New article by Thomas Kiørboe and Andrew Hirst in American Naturalist: Marine organisms, from microscopic flagellates to large fish, are strikingly similar in their feeding performance and metabolic rates.
Big organisms grow slower than small ones: a unicellular flagellate may divide several times in one day, while a fish spends weeks or even years to double its mass. It has been assumed that other aspects of the ‘pace of life’, such as metabolism and feeding performance, all decline with size in a similar way. This would suggest that for two organisms, one 10 times heavier than the other, the bigger one will have a metabolic rate, a growth rate, and a feeding performance that, relative to its size, is only about half of that of the smaller organism. It has further been argued that this ‘rule’ applies universally.
To test this idea, Kiørboe and Hirst compiled thousands of measurements from the literature on metabolism, feeding performance, and growth rates of marine organisms ranging in size from microbes to large fish. They show that metabolism and feeding performance per unit of living tissue are strikingly constant. Within groups of related organisms, however, the rule of declining rate with mass applies, but there are concurrent upward shifts with transitions between taxa of increasing size. Relative consumption and the growth rates, in contrast, decline over the entire size range of organisms examined.
The authors suggest that the transitions are associated with changes in body plan and feeding efficiency that are necessary to compensate for the decline in food availability in the ocean as organisms get larger. More efficient ways of gathering food implies higher costs and, hence, higher metabolism. But it implies that, independent of size, they are all able to search daily for food through a volume of water corresponding to 1 million times their own body volume. This is the rate required to make a living in the nutritionally dilute ocean.
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