Marine life typically thrives in the tropics – so why do whales prefer the poles?

February 12, 2019 by Sarah Marley, The Conversation

Life in the sea isn't easy. Talk to most people about the ocean and they are likely to imagine a tropical scene with a stretch of golden sand and warm, clear water. The reality is often quite different – the marine environment can be a surprisingly cold place.

Water conducts heat far more effectively than air, which means that submerged animals quickly lose their body heat. It's also harder to warm up again than on dry land, where animals often have the option of basking in the sun or on hot rocks. Finally, many aquatic animals use gills to get oxygen – great for breathing, but essentially another source of heat loss due to all the of water flowing across them and sucking away warmth.

All this contributes to making it much harder for aquatic animals to regulate their body temperature. So surely it would make sense to find more marine animals in warmer waters rather than in colder ones?

Not necessarily. A new study published in Science by a team of US researchers led by John Grady reports higher levels of marine biodiversity in polar waters than tropical ones – but only for some types of animals.

This goes against the longstanding idea that species richness is always highest in the tropics. The theory was that tropical waters provide a less thermally-extreme environment than polar ones – compare, for example, the Caribbean with Antarctica. Tropical areas often provide greater stability and productivity, giving benefits such as a predictable environment and plenty of food. As a result, scientists typically report high levels of biodiversity in warmer waters, with a multitude of species making the most of these favourable areas.

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