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A Question of Balance :: 7:30pm 31st Oct 2017

Deep Space: John Turnbull, President of the Underwater Research Group of NSW (URG), looks at more of the marvels of our underwater world.
It is a misconception that temperate reefs are any less spectacular than tropical ones, especially if you carry a light. The first 10-15m has big kelp underwater forests, then the next 15-20m has colourful sponge habitat. In Sydney Harbour there is less to see past 40m, apart from wrecks. One steam-driven wreck between North and South Head has its old boiler and a big flywheel. 

At deep levels fish have very big eyes which let them capture more light. They are often red-coloured which effectively makes them invisible as red light does not penetrate as deep down as other colours. Some fish at those depths make their own light using bioluminescence.

The most powerful and disruptive weather is a deep swell. The longer a wave is the deeper it goes underwater. A 2m high wave of short peak length is not noticed 5-6m below the surface whereas 2m swell with a long period between peaks (like 12-15 seconds) creates a lot of surge on the bottom like a washing machine.
There is quite a lot of sound underwater. Tropical reefs have a background hum, with clicks, scratches, snaps, pops and bubbles. Temperate waters get deeper and booming sounds such as ships and outboard motors and memorably humpback whales singing.

According to John most sharks are not dangerous and he has never felt threatened by one. He also enjoys cuttlefish which, with the related octopus, are among the most intelligent invertebrates on the planet. Cuttlefish have an amazing ability to change colour and texture. The giant cuttlefish, endemic to Australia, are very effective stealth hunters. A giant cuttlefish has two tentacles that shoot out, like harpoons, to catch its prey. It will investigate divers, and might even put on a display of coloured stripes running down its body, stripes that change from white to yellow to red to brown in a flash.
Such a rippling flow of neon flashing display is one of the many astonishing features of deep space, except this deep space is all under water.

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