic and near anoxic conditions, some individuals remained active. The biomass-based model was successfully calibrated and verified, using independent data, to accurately predict B annually. Simulation analysis of the DO formulation showed B strongly linked to DO concentration, with fluctuations in biomass significantly correlated with the duration and severity of hypoxia.