Cuttlefish are marine mollusks in the order Sepiida, with around 100 species known to exist on Earth. These mollusks are closely related to their fellow Cephalopods like octopi and squid, but they have a few unusual traits which set them aside from their relatives and other marine animals. Humans are interested in cuttlefish for a number of reasons, ranging from a potential food source to a rich field of research, although many laypeople are not familiar with these unusual and actually rather fascinating mollusks.
Although the common name includes the word “fish,” cuttlefish are not fish at all. They more closely resemble their relatives the squid, with 10 grasping arms and a unique calcium-rich inner shell known as the cuttlebone. The cuttlebone can be filled with varying levels of gas to adjust the buoyancy of the cuttlefish, and these animals move with the use of jet propulsion, using their complex fins to control their path through the water. Although cuttlefish are not quite as rapid as squid, they are very fast movers.
Speed isn't the only weapon of the cuttlefish. These creatures are capable of ejecting ink in self defense, like their relatives, and they can also change the color and texture of their skins for camouflage. In addition to changing color to blend in, cuttlefish also change their color to send signals to other cuttlefish, and some species are quite flamboyant, generating intense displays of red and orange.
Cuttlefish have three hearts to pump their copper-rich blue to green blood. Each gill has a dedicated heart, with the third heart handling general circulation in the body. These animals require three hearts because their blood is not very efficient at oxygen transport, so they need to work harder to get oxygen through their bodies. The eyes of a cuttlefish are also unusual, with very distinctive W-shaped pupils, rather than the classic rounded or vertical pupil most people would expect to see.
Cuttlefish ink, known as sepia, has been used for centuries by humans for writing, and cuttlefish are used as a food source in some regions of the world. The cuttlebone is also useful as a dietary supplement for pet birds, many of whom need additional calcium in order to thrive. Biologists are very intrigued by the cuttlefish, because these mollusks are extremely intelligent and very social. By studying cuttlefish intelligence and behavior, researchers hope to learn more about the different kinds of intelligence on Earth, and how they interact.