No species of dumbo octopus discovered has a functional ink sac. This is very peculiar because the ink sac is a trait that is found in most species of octopus and squid. In fact, it is a defining characteristic of the entire class Cephalopoda.For most cephalopods, the ink sac is an indispensable adaptation because it allows them to confuse and hide from predators. Without the ink sac, most cephalopods would be much more vulnerable to sharks, whales, and large fish that try to eat them. But, there are few large predators in the depths of the ocean. Part of this is due to the fact that there is very little prey at extreme depths. So, without many sharks, whales, or large fish, the dumbo octopus would rarely (if ever) be required to use an ink sac to escape from predators. Since the ink sac costs energy to grow, maintain, and fill with ink, it would be a complete waste for the dumbo octopus to keep its ink sac. Some time ago, an octopus mutated to not have an ink sac and was, therefore, able to produce more offspring than its relatives with ink sacs. This adaptation was very good, which is likely why all dumbo octopuses are without ink sacs!