Cephalapods have not always had the ability to produce ink. Instead, they used their shells to defend themselves. According to the fossil record, the first ink sacs appeared around 330 million years ago. This shift distinguished cephalopods into different subclasses; Caleodia is the class that produces ink, rather than uses a shell, for defense.
Octopuses, squids and cuttlefish fall into the Caleodia subclass. These cephalopods evolved to include the ink sac in their biological repertoire of defenses. Today we know that ink sacs are used by these creatures to hold a mixture of melanin, enzymes related to melanin production, catecholamines, peptidoglycans, free amino acids and metals. Ink is used by most cephalopods to create a distraction, allowing them to escape from predators. There are also some studies that show that ink, in its mucus state, is irritating to fish gills, which adds an additional level of defense for these cephalopods.
This evolutionary feat is one of many traits that make cephalopods unique! Stay tuned all week long as we share more fun facts about cephalopods.