What are Cephalopods?
Cephalopods are known to release ink as a defense mechanism. Usually people only think that Octopi and Squids do this but there are many other species of cephalopods. In fact, all cephalopods except Nautilidae and Octopi from the suborder Cirrina have an ink sac and are able to expel ink as a defense mechanism. The dumbo octopus is one example of a deep sea octopus that does not produce ink. A marine organism is considered a cephalopod if they have bilateral body symmetry, a prominent head, and a set of arms or tentacles. There are about 800 known species of cephalopods. Nautiluses are the only extant cephalopods that have an external shell.
Sea Hares are another species that uses ink as a defense mechanism. They are gastropod mollusks and they have a soft internal shell. A sea hares ink is usually more colorful than the typical black ink that you see in Squid and Octopi.
“Sea Hare” By Andrew David
The video below shows a sea hare in a tank with a lobster which makes it secrete its defensive purple ink.
The Ink Sac
The ink sac is what produces the cephalopods ink. It is a muscular bag that was originally just an extension of the hind gut. It can be found under the gut and it opens into the anus. That is where the ink can be expelled from. The ink is made up of almost pure melanin. Melanin is a Greek word that means dark or black. It is a vast term for a natural pigments that are found in most organisms. The cephalopod is able to expel the ink while it uses its jet propulsion. The ink is bound together by mucas which allows it to be around the shape and size of the cephalopod. This distracts the predator long enough for the cephalopod to get away.
Fun Fact About Squid Ink
Humans occasionally use squid ink for cooking. It is commonly used in sauces and for pasta. It creates a rich flavor and turns whatever it is in a dark black. It also has some health benefits. It has lotsof antioxidants and is rich in iron.