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An Empire Lacking Food

Once viewed as a barren expanse, the deep seafloor is a biologically elaborate ecosystem whose fate is tied to life above, near the sea surface

Craig McClain

Insights into the Past

2010-11McClainF9.jpgClick to Enlarge ImageStudying carbon or energy limitation at its extreme, like that found in the deep ocean, can also provide broad insights into how carbon flows through biological systems in the present and in the deep past. In the aforementioned island rule example, the examination of body-size evolution among deep-sea invertebrates shed light on what drives evolutionary novelty on islands. How the modern deep sea responds to gradients of food across the seafloor also offers evidence regarding how marine fauna have developed over the entire history of life on Earth.

Some 200 to 100 million years ago, the Mesozoic marine revolution (MMR) was one of the most spectacular reorganizations of life on Earth. In 1995 Richard Bambach of Virginia Polytechnic Institute and State University suggested in another cleverly titled paper, “Seafood Through Time,” that food requirements of marine animals during this period increased based on evidence of increased predation, motility, burrowing, antipredator morphologies, fleshiness, bioencrustation, bioerosion and body size. So increased productivity of the oceans provided the energy needed to drive innovation during the MMR.

A recent paper by Seth Finnegan of Stanford University and California Institute of Technology and his collaborators, including myself, tried to explain the MMR from another angle. We explored what would have been the likely energy or food demands through time by one of the most dominant groups in the fossil record: snails. We approached this by combining information from the marine fossil record, modern oceans both shallow and deep, physiological data from living species and mathematical equations to estimate energy consumption—and thus the amount of energy—needed to sustain life. Calculated with equations derived a century ago, the temperature and body size of an individual animal can tell much about its basic energy requirements, or metabolism. Larger and warmer organisms utilize more energy. The metabolic requirement for entire assemblages of organisms is simply the individual need multiplied by the number of individuals.

To obtain the demands for communities of snails in the distant past, we extrapolated from our knowledge of modern snails. Comparing snails before and after the MMR, we found that per capita metabolic rates rose by about 150 percent, a change largely attributed to increases in body size. If we account for the increased presence of carnivores, metabolically more expensive organisms, the percentage of increase in the metabolic rate after the MMR would be substantially higher. By demonstrating that a similar shift in metabolic demand occurs from the food-limited deep into productive coastal waters, we provided evidence that ocean productivity 200 to 100 million years ago would have needed to increase to support this change.

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