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The population dynamics and the feeding of the scyphomedusa Aurelia aurita in Tapeng Bay, southwestern Taiman, were investigated from April, 1999 to April, 2000 and May, 2001 to April, 2002. A. aurita distributed mainly in the inner water of the Bay. The average abundance of A. aurita was 71¡Ó256 ind.100m¡Ð3, with higher abundance in winter and spring than in summer and autumn. The abundance of A. aurita showed no significant correlation with hydrographic features, but it seems to have one or two month¡¦s time lag with the seasonal distribution pattern of copepods. The main reproduction period of A. aurita was form autumn to next spring. The occurrence of ephyra was mainly in winter and spring, with maximum abundance of 328 ind./100m3. The average bell diameter of A. aurita was 13.9¡Ó4.2 cm. The size of bell diameter varied seasonally, generally had larger size in autumn and smaller size in spring. Seventeen zooplankton taxa were found in the stomach contents of A. aurita, copepods were the most dominant (70.3%), followed by copepods nauplius (20.1%), bivalve larva (3.0%) and fish eggs (2.3%). The average ingestion rate of A. aurita was 2165¡Ó2673 prey ind.-1 day-1 , the feeding impact of A. aurita on zooplankton was 14.69 % ~ 40.84 % %, with no significant difference among sizes.

Gelatinous zooplankton are characterised as different from other planktonic taxa due to the high relative water content of their tissues. This thesis investigates whether elevated somatic water content (expressed here as carbon percentage) has effects on the biology of zooplankton. My approach was to examine this at a range of scales with a variety of approaches, ranging from experiments on individual ephyra larvae of Aurelia aurita, through analysis of a zooplankton time series at the Plymouth L4 station, up to a large scale meta-analysis of zooplankton growth and body composition data. In this meta-analysis, carbon percentage varied continuously across the range of the zooplankton, ranging from 0.01% to 19.02% of wet mass, a difference of over three orders of magnitude. Specific growth rate (g, d-1) was negatively related to carbon percentage, both across the full range of zooplankton species, and within the subset of taxa traditionally classified as gelatinous. The addition of carbon percentage to models of zooplankton growth rate based on carbon mass alone doubled explanatory power. I present an empirical equation of maximum (food saturated) zooplankton growth that incorporates carbon mass and carbon as a percentage of wet mass. Applying this equation to a natural assemblage near Plymouth yielded sometimes double the secondary production, as compared to a simpler model based on crustacean growth. Both interspecifically and intraspecifically, carbon percentage was negatively related to carbon mass; more gelatinous taxa tended to have higher carbon masses. During the early development of Aurelia aurita ephyrae, carbon percentage was found to decrease from 2.36% (an intermediate value between crustaceans and classical gelatinous zooplankton) down to 0.1%, the adult value for Aurelia aurita. Juvenile forms of gelatinous taxa are often poorly sampled and their intermediate carbon percentages may help to form a continuum between those of crustaceans and adult cnidarians and ctenophores. As ingestion in the ephyrae was related to their diameter, models suggest that this dilution resulted in an increase in carbon-specific ingestion rate by an estimated 28% relative to an ephyra that did not dilute through development. At the species level, carbon percentage was negatively related to indices of temporal variation in numerical density but not related to rate of population increase. A wide variety of zooplanktonic taxa of different carbon percentages were found to increase in population at a rate that could be considered as forming a bloom. Likewise many gelatinous taxa at L4 did not form blooms. Thus the frequent reference to “jellyfish blooms” reflects, in part, the fact that unlike the other zooplankters that regularly reach even higher carbon concentrations, gelatinous taxa are simply more noticeable to the eye when at these concentrations. Calculating the carbon percentage of whole assemblages could be useful for investigating the influence of environmental parameters on zooplankton. Taken together, these results demonstrate the benefits of explicitly recognising the decoupling of metabolic and ecological body size seen in the gelatinous zooplankton.

Effective locomotion and maneuvering in aquatic environments is important for survival for marine fauna. The ability to move quickly, change direction, and tune energy consumption for long migrations is critical for escape from predators and pursuit of prey. This controlled propulsion in terms of varying speed, turning rates, and actuation effort is of interest for the next generation of underwater vehicle design. Integration of biological functional simplicity, robustness, and superior performance enables robotic vehicles to successfully complete difficult and dynamic operational goals. Gelatinous animals known as Cnidarians employ a wide variety of propulsive methods, ranging from the simple but efficient propulsion of large jellyfish to the rapid and highly maneuverable multi-jet propulsion of colonial animals known as siphonophores. This dissertation studies how these two extremes of underwater soft body propulsion are able to achieve simple yet effective control and locomotion, and thus inform the design of effective vehicle propulsion control and actuation. Two large single bell jellyfish robots, Cyro 2 and Cyro 3, were designed and constructed to implement the simple body form and propulsive methods of large jellyfish to study the unique locomotive characteristics and fluid interactions that generate straight swimming and turning maneuvers. The other extreme of small soft-body colonies moving by multi-jet propulsion was subsequently investigated in-depth, starting with a characterization of the biological fluid jetting actions and gaits. The results of these performance capabilities were then applied to an experimental robotic model with bio-inspired construction and controls to verify an elegant but highly functional neurological control scheme and the kinematic capabilities from varying jetting gait patterns. / PHD / The ability to move rapidly in any direction is a primary characteristic of successful animal species. Evasion of predators, as well as pursuit of prey, is paramount for survival. Jellyfish are excellent examples of animals that have thrived for millions of years with varied methods of moving in their diverse environments. However, the propulsion methods of large jellyfish in straight swimming and turning have not been well understood until recent years. This dissertation focuses on the fundamental understanding of the locomotion and fluid interaction that jellyfish use for propulsion. A large jellyfish robot, named Cyro 2 (“Cy” for the species Cyanea, “ro” for robot, and the second generation of the design), was constructed to explore the role of various structural and fluidic parameters on the locomotion characteristics of the largest jellyfish species Cyanea. The successor Cyro 3 was designed to mimic the complex motions of large jellyfish during maneuvering. Motion tracking and fluid analysis of the robot during turning was utilized to explain how jellyfish dynamically control their orientation. These results inspired further study of a unique relative of jellyfish, the siphonophore, which can swim with a modular chain of soft pumping bodies that coordinate without a central nervous system. This unique control strategy and method of movement underwater was studied by analyzing specimens of the siphonophore Nanomia. Development and modeling of elegant control techniques inspired by this species is presented and implemented on an experimental model that uses this unique propulsion method to validate and expand upon observations of live specimens. Combined, the results obtained in this dissertation open the possibility of designing advanced underwater vehicles.

Unmanned underwater vehicles (UUVs) have long been in use but increasingly there has been a wave of biomimetic robots taking over the duties and functions of traditional vehicles. A robotic jellyfish, inspired by the species Aurelia aurita was developed and characterized. In addition to the body of the main robotic vehicle, supporting technologies were developed including polymeric artificial muscles, hydrogel-based artificial mesoglea, and an inclinometer inspired by the jellyfish statocyst organ. Through multiple versions, the vehicle was able to attain an order of magnitude increase in proficiency from 0.022 s?? to 0.21 s?? and robustness not found in initial prototypes. A polyvinyl alcohol hydrogel reinforced with ferritin nanoparticles was found to accurately mimic the stress and strain characteristics of natural Aurelia mesoglea while maintaining a high water content similar to the animal. In addition, the optical properties were shown to be controlled by water to DMSO ratio. A five layer PPy-Au-PVDF-Au-PPy actuator stored in 0.5M KCl solution actuated at 4 VDC potential and produced an impressive 90% tip deflection. In addition, the rate of change was extremely high at 50% deflection of initial actuator length per second. The artificial jellyfish statocyst was found to produce the required highly linear voltage divider output. This sensor will provide the vehicle with biomimetic self-awareness of its own body position. Future directions are proposed for the biomimetic robotic jellyfish such as on-board power and computing, multi-material mesoglea with a dermal layer, a MEMS-based statocyst, and polymeric muscles with increased force production and time response. / Master of Science

The design of unmanned underwater vehicle (UUV) was inspired by the form and functionality of Jellyfish. These natural organisms were chosen as bio-inspiration for a multitude of reasons including: efficiency, good room for payload, and a wide range of sizes and morphology. Shape memory alloy (SMA) actuators were selected as the primary source of actuation for the propulsion of the artificial jellyfish node. These actuators offer high power density which enables a compact system size and silent operation which is preferred for surveillance.  SMA wires mimic the form and function of natural muscles; allowing for a wider range of applications than conventional actuators. Commercial SMA wires (100 um in diameter) can exhibit a 4% deformation of the initial actuator length with a blocking stress of over 200 MPa. The deformation of SMA wire is not enough to mimic the bell contraction of jellyfish. In order to resolve this problem, a beam-shape composite actuator using SMA wires as the active component, termed as BISMAC, was designed to provide large curvature. The BISMAC design was inspired by rowing jellyfish bell contraction. Characterization of maximum deformation in underwater conditions was performed for different actuator configurations to analyze the effect of design parameters that include silicone thickness, flexible steel thickness and distance between SMA and flexible steel. A constant cross-section (CC) BISMAC of 16 cm in length was found to achieve deformation with a radius of curvature of 3.5 cm. Under equilibrium conditions, the CC-BISMAC was found to achieve 80% of maximum deformation consuming 7.9 J per cycle driven at 16.2 V/0.98 A and frequency of 0.25 Hz. Using the a developed analytical model, an actuator design was fabricated mimicking the maximum deformation profile of the A. aurita. The optimized AA-BISMAC achieved a maximum curvature of 0.428 1/cm as compared to 0.438 1/cm for the A. aurita with an average squared root error of 0.043 (1/cm), 10.2% of maximum A. aurita curvature.   BISMAC actuators are unidirectional flexible actuators capable of exhibiting high curvature. To extend the application range of these actuators, they were modified to achieve bidirectional deformation. The new bidirectional actuators termed as "BiFlex" actuators had the capability to achieve large deformation in two directions. The FlexLegs consist of six segments which can be actuated individually. Two different sets of legs were constructed to determine the effect of size. The small legs measured 35.8 mm in height and 63.2 mm in width and the large legs were 97.4 mm in height and 165.4 mm in width. The small FlexLegs achieved a maximum deformation of 12 % and 4 % in the x- and y-direction respectively using a power of 0.7 W while producing a maximum force of 0.023 N. They were also able to withstand a load of 1.18 N. The large FlexLegs had a maximum deformation of 57 % and 39 % in the x- and y-direction respectively using a power of 3 W while producing a force of 0.045 N. They were able to withstand a load of 0.25 N. The legs were also able to perform several walking algorithms consisting of stepping, crabbing and yawing. In order to reduce the power consumption and contraction time of SMA wires, a feedback control scheme using wire resistance was developed. The controller required the knowledge of threshold resistance and safe current inputs which were determined experimentally. The overheating effect of SMA wires was analyzed for BioMetal Fiber (BMF) and Flexinol 100 "m diameter wires revealing an increase in resistance as the wires overheated. The controller was first characterized on a SMA wire with bias spring system for a BMF 100 using I_hi=0.5 A and I_low=0.2 A, where hi corresponds to peak current for fast actuation and low corresponds to the safe current which prevents overheating and maintains desired deformation. A contraction of 4.59% was achieved in 0.06 s using the controller and the deformation was maintained for 2 s at low current. The BISMAC actuator was operated using the controller with I_hi=1.1 A and I_low=0.65 A achieving a 67% decrease in contraction time compared to using a constant driving current of I_low=0.2 A and a 60% decrease in energy consumption compared to using constant I_hi=0.5 A while still exceeding the contraction requirements of the Aurelia aurita. Two fundamental parameters at the composition level were associated with the power consumption of SMA: i) martensite to austentite phase transition temperature and ii) thermal hysteresis. Ideally, one would like to reduce both these quantities and for this purpose an equiatomic Ni-Ti alloy was modified with Cu. Replacing nickel with 10 at% copper reduces the thermal hysteresis by 50% or more. For Ni-Ti alloys with nickel content greater than 50 at%, transition temperature decreases linearly at a rate of 100 "C/Ni at%. Given these two power reducing factors, an alloy with composition of Ni40+xTi50-xCu10 was synthesized with x = 0, ±1, ±2, ±3, ±4, ±5. Metal powders were melted in an argon atmosphere using an RF induction furnace to produce ingots. All the synthesized samples were characterized by differential scanning calorimetric (DSC) analysis to reveal martensite to austenite and austenite to martensite transition temperatures during heating and cooling cycles respectively. Scanning electron microscopy (SEM) was conducted to identify the density and microstructure of the fractured samples. The results show the possibility of achieving low power consuming high performance SMAs. Using the BISMAC actuator and feedback control system, a robotic jellyfish called Robojelly that mimics the morphology and kinematics of the Aurelia aurita species was created. A systematic fabrication technique was developed to replicate the essential structural features of A. aurita. Robojelly's body was fabricated from RTV silicone having a total mass of 242 g and bell diameter of 16.4 cm. Robojelly was able to generate enough thrust in static water conditions to propel itself and achieve a proficiency of 0.19 s-1 while the A. aurita achieves a proficiency of around 0.25 s-1. A thrust analysis based on empirical measurements for natural jellyfish was used to compare the performance of the different robotic configurations. The configuration with best performance was a Robojelly with segmented bell and a passive flap structure. Robojelly was found to consume an average power on the order of 17 W with the actuators not having fully reached thermal steady state. A comparative kinematics analysis was conducted between a natural Aurelia aurita and Robojelly. The resistance feedback controller was implemented to tailor the deformation profile of BISMAC actuators embedded in Robojelly. Robojelly's performance was quantified in terms of thrust production and power consumption during vertical swimming experiments. A maximum average instantaneous thrust production of 0.006 N was achieved at a driving current (Ihi) of 1.5 A with 35% duty cycle. Rapid heating of SMA wires was found to reduce power consumption and increase thrust. The bell kinematic analysis revealed resemblance and differences in bell deformation trajectories of the biomimetic and natural jellyfish. The inflexion point of the A. aurita was found to convert an inner bell trajectory into an outer one during contraction which assists the thrust production. A biomimetic robot inspired by Cyanea capillata, termed as "Cyro", was developed to meet the functional demands of underwater surveillance in defense and civilian applications. The design of Cyro required kinematics of large C. capillata which are elusive creatures. Obtaining accurate kinematic data of animals is essential for many biological studies and bio-inspired engineering applications. Many animals such as the C. capillata however, are either too large or too delicate to transport to controlled environments where accurate kinematic data can easily be obtained. Often, in situ recordings are the only means available but are often subject to multi-axis motion and relative magnification changes with time, which lead to large discrepancies in animal kinematics. In Chapter 5, techniques to compensate for magnification and body rotation of animal footage were developed. A background reference point and animal dimensions were used to account for magnification. A linear fit of body points were used to measure body rotation. These techniques help resolve animal kinematics from in situ video footage. The techniques were applied to a large jellyfish, Cyanea capillata, swimming in ocean waters. The bell kinematics were captured by digitizing exumbrella profiles for two full swimming cycles. Magnification was accounted for by tracking a reference point on the ocean floor and by tracking the C. capillata exumbrella arclength in order to have a constant scale through the swimming cycles. A linear fit of the top bell portion was used to find the body angle with respect to the camera coordinate system. Bell margin trajectories over two swimming cycles confirm the accuracy of the correction techniques. The corrected profiles were filtered and interpolated to provide a set of time-dependent points along the bell. The ability to use in situ footage with significant multi-axis motion provides an opportunity to analyze previously impractical footage for gaining a better understanding of large or delicate organisms. The swimming kinematics of the C. capillata were analyzed after extracting the required kinematics from the in situ video. A discrete model of the exumbrella was developed and used to analyze the kinematics. The exumbrella discretization was done using three different methods. The first method consists of analyzing the animal anatomy for structural and mechanical features. The second method consists of analyzing the bell kinematics for areas of highest deformation over time. The third method consists of optimizing node locations that can provide minimal error with comparison to the digitized profiles. Two kinematic models of the C. capillata swimming motion were developed by fitting Fourier series to the discretized segments and angles formed by each segment. The four-segment anatomical kinematic model was used to analyze the bell kinematics of the C. capillata. It was found that the bell does not deform uniformly over time with segments lagging behind others. Hysteresis between contraction and relaxation was also present through most of the exumbrella. The bell margin had the largest hysteresis with an outer path during contraction and inner path during relaxation. The subumbrella volume was approximated based on the exumbrella kinematics and was found to increase during contraction. Cyro was designed to mimic the morphology and swimming mechanism of the natural counterpart. The body of the vehicle consists of a rigid support structure with linear DC motors which actuate eight mechanical arms. The mechanical arms in conjunction with artificial mesoglea create the hydrodynamic force required for propulsion. The full vehicle measures 170 cm in diameter and has a total mass of 76 kg. An analytical model of the mechanical arm kinematics was developed. The analytical and experimental bell kinematics were analyzed and compared to the C. capillata. Cyro reached the water surface untethered and autonomously from a depth of 182 cm in five actuation cycles. It achieved an average velocity of 8.47 cm/s while consuming an average power of 70 W. A thrust stand was developed to calculate the thrust directly from a single bell segment yielding an average thrust of 27.9 N for the whole vehicle. Steady state velocity during Cyro's swimming test was not reached but the measured performance during its last swim cycle resulted in a cost of transport of 10.9 J/kg m and total efficiency of 3%. It was observed that a passive flexible margin or flap, drastically increases the performance of the Robojelly. The effects of flap length and geometry on Robojelly were analyzed using PIV. The flap was defined as the bell section which is located between the flexion point and bell margin. The flexion point was established as the location where the bell undergoes a significant change compliance and therefore in slope. The flap was analyzed in terms of its kinematics and hydrodynamic contribution. An outer trajectory is achieved by the flap margin during contraction while an inner trajectory is achieved during relaxation. The flap kinematics was found to be replicable using a passive flexible structure. Flaps of constant cross section and varying lengths were put on the robotic vehicle to conduct a systematic parametric study. Robojelly's swimming performance was tested with and without a flap. This revealed a thrust increase 1340% with the addition of a flap.  Velocity field measurements were performed using planar Time Resolved Digital Particle Image Velocimetry (TRDPIV) to analyze the change in vortex structures as a function of flap length.  The robot input parameters stayed constant over the different configurations tested thus maintaining a near constant power consumption. Non-dimensional circulation results show a dependence on flap kinematics and geometry. The robot was approximated as a series of pitching panels circularly oriented around its apex. The first circulation peak of the pitching panel approximation revealed a normalized standard deviation of 0.23. A piston apparatus was designed and built to test different flexible margin configurations. This apparatus allow the isolation of the flap parameters and remove the uncertainties coming from the robotic vehicle. / Ph. D.

Underwater surveillance missions both for defense and civilian applications are continually demanding the need for unmanned underwater vehicles or UUVs. Unmanned vehicles are needed to meet the logistical requirements for operation over long distances, greater depths, long duration, and harsh conditions. In order to design UUVs that not only satisfy these needs but are also adaptive and efficient, there has been increasing interest in taking inspiration from nature. These biomimetic/bio-inspired UUVs are expected to provide significant improvement over the conventional propeller based vehicles by taking advantage of flexible bodies and smart actuation. In this thesis, jetting jellyfish were utilized as the inspiration to understand the fundamentals of this new form of propulsion and subsequently translate the understanding onto the engineered platform to validate the hypothesis and construct robust models. Jetting jellyfish species are generally smaller in dimensions than rowing jellyfish, consume lower energy for transport, and exhibit higher proficiency. In the second chapter, a bio-inspired stationary jet propulsion mechanism that utilizes an iris diaphragm actuation system was developed. Detailed discussion is provided on the design methodology and factors playing the leading role in controlling the vortex formation. The propulsion mechanism was intended to mimic the morphological and deformation features of Sarsia sp. jellyfish that measures approximately 1 cm in diameter. The performance of experimental model was analyzed and modeled to elucidate the role of structure and fluid displacement. Utilizing the results from Chapter 2, a free-swimming jellyfish-inspired robot (named JetPRo) was developed (also utilizing an iris diaphragm) in Chapter 3 and characterized for relevant propulsive metrics. A combination of theoretical modeling and experimental analysis was used to optimize the JetPRo's gait for maximum steady-state swimming velocity. Next, an attempt was made towards creating a free-swimming jetting robot (named JP2) using a guided cable mechanism to achieve the desired actuation and improve the propulsion while simplifying the drive mechanism. Using JP2 robotic model, a systematic set of experiments were conducted and the results were used to refine the theory. Based upon the comprehensive computational analysis, an optimized swimming gait was predicted and then validated. A modular robot inspired by siphonophores was developed and initial efforts were made in laying down the foundation for understanding of this complex locomotion mechanism. Siphonophores are colonial organisms consisting of several jetting bodies attached to a central stem. An experimental model was developed mimicking the multimodal swimming propulsion utilized by Siphonophores. Several swimming gaits inspired by the natural animal were replicated and the preliminary performance of the experimental model was quantified. Using these results, an analysis is presented towards further improving the design and assembly of a siphonophore-inspired robot. / Master of Science

Philosophiae Doctor - PhD / Although jellyfish are recognised recently as key components that can influence ecosystem functioning and trophic flows in the northern Benguela upwelling ecosystem, the number of published studies on their abundance, seasonality, life history and ecological roles off Namibia is strictly limited. Chrysaora fulgida is one of the most common and conspicuous medusae in the plankton off Namibia, and has flourished in the region, following the decline of the pilchard fishery at the end of the 1960s. It is said that their biomass (together with Aequorea forskalea) exceed that of the commercially important fish stocks off Namibia. In addition, this species is also capable of forming large swarms in northern Benguela where they are a nuisance to fisheries operations. The objective of this study is to try and fill gaps regarding our knowledge of the biology and ecology of Chrysaora fulgida off Namibia, with a view to improve our understanding of its success in the northern Benguela ecosystem. In the Chapter 1, a general overview on the current knowledge and population dynamics of jellyish blooms and their ecology is compiled. Other key topics of the thesis such as jellyfish life cycles and their reproduction are also introduced. Chapter 2 investigates the temporal changes in the jellyfish community in Walvis Bay over a 23-month period from biweekly plankton samples. All twelve of the recovered taxa were characteristically neritic, and included meroplanktonic Hydrozoa and Scyphozoa, as well as cydippid ctenophores and shallow water siphonophores. Whilst, ephyrae of Chrysaora fulgida were dominant overall, and peaked in abundance during mid-spring (Year 2012: 168 933 ind. 100 m-3) and late winter (Year 2013: 23 389 ind. 100 m-3), they were not present all year round, being replaced (in part) by Obelia in summer and autumn, Bougainvillia in spring and summer, and Muggiaea atlantica in summer. Seasonal changes in the composition and structure of the community were driven primarily by bottom water temperature and day length (explaining 24% of the variability in community structure), with wind speed and moon illumination playing a secondary role. The recruitment of ephyrae of C. fulgida to the plankton off Walvis Bay is confirmed not to be continuous throughout the year. Chapter 3 present the first detailed investigation on the identification, morphological development and growth of wild caught ―ephyrae‖ of the scyphozoan Chrysaora fulgida and Chrysaora africana in Walvis Bay, off Namibia. Concrete morphological dissimilarities are documented to distinguish C. africana from C. fulgida, despite the limited sample size of C. africana: coloration differences and the presence/absence of branched canals on the periphery of velar and rhopalial canal tips. In the case of C. fulgida the morphological development from an ephyra (Stage 0) to a juvenile medusa could be described successfully in six stages, whilst missing stages were noted for C. africana. In general, the development of ephyrae described here agrees with patterns described for other species in the genus from elsewhere. The ephyrae stages of C. fulgida illustrated a low overall growth rate (4.33 and 3.45% d-1, respectively) and longer ontogenic development (~164 days), respectively, than most other jellyfish species. Through the histological examination of medusa gonads, Chapter 4 investigates the sexual reproduction and maturation of both Chrysaora species, collected off Walvis Bay, Namibia. Both species were non-brooding, gonochoristic, displayed a 1:1 sex ratio and exhibited no clear sexual dimorphism features. Gametogenesis in both species was similar to that displayed by other Discomedusae, whilst some differences in gonad maturity were evident between them – Chrysaora fulgida displayed aseasonal, reproductive heterogeneity (maturing at ~300 mm diameter) and individuals were semelparous, whilst C. africana appeared strongly seasonal but iteroparous. Through stable isotope analysis (𝛿13C, 𝛿15N and C:N ratios), Chapter 5 examines the presence of tissue, ontogenetic, seasonal, spatial and interspecific variability in medusae of Chrysaora fulgida and Chrysaora africana off Walvis Bay, in the northern Benguela, Namibia. This study did not only illustrate size-associated shifts in trophic ecology, but also revealed spatial, inter-species and some tissue differences in the northern Benguela upwelling system. Size would appear to be the over-riding factor that influences the isotope signatures of Chrysaora fulgida; size being linked in turn to space. A clear negative relationship is illustratred between 𝛿15N and individual size for two scyphozoans (C. fulgida and C. africana) off central Namibia, indicating that larger jellyfish feed lower down the food chain than smaller ones in both species. This is explained by the need and ability of ephyrae and small medusae to access the microbial food web which consists of many trophic steps and hence numerous opportunities for enrichment of nitrogen isotopes, resulting in higher 𝛿15N values of smaller individuals. Chapter 6 provides a synthesis of the main findings of the thesis, and makes recommendations on ways that the research can be carried forward. / 2020-08-31

Few data are available on distribution, abundance, and ecology of scyphozoans in the Northern California Current (NCC). This thesis is divided into four chapters, each of which contributes to our understanding of a different stage of the scyphozoan life history. The first study describes the settlement preferences of Chrysaora fuscescens planulae in the laboratory. Planulae were found to respond to the interaction of substrate and orientation. Artificial substrates were identified as viable habitat for C. fuscescens. In the second chapter, a population of scyphistomae in Charleston, Oregon were identified to species-level using DNA barcoding techniques. The third and fourth chapters focus on the medusa stage of the life history. Using surveys mailed to fishers in the Pacific Region, this study provides baseline data on the types and magnitudes of economic damages caused by jellyfish on different fisheries and helps assess fishers' perceptions of jellyfish population trends in the NCC. This thesis includes previously unpublished co-authored material.