Functional differences in the allometry of the water, carbon and nitrogen content of gelatinous organisms

Abstract

We have supplemented available, concurrent measurements of fresh weight (W, g) and body carbon (C, g) (46 individuals, 14 species) and of body nitrogen (N, g) and C (11individuals, 9 species) of marine gelatinous animals with data obtained during the global ocean expedition MALASPINA 2010 (totalling 267 individuals and 33 species for the W vs. C data; totalling 232 individuals and 31 species for the N vs. C data). We then used those data to test the allometric properties of the W vs. C and N vs. C relationships. Overall, gelatinous organisms contain 1.13±1.57% of C (by weight, mean±SD) in their bodies and show a C:N of 4.56±2.46, respectively, although estimations can be improved by using separate conversion coefficients for the carnivores and the filter feeders. Reduced Major Axis Regression (RMA) indicates that W grows isometrically with C in the carnivores (cnidarians and ctenophores), implying that their water content can be described by a single conversion coefficient of 173.78 gW[gC]-1, or a C content of 1.17±1.90% by weight, although there is much variability due to the existence of carbon-dense species. In contrast, W grows faster than C in the filter feeders (salps and doliolids), according to the power relationship W=446.68)C1.54. This exponent is not significantly different from 1.2, which is consistent with the idea that the watery bodies of gelatinous animals represent an evolutionary response towards increasing food capture surfaces, i.e. a bottom up rather than a top down mechanism. Thus, the available evidence negates a bottom up mechanism in the carnivores, but supports it in the filter feeders. Last, N grows isometrically with C in both carnivores and herbivores, with C:N ratios of 3.89±1.34 and 4.38±1.21, respectively. These values are similar to those of compact, non-gelatinous organisms and reflect a predominantly herbivorous diet in the filter feeders, what is confirmed by a difference of one trophic level between filter feeders and carnivores, according to stable N isotope enrichment data.