If an animal doubles in size, why doesn't it need twice as much food? This question has been bugging animal physiologists for years, particularly regarding aquatic invertebrates such as sea anemones and corals. In a recent paper in the weekly journal Science, UC Davis marine ecologist Mark R. Patterson explained why the relationship between body size and metabolic rate is considerably different for algae and sea urchins than for hummingbirds and elephants. Patterson's paper may reach beyond commercial applications to shake up one of biology's accepted principles known as Kleiber's Rule. Simply put, the rule says that the metabolic rate doesn't increase in direct proportion to body size and that this scaling relationship is amazingly constant for warm-blooded animals. Researchers have been puzzled about why algae and aquatic invertebrates such as corals, sea anemones and sea urchins varied from the rule. For these organisms, the metabolic rate appears to be regulated by the very thin layer of water directly surrounding the organism known as the boundary layer. The oxygen or carbon dioxide necessary for aquatic organisms' metabolic processes must pass through this boundary layer. Borrowing theory from chemical engineering, Patterson discovered that the metabolic rate increases as water turbulence increases and the boundary layer thins. The shape of the organism also affects the water flow and boundary layer thickness and thus the metabolism. Patterson refers to the phenomenon as "flow-modulated metabolism." Practical applications range from calculating the photosynthetic requirements for algae to better understanding how water flow affects the population biology of a wide range of organisms including sea urchins and kelp. He intends to use the information in his own studies on the health of coral reefs.