How The Squid Lost Its Shell?

Squid Evolution

Mollusks are usually the simplest creatures, do not have blood vessels, and sometimes lack a proper brain. But one group of animals challenges this: Squid and octopus. Many scientists think that squid and all cephalopods are closely related to a group of mollusks not too different in appearance from limpets called monoplastophorans.

Specifically, a monoplastophoran lived in modern-day Antarctica over 500 million years ago in the Cambrian period called nitoconus. That had an extremely exaggerated conical shell.

How the squid lost its shell?

Squids belong to cephalopods called coleoids, including octopuses and cuttlefish. Unlike their distant relatives, the nautiloids, squids lost their external shells throughout their evolutionary history. The process of losing the shell and developing a more streamlined body involved several adaptations:

Enhanced Mobility: Losing a heavy, external shell allowed squids to become more agile and maneuverable in the water. With greater freedom of movement, they could actively swim and navigate their surroundings more efficiently. This change in body structure provided advantages in capturing prey, evading predators, and responding to environmental changes.

Hydrodynamic Efficiency: Squids acquire a streamlined body shape by shedding their shells. Their bodies became more elongated and streamlined, which reduced drag and enabled faster swimming speeds. This adaptation allowed squids to move swiftly through the water and use their speed to outmaneuver predators and catch prey.

Internal Support Structures: Although squids lack an external shell, they still possess internal structures to provide support. Within their bodies, squids have a rigid structure known as the pen or gladius. The pen is made of a flexible, cartilaginous material and is an internal backbone-like structure. While not as protective as a shell, the pen provides some structural support and aids in maintaining the squid’s streamlined shape.

Increased Camouflage Abilities: The absence of a bulky shell allows squids to employ a wide range of camouflage strategies. They can rapidly change their body color, patterns, and texture to blend in with their surroundings. This remarkable camouflage enables squids to hide from predators and stealthily approach prey.

About 500 million years ago, a pioneering little mollusk floated off the ocean floor in the Cambrian Period. It had developed a way to use its defensive shell for a new purpose buoyancy. It turned out that by filling its shell with gas. This mollusk could reach new heights, gaining a key advantage over its relatives on the seafloor.

Scientists believe this was the first cephalopod, which now includes squids, octopuses, cuttlefish, and nautiluses. We might think of the nautilus with its shell as an oddity today. The ancestors of modern, squishy cephalopods like octopuses and squid all had shells. Early cephalopods are defined by their shells or, more specifically, by how their shells are adapted to suit their needs.

Some cephalopods truncated their shell. Others acquired a different shape. Some of them internalized their shell like a backbone. In some cases, they got rid of the thing altogether. In ancient times, the shell was cephalopods’ greatest asset. But it also proved to be their biggest weakness. Mollusks were the first truly complex animals, probably appearing in the late Ediacaran Period.

Although there is some evidence of hard mineralized shells from this time, shells became much more common after the Cambrian Explosion as we know them today. It is not coincidentally when the first evidence of predators appears. So, early shells worked like shields, protecting the animal’s soft body, or mantle, from predators lurking above. By the late Cambrian, one mollusk known as Plectronoceras had acquired a couple of adaptations that marked the beginning of a new form of transportation and a new kind of mollusk.

For one thing, its shell was divided into sealed-off chambers by thin walls called septa. As the animal grew, it added new chambers to its shell. This wasn’t new, but it was instrumental in another adaptation. As Plectronoceras added septa to its shell, it left behind a small, tube-like part of its mantle in each chamber. This little tube of tissue is known as a siphuncle.

As unassuming as it seemed, it helped Plectronoceras perform a trick the world had never seen before. Plectronoceras could absorb all the water from the chambers in its shell by making the blood flow through the siphuncle super salty.

As water diffused from the shell and into the salty blood, gas seeped in, and what was once a suit of armor became a personal floatation device. The first true cephalopods had arrived, and they looked like tiny, adorable, upside-down ice cream cones. This development of a gas-filled chambered shell. It, also known as a phragmocone, was a triumphant, history-making adaptation. Cephalopods had entered a golden age when the Cambrian segued into the Ordovician.

There were few predators to threaten them. Rising ocean oxygen levels caused life to flourish, diversify, occupy new habitats, and provide abundant food. It is known as the Great Ordovician Biodiversification Event. That’s when they got huge. The Ordovician endoceratids cephalopods were the biggest animals of their time, reaching an impressive 6 meters in length. As the Ordovician progressed, cephalopods began to leave the shallows to explore the open ocean.

So they had to find ways to become faster and more agile. Some species developed coiled shells, forming a more compact and maneuverable form, like the modern nautilus. By the Silurian Period, a genus called Sphooceras tried a different tactic: Instead of coiling its shell, it broke off its end. Sphooceras periodically wrapped part of its soft mantle around the outside of its shell. Then secreted, enzymes that helped break off the chambers at the end. This made the end blunter, shorter, and sturdier.

In turn, it made the shell less vulnerable to breaking and easier to maneuver. Sphooceras might be the very first cephalopod that kept its shell inside its mantle for any time. It was an experiment about to be taken to a whole new level.

A new evolutionary pressure awaited cephalopods in the Devonian Period: fast, jawed fish. While fish with jaws first appeared in the Silurian, they increased in the Devonian. That kicked off an evolutionary arms race between fish and cephalopods.

Until this point, all cephalopods had been members of the slow and steady group known as nautiloids, from the pioneering little Plectronoceras to the imposing Cameroceras. This ancient lineage still survives today in the form of the modern nautilus. But in the Devonian, a new branch of the cephalopod family tree appeared: ammonites. They coped with the rise of fish with a live-fast, die-young strategy. Unlike nautiloids, which grew slowly and invested a lot of energy into making a few offspring, ammonites grew quickly and had many offspring.

They were so successful, diverse, and numerous that their shells are now used as index fossils to define Periods in the Mesozoic. Ammonites developed many shell sizes and shapes, growing shells resembling hooks, knots, or paper clips.

Then, around the beginning of the Carboniferous, a new lineage appeared with an even more radical strategy to deal with the fish problem. They were the first coleoids like Sphooceras millions of years before them. Coleoids wrapped their soft mantles around their hard shells.

Unlike Sphooceras, they kept it there permanently. Hematites, for example, were one of the earliest coleoids, with a cone-shaped shell inside their soft body. Then, the shells began to shrink over millions of years, and what remained was built with lighter-weight material.

After all, internal shells no longer offered protection. So there was no reason to keep lugging around all that extra weight. They lost the gas-filled chambers that had kept them afloat and developed new ways to stay buoyant. In time, the internal shell was streamlined to a long, chitinous structure, like a backbone called a gladius.

All squid alive today still have some gladius, while octopuses have a pair of similar structures called stylets. With these adaptations, coleoids began taking advantage of a new niche: the deep sea. While the old gas-filled phragmocone couldn’t withstand the pressure of the deep ocean, the gladius had no such problem.

Their ability to live in the deeply saved coleoids from extinction. At the end of the Cretaceous Period, a fatal blow struck the ammonites and most nautiloids: the Cretaceous-Paleogene extinction event. The same event killed the non-avian dinosaurs.

Acid rain changed the pH of the oceans, compromising the integrity of the shells these animals needed to survive. It hit baby ammonite, which relied on thin, fragile shells to float near the ocean surface, especially hard passively.

At the same time, there was likely a massive die-off of ammonites’ primary food source, plankton. Their slow and steady lifestyles probably saved the nautiloids, and six species in two genera have survived today. But the coleoids could take refuge in the deep sea and no longer depend on their shells.

So with the ammonites gone, the coleoids rose and took their place when conditions improved. Today, coleoids have colonized every marine ecosystem on the planet and play a vital role in ocean food webs. Instead of relying on a suit of protective armor, they now use intelligence, camouflage, and agility to outsmart predators and prey alike.

Their journey from small, passive mollusks to sleek, voracious predators took hundreds of millions of years of trial and error, from developing shells to survive to finally learning to thrive without them. The squid still swims around with its gladius intact, and the octopus with its stylets reminds us of history. They share with the shelled creatures of the past.

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“Squids, cuttlefishes, and their relatives.”
“Genus-level phylogeny of cephalopods using molecular markers: current status and problematic areas.”
“Molecular clocks indicate turnover and diversification of modern coleoid cephalopods during the Mesozoic Marine Revolution.”

Julia Rose

My name is Julia Rose. I'm a registered clinical therapist, researcher, and coach. I'm the author of this blog. There are also two authors: Dr. Monica Ciagne, a registered psychologist and motivational coach, and Douglas Jones, a university lecturer & science researcher.I would love to hear your opinion, question, suggestions, please let me know. We will try to help you.

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