Book on a Friday: Squid Empire: The Rise and Fall of the Cephalopods by Danna Staaf

Danna Staaf’s Squid Empire: The Rise and Fall of the Cephalopods is about the evolution of squids and their relatives–nautiluses, cuttlefish, octopuses, ammonoids, etc. If you are really into squids or would like to learn more about squids, this is the book for you. If you aren’t big on reading about squids but want something that looks nice on your coffee table and matches your Cthulhu, Flying Spaghetti Monster, and 20,000 Leagues Under the Sea decor, this is the book for you. If you aren’t really into squids, you probably won’t enjoy this book.

Squids, octopuses, etc. are members of the class of cephalopods, just as you are a member of the class of mammals. Mammals are in the phylum of chordates; cephalopods are mollusks. It’s a surprising lineage for one of Earth’s smartest creatures–80% mollusk species are slugs and snails. If you think you’re surrounded by idiots, imagine how squids must feel.

The short story of cephalopodic evolution is that millions upon millions of years ago, most life was still stuck at the bottom of the ocean. There were some giant microbial mats, some slugs, some snails, some worms, and not a whole lot else. One of those snails figured out how to float by removing some of the salt from the water inside its shell, making itself a bit buoyant. Soon after its foot (all mollusks have a “foot”) split into multiple parts. The now-floating snail drifted over the seafloor, using its new tentacles to catch and eat the less-mobile creatures below it.

From here, cephalopods diversified dramatically, creating the famous ammonoids of fossil-dating lore.

Cross-section of a fossilized ammonite shell, revealing internal chambers and septa

Ammonoids are known primarily from their shells (which fossilize well) rather than their fleshy tentacle parts, (which fossilize badly). But shells we have in such abundance they can be easily used for dating other nearby fossils.

Ammonoids are obviously similar to their cousins, the lovely chambered nautiluses. (Please don’t buy nautilus shells; taking them out of their shells kills them and no one farms nautiluses so the shell trade is having a real impact on their numbers. We don’t need their shells, but they do.)

Ammonoids succeeded for millions of years, until the Creatceous extinction event that also took out the dinosaurs. The nautiluses survived–as the author speculates, perhaps because they lay large eggs with much more yolk that develop very slowly, infant nautiluses were able to wait out the event while ammonoids, with their fast-growing, tiny eggs dependent on feeding immediately after hatching simply starved in the upheaval.

In the aftermath, modern squids and octopuses proliferated.

How did we get from floating, shelled snails to today’s squishy squids?

The first step was internalization–cephalopods began growing their fleshy mantles over their shells instead of inside of them–in essence, these invertebrates became vertebrates. Perhaps this was some horrible genetic accident, but it worked out. These internalized shells gradually became smaller and thinner, until they were reduced to a flexible rod called a “pen” that runs the length of a squid’s mantle. (Cuttlefish still retain a more substantial bone, which is frequently collected on beaches and sold for birds to peck at for its calcium.)

With the loss of the buoyant shell, squids had to find another way to float. This they apparently achieved by filling themselves with ammonia salts, which makes them less dense than water but also makes their decomposition disgusting and renders them unfossilizable because they turn to mush too quickly. Octopuses, by contrast, aren’t full of ammonia and so can fossilize.

Since the book is devoted primarily to cephalopod evolution rather than modern cephalopods, it doesn’t go into much depth on the subject of their intelligence. Out of all the invertebrates, cephalopods are easily the most intelligent (perhaps really the only intelligent invertebrates). Why? If cephalopods didn’t exist, we might easily conclude that invertebrates can’t be intelligent–invertebrateness is somehow inimical to intelligence. After all, most invertebrates are about as intelligent as slugs. But cephalopods do exist, and they’re pretty smart.

The obvious answer is that cephalopods can move and are predatory, which requires bigger brains. But why are they the only invertebrates–apparently–who’ve accomplished the task?

But enough jabber–let’s let Mrs. Staaf speak:

I find myself obliged to address the perennial question: “octopuses” or “octopi”? Or, heaven help us, “octopodes”?

Whichever you like best. Seriously. Despite what you may have heard, “octopus” is neither ancient Greek nor Latin. Aristotle called the animal polypous for its “many feet.” The ancient Romans borrowed this word and latinized the spelling to polypus. It was much later that a Renaissance scientists coined and popularized the word “octopus,” using Greek roots for “eight” and “foot” but Latin spelling.

If the word had actually been Greek, it would be spelled octopous and pluralized octopodes. If translated into Latin, it might have become octopes and pluralized octopedes,  but more likely the ancient Roman would have simply borrowed the Greek word–as they did with poly pus. Those who perhaps wished to appear erudite used the Greek plural polypodes, while others favored a Latin ending and pluralized it polypi.

The latter is a tactic we English speakers emulate when we welcome “octopus” into our own language and pluralize it “octopuses” as I’ve chosen to do.

There. That settles it.

Dinosaurs are the poster children for evolution and extinction writ large…

Of course, not all of them did die. We know now that birds are simply modern dinosaurs, but out of habit we tend to reserve the word “dinosaur for the hug ancient creatures that went extinct at the end of the Cretaceous. After all, even if they had feathers, they seem so different from today’s finches and robins. For one thing, the first flying feathered dinosaurs all seem to have had four wings. There aren’t any modern birds with four wings.

Wesl… actually, domestic pigeons can be bred to grow feathers on their legs. Not fuzzy down, but long flight feathers, and along with these feathers their leg bones grow more winglike. The legs are still legs’ they can’t be used to fly like wings. They do, however, suggest a clear step along the road from four-winged dinosaurs to two-winged birds. The difference between pigeons with ordinary legs and pigeons with wing-legs is created by control switches in their DNA that alter the expression of two particular genes. These genes are found in all birds, indeed in all vertebrates,and so were most likely present in dinosaurs as well.

…and I’ve just discovered that almost all of my other bookmarks fell out of the book. Um.

So squid brains are shaped like donuts because their eating/jet propulsion tube runs through the middle of their bodies and thus through the middle of their brains. It seems like this could be a problem if the squid eats too much or eats something with sharp bits in it, but squids seem to manage.

Squids can also leap out of the water and fly through the air for some ways. Octopuses can carry water around in their mantles, allowing them to move on dry land for a few minutes without suffocating.

Since cephalopods are somewhat unique among mollusks for their ability to move quickly, they have a lot in common, genetically, with vertebrates. In essence, they are the most vertebrate-behaving of the mollusks. Convergent evolution.

The vampire squid, despite its name, is actually more of an octopus.

Let me quote from the chapter on sex and babies:

This is one arena in which cephalopods, both ancient and modern, are actually less alien than many aliens–even other mollusks. Slugs, for instance, are hermaphroditic, and in the course of impregnating each other their penises sometimes get tangled, so they chew them off. Nothing in the rest of this chapter will make you nearly that uncomfortable. …

The lovely argonaut octopus

In one living coleoid species, however, sex is blindingly obvious. Females of the octopus known as an argonaut are five times larger than males. (A killer whale is about five times larger than an average adult human, which in turn is about five times large than an opossum.)

This enormous size differential caught the attention of paleontologists who had noticed that many ammonoid species also came in two distinct size, which htey had dubbed microconch (little shell) and macroconch (big shell). Bot were clearly mature, as they had completed the juvenile part of the shell and constructed the final adult living chamber. After an initial flurry of debate, most researchers agreed to model ammonoid sex on modern argonauts, and began to call macroconchs females and microconcs males.

Some fossil nautiloids also come in macroconch and microchonch flavors, though it’s more difficult to be certain that both are adults…

However, the shells of modern nautiluses show the opposite pattern–males are somewhat large than females… Like the nautiloid shift from ten arms to many tens of arms, the pattern could certainly have evolved from a different ancestral condition. If we’re going to make that argument, though, we have to wonder when nautliloids switched from females to males as the larger sex, and why.

In modern species that have larger females, we usually assume the size difference has to do with making or brooding a lot of eggs.Female argonauts take it up a notch and actually secrete a shell-like brood chamber from their arms, using it to cradle numerous batch of eggs over their lifetime. meanwhile, each tiny male argonaut get ot mate only once. His hectocotylus is disposable–after being loaded with sperm and inserted into the female, it breaks off. …

By contrast, when males are the bigger sex, we often guess that the purpose is competition. Certainly many species of squid and cuttlefish have large males that battle for female attention on the mating grounds. They display outrageous skin patterns as they push, shove, and bite each other. Females do appear impressed; at least, they mate with the winning males and consent to be guarded by them. Even in these species, though, there are some mall males who exhibit a totally different mating strategy. While the big males strut their stuff, these small males quietly sidle up to the females, sometimes disguising themselves with female color patterns. This doesn’t put off the real females, who readily mate with these aptly named “sneaker males.” By their very nature, such obfuscating tactics are virtually impossible to glean from the fossil record…

More on octopus mating habits.

This, of course, reminded me of this graph:

In the majority of countries, women are more likely to be overweight than men (suggesting that our measure of “overweight” is probably flawed.) In some countries women are much more likely to be overweight, while in some countries men and women are almost equally likely to be overweight, and in just a few–the Czech Republic, Germany, Hungary, Japan, and barely France, men are more likely to be overweight.

Is there any rhyme or reason to this pattern? Surely affluence is related, but Japan, for all of its affluence, has very few overweight people at all, while Egypt, which is pretty poor, has far more overweight people. (A greater % of Egyptian women are overweight than American women, but American men are more likely to be overweight than Egyptian men.)

Of course, male humans are still–in every country–larger than females. Even an overweight female doesn’t necessarily weigh more than a regular male. But could the variation in male and female obesity rates have anything to do with historic mating strategies? Or is it completely irrelevant?

Back to the book:

Coleoid eyes are as complex as our own, with a lens for focusing light, a retina to detect it, and an iris to sharpen the image. … Despite their common complexity, though, there are some striking differences [between our and squid eyes]. For Example, our retina has a blind spot whee a bundle of nerves enters the eyeball before spreading out to connect to the font of every light receptor. By contrast, light receptors in the coleoid retina are innervated from behind, so there’s no “hole” or blind spot. Structural differences like this how that the two groups converged on similar solution through distinct evolutionary pathways.

Another significant difference is that fish went on to evolve color vision by increasing the variety of light-sensitive proteins in their eyes; coleoids never did and are probably color blind. I say “probably ” because the idea of color blindness in such colorful animals has flummoxed generations of scientists…

“I’m really more of a cuddlefish”

Color-blind or not, coleoids can definitely see something we humans are blind to: the polarization of light.

Sunlight normally consists of waves vibrating in all directions. but when these waves are reflected off certain surface, like water, they get organized and arrive at the retina vibrating in only one direction. We call this “glare” and we don’t like it, so we invented polarized sunglasses. … That’s pretty much all polarized sunglasses can do–block polaraized light. Sadly, they can’t help you decode the secret messages of cuttlefish, which have the ability to perform a sort of double0-talk with their skin, making color camouflage for the befit of polarization-blind predators while flashing polarized displays to their fellow cuttlefish.

That’s amazing. Here’s an article with more on cuttlefish vision and polarization.

Overall, I enjoyed this book. The writing isn’t the most thrilling, but the author has a sense of humor and a deep love for her subject. I recommend it to anyone with a serious hankering to know more about the evolution of squids, or who’d like to learn more about an ancient animal besides dinosaurs.

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