Abstract: Genome size is often tied to respiration rate and metabolism, with lower genome sizes commonly occurring in organisms with high metabolism. This relationship is called the metabolic theory of genome size. Insects and large crustaceans have patterns of genome size metabolism dependent on habitat, with coldwater marine crustaceans following Bergmann’s rule, and terrestrial insects possessing patterns in opposition to Bergmann’s rule. Inland aquatic systems represent an intermediate habitat between terrestrial and marine. Organisms living in these systems may enlighten our understanding of how metabolism and genome size relate to one another. Rotifers are common and diverse members of planktonic and littoral communities of inland waters, making them an ideal group in which to test the metabolic theory of genome size in this system. To investigate this theory in rotifers we estimated size-specific respiration rates for eight species using a Loligo microplate and microplate reader equipped with oxygen sensors and compared these with genome sizes obtained from the literature. Rotifer sizes were estimated from maximum lorica length or maximum body length for illoricate species. Genome sizes ranged from 0.046 to 0.25 pg. Respiration rates ranged from 0.76 to 5.42 pmol/min, within the range known for rotifers. We found size-specific respiration rate to be significantly positively correlated with genome size (marginal r-squared: 0.53, P-value:0.03, coefficient: 4.3). This contrasts with the expectation that animals with higher metabolism possess smaller genome sizes. This may mean that factors other than metabolism are important for limiting genome size in this group. One species measured, a novel Hexarthra, had among the smallest genome sizes measured. This rotifer has the fastest observed life cycle in the phylum. For rotifers perhaps, life cycle speed overrides the importance of metabolism in limiting genome size.
