Colonial lifestyle is widespread in metazoans, arising independently many times within this clade. This repeated occurrence strongly suggests it confers an adaptive advantage and several hypotheses have been invoke as an explanation. In rotifers, common aquatic animals with diverse lifestyles, including both colonial and solitary forms, two main explanations for coloniality have been proposed: (1) colonies provide an antipredator defense and (2) that colonial life offers an energetic advantage to the colony members. To address the energetic advantage hypothesis, we measured respiration in three species: the sessile, colonial Sinantherina socialis, and two solitary, free-swimming species, Euchlanis dilatata and Asplanchna girodi. Rates were determined under three temperatures (20C, 25C, 30C) and at a variety of colony sizes/population densities. Both of these factors may impact respiration rates. Oxygen consumption was measured using a 24-channel oxygen sensor coupled with a 24-well glass microplate. For S. socialis, a single colony was placed in each well (n=68), for solitary rotifers, animals were counted and placed into the wells at a variety of densities (n=40, 102 for Euchlanis and Asplanchna, respectively). Media only controls (total n=109) were included in all experiments. Results were interpreted using multiple linear regression analysis.
Results/Conclusions
Individual respiration scaled with colony size/density for both A. girodi and S. socialis, decreasing by 0.03 pmol/min/animal (p <0.001) and 0.009 pmol/min/colony member (p = 0.012), respectively. E. dilatata respiration was not significantly impacted by population density. Respiration rate increased at higher temperatures with a 0.26 pmol/min/C increase in individual respiration in E. dilatata (p < 0.001), a 0.16 pmol/min/C increase in A. girodi (p= 0.002), and a non-significant increase of 0.03 pmol/min/C in S. socialis (p= 0.37). In past studies, solitary rotifer respiration was independent of density, similar to our results for E. dilatata. A. girodi were observed at higher densities than past studies (75 – 300 ind/mL), which may account for the discrepancy in respiration scaling. A. girodi bumped into each other while swimming, consequently they retracted their coronae and stopped swimming. This behavior may have resulted in the negative relationship between density and respiration found for this species. To further our understanding of the impact of lifestyle on rotifer respiration rates, we will include additional colony-forming and solitary rotifers. Our preliminary findings show that at, least for S. socialis, an energetic advantage may explain, in part, the adaptive origin of coloniality within rotifers.