Monarchs of the Sea: The Extraordinary 500-Million-Year History of Cephalopods

by Danna Staaf

When they hatch from their eggs, they’re smaller than your fingernail, so Baby’s First Predators are small too: fish larvae and aquatic worms.

But squid grow quickly, and in a matter of days or weeks those that survive turn the tables. Growing fat on their onetime adversaries, squid start to attract larger predators: seals and seabirds, sharks and whales.

Scientists once pumped the stomachs of sixty elephant seals from South Georgia and found 96.2 percent squid by weight.1 They calculated that the island’s elephant seal population scarfs down at least 2.5 million tons (...

a single sperm whale can eat 700 to 800 s...

“ecological keystones.” They start so small, grow so fast, and get so big that they provide abundant food for marine predators of every size—a “one-prey-fits-all” solution.

Humboldt squid can grow up to 6½ feet (2 m) long and spawn millions of eggs. Their size and abundance support the world’s largest invertebrate fishery.

Admittedly the ocean is full of other boneless, unshelled animals, like jellyfish, but most of them are gelatinous, containing up to 95 percent water. Squid, on the other hand, are solid muscle and make a far more nourishing meal.

In a living squid, one end of the mantle is sealed closed and adorned with two flexible fins that can flap like wings. The other end is open to the sea. Here the squid sucks water in and squeezes it out through a siphon, creating a jet stream that propels the animal through the sea and even into the air. (Not all squid can fly, but the ones that do can travel up to 165 airborne feet/50 m before splashing down.)

The distinction between arms and tentacles is easy to remember from the words themselves: arms are shorter than tentacles.

(“Mollusk” comes from the Latin word for “soft”; one has to wonder if ancient Romans used the same word to denote “squishy” or “slimy.”)

Molluscan bodies are broadly divided into two parts: a muscular foot and a shell-secreting mantle. Like different poets improvising with the same poetry form, different mollusks have adapted the same body plan for various lifestyles. Snails ooze along on their foot and carry a coiled shell on their back; clams dig into the mud with their foot and hide in a hinged shell; squid divide their foot into arms and tentacles and repurpose the mantle for jet propulsion, shrugging off its shell-producing capabilities.

the far-distant ancestors of today’s squid filled their shells with gas and floated up through the water. They were slow swimmers, but they had no need for speed. They could drift over the bottom-dwelling buffet like deadly dirigibles, selecting their prey at leisure.

Over time their lineage separated into three main branches: nautiloids, coleoids, and ammonoids. The “-oid” suffix is common in zoological nomenclature. It sounds a bit goofy, but it’s important because it tells you that I’m talking about a whole group of animals.

despite shell similarities with early fossils, today’s nautiluses have evolved their own contemporary peculiarities. Their heads bear not eight, not ten, but anywhere between sixty and ninety tentacles. The number can vary even within a single species.

at some point in the nautilus’s evolution the two uppermost tentacles seem to have enlarged and fused to form a protective hood over the animal.

cephalopod appendages are really modified feet. If you watch baby squid and nautiluses develop in their eggs, it’s easy to see the connection. In the same way that human embryos retain tails from our evolutionary history, cephalopod embryos show off their molluscan heritage with a single foot that eventually differentiates into ten arm buds.

nautiloids are only a little bit older—by geologic standards, anyway—than the first coleoids and ammonoids. These latter two groups seem to have evolved in response to competition from and predation by the Paleozoic nouveau riche: fish.

Fish could grow several times bigger than the biggest cephalopods, swim significantly faster, and break shells with their jaws.

The word “coleoid” comes from the Greek word for “scabbard.” A scabbard covers a sword, and the bodies of coleoids cover their shells (or lack thereof). Coleoids include all the modern non-nautilus cephalopods—octopuses, squid, cuttlefish, and more—as well as numerous fossils.

A thin internal rod called a gladius stiffens the squid’s body and gives its muscles something to work against.

A cuttlefish, which looks almost exactly like a squid on the outside, has a more complex calcified internal structure called a cuttlebone.

For our purposes here, we need consider only one eon, which is the eon we still live in today: the eon of “visible life” or Phanerozoic, which is just half a billion years long.

This eon contains three eras: “old life” (Paleozoic), “middle life” (Mesozoic), and “new life” (Cenozoic), each with its own constituent periods.

The word “worm” is almost uselessly vague, as it describes so many different animals. There are the segmented worms, a group that includes both the humble earthworm and the ornate Christmas tree worm. There are the aggressively carnivorous ribbon worms, the usually plant-sucking roundworms (but the parasitic hookworms and heartworms are also types of roundworms) and even slow worms, which are really legless lizards.

penis worms.

Priapulida, and Priapus was a Greek fertility god

both worm and phallus derive their shapes from the same anatomical structure, a hydrostatic skeleton.

Penis worms today use their hydrostatic skeletons to make burrows with a distinctive shape, and paleontologists have discovered similar shapes in rocks 542 million years old.

The actual burrower has never been found; probably it was too soft to fossilize. The burrows have been given their own scientific name, Treptichnus pedum, and the honor of marking the onset of the Cambrian

Treptichnus is the first fossil that shows a creature moving down into the ground, creating a three-dimensional space with its body.

Cambrian Explosion.7 This sudden profusion of life may have been kicked off by changes in the earth’s physical environment. Various lines of evidence suggest that animal evolution may have initially been constrained by low oxygen levels. During the Ediacaran, oxygen began to increase, and when it reached critical levels in the Cambrian, animals were finally free to get big and interesting.

Their burrows brought food and water deeper underground, which allowed new kinds of microbes to grow—and become food for still newer animal forms. And as animals continued to explore new food sources, it was inevitable that they turned on each other.

arrow worm. These small but ferocious predators still terrorize the sea’s wee beasties today. Arrow worms and their ilk may even have driven much of the early Cambrian diversification, as animals tried every trick to avoid getting eaten.

Shells, the hallmark of mollusks, probably came into existence as armor. The first Cambrian fossils after Treptichnus are called the “small shellies,”

calcium carbonate. Calcium is life’s preferred material for hardening both external and internal skeletons—it shows up in bones as calcium phosphate.

Calcification cropped up quickly in numerous Cambrian groups, including early relatives of starfish and corals as well as mollusks, and defense seems the most likely reason.

for anyone without the advantage of armor or size is to get out of town: if you can’t defend yourself in one place, go colonize a new habitat.

predators can adapt too. It wasn’t very long at all before much larger predators made their debut—notably, the “weird shrimp” Anomalocaris.

the very first cephalopods—escaped Anomalocaris’s tyranny of the seafloor by rising up into the water column.

At one time scientists thought they could learn how an animal evolved over geologic time simply by watching it develop over its lifetime.

Modern humans, lizards, and fish all evolved from a common ancestor, not one from another. But observing the similarities between their embryos still helps us to understand how it happened.

embryos don’t yet look much like either squid or nautiluses, but they do look like mollusks of another kind: an odd little group of not-quite-snails called Monoplacophora. The name is Greek for “single shell bearers,”

Monoplacophorans have no common name (like “snail” or “clam”) because they are so rare today.

Unique among mollusks, monoplacophorans turned out to have multiple repeating sets of shell-attachment muscles, kidneys, and gills. Such repetition can be an adaptation to deal with low oxygen levels and is also a prominent feature of the trilobites.

One of the current best guesses as to what links the two groups is the fossil Knightoconus (probably not itself a cephalopod, though its descendants may have been), whose shell grew into a tall cone rather than a flat cap. Critically, this shell was roomy enough to be divided into chambers.

First: some monoplacophorans began secreting a liquid into their shells that was less salty than seawater.

Second: some descendants of these first shell lighteners began to alternate the secretion of liquid with the secretion of more shell.

Third: descendants of the descendants used a thin tube of flesh, stretching back through every chamber in the shell, to extract liquid and replace it with gas.

It is generally accepted that Plectronoceras cambria was the first fossil cephalopod, with its shell divided into chambers and a narrow tube available to adjust fluid content.

create a high internal pressure to expel a large mass through a narrow opening at great speed. Squid accomplish this by drawing water into the mantle through wide openings around the head, then sealing these openings and forcing the water out through the much narrower siphon.