Sir David Attenborough takes a look at the twisted tusk of the narwhal and the spiral shells found on snails, nautiluses and ammonites.
The shell of a nautilus
A Spiralled Shell
The diversity and shape of invertebrate shells is breath-taking and they have adapted the spiral into a multitude of variations.
Snail shells are either left or right-handed, meaning they turn to the left or to the right. Left turning snails can only mate with other left turning snails and the same for right turning snails.
The nautilus is a ocean-living cephalopod, a relative of squid and octopuses, and has adapted their spiral shell in a way very different, yet superficially similar, to that of the snail.
Sir David Attenborough and a narwhal's tusk
Narwhal
The narwhal is a mysterious creature from the cold waters of the Arctic.
This species of whale is rarely seen but has inspired the myths and tales of unicorns.
Even today the reason why it only has one tusk and why it is spiralled is still something of a conundrum, and continues to puzzle scientists. However, recent techniques has allowed the scanning of their tusks revealing more clues to the answer.