Basic Science

How squids outsmart their predators

Scientists know this because on the bodies of washed-up whales, they frequently find huge, round suction scars, emblazoned there by large, grasping tentacles. Ranging in size from this giant’s impressive 14 meters to the 2.5 centimeters of the southern pygmy squid, these creatures fall into the group of animals known as cephalopods. There are about 500 squid species worldwide, and they live in all the world’s oceans, making them a reliable food source for whales, dolphins, sharks, seabirds, fish, and even other squid. Indeed, squid themselves are fearsome ocean predators.

But their most extraordinary adaptations are those that have evolved to help them thwart their predators. Squid, which can be found mainly in estuarine, deep-sea, and open-water habitats, often swim together in shoals. Being out in the open without anywhere to hide makes them vulnerable, so as a first line of defense, they rely on large, well-developed eyes. In the colossal squid, these are the size of dinner plates, the largest known eyes in the animal kingdom.

When it’s dark or the water is murky, however, squid rely on a secondary sensory system, made from thousands of tiny hair cells that are only about twelve microns long and run along their heads and arms. Each of these hair cells is attached to axons in the nervous system. Swimming animals create a wake, so when the hairs on the squid’s body detect this motion, they send a signal to the brain, which helps it determine the direction of the water’s flow.

This way, a squid can sense an oncoming predator in even the dimmest waters. Aware of the threat, a squid can then mask itself from a predator. Squid skin contains thousands of tiny organs called chromatophores, each made of black, brown, red or yellow pigments and ringed in muscle. Reflecting cells beneath the chromatophores mirror the squid’s surroundings, enabling it to blend in. So, when the muscles contract, the color of the pigment is exposed, whereas when the muscles relax the colors are hidden.

Each of these chromatophores is under the individual control of the squid’s nervous system, so while some expand, others remain contracted. That enables countershading, where the underside of the squid is lighter than the top, to eliminate a silhouette that a predator might spy from below. Some predators, however, like the whales and dolphins, get around this ruse by using sound waves to detect a squid’s camouflaged form. Not to be outfoxed, the squid still has two more tricks up its sleeve.

The first involves ink, produced inside its mantle. Squid ink is made mostly of mucus and melanin, which produces its dark coloring. When squid eject the ink, they either use it to make a large smokescreen that completely blocks the predator’s view or a blob that roughly mimics the size and shape of the squid. This creates a phantom form, called a pseudomorph, that tricks the predator into thinking it’s the real squid.

As a final touch, squid rely on jet propulsion to rapidly shoot away from their hunters, reaching speeds of up to 25 miles per hour and moving meters away in mere seconds. This makes them Earth’s fastest invertebrates. Some squid species have also developed unique adaptive behaviors. The deep-sea vampire squid, when startled, uses its webbed arms to make a cape it hides behind.

The tiny bobtail, on the other hand, tosses sand over its body as it burrows away from prying eyes. The Pacific flying squid uses jet propulsion for another purpose: to launch itself right out of the water. Squids’ inventive adaptations have allowed them to proliferate for over 500 million years. Even now, we’re still uncovering new species. And as we do, we’re bound to discover even more about how these stealthy cephalopods have mastered survival in the deep and unforgiving sea.

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