Figure by Ainsley Seago.
As well as zeppelins, a range of cephalopod hovercrafts and tanks probably prowled the sea floor – some of the shells from this time seem too unwieldy to carry in the open water. All these animals are now extinct, with one non-fearsome exception, the nautilus. Many of the losses occurred as part of the mass extinctions that punctuate the history of life, but it’s also likely that some predatory cephalopods were slowly outcompeted by fish, as those fish became larger and better armed. The zeppelins were challenged, and eventually vanquished, by airplanes.
The nautilus, however, made it through. No one knows why. At the start of this book I cited a Hawaiian creation myth that judges the octopus a “lone survivor” from an earlier world. The real survivor is indeed a cephalopod, but nautilus rather than octopus. Still living in the Pacific, present-day nautiluses are little changed from 200 million years ago. Living in coiled shells, they’re now scavengers. They have simple eyes and a cluster of tentacles, and they move up and down, from the deep sea to shallower water, in a rhythm that’s still being studied. They seem to stay higher in the water at night, deeper in the day.
Another shift was to come in the evolution of cephalopod bodies. Sometime before the age of the dinosaurs, it seems, some cephalopods began to give up their shells. The protective casings that had become buoyancy devices were abandoned, reduced, or internalized. This enabled more freedom of movement, but at the price of greatly increased vulnerability. It seems quite a gamble, but this was a path taken several times. The last common ancestor of “modern” cephalopods is not known, but at some stage the lineage split into two main branches, an eight-armed group including octopuses and a ten-armed group including cuttlefish and squid. These animals reduced their shells in different ways. In the cuttlefish, a shell was retained internally, and still helps the animal remain buoyant. In squid, a sword-shaped internal structure called a “pen” remains. Octopuses have lost their shell entirely. Many cephalopods began to live as soft-bodied, unprotected animals on reefs in shallow seas.
The oldest possible octopus fossil dates from 290 million years ago. I emphasize the uncertainty – it’s just one specimen, and little more than a smudge on a rock. After this there is a gap in the record, and then at around 164 million years ago there is a clearer case, a fossil that looks undeniably like an octopus, with eight arms and an octopus-like pose. The fossil record of octopuses remains skimpy because they don’t preserve well. But at some stage they radiated; around 300 species are known at present, including deep-sea as well as reef-dwelling forms. They range from less than an inch in length to the giant Pacific octopus, which weighs in at 100 pounds and spans twenty feet from arm tip to arm tip.
That’s the journey of the cephalopod body, a path from Ediacaran macaron through limpet-like shellfish to predatory hovercraft and zeppelin. The encumbrance of the external shell is then abandoned, as the shell is brought inside the body or, in an octopus, lost completely. With that step, the octopus loses almost all definite shape.
To completely forgo both skeleton and shell is an unusual evolutionary move for a creature of this size and complexity. An octopus has almost no hard parts at all – its eyes and beak are the largest – and as a result it can squeeze through a hole about the size of its eyeball and transform its body shape almost indefinitely. The evolution of cephalopods yielded, in the octopus, a body of pure possibility.
During the time I was writing an early version of this chapter, I spent a few days watching a pair of octopuses in the rocky shallows. I saw them mate once, and then spend much of the next afternoon just sitting, it seemed. The female moved off a little way, but returned to her den as the sun got low. The male had spent the day in a more exposed spot, less than a foot from her den. He was there when she came back.
I watched them, off and on, for two afternoons, and then storms came. Winds of sixty miles per hour lashed the coast, and waves rolled in from the south. The bay where the octopuses live has some protection from this onslaught, but not much. Waves swept around the entrance and turned the water into a boiling white soup. The shore was beaten by these storms for the next four days. Where do the octopuses go when the waves are pounding their rocks? It was impossible to get into the water to see. The cuttlefish have no problem. They disappear for weeks when the weather is bad. They fire up their jet propulsion and move off to some unknown deeper place. Perhaps the octopuses also move further out to sea, but more likely they climb into a crevice and hang on, for days at a stretch, recalling their ancestors who gripped rocks from inside cap-shaped shells.
Evolution of the Cephalopods: The figure is not to scale (far from it), and doesn’t represent actual descent relations between species. It presents a chronological sequence of forms seen in cephalopod evolution from over half a billion years ago to the present, with a few of the most important branchings marked along the way. I have included the controversial Kimberella as a possible early stage. The capped limpet-like shellfish is a monoplacophoran. The next animal, with a shell divided into compartments, is something like Tannuella. Opinion seems divided on whether the next in line, Plectronoceras, had lifted off the ground or was still on the sea floor, but this animal is often regarded as the first “true” cephalopod, because of various internal features. Cameroceras is the giant of the large predatory cephalopods, with conservative length estimates of up to eighteen feet. The octopus and squid are descended from unknown cephalopods that gave up their external shells and are now extinct, unlike the nautilus, which kept its shell and lived on. Figure by Eliza Jewett.
~ Puzzles of Octopus Intelligence
As the cephalopod body evolved toward its present-day forms, another transformation occurred: some of the cephalopods became smart.
“Smart” is a contentious term to use, so let’s begin cautiously. First, these animals evolved large nervous systems, including large brains. Large in what sense? A common octopus (Octopus vulgaris) has about 500 million neurons in its body. That’s a lot by almost any standard. Humans have many more – something like 100 billion – but the octopus is in the same range as various smaller mammals, close to the range of dogs, and cephalopods have much larger nervous systems than all other invertebrates.
Absolute size is important, but it is usually regarded as less informative than relative size – the size of the brain as a fraction of the size of the body. This tells us how much an animal is “investing” in its brain. This comparison is made by weight, and only counts the neurons in the brain. Octopuses also score high by this measure, roughly in the range of vertebrates, though not as high as mammals. Biologists regard all these assessments of size, though, as only a very rough guide to the brainpower an animal has. Some brains are organized differently from others, with more or fewer synapses, and those synapses can also be more or less complicated. The most startling finding in recent work on animal intelligence is how smart some birds are, especially parrots and crows. Birds have quite small brains in absolute terms, but very high-powered ones.
When we try to compare one animal’s brainpower with another’s, we also run into the fact that there is no single scale on which intelligence can be sensibly measured. Different animals are good at different things, as makes sense given the different lives they live. An analogy can be drawn with tool kits: brains are like tool kits for the control of behavior. As with human tool kits, there are some elements in common across many trades, but much diversity also. All the tool kits found in animals include some kind of perception, though different animals have very different ways of taking in information. All (or almost all) bilaterian animals have some form of memory and a means for learning, enabling past experiences to be brought to bear on the present. The tool kit sometimes includes capacities for problem solving and planning. Some tool kits are more elaborate and expensive than others, but they can be sophisticated in different ways. One animal might have better senses, while another may have more sophisticated learning. Different tool kits go with different ways of making a living.
When comparing cephalopods with mammals, the difficulties are acute. Octopuses and other cephalopods have exceptionally good eyes, and these are eyes built on the same general