Role of colony integration in metabolic scaling of colonial animals

METABOLIC SCALING IN COLONIAL ANIMALS: THE ROLE OF METABOLIC ACTIVATION AND COLONY INTEGRATION Maram Almegbel, MS Department of Biological Sciences Northern Illinois University, 2018 Neil W. Blackstone, Director Metabolism, one of the canonical features of life, shows a clear correlation to organismal size. Metabolic scaling is explaining the relationship between organism body mass and metabolic rate. To explain the relationship between the metabolism and body size, several factors have to be examined, including resource uptake, usage, and transportation of resources throughout the body. Empirically, metabolism usually scales as approximately mass to the 3/4 power. Many theories seek to explain this relationship in terms of surface area to volume. These theories typically model unitary organisms as spheres, with surface area to volume increasing as roughly volume to the 2/3 power. In this context, the study of metabolic scaling in colonial or modular animals may provide insight. These organisms differ significantly from unitary organism, e.g., in geometry and hence surface -tovolume relationships. Modular organisms can be modeled as “sheet,” with surface area increasing area roughly in proportion to volume. Nevertheless, the biology of colonial organisms remains relatively poorly known. For instance, it is widely recognized that metabolic state affects metabolic scaling in unitary organisms, but the effects of metabolic state on scaling in modular organisms is untested. In this context, the intraspecific metabolic scaling relationships were examined in two colonial cnidarians, Hydractinia symbiolongicarpus and Sympodium species. The former is heterotrophic and circulates gastrovascular fluid using muscular contractions, while the latter has photosynthetic symbionts and circulates gastrovascular fluid using cilia. Nevertheless, both are “sheet-like,” encrusting forms. Experimental colonies of each species were grown on 12 mm diameter cover glass, and metabolism was measured by oxygen uptake in the dark (Hydractinia 20.5 C, Sympodium 27 C). Several size variables were measured using a protein assay and image analysis (total protein, total area, feeding polyp area, number of feeding polyps, and number of reproductive polyps). Principal components analysis of the log-transformed variance covariance matrix in Hydractina suggests that the best size measure is based on total protein, feeding polyp area, and number of feeding polyps. Using oxygen uptake regressed against this size measure, the slope was not significantly different from 2/3 or 3/4 but was significantly less than 1. Using the individual variables as size measures yielded various exponents, yet in every case there was no difference between fed (active) and unfed (resting) colonies. The differences in the intercept suggests that feeding activates metabolism by roughly 2.5 times, and a paired comparison t test shows that this difference is highly significant. In corals containing photosynthetic symbionts, light has been shown to activate metabolism in the same manner as feeding activates a heterotroph. Nevertheless, with Sympodium, colonies that were dark adapted exhibited similar oxygen uptake as colonies that were light treated (140 μm) for 90 min. With regard to metabolic scaling in this species, several size variables were measured using a protein assay and image analysis (total protein, total area, polyp area, number of feeding polyps and weight). Principal components analysis of the logtransformed variance covariance matrix in Sympodium suggests that the best size measure is based on total weight, followed by total polyps area and total area. Using oxygen uptake regressed against this size measure, the slope was not significantly different from < 3/4 but was significantly less than 1. As colonial animals can provide more understanding of metabolic scaling, it will possibly need more investigation for these colonial animals and the factors that might be affecting metabolic scaling. NORTHERN ILLINOIS UNIVERSITY DEKALB, ILLINOIS