Have you heard of a floating fortress? Here are some hints: It's a sea creature. It has armor made of shell. It stays afloat. And, it's propelled along by jet propulsion. We call it the Nautilus. It's called a floating fortress because its armor is strong enough to protect against attack. Also, the air inside its chambers keeps it afloat. This amazing species has a brilliant design that has helped it to survive for over 500 million years! About six different species of nautilus survive to this day living in the Western Pacific Ocean.

Not many animals and plants have lived for over 500 million years. Nautilus species were swimming in the ocean before it even had fish! Scientists know this because they've found fossils with the same design as the Nautilus shell in rocks dated millions of years old. Other marine animals are long extinct, but not the Nautilus. Its design has to be very efficient to have lasted so long. Because the Nautilus has lived for such a long time, scientists call it a living fossil.

The Nautilus is a mollusk such as the cuttlefish, the octopus, and the squid. Mollusks usually have a bony structure; though in some mollusks like the octopus, the shell has disappeared completely. The Nautilus is one of the few mollusks to have a well-developed shell. In other mollusks like the cuttlefish the "shell" has become an internal tusk. You may have seen a cuttlefish tusk inside bird cages. It makes good bird food and is used by birds to sharpen their beaks.

The Nautilus has an efficient design with many features to help protect it in the race for survival.

• Like a battleship, its hard shell stops predators from biting. Inside, the soft-bodied creature is safe and sound.
• It has a flexible tube that shoots out jets of water to push the animal along. Water goes in through a "mouth" into a tube. This spout-like tube acts like a siphon, pumping the water out below the tentacles.
• Its nervous system is highly developed. In fact, scientists consider that it has a "brain".
• It is able to adapt to a range of temperatures and pressures. Most of its life it lives at depths of one mile deep (1,000 meters), where the pressure of water is great.
• Inside the chambers of its shell are gases that control its buoyancy—the way it floats and sinks.
• It has between 38 and 90 tentacles to catch and bring back food into its "mouth".
• Its tentacles have grooves and ridges to grip the prey.
• It has a beak-like jaw for chomping. Inside the Nautilus, a structure called the radula shreds the food into smaller pieces.

Nautiluses are usually about 8 inches (20 centimeters) long and they have four gills. Though lacking good vision—because of the absence of lens in their stalk-like eyes—they can see shadows, light, and dark.

Their shell is made of calcium carbonate, which is secreted by the mantle—just like its relative the snail. It coils around and around and is one of the most remarkable mathematical shapes in nature because it is a complex spiral. It is both beautiful and functional. The spiral shell grows from the day it is born with four compartments, or chambers, until it is a fully-grown adult when it has about 30 compartments. The animal lives in the last chamber on the outside.

Some animals became extinct very quickly; others live longer because their design is close to perfect. Nautiluses have many efficient adaptations that have helped them to survive: speed and buoyancy are just two of them. The speed is provided by its efficient jet propulsion. The buoyancy comes from the gas-filled chambers in its spiral shell. In the survival stakes, it's a winner because its special adaptations make it a floating fortress.

Bibliography
Forsyth, Adrian. The Architecture of Animals: The Equinox Guide to Wildlife Structures. Ontario: Camden House Publishers, 1989.

Parish, Steve. Amazing Facts about Australian Marine Life. Queensland: Steve Parish Publishing Pty Ltd, 1997.

Internet sources
http://www.clas.ufl.edu/users/pciesiel/gly3603c/taxon.html
http://www.waquarium.org/MLP/root/pdf/MarineLife/Invertebrates/Molluscs/Nautilus.pdf
http://animaldiversity.ummz.umich.edu/site/accounts/information/Nautilus_pompilius.html

Margaret Etherton is a teacher, tutor, and writer. She has taught a range of subjects such as reading, writing, mathematics, and computers to people of all ages—from small kids to seniors! Her publishing credits include many non-fiction articles for Australian School Magazines and educational resources for teachers. Margaret lives close to the beach in Sydney with her husband, her children, and her cat.

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