© 1975-1979, 2008 Robert A. Freitas Jr. All Rights Reserved.

Robert A. Freitas Jr., Xenology: An Introduction to the Scientific Study of Extraterrestrial Life, Intelligence, and Civilization, First Edition, Xenology Research Institute, Sacramento, CA, 1979; http://www.xenology.info/Xeno.htm

11.3.1  Aquatic Locomotion

The most ancient and respected form of aquatic locomotion is by undulatory movements of the body. Even at the level of the protozoans we find this to be true. Alien fishes in watery oceans of other worlds must often have evolved some similar technique.

Earth’s first fish species -- eels, lampreys, and so forth -- swam in a serpentine fashion using sinuous motions of the body. More evolved creatures of the sea, such as the stingray and the skates, drive forward through the medium by rippling their bodies in a series of back-moving up-and-down waves (rather than the sideways motions of the eel).

As time went on, still more sophisticated methods came into general usage. Aquatic lifeforms developed paired fins for added power and control. Stabilizing vanes appeared. As in the case of the alligator and several other water-dwelling lizards, many fish use their tails alone as the main source of thrust.*

When traveling at a steady pace, the output of propulsive energy is proportional to the resistance of the surrounding medium. Therefore, it is in the best interests of the seagoing organism to reduce that resistance as much as possible. How might alien lifeforms accomplish this?

We might take a tip from the dolphins. These lissome marine mammals have a unique conformable skin which changes shape slightly as the animal slices through the water. At higher speeds, the cetacean varies its skin surface to maintain an exactly hydrodynamic streamlined form. The smooth, laminar flow of water over the dolphin’s body minimizes resistance and saves large amounts of energy. Indeed, porpoises are known capable of steady speeds of 55 kph, and still higher velocities are probably attainable by schooling.¹⁷⁰⁸

But there is no need to hurry so. Most animals manage quite well with far less speed at their command. Earthworms, nematodes and others with hydrostatic skeletons slither along the ocean bottom like tiny accordions.

Jellyfish, squids, cuttlefish and octopuses all use a kind of biological ramjet for propulsion. After water is passed over the gills for respiratory purposes, the exhaust is rapidly expelled. The siphon-like apparatus is such that, by merely gasping harder and faster, the organism can propel itself backwards in a series of sharp bursts. Octopuses can manage 8 kph or better in this fashion, and there is no reason why extraterrestrial biological ramjets could not do far better than this.

There are more exotic possibilities. One highly unusual technique which has never been exploited on Earth is osmotic power.

A permeable sac filled with salty water and placed in a beaker of fresh water will expand. This is the process of osmosis at work: The pure solvent flows through the membrane into the region of higher salt content. This represents a force which, when distributed over the surface of the sac, becomes a pressure.

Pressure is a force which can be harnessed to do useful work. And osmotic pressures are usually quite high: The pure water in the example above tries to dilute the seawater with a pressure of nearly 25 atm.²³⁰ This osmotic force is known to increase directly both with temperature and with salt concentration in the sac.

Could osmosis drive an alien fish through some faraway ocean?

Imagine a "freshwater" ET constructed in three sections: Head, torso, and hindbulb. The head faces forward, exposing a sac of highly concentrated salts to the surrounding water. An osmotic pressure of many tens of atmospheres forces the pure liquid into the sac in an attempt to dilute the salts. But this working fluid is continuously filtered through a large organ in the torso. There, energy is expended to reconcentrate the saltwater and to extract the excess water, which is stored in the hindbulb. When the creature wants to move, this liquid is rapidly exhausted from the hindbulb through a small nozzle in the tail.

There are three evolutionary preconditions for the emergence of the osmofish.

First, there must be some reason why the animal cannot simply inhale the surrounding fluids directly and jet them out again. Perhaps the osmofish inhabits a sea filled with poisons, or maybe the creature’s organs might be thermally or ionically damaged if contact with the outside were permitted. There is some terrestrial precedent for this: To this day, cell nuclei exposed to life-giving oxygen are poisoned by it.

Second, the medium must be so viscous that ordinary fin-flapping and body-wriggling are wholly inadequate. On a sulfur thalassogen world, where the sea alternated regularly from very thick to very thin, osmotic propulsion might evolve as a backup system for when the oceans became too gluey to swim in.

Third, the diffusion process across the membrane must be sufficiently fast to render the osmotic drive competitive with other forms of locomotion. This is probably do-able.

The osmofish is thus a very real and plausible possibility.

The surfaces of seas and inland pools of other worlds may harbor still more surprises. There are many forces that may be tapped for motive power. Let us consider just a few of these.

The swamps of Earth are inhabited by a fascinating variety of water beetle known to zoologists as Stenus. Thrown to the middle of a pond, these tiny creatures shoot to the safety of the banks at the water’s edge. The method of propulsion is not unlike that used by toy camphor boats: surface tension.

Surface tension is a property of all liquids, causing them to adhere to themselves at the interface between fluid and atmosphere. This tension represents a considerable force, and many insects such as water striders are elevated above water entirely by this support.

Stenus not only uses the surface for support, but for propulsion as well. By secreting a substance similar in action to camphor, the surface tension behind the organism is lowered. The resulting imbalance of forces causes the beetle to be drawn forward rapidly.

The data in Table 11.1 suggest that few liquids can compete with water in terms of providing useful energy for surface tension locomotion. Picture a propulsive appendage with reduced tension on one side only. Table 11.1 gives the maximum available force along each meter of appendage for which the surface tension is lowered from its normal value down to zero.

How large could an alien surface-sprinter be?

Let’s assume 100% efficiency, propulsive appendages one millimeter in radius with the approximate density of water, and a total length of all propulsive strands of a hundred meters when unfurled.

If such a creature totaled 2 kg in mass, it would theoretically be capable of accelerative bursts on the order of 0.4 gees -- the approximate rate achieved by Olympic-class human sprinters. The bundled threadlike appendages would represent 16% of the total body mass in this case. If only a tenth as much acceleration is required, our surface tension beast may weigh as much as 20 kg, with appendages a mere 1.6% of the total.

There are other forms of surface locomotion. The basilisk lizard of Central America has the unique ability literally to walk on water. (Locally, this has earned it the appellation "lagarto Jesus Cristo.")

This small but active reptile, biologically related to the desert iguana, speeds across lakes with a gait suggestive of the hasty canter of a terrestrial biped. This ability is of tremendous selective value, since by scurrying along the water’s surface the animal avoids aquatic predators as well as its enemies back on shore.²⁴³³

The basilisk is a cold-blooded animal, so a warm-blooded ET might be expected to do much better. The coordination and agility required at such high speeds could provide the challenge necessary for the evolution of intelligence on another world.

What about the principle of the rowboat? Rowing consists of a series of power strokes by which oars move backwards relative to the boat, thus driving it forward. Is it possible that extraterrestrial animals could copy this idea and, fantastic as it may sound, row across the surface of the sea?

Mother Nature has provided ample precedent. The hexapodal water beetles Dysticus and Acilius, and an insect known as the water boatman (Notonecta), use their middle and hind legs to row themselves along near the water’s surface. Hairs on their appendages have a distinctive oarlike appearance, and it has been calculated that this method of locomotion could be as much as 45% efficient energetically.²³⁰ The platypus, a far larger animal, may also be said to "row."

It is often asserted that rotating structures such as flywheels and paddlewheels cannot be used in living organisms, because "all parts of the body must be connected by blood vessels and nerves." This simple objection contains two hidden fallacies.

First, it is now known that certain spirochetes have flagellae driven by tiny ionic motors complete with rotor, stator, bushings and drive shafts.²⁴³² The feisty bacterium E. coli, for instance, comes equipped with a rotor spinning at roughly 60 cycles per second (just like the alternating current from a wall socket). Although admittedly small in size, this micromotor involves an axle which spins around its long axis through a kind of universal joint. It is in no way connected directly to the main body -- save through the rotating armature itself.¹⁶⁶¹

This finding contradicts the common assertion that living organisms may not contain detached, self-rotating parts.

Second, there really is no need to design a paddlewheel as a true rotating assembly. Fins may be stuck out, scooped through the water, and retracted again in regular sequence. This would provide both the appearance and the motive power of a true paddlewheel.

Many strong paddlers have evolved on this planet. The duck, for example, is efficient enough to propel herself through the water at a stately three kilometers per hour.¹⁰⁰⁶

Paddlewheel aliens thus may not be excluded a priori.

Wind power is yet another largely unexplored avenue of water surface locomotion. The possibilities are barely hinted at by the fauna of this planet. Whales are known playfully to dive underwater, leaving only their giant broadleaf tails exposed above the surface. These lighthearted leviathans then "sail," catching gusts of air on their huge tail vanes and drifting with the wind for hundreds of meters before they’re forced to come up for air.⁵⁵²² (Since it is far more efficient to swim than to sail, this is presumably a form of play.)

Let’s carry the idea to its logical conclusion.¹⁹⁴⁶

Imagine a mollusc-like organism with a moderately thick concave shell which inhabits the coastal shallows of another world. Over the years this alien species acquires the ability to float boatlike on the inverted shell, drifting with the shore currents slowly across the face of the planet. Such creatures might feed on floating plant life such as surface scum or the tops of seaweed stalks.

With evolution, the shell might become better adapted for navigation, perhaps developing a more streamlined underbelly. This would allow the ET to better chart its course between patches of food. Eventually it would discover that its speed could be significantly augmented with a crude "sail," a thin membrane growing up out of the animal’s back.

In time, the membrane could become retractable, or even delicately manipulable by muscles. With the emergence of a brain and sensory organs strictly comparable to molluscs on this planet, a kind of living clipper ship might evolve -- complete with masthead (forward sensors), jib, mainsail and riggings (extensible tendon), and probably a rudder. The minds of such lifeforms could be truly awesome.

What other glorious mysteries may await us in the star-dusted depths of space?

* As a general rule of thumb, the top swimming speed of aquatic lifeforms on Earth is roughly ten body-lengths/second. Normal cruising speeds check in at about four body-lengths/second.^224,2424

Last updated on 6 December 2008