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Title: Population variability in Aurelia aurita
Author: Hoehn, Danja Persephone
ISNI:       0000 0004 6347 7674
Awarding Body: University of Southampton
Current Institution: University of Southampton
Date of Award: 2017
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Jellyfish blooms are increasing in some parts of the world, influencing marine ecosystem function and services for humans. Concerted research efforts have focused on understanding the causes and mechanisms of bloom events, particularly in response to environmental change. Most bloom forming species are characterised by a metagenic life cycle involving sexually reproducing medusae and asexually reproducing polyps. In comparison with the pelagic medusa little is known about polyp biology and physiology. However, the polyp is thought to be a key factor in the formation of jellyfish blooms, through the success of medusa recruitment via asexual reproduction. The study species Aurelia aurita is widely distributed in northern Europe and often forms bloom populations. The adaptive ability to environmental variability and change of the medusa and polyp life stage is the topic of this study. Specifically, this PhD thesis aims to 1) understand population dynamics of the common jellyfish A. aurita medusa and polyp populations, 2) gain information about the physiology and reproduction of A. aurita polyps and 3) investigate polyp diet and metabolism under natural conditions. This study involved sampling and monitoring of natural medusa and polyp populations in southern England, and the design of ecological experiments on laboratory maintained polyps. I demonstrate that A. aurita follows density driven population dynamics in the Beaulieu River. Short longevity, large medusa sizes of small abundance and high mesozooplankton levels alternated with, prolonged life span, small medusae sizes of high abundance and low mesozooplankton levels. Three closely located populations of A. aurita in southern England varied in abundance, longevity and medusa size, thus demonstrating sitespecific adaptations. Experimental studies on the effect of temperature on polyp respiration rates showed increased sensitivity to temperatures above 14°C. The respiratory response of three populations to a range of temperatures revealed evidence of acclimation to their natural thermal tolerance window. Furthermore, the asexual reproduction in response to temperature of three polyp populations differed with the lowest bud production and survival in Norwegian polyps and the greatest bud production in the southern England populations at 12 and 16°C, demonstrating polyps sensitivity to higher temperatures and adaptation to their natural environment. Furthermore, a photographic survey of in situ populations showed that strobilation is prolonged in Horsea Lake (Dec-Apr) with peak strobilation in January (50%). A lower number of discs were produced in nature compared to laboratory maintained polyps (4 instead of 7). Finally, I show that natural polyps are not primarily supported by zooplankton prey and switch their diet from a terrestrial and sediment derived winter diet to sediment and plankton dominated summer diet. The implications of this study were to examine adaptation in A. aurita populations in response to environmental parameters. Temperature was especially a critical factor on the respiratory and the reproductive rates of A. aurita polyps indicating a degree of sensitivity to warmer temperatures. Furthermore, reproductive rates measured on Artemia-fed polyps possibly overestimate medusa recruitment as polyps of the Beaulieu River were not supported by zooplankton but by (lower energy) terrestrial and sediment derived food sources. In conclusion, this thesis highlights the importance of temperature and food availability on the physiology and reproductive ability of A. aurita polyps, especially in the context of climate change scenarios.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available