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.

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