Abstract
Jellyfish form spectacular blooms throughout the world’s oceans. Jellyfish body plans are characterised by high water and low carbon contents which enables them to grow much larger than non-gelatinous animals of equivalent carbon content and to deviate from non-gelatinous pelagic animals when incorporated into allometric relationships. Jellyfish have, however, been argued to conform to allometric relationships when carbon content is used as the metric for comparison. Here we test the hypothesis that differences in allometric relationships for several key functional parameters remain for jellyfish even after their body sizes are scaled to their carbon content. Data on carbon and nitrogen contents, rates of respiration, excretion, growth, longevity and swimming velocity of jellyfish and other pelagic animals were assembled. Allometric relationships between each variable and the equivalent spherical diameters of jellyfish and other pelagic animals were compared before and after sizes of jellyfish were standardised for their carbon content. Before standardisation, the slopes of the allometric relationships for respiration, excretion and growth were the same for jellyfish and other pelagic taxa but the intercepts differed. After standardisation, slopes and intercepts for respiration were similar but excretion rates of jellyfish were 10× slower, and growth rates 2× faster than those of other pelagic animals. Longevity of jellyfish was independent of size. The slope of the allometric relationship of swimming velocity of jellyfish differed from that of other pelagic animals but because they are larger jellyfish operate at Reynolds numbers approximately 10× greater than those of other pelagic animals of comparable carbon content. We conclude that low carbon and high water contents alone do not explain the differences in the intercepts or slopes of the allometric relationships of jellyfish and other pelagic animals and that the evolutionary longevity of jellyfish and their propensity to form blooms is facilitated by their unique body plans.
Highlights
Jellyfish have been forming blooms in the world’s oceans for 500 million years and they frequently represent a large proportion of the pelagic consumer biomass
The high water content of jellyfish has caused them to differ from other metazoans when they are incorporated into general allometric relationships based on wet or dry mass and the degree of difference varies with the unit chosen
Respiration rates of jellyfish were 28 times slower than those of other pelagic animals of comparable equivalent spherical diameter (ESD); when jellyfish ESD was adjusted for carbon content, the differences in respiration rates were negligible (Table 1, Figure 1C)
Summary
Jellyfish (cnidarian medusae and ctenophores) have been forming blooms in the world’s oceans for 500 million years and they frequently represent a large proportion of the pelagic consumer biomass. The high water content of jellyfish has caused them to differ from other metazoans when they are incorporated into general allometric relationships based on wet or dry mass and the degree of difference varies with the unit chosen. Differences in the size-dependence of some functional responses of jellyfish relative to those of other metazoans, disappear once their carbon mass, rather than wet or dry mass is used to characterize their body size [2,5]. Due to the very dilute carbon present in gelatinous organisms, scaling to carbon will under-represent the physical size of jellyfish relative to other animals and, may hinder consideration of many of the physiological and ecological functions and benefits associated with large body size which do not directly depend on carbon
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