When threatened by predators, squid, cuttlefish, and octopuses respond by secreting a stream of dark pigment. A jet of water is discharged from the funnel in conjunction with ink release, diffusing ink into a large, dark cloud. The ink may also mix with mucus from the funnel and form smaller blobs.
Cephalopods have two main methods of inking. They may obscure themselves from view of predators by releasing an immense smokescreen of ink, or they can squirt out a succession of cephalopod-sized ink blobs called pseudomorphs (“false body”), which serve as decoys. Often, when using psuedomorphs as a diversionary tactic, the animal will blanch white, change direction erratically, and jet off, leaving only a black blob for the disoriented predators to attack.
Ink contains a chemical compound called tyrosinase that impairs a predator’s sense of smell and taste. Cephalopod ink also contains dopamine and L-DOPA, neurotransmitters that, according to some research, may serve to alert other cephalopods to the presence of predators.
Different cephalopod species have been observed using their ink in a creative assortment of deceptive techniques.
- The black eggs of several cuttlefish species, including the common cuttlefish (Sepia officinalis) contain ink in their outer coating, making them less visible to predators.
- The Atlantic cranch squid (Teuthowenia megalops) and some species of glass squid inject ink inside their own bodies. When disturbed, the squid will ball up like a balloon, tucking its head and arms inside the mantle, and ink into the mantle cavity. Scientists don’t know the purpose of this behavior. Perhaps inflating itself into a big black ball makes the cranch squid look deceptively large.
- In the ultimate self-tribute, the Caribbean reef squid (Sepioteuthis sepioidea) may imitate its own ink. After inking a predator and jetting a short distance away, the squid tints itself black and hangs in the water, mimicking the ink cloud.
- In deeper, darker waters, where black ink would be useless for defense, many cephalopod species no longer have ink sacs. Some bobtail squid, which do possess ink sacs, have evolved a strategy for concealing themselves in dark water. They secrete a glowing curtain composed of mucus and luminescent bacteria from their unique light organs. They may use luminescent clouds for concealment in low light and ink clouds in well-lit waters.