Sand Flats

When the tide goes out on some beaches it exposes miles of sand flats, often gently undulating, across which the rising waters come in very quickly.

This is the beach at Saadani in Tanzania. The huge quantities of sand are brought down by the Wami River which enters the sea nearby. Unlike many East African beaches, where the sand is formed from the remains of the calcitic hard parts of millions of sea creatures....

....these beaches are largely formed of mineral grains, mainly quartz.

The river also brings down nutrients, so sand flats are home to a wide variety of marine life such as worms, echinoderms and molluscs. Examples include this turritella, a snail which, given the chance, will burrow under the sand - and....

....these strange, flattened sea urchins which are sand dollars, very similar in lifestyle to the British sea potato except that they operate just under the sand surface. 

This rich habitat is exploited by the local people, who appear as soon as the tide turns and follow it out as it falls. Working in pairs, their main catch is....

....king prawns, the largest prawns I have ever seen.