Defensive Adaptations for Cephalopods

By Ben Team

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Contrary to the frightening beast described in Jules Verne’s 1869 classic “Twenty Thousand Leagues Under the Sea,” real cephalopods spend most of their time trying not to become lunch for one of their many predators. Scientists suspect that predator avoidance was a key driving force in cephalopod evolution, and this advanced clade of mollusks has devised a number of defensive adaptations.

Ink

One of the most famous defensive adaptations of cephalopods is their ink. Used to distract a predator and allow a squid or octopus to escape quickly, ink not only provides a visual distraction or barrier, but it also disrupts a predator’s sense of smell and taste. In 2012, researchers at the University of Virginia examined the preserved ink sack of a 160-million-year-old squid. Publishing their findings in the “Proceedings of the National Academy of Sciences,” the team found that the preserved ink was almost identical to the ink of modern squid, which suggests that squid perfected the formula for their ink early in their evolution. Often, ink is used in conjunction with jetting away and changing color.

Shells

The oceans once swarmed with chambered nautiluses, but today only two types survive -- Nautilus and Allonautilus. The only living cephalopods with an external shell, nautiluses catch fish with their short tentacles. Often called the chambered nautilus in reference to their shells that have up to 30 internal partitions, these cephalopods maintain their buoyancy by adjusting the gas pressure inside their shells. The shell protects the nautilus from many predators, and unlike most other cephalopods that live a few years at best, the chambered nautilus may exceed 16 years of age.

Crypsis and Color Change

Many cephalopods, including squids, cuttlefish and octopi, are capable of remarkable and rapid color change. When they are trying to avoid a predator, they may swim over sand, rocks and grass in quick succession; with each change of substrate, the creature’s color instantly changes to match it. Sometimes, cephalopods use their color-changing capabilities during or immediately after ejecting ink into the water. For example, a squid may turn black, eject a cloud of black ink, rapidly jet away and quickly change to a white color.

Jet Propulsion

Octopi spend a lot of time crawling on the substrate, while cuttlefish and squid use their small lateral fins to hover or swim slowly. However, when a predator appears, all three of these cephalopods can use “jet propulsion” to escape. By forcing water out of or through their body cavity, squids and their kin can shoot themselves backwards at high speed.

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