In the case of most groups of marine animals, then, it is unlikely that significantly larger members will evolve any time soon. These include the risk of tentacle tangling in jellyfish, metabolic constraints on giant clams, physiological limitations of pumping water over gills in large bony fish, or the reliance of blue whales on dense concentrations of their crustacean prey. In their fascinating study Sizing Ocean Giants published in the journal PeerJ earlier this year, marine biologist Craig McClain and colleagues document the factors limiting size in many of the most conspicuous large marine species. In fact, many ocean giants are already more or less as big as they could be, given physical and physiological limits. There is rather less of a drive towards larger sizes within any existing group. Second, Heim and colleagues show that most of the overall increase in body size across all marine animals is explained by the evolution of major new groups, with all of the anatomical and physiological innovation that implies. It takes a lot of small fish to meet the energetic demands of a big fish, and so the only way these food webs can work is if small organisms substantially outnumber their larger predators. In the seas this is especially pronounced, because marine food webs are typically highly size structured – that is, big things eat small things. First, the minimum size has not changed, and – moving for a moment from evolution to ecology – it is well known that most species are small. Instead, they show that only persistent directional selection for larger body sizes – due to the many advantages to being large – can explain the observed trends.ĭoes this mean that sea creatures are all inexorably getting bigger, and will continue to do so until the oceans are full of behemoths? Not really. Neither of these models fitted the observed data well. So the re-invasion of the seas by the ancestors of today's marine mammals imposed a new hard boundary on the minimum size within this group, which in turn affects the average size across groups. Likewise, warm-blooded marine animals like whales can only stave off hypothermia if they are more than about a metre long. And although evolutionary biologists are always wary of narratives of "progress", many innovations in evolution require a large body size – for example, the smallest vertebrates are inevitably larger than the smallest invertebrates, because it takes a certain size of organism to pack in all the stuff that vertebrates have. To some extent this may seem inevitable: if life starts small, the only way to go is bigger. The results are clear: both the average and maximum sizes of marine organisms have increased substantially over this period, whereas the minimum size has remained reasonably constant. The team, led by Noel Heim from Stanford University, delved into the fossil record to compile information on the body sizes of more than 17,000 kinds of marine animals that have existed since the start of the Cambrian period, 542 million years ago. Cope's rule has been documented or disputed in hundreds of studies of numerous animal lineages over the last century, but a new study in the journal Science provides perhaps the most comprehensive test yet of its existence. This trend towards larger body sizes through evolutionary time has become known as Cope's Rule, after the American palaeontologist Edward Drinker Cope. Indeed, the largest animal ever to live, the blue whale, is still very much with us, and has been swimming the world's oceans for only a couple of million years – a mere blink of the eye in the long, long history of life in the sea. Since then, things have accelerated a bit and – along with the great diversification of body forms – animals have tended to get bigger.
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