Deadly snails & freaky fish up a cucumbers bum - Great Barrier Reef starts New Years Day BBC TWO

For 10,000 years, more than 400 different types of coral have built thousands of individual reefs in the coral sea off the north-eastern coast of Australia. Covering over 44,400 square kilometres (133,000 sq mi) they, and more than 1000 islands, have come to define the biodiversity and character of this part of the world. Together, they form the Great Barrier Reef - the largest living structure on the planet and the only living thing visible from space.

Uncovering the secrets of this 2000km long super-reef, marine biologist Monty Hall travels from the wild outer reefs of the coral sea to the tangle of mangrove and rainforest on the shoreline, and from the large mountainous islands to tiny coral cays barely above sea level. Along the way he experiences the reef at its most dangerous and its most intriguing, and visits areas that have rarely been filmed, from the greatest wildlife shipwreck on earth to the mysterious seafloor of the lagoon, where freakish animals lurk under every rock.

The freakiest behaviour of the series can be seen in episode 2, when we see the bizarre 'pearl fish' moving in to take residence up a sea cucumber's bottom!

They usually have a commensal relationship, not harming their hosts. However, some species are parasitic and not only squirm inside a sea cucumbers anus but as an added blow to the cucumbers self-esteem they also eat their gonads - some lodger!

I saw a preview of this sequence recently, it sent shivers down my spine...

The first film, shown at 8pm on New Years day, sees Monty explore the complex structure of the coral reef itself and the wildlife that lives on it. So vast it is visible from space, the reef is actually built by tiny animals in partnership with microscopic plants. It is a place full of surprises, always changing, responding to the rhythms of weather, tide, sun and moon.

Within this magical and intensely crowded world this episode reveals how the amazing reef creatures compete and co-operate - from deadly fish-hunting snails to sharks that can walk on land, fighting corals and parrot fish that spin sleeping bags every night.

My favourite, and most surprising behaviour, is that of the deadly snails. They might not seem like deadly predators, but cone snails are equipped with a battery of toxic harpoons which can fire in any direction, even backwards. They await the cover of darkness to prey on sleeping fish. First, they waft  paralysing chemicals towards the unsuspecting prey, next they start to suck the subdued fish into their expanding mouths, and finally they use a venomous barb to deliver the killer blow... Don't mess with snails!

(Read more about this incredible hunting technique on BBC Nature)