Squids, octopuses, and their relatives
Although squids and octopuses can swim and breathe underwater like fish and in some cases even have fins, they are part of the mollusk family, making them more closely related to snails than to fish. Mussels also are part of the mollusk family.
Squids and octopuses are also called "cephalopods", meaning that their arms are directly attached to their heads. The organs are contained in a sac behind the head. If humans wanted to adopt this design, we would have to change the place of the head and the abdomen.
Cephalopods have a long history. Scientists assume that they have lived on earth for 500 million years. In comparison, the first dinosaurs appeared some 235 million years ago. While dinosaurs are now extinct, the cephalopods thrive in seawater almost everywhere in the world.
They are astonishingly intelligent, fast, and have excellent hunting skills. They can survive in great depths and a diving robot has filmed a squid at a depth of 7,000 meters. Almost all species have three hearts, one in the center of the body and one at each of the two gills. The gills allow them to breathe in water like fish. Squids have eight arms and two tentacles. These are arms that have evolved into especially long gripping tools. Octopuses have no tentacles.
To date, some 900 different species have been recorded, but more are discovered all the time. Some are tiny and would fit in the hand of a child, while others, such as the giant squid or the colossal squid, are enormous. The giant and colossal squid have the largest eyes known in the animal world. Their eyes are as large as volleyballs to make sure the animals can see in the dark.
There are female and male cephalopods. Reproduction is difficult for these animals. The eggs are usually fertilized in the abdomen of the female. These eggs are then hidden in caves, on the bottom of the sea, or simply "glued" somewhere. This task is so energy-intensive for many squids that they die shortly afterwards. The smaller species are about one or two years old at that point, and giant squids probably four or five years. Marine biologists are not exactly sure about life expectancy and still have to do further research.
Of course, the ink is also very special. Most cephalopods carry ink in an abdominal pouch. When they are attacked, they spray the dye into the water, using a funnel in their own body. They can also use this funnel to create a strong water jet and push themselves off like a rocket. While the attacker is still swimming through the ink, the squids are long gone. However, that does not work all the time. Squids and octopuses are hunted by larger predatory fish or whales. Giant squids are the favorite food of sperm whales, who eat them almost exclusively.
Many octopuses have another trick up their sleeves–they can make themselves invisible by assuming the color of their surroundings of stone or sand. For that purpose, their skin has many colorful spots. When the squid sits on a grey-green surface, it can open the skin over the grey and green spots. There are even species that collect fluorescent bacteria in their belly to eliminate their own shadow when they swim. The world of the deep sea is full of wonders.