Scaling and Complexity in Simple Multicellular Animals

Abstract

The earliest-diverged multicellular animals are decentralized organisms capable of growing to indeterminate sizes and highly variable morphologies. These organisms must coordinate activity among their constitutive cells at the scale of the organism in order to to leverage the benefits of multicellularity, and must do so using decentralized mechanisms that are robust to uncertainty in size and shape. This thesis investigates how coordination within the Placozoa - arguably the simplest animals - scales with organism size, quantifies the extent to which different developmental processes affect size regulation, and creates a framework for measuring morphological variability in what had been considered amorphous animals. In Chapter \ref{Chapter_Criticality} I develop a method by which one can measure coordination and information propagation within an animal's body plan, and investigate how this propagation is affected by changes in size. I argue that such animals are poised at criticality, with evidence presented to suggest that this facilitates optimal information transmission, but that the physical constraints of multicellularity create a size-coordination trade-off in such decentralized organisms. The presence of size-induced trade-offs brings forth the question of how size is regulated, which in Placozoa occurs through growth and asexual fission. In Chapter \ref{Chapter:Size} I investigate whether size is regulated in response to changing environmental nutrient conditions and find that animals adjust their sizes to match their environments. I further find that this change comes about primarily due to changing dynamics of growth rather than fission, and identify that growth is highly dependent on nutrient conditions, but find evidence that asexual fission could be an emergent phenomenon of poor coordination beyond certain sizes. Finally, in Chapter \ref{Chapter_Morphology} I investigate the morphological variability in Placozoa and find evidence for allometric growth in such animals. In addition, Chapter \ref{Chapter_Morphology} sets the groundwork for future comparative morphological studies between individuals and for behavioral stereotyping by developing a size and rotation invariant shape representation, which I use to identify the presence of idiosyncratic morphologies. I close the thesis with some remarks regarding future directions in exploring the effects of scaling on coordination, morphology, and behavior in this small yet evolutionarily significant Metazoa phylum.