The size of an individual organism is a key trait to characterize its physiology and feeding ecology, often through power-law relationships. However, such size-based scaling laws may have a limited size range of validity or undergo a transition from one scaling exponent to another at some characteristic size. I review size-based scaling laws for resource acquisition, mobility, sensory range, and progeny size for all pelagic life in the ocean, from bacteria to whales. Further, I develop simple theoretical arguments for observed scaling laws and the characteristic sizes of a change or breakdown of power laws. The characteristic sizes of a change are used to divide life in the ocean into seven major realms based on trophic strategy, physiology, and life history strategy. Such a categorization represents a move away from a taxonomically oriented description toward a trait-based description of life in the oceans. Finally, I will discuss life forms that transgress the simple size-based rules and identify unanswered questions.
I have a background in theoretical physics. Currently I hold a professorship in theoretical marine ecology. My research interests are to understand how life in the ocean is organised, why marine organisms look and act the way they do, and how marine ecosystems react to perturbations like fishing, species removals/invasions or climate change.