Sunday, March 7, 2010
Aliens Among Us
(Picture of Odontodactylus scyllarus, from here.)
Popkes' third rule: If under examination it appears simple, you probably haven't examined it closely enough.
Corollary to Popkes' third rule: If under examination it appears that something is too complex to understand, you certainly haven't examined it closely enough.
First, a little history.
I took my undergraduate degree in Zoology with a minor in chemistry. Later, I went on and took a MS in Neurophysiology. It was a hard decision.
The problem arose out of a course I took in Invertebrate Zoology and came out of the first sentences of the first lecture: "Humans like to think they're important and so we divide the animal kingdom into vertebrates and invertebrates, animals with and without backbones. I.e., like us and not like us. The fact that nearly all the world of animals is not like us has no bearing in our attempt to divide the world in half."
Studying animals and their existing relationships is, essentially, natural history. Studying how they work is physiology. Studying where they came from is evolution. Eventually, I chose to to try to figure out how they work and eventually left biology entirely, ending up in software engineering.
But my heart still lies in biology.
I write mostly science fiction and I like to write about aliens. It's not a coincidence that many of my aliens (notably Bishop 24 in the Future Boston anthology) are based on invertebrates.
Evolution determines the range of possibilities an organism has. Remember, evolution favors the most fit organism in a local environment. It has no real clue on what comes later. Therefore, paths that have been taken by an organism in its past become a limiting factor in its future. The streamlining of a parasitic organism's organs, such as a tapeworm, are extreme. Such an organism has no need of a true gut, eyes, brain, etc., and sheds those as expensive luxuries. But, once shed, the organism cannot simply regrow them under different circumstances.
That said, evolutionary reversals can happen (See here and here.) and the degree of which depends on the method by which the original structures were suppressed. But that is not our subject for today.
The aliens I've developed have often been extensions of life here on earth-- a biological "what if" scenario. What if invertebrates were not limited to the structural strength of chitin but were able to biologically synthesize stronger materials? (The Bishop, mentioned before.) What if the dominant life form descended from a Parasaurolophus ancestor? (Whistle in the Dark, Asimov's, June 1994.)
How alien can life on earth be? Let's find out. Let's talk about mantis shrimps.
Mantis shrimp are neither related to mantids or shrimps, except that mantids, shrimps and mantis shrimps are crustaceans. That is, they comprise a group of arthropods that have an exoskeleton, a ventral nerve cord, compound eyes, etc. Flies, crabs and barnacles are all crustaceans.
Mantis shrimp the only existing group of stomatopods. There are other groups in stomatopoda but they're all extinct. (See here.) They've been around for nearly 400 million years. Stoma means mouth and poda means foot. Stomatopoda have abdominal gills, hence the grouping.
This ordering of mantis shrimp puts them square in the invertebrates and fairly far from the crustaceans (insects and shrimp, for example) that we normally see.
Mantis shrimp have a number of common names: prawn killer, thumb splitter. They are aggressive predators on smaller crustaceans. They are often inadvertently found in salt water aquariums (here). They get the term "mantis shrimp" from the claws. (Picture from here.) The different species within the group are differentiated largely by how they use the claw: spearing or smashing. We're going to talk about smashing.
Here's a video of a Odontodactylus scyllarus smashing its prey. Here's another. Notice you can hear it through the glass. Outside. There's no mike in the tank. Given that the sound is audible, you can imagine that the force involved is fairly strong.
You have no idea.
The acceleration of the blow for a mantis shrimp is on the order of 10,000g-- similar to the acceleration of an artillery shell. What's more, it hits with about 1,500 newtons. A newton is the force required to accelerate a mass of 1 kg 1 meter/second. To put it in perspective, a newton is the force of gravity on something of a small mass-- 102 g-- like an apple. 1,500 newtons is the force of gravity on an apple on a planet about 1/2 the mass of the sun. (See here.) The "club" of the mantis shrimp has evolved to take this. In the picture at left, the brown limb is the mantis club compared to a non-striking species. (Picture from the evolution article from Berkeley here.)
But wait. There's more.
The force applied by a mantis shrimp is so great that it causes the creation of vacuum bubbles next to the surface of the prey, a phenomenon known as cavitation. (See here and here.) The collapse of the bubbles approximates 1/2 the original force of the blow. The energy released is so great it can heat the water to the point it glows with visible light. (Here's another video.)
How does it do this?
We might do it with an explosive or a rocket. Animals who pack this sort of punch (fleas, pistol shrimp) do it by storing energy in a limb or muscle, cocking the limb like a trigger and then releasing it all at once. Fleas store the energy in resilin and then release it for their hops.
Mantis shrimps use a saddle shaped piece of chitin that is bent by muscular energy and then released. (Additional mechanisms are discussed here.) Because of the lever action, the striking limb (called a dactyl) moves much faster than the releasing chitin and attains the tremendous acceleration. The dactyl degrades over time but is rebuilt when the shrimp molts and creates a new skin. (See here, as well.)
Since the stomatopods are a single surviving group, we have only fossil stomatopods to compare to. However, an analysis of the full mechanical mechanism (here) show that the spearing mantis shrimp and the smashing mantis shrimp have similar anatomy. The smashing mantis shrimp have increased the amount of energy they can store in the chitin. The specifics of how the energy mechanics first evolved in the stomatopods aren't well known as yet.
Mantis shrimp aren't just alien in their mechanics. They see differently as well.
Humans have three retinal pigments that approximate red, blue and green colors. A good discussion of human color vision is here. But I'm just going to talk about pigments for a bit.
If, say, we saw colors as on or off we could represent the field of color vision as three bits. Red would be on, no red would be off. Red and Green would be red bit on and green bit on with blue bit off. Etc. With this sort of representation we would only be able to see 8 colors: 8 possible combinations of bits to represent a color. This represents base 2 to the third power. On/off is binary-- base 2-- and 3 bits represents the power. If we were able to see 10 values for each color we would have a 1000 colors: 10 cubed. In point of fact, we can see many more gradations of color than 10. Humans see about 7,000,000 colors. The cube root of that is about 191. So, let's say in our model that humans can see about 191 gradations of pigment color.
(In point of fact this is wildly wrong since color vision is extremely complex. But this is just to make a point.)
Mantis shrimp have 16 pigments, twelve of which have been attributed to color vision. (See here.) Let's say, for the moment, that the same 191 grades of sensitivity apply to mantis shrimps. That means 191 to the 12th (not cubed) power. Or:
2, 357, 221, 572, 577, 185, 690, 065, 114, 881 colors (2 * 10**27)
336,745,938,939,597,955,723 as many colors as we have.
These numbers don't make any sense, do they? Clearly, the mantis shrimp doesn't see colors the way we do. Even if it was on/off representation, as we discussed a moment ago, that would be 4000 colors. Some scientists believe that mantis shrimp do hyperspectral imaging and don't see color as we do at all. Instead, they perceive color patterns rather than colors.
However, mantis shrimp vision is even weirder. Mantis shrimps have a compound eye. Each eye in effect has trinocular vision and depth perception. (A full discussion is here.) They see into the ultraviolet and down into the infrared and can even see polarized light. (See here, here and here.)
Remember these creatures evolved. What would cause such a radical approach to vision?
One idea has to do with the prey of mantis shrimp. Many of the species the mantis shrimp preys on are transparent-- at least they are to our eyes. But to the mantis shrimp they may not be. Also, the speed of the blow may require high precision eyesight.
However, there's an interesting correlation. The shrimp with the most advanced color vision are also those who are themselves most colorful, suggesting that the vision and coloration are sexually selected.
I think probably all of these factors contribute. Traits aren't just selected for by a single selection criteria. The same color vision that lets us detect red hair allows us to detect red lights. The same could be true for mantis shrimp. Predation and sexual selection could both be in play.
So: here's an animal that has been on the earth a long time-- longer than land based vertebrates. As truly alien to us as anything from another star system.
Living in our aquariums.
More on mantis shrimp:
Evolution of Mantis Shrimp
Mantis shrimp shattering aquariums.
Mantis shrimp see all forms of polarized light.
Mantis shrimps show us a new way to make DVDs. And here.