Why do no other aquatic animals grow to a huge size like whales do?
Richard Swifte
Darmstadt, Germany
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Although the largest fish is the whale shark, which can typically weigh 20 tonnes, closely followed by the basking shark, this is far exceeded by various whale species. The largest is the blue whale, which averages 150 tonnes.
The buoyancy of water enables ocean animals to grow to greater sizes than land animals, which need legs sturdy enough to support them. But several dinosaur species are estimated toÌýhave reached 70 or 80 tonnes, soÌýit seems surprising that no modern sea creatures other than whales have come anywhere near what should be possible.
The largest whales and fish aren’t active predators, which would require speed and agility, but feed by passively taking in water and filtering out plankton orÌýother small organisms.
Various possible reasons have been suggested for why fish don’t reach the size of whales, which are mammals. Whales have lungs and breathe in air with its 21 per cent oxygen content, while fish use gills to absorb oxygen from water at much lower concentrations. Less oxygen intake could limit theÌýmaximum size achievable.
Another idea is that mammal hearts have a double circulation whereby blood goes to the lungs atÌýlow pressure to obtain oxygen, and is then pumped separately round the body at higher pressure. Fish hearts are simpler and drive a single, lower-pressure heart-gills-body-heart circulation that is slower at providing nutrients toÌýthe tissues. In truth, though, the exact reasons for smaller fish sizes are still uncertain.
Stephen Johnson
Eugene, Oregon, US
While it is true that the biggest oceanic creatures are the large baleen whales, there are other creatures in the ocean whose size overlaps with many whale species.
The order Cetacea, to which allÌýwhales belong, has around 89Ìýspecies and ranges from the world’s largest animal, the blue whale, with a length of up to 30Ìýmetres and a weight of 200Ìýtonnes, down to river dolphins under 2 metres. Many fish overlap the size range of whales and dolphins, in particular the filter-feeding sharks.
These include the whale shark, which can reach 13Ìýmetres and 22Ìýtonnes. This fish is bigger than all but a few of the whale species. In addition, manta rays, great white sharks and sunfish can all reach 3 metres and weigh around 3Ìýtonnes, again more than most cetaceans. Finally, let’s not forget the elephant seal, which can reach 5 metres and weigh over 3 tonnes.
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Rebecca Helm
Georgetown University, Washington DC, US
Why stop at the size of a whale? – entities formed of a colony of tiny clone animals called zooids – and jellyfish can beÌýlonger than whales. And , though we often don’t think of any of these life forms when we think of big things in the ocean.
[Ed. – shows that the world’s largest whales (rorqual whales, including blue and minke whales, that carry out a lunging movement to engulf huge amounts of water) need to be very big to consume enough food to survive.]
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