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Invasive species can severely disrupt biological communities through their interactions with native organisms, yet little is known about the response of marine predators to the establishment of a competitive invasive fish. In the western Atlantic, invasive Indo-Pacific lionfishes (Pterois spp.) may represent a novel competitor to several commercially and ecologically important native species. However, there is a scarcity of empirical research documenting comparative resource use of cohabitant lionfish and native fishes, as well the physiological consequences that may result from interspecific interactions with the invasive species. For this thesis, I conducted two studies designed to elucidate the strength of resource use overlap and potential competition among invasive lionfish and an ecologically similar serranid, the Graysby (Cephalopholis cruentata), along a contiguous coral-reef ledge in Biscayne National Park, South Florida. My first study aimed to determine whether lionfish and Graysby could be classified as competitors through comparisons of Graysby population size, diet, and condition across a range of ambient lionfish biomass. Using stable isotope and gut content analyses, I measured a difference in Graysby diet on sites with larger populations of lionfish, specifically a smaller breadth of resource use and lower consumption of teleost fish prey. Despite a shift in diet, Graysby condition did not vary with lionfish biomass, and thus this study did not provide unequivocal evidence of competition between the two species. However, based on a high amount of apparent overlap in interspecific resource use, competitive interactions between lionfish and species such as Graysby remain likely in systems with more limiting prey or shelter. For my second study, I measured stable isotope values of muscle, liver, and eye lens layers in lionfish and Graysby to further compare individual and population-level patterns of diet and habitat use. The use of eye lenses as metabolically stable chronological recorders of stable isotopes has vast potential to provide insight about animal life history, but has not yet been applied to describe trends in resource use among invasive and native species. To aid these analyses I created a rudimentary map of spatial isotopic variation along the reef ledge of Biscayne National Park, which could serve as a frame of reference to study local-scale animal movements. Isotopic differences between liver and muscle samples suggested a broader range of movement in lionfish than Graysby, important for understanding the relative scale of habitat use in these species. In eye lenses, stable isotope values increased logarithmically with lens radius (i.e. fish size), likely reflecting patterns of trophic growth. There was a high amount of variability among the shapes of eye lens isotopic chronologies, particularly those of lionfish, yielding further information about movement and individual resource use specificity in these species. The results of this thesis are the first to compare native predator diet and condition across a range of invasive lionfish biomass, as well as the first to measure size-structured trends in the resource use of individual lionfish. Together, these results enhance our understanding of the potential for competition among lionfish and native mesopredators, an important objective for researchers studying how this highly invasive species interacts with surrounding ecological communities.

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Yes / Large changes in spectacle prescription can increase falls risk in older people. We investigated the effect of induced astigmatism (a common cause of distorted or blurred vision in older people) on locomotor stepping patterns to determine whether the orientation of astigmatic changes could have differential effects on gait safety when negotiating steps and stairs. Methods 10 older adults (mean age 76.0±6.4 years) walked up to and stepped onto a raised block whilst wearing their spectacle prescription and when blurred with ±3.00D cylinders at axes 45°, 90°, 135° and 180°. Gait measurements included foot placement before the block, toe clearance over the block edge and foot placement on the block. Results Induced astigmatism with axes at 90°, providing magnification in the horizontal meridian only, caused no change in stepping pattern. Induced astigmatism with axes at 180° caused foot placement changes in the anterior or posterior direction according to whether magnification was positive or negative in the vertical meridian (block perceived higher or lower respectively). Induced astigmatism with axes oblique at 45° and 135° (causing the block to be perceived as a parallelogram sloping downwards either to the right or left) caused gait changes in the anterior and posterior, vertical and lateral directions. Changes in lateral foot placement appeared to be an attempt to maintain constant foot clearance levels over the block edge by stepping over the perceived ‘lower’ side of the ‘sloping’ block. Conclusions Astigmatic changes with oblique axes had the greatest effect on gait. Clinicians, including optometrists, physiotherapists, occupational therapists and nurses should counsel older patients about the effects of astigmatism on gait safety. Furthermore, partial prescribing of astigmatic corrections should be considered to reduce the risk of falling.

The focus of this work was on the use of otolith microchemistry and fish eye lens chemical profiles to measure fish movement and provided indirect support for the use of otolith microchemistry to examine exposure to crude oil. Chapter 1 provides an introduction to the applications of otolith microchemistry and eye lens isotopic profiles. In the second chapter, which examined associations between metal exposure and lesion formation in fishes collected after the Deepwater Horizon (DWH) oil spill, I did not observe any change in oil-associated metal concentrations in otoliths coinciding with the timing of the DWH oil spill. This suggests that either the technique used is not sensitive enough to detect any transient changes that may have occurred because of exposure to the oil spill or that the fish examined were not exposed to the oil spill. However, I did find that lesioned fish may have been exposed to a persistent source of trace-metals in the GoM prior to, during, and after the oil spill, and metal-induced immunomodulation may have occurred in these fish. These interactions between the physiological and environmental modulation of otolith element incorporation were explored further in Chapter 3 in which multiple tests demonstrated that physiology explained more of the variation in otolith chemical tags than ambient water chemistry. These findings suggest that the use of otolith microchemistry alone to track fish movement and potential exposure to harmful metals may be complicated by physiological control of otolith microchemistry. Thus, in Chapter 4, I pursued a novel method to evaluate the movement of fish across isoscapes of varying δ15N. I validated the use of fish eye lenses as potential lifetime recorders of isotopic histories and in Chapter 5 compared the use of fish eye lens δ15N profiles to otolith microchemistry profiles to examine fish movement. Both techniques suggested similar patterns of movement in Red Snapper from the northern GoM to the West Florida Shelf. This is the first study to use these complimentary techniques to track fish movement.

The aim of this project is to design a cost-effective planar Maxwell fish-eye lens in PCB technology operating at the center frequency 5 GHz with a bandwidth of 20 %. An approach to design a cost-effective lens is to use a metasurface, which is commonly realized as a periodic structure of unit cells. In this project, a study was made by comparing different unit cells and considering the effect of applying glide symmetry to the unit cells. Comparing different unit cells withand without glide symmetry demonstrates that glide symmetry is necessary to achieve a 20 % bandwidth. Introducing glide symmetry showed a reduction in dispersive behavior, an increaseof isotropy and effective refractive index. Simulations of the full lens show a functioning lens with a power transfer of 67 % at 5.46 GHz. In conclusion, glide symmetry will improve the performance of the lens and is necessary to fulfill the requirement of a 20 % bandwidth.

The eye lens grows continuously throughout life and changes its shape as the eye changes focus from a distant to a near object (the process of accommodation). These changes are complex because they may affect not only the shape of the lens, but also its refractive index distribution. To date there has been no satisfactory technique for directly and non-invasively measuring these changes. In this study the refractive index distribution through the isolated lens was measured non-invasively using a novel MRI technique. The dependence of the refractive index value of lens tissue on its transverse relaxation rate (R2) was determined empirically from measurements on lens homogenate samples. Using a multi-spin-echo imaging sequence, data were acquired for constructing R2 maps of a central slice through the isolated lens. These R2 maps were transformed to refractive index maps using the empirically determined dependence of refractive index on R2. Using a standard algorithm for ray tracing through gradient index media, the propagation of light rays through the index map were simulated. The optical properties of the lens, such as focal length, were then measured. The technique was validated by also directly measuring the focal length of each lens using laser ray tracing. The subtle changes in refractive index distribution that are responsible for the dramatic change in the optical properties of the isolated lens with age, were observed for the first time. The decrease in surface power of the isolated lens with age accounted only partially for the decrease in total lens power with age, the remainder resulting from a reduction in the gradient of refractive index (GRIN) power. It is likely that this reduction in GFUN power is the mechanism by which the eye maintains emmetropia (good distant vision) with age despite the increasing curvature of its surfaces. The reduction in the GRIN power of the lens was found to be mainly due to a flattening of the refractive index profile in the central region of the lens, accompanied by steepening of the profile near the edge of the lens. In agreement with a previous MRI study of the isolated human eye lens, this study found a decrease in the refractive index of the nucleus with age. However the age related change in this study was not as large and not found to be statistically significant. The results demonstrate that existing simple models for the optics of the eye lens are inadequate to accurately describe its properties. Several more sophisticated models were considered in an attempt to describe better the age-dependent changes that occur in both the power of the lens and its longitudinal aberration. Mathematical modelling was also used to simulate the accommodative process and investigate possible changes in the index distribution of the lens that may occur with accommodation. A preliminary in vivo study was performed aimed at observing the change in the refractive index distribution of the eye lens with age and accommodation. These results demonstrated the feasibility of the technique for in vivo applications and showed that within experimental error there is little change in the central refractive index of the lens with age. However the resolution achievable with standard clinical imaging sequences and signal detection hardware was not optimal for in vivo refractive index mapping of changes in the human eye lens with accommodation. Finally therefore, methods for refining the technique for in vivo applications are discussed which may make it possible to directly and simultaneously measure both the shape and refractive index distribution of the lens with age and accommodation.

Patterned neuronal activity driven by the sensory environment plays a key role in the development of precise synaptic connectivity in the brain. It is well established that the action potentials (‘spikes’) generated by individual neurons are crucial to this developmental process. A neuron’s spiking activity is jointly determined by its synaptic inputs and its intrinsic excitability. It is therefore important to ask how a neuron develops these attributes, and whether the emergence of spiking might itself be governed by activity-dependent processes. In this thesis, I address these questions in the retinotectal system of Xenopus laevis. First, I investigate the extent to which visuospatial information is available to the developing retinotectal system. I show that the eyes of developing Xenopus larvae are hyperopic at the onset of vision, but rapidly grow towards correct vision. Despite its imperfect optics, the Xenopus eye is able to generate spatially restricted activity on the retina, which is evident in the spatial structure of the receptive fields (RFs) of tectal neurons. Using a novel method to map the visually driven spiking output and synaptic inputs of the same tectal neuron in vivo, I show that neuronal spiking activity closely follows the spatiotemporal profile of glutamatergic inputs. Next, I characterise a population of neurons in the developing optic tectum that does not fire action potentials, despite receiving visually evoked glutamatergic and γ-aminobutyric acid (GABA)ergic synaptic inputs. A comparison of visually spiking and visually non-spiking neurons reveals that the principal reason these neurons are ‘silent’ is that they lack sufficient glutamatergic synaptic excitation. In the final section of the thesis, I investigate whether visually driven activity can play a role in the ‘unsilencing’ of these silent neurons. I show that non-spiking tectal neurons can be rapidly converted into spiking neurons through a visual conditioning protocol. This conversion is associated with a selective increase in glutamatergic input and implicates a novel, spike-independent form of synaptic potentiation. I provide evidence that this novel plasticity process is mediated by GABAergic inputs that are depolarising during early development, and can act in synergy with N-methyl-D-aspartate receptors (NMDARs) to strengthen immature glutamatergic synapses. Consistent with this, preventing the depolarising effects of GABA or blocking NMDARs abolished the activity-dependent unsilencing of tectal neurons. These results therefore support a model in which GABAergic and glutamatergic transmitter systems function synergistically to enable a neuron to recruit the synaptic excitation it needs to develop sensory-driven spiking activity. This represents a transition with important consequences for both the functional output and the activity-dependent development of a neuron.