Locomotion in Water
First, get a rubber band.
Movement is by no means a given for aquatic organisms. Many organisms are sessile for all of their lives. Some divide their life cycles into sessile and motile stages, and still others must remain constantly in motion. Some organisms move actively, expending energy to impart motion, while others drift passively, relying on water or wind currents, or density differences, to carry them. Many organisms, such as the plankton, are able to move their bodies within the currents they find themselves in, but are unable to oppose those same currents and move independently of them. The topic of aquatic locomotion is complex, and we will only be able to deal with a portion of that complexity here. We will start by examining the Newtonian physics that describes the rules of motion; then we will review some of the physical properties of water that have an effect on locomotion in the aquatic realm; and we will finish by examining the main modes of locomotion used by organisms ranging in size from microscopic plants to blue whales, the largest organism to have ever lived.
Newton's Laws and Basic Units
Three of Newton's "Laws" of motion will suffice for our understanding of locomotion. These are: I. a body at rest stays at rest unless acted on by a force; II. a body in motion continues in a straight line unless acted on by another force; and III. for every action there must be an equal and opposite reaction. Taken together, these laws can be restated as the principles of inertia and momentum. Before going further, we have to also consider the basic units of measurement that scientists employ to express Newton's Laws mathematically.
We must first define four basic quantities from which we will derive all the other properties of objects in motion. The four basic quantities are length, mass, time and temperature; which we measure as meters (m), kilograms (kg), seconds (s) and degrees Kelvin (K) respectively.
We can now answer a number of questions concerning the organism about to go into motion. We can measure its length (or width or height) in meters and its weight in kilograms. We can go a little further, and by multiplying and adding specific measurements of its size in meters, we can calculate its surface area in meters squared, or its volume in meters cubed. All of these are pure measurements with only one of our basic units involved at any one time. We can also describe some derived units, which call for measurement of more than one of the basic quantities at a time, For instance, we can determine the mass in kilograms and calculate volume in terms of meters cubed, and come up with density, which is kilograms per meter cubed.
Pure units describe our organism only at rest; once set in motion, we must necessarily use derived units to describe our organism because motion requires that we keep track of how far (meters) our organism moves and how long (seconds) it takes to get there. The basic unit of motion is speed, or velocity, which is expressed as length/time or meters/second. We can also talk about acceleration, which is the change in velocity over time, velocity/time or meters/second/second. If the change in velocity is from a higher velocity to a lower one, we speak of negative acceleration or deceleration.
We were able to describe velocity and acceleration by using only two basic units, length and time. Other derived units bring mass into the picture. You know that catching a softball is different from catching a hardball, mainly because the softball weighs more. This introduces the concept of momentum, which is the product of mass times velocity or kilograms x meters/second. By itself, momentum is kind of an abstract thing; we are more interested in what it takes to get that mass up to the velocity we are interested in, or what kind of wallop, or impact, that softball will have when it suddenly decelerates in our mitt. The push or impact is another derived unit, force, which is defined as mass times acceleration, or kilograms x meters/second x second. This formulation is both awkward to say and useful to know, so it is given its own unit in the SI, the newton.
Now, it's one thing to know how many newtons it will take to get a 1.5 kg squid moving 3 m/s in 4 seconds (1.125 newtons), but its also good to know how much energy an organism invests in locomotion. This is the realm of work, which is the product of force times the distance it is applied over, or length. In SI units it comes out kilograms x meters x meters/seconds x seconds; another mouthful given its own SI unit, the joule. If you keep track of how long you apply power, the next unit to appear is the watt, which is joules/second or kilograms x meters x meters/seconds x seconds x seconds. Another way to look at these units is to consider joules as energy and watts as energy expended over time. In this light, many relationships can be seen. A 60-watt lightbulb uses 60 joules/second, one calorie is 4.2 joules, one horsepower is 746 watts; to run a horse for an hour takes 0.746 kilowatt-hours or 2.6856 megajoules or 639,429 calories or 639 dietetic calories.
Finally, let us consider one last, special case of acceleration. Gravity is the force that attracts two bodies, and the Earth, acting as one of those bodies, attracts 1-kg bodies on its surface with a force equal to 9.8 newtons. We speak of the gravitational constant (for the Earth) of 9.8 m/s2, that is the acceleration due to gravity. The weight of an organism is equal to the force it applies to the ground; a one-kg organism has a weight of 9.8 newtons. Remember that the kilogram is a unit of mass, not weight! Weight is simply the measure of the Earth's attraction for an object, and since that relationship is proportional to an object's mass, our systems of scales utilize that relationship to estimate mass by calculating weight.
Physical Aspects of Moving through Water
When an organism moves through water, there are two basic forces it must overcome. As we have seen, for aquatic organisms the force of gravity is a minor concern. Because organisms are mostly made up of water, their weight is largely offset by the weight of the water around them. Organisms that want to maneuver vertically usually have body densities approaching neutral buoyancy, that is, the density of the surrounding water. Organisms that want to remain on the bottom have very dense bodies (negative buoyancy); those who wish to remain near the surface have less dense bodies (positive buoyancy).
The other force that organisms wanting to move (or stay still in a current) have to contend with is drag. Drag, as we saw earlier, is the force imparted by a moving fluid (or the force needed to move an object through a stationary fluid); drag has two components, friction drag and pressure drag. Friction drag may be visualized as the force needed to overcome the inherent cohesion of the water molecules; pressure drag may be visualized as the force needed to push the water molecules out of the way. These two types of drag differ in their importance. Smaller organisms or slower speeds shift the emphasis to friction drag, which is proportional to wetted surface area; larger organisms or higher speeds will emphasize the importance of pressure drag, which is proportional to volume and shape.
The Reynolds number determines which type of drag is more important given the conditions. At Re<1, friction drag predominates; at Re>1000, pressure drag is the major component of overall drag; the area from 1<Re<1000 is a transitional zone where neither factor can be ignored. The formula for Re is:
Where p = density of the fluid; l = characteristic length of the object; U = the speed (of the object or the fluid or both); and m = the dynamic viscosity. The length is somewhat arbitrary; it is usually the length normal to the flow, i.e. the diameter of a sphere, the diameter of a cylinder normal to the flow, the length of a cylinder parallel to the flow. Now, another way of looking at Re is in terms of its basic units. In the numerator, we have density (kg/m3), length (m), and velocity (m/s); in the denominator we have viscosity, which is the momentum of the water (kg/m/s). Multiplying across the numerator, kg/m3 * m * m/s, we get kg/m/s, and these units cancel with the kg/m/s in the denominator, making Re a dimensionless index.
The type of drag that predominates can have dramatic consequences for the organism. For instance, we live in a world dominated by pressure drag, and the way to reduce pressure drag (thus increasing efficiency of locomotion) is to streamline an object, making it present a small profile to the fluid the object is moving in - a Corvette has less pressure drag than a Mack truck, even if both were the same size. However, streamlining also increases friction drag. In our world of big things and high speeds, friction drag is by far the smaller coefficient of overall drag and can be ignored, but many small organisms live under the reverse conditions and cannot afford to overlook friction drag.
Organisms give us many clues about their lifestyles with the forms of their bodies, and this is particularly true in locomotion. One look convinces us that the streamlined squid is a faster swimmer than the bulky octopus; the squid's body is adapted to life at higher Re. We must be careful with this predictive tool, however, since other factors may also enter in. In particular, it is important for us not to be prejudiced about how we view the world of very small organisms, extrapolating our own experiences with water at our scale down to microscopic scales. To microscopic organisms, water (or even air) is more like molasses. The weight of individual molecules begins to have a measurable effect, for instance, the phenomenon called Brownian motion, where small particles vibrate randomly, is caused by an imbalance in the number of molecules vibrating into the different sides of the particle. We would laugh at people trying to propel themselves through water by wriggling a length of rope, yet at small scales flagellar movement is quite effective.
Movement by Microorganisms
Almost all movement by organisms (aside from moving with currents or gravity) is ultimately performed by using ATP to alter the shape of protein molecules in such a way that the protein molecule moves. The actions of thousands of protein molecules in concert magnifies the movement enough to do usable work for the organism, whether that be flexing a muscle, moving a flagellum, or moving a chloroplast along a microtubule.
Flagellar propulsion (we will, for now, consider flagellar and ciliary movement together, with some distinctions to follow shortly) is one of the simplest means of propulsion used in aquatic systems. Organisms from bacteria to flatworms use it to move the whole organism through the water, and even large multicellular organisms are not above using cilia to create currents in respiratory and digestive tracks. The differences between prokaryotic and eukaryotic flagella need not bother us here; we will concern ourselves only with the final result.
Flagellar propulsion works in a manner counterintuitive to us. Rather than acting by moving water in a rearward direction, and pushing the organism forward (from Newton's third law), the flagellum acts because as it moves the drag on one side of the flagellum is different than on the other side. Basically, what happens is that anytime a cylinder is moved with respect to a fluid at any angle other than straight, a force is developed that pulls it to one side, and that force is what is exploited for locomotion. In the case of short cilia, where the wavelength of the cilium is less than the wavelength of the motion imparted to it, the force is at a 90o angle to the cilia when it stands straight up. On a flagellum, where the ATP induced motion forms a wave shorter than the long length of the flagellar fiber, the force is either towards or away from the base of the flagellum.
Organisms typically have four or fewer flagella, usually just one. On organisms with flagella, the flagella usually make a rotating motion, much like a corkscrew, and either draw the organism forward or push it forward from behind. Organisms with cilia typically have many more - often thousands - and each cilium is much shorter than the typical flagellum. The cilia beat in coordination with each other, much like 'the wave' at a stadium. Often the cilia are arranged in discrete bands, which in turn offer more control. For instance, if the cilia on one side of the organism beat 'forward', and on the other side they beat 'reverse', then the organism will spin quickly. Usually flagella are found on smaller organisms and cilia on larger ones, and where these size ranges overlap the ciliated organisms are much faster, indicating that they are potentially the more efficient system (neither is very efficient, though). Cilia and flagella are ideal for small organisms moving relatively slowly; they require no elaborate muscles and skeleton. They only work at very low Re, however, and would be extremely inefficient on something like a whale. If nothing else, consider the old problem of decreasing surface/volume ratios as size increases. Remember that mass - one of the prime factors in determining how much force will be needed to set an object into motion - will increase with volume, and that the propulsion system, the cilia, are limited to the surface for attachment. Hydrodynamics aside, large organisms cannot use cilia or flagella as means of propulsion simply because they do not have enough surface area to attach enough cilia to move the increased mass.
Movement by Macroorganisms
With cilia not suitable as a propulsive force for larger (<2mm) organisms, some other method must be found. As we saw, part of the problem with cilia is due to the old problem of surface/volume ratios. Consider muscles, however. The fibers making up the muscle and doing the work are a large portion of the volume, and thus increase along with the size of the organism. Of course, muscles also bring on problems, such as a greatly increased need for O2 and nutrients; muscles that are in action for long periods of time require efficient circulatory systems.
Take a rubber band and stretch it. The force resisting you is comparable to that of contracting muscles - exactly comparable to the muscles in your arms that are stretching the rubber band. Now let go of both ends at the same time. Is any useful work done? Stretch the rubber band again; hold one end against your leg; and let go the other end. The impact you felt was some useful work (it woke you up, didn't it? - if you were sleepy enough to follow those instructions you needed it). More to the point, shoot the rubber band across the room at your roommate or a librarian or whatever. Now you have locomotion.
For a muscle to do any useful work, it must be anchored against something. That something is usually a skeleton, which come in three basic flavors: hydrostatic skeletons, where the muscles act against pressure imposed on body fluids (by other muscles); endoskeletons, where the skeleton is internal to the muscles; and exoskeletons, where the skeleton lies outside the muscles. Of course, some organisms use these in combination, and there are a few places where no skeleton at all is used, but rather the muscles work as antagonists against each other. This latter situation occurs in a number of situations, from flatworms to mollusk feet to human tongues (you didn't think there was a bone in there did you?). Reportedly, politician's spines also work on this principle. Even where skeletons exist, antagonistic muscles are often found, with one muscles responsible for movement in one direction, and the other muscle responsible for movement in another direction. The need for antagonists is a natural consequence of the fact that ATP induced contraction of muscle fibers is a one-way affair. The antagonist, by contracting, stretches its partner back out so it can contract again. In many situations, of course, gravity may also act as an antagonist.
Hydrostatic skeletons are common among aquatic invertebrates. In its basic incarnation, the fluid-filled body cavity is put under pressure by the contraction of muscles around the body cavity. When these muscles attempt to contract against the incompressible body fluids, the result is a rigid sac that forms a useful base for the muscles attached to it. A familiar example of the rigidity afforded by a hydrostatic skeleton is a balloon; it starts out limp, but once enough air is added, the rubber encircling the air space stretches and puts the air under pressure, forming a rigid structure. I'm sure you can think of other common examples of this principle.
Locomotion generally takes advantage of Newton's third law: for every action there is an equal and opposite reaction. In most cases, the action takes the form of pressing backwards against the substrate or water, with the reaction being forward movement of the organism. The points at which that backward pressure is applied are known as pivot points. On many substrates, the force that the organism exerts is minuscule compared to the mass of the substrate, and the substrate does not move; in these cases all of the force is used to move the organism forward and locomotion is very efficient. In loose substrates, or in water, the pivot points are on media that "give" and move backwards, wasting some of the energy and making locomotion less efficient. It is harder to run in deep sand than on gravel or a hard surface in large part because part of your effort goes to moving sand rather than your body.
Hydrostatic skeletons also make use of pivot points. We will look at several examples; the first is that of the nematode worms. These simple animals are only capable of thrashing back and forth. This motion throws their bodies into wriggling S-curves, but is highly inefficient for locomotion. As you recall from our discussion of flagella, when a cylinder is moved through water, there is more drag on one side than on the other, and the resulting imbalance of force can be used for propulsion. The flagellum is anchored to a larger body, however, and the forces can be resolved to do useful work. In the nematode, the cylinder is the body, and the drag forces largely cancel each other out, with only a small net movement resulting from thrashing around. On or in a substrate, however, efficient pivot points can be set up on the outside curves of the 'S'; and the body can thrust back against these points to move forward. It's still not very efficient, though.
Other organisms making use of hydrostatic skeletons include the segmented annelid worms (like the earthworm). These organisms have divided their hydrostatic skeleton into many small parts - one for each segment. This allows some parts of the body to be rigid while others are relaxed. It also allows for parts of the body that are relaxed to be wider than parts where the circular muscles are contracted. The combination of circular and longitudinal muscles allows for great control of the body (and requires a more advanced nervous system). It allows for more efficient locomotion, both in the pivot-crawling (described above for nematodes) and in thrusting, where one portion of the body is narrowed by contractions of circular muscles and forced forward through the substrate, where it subsequently anchors itself by expanding as a result of contraction of longitudinal muscles.
Marine polychaete worms (a type of annelid) are optimized for locomotion on or near the ocean bottom. They have the sides of each segment expanded into lobes or parapodia, which act as paddles when the worm swims. A paddle makes a more efficient pivot point in water since its wide end presses against a greater mass of water than a cylinder would. With these more efficient pivot points, these worms can swim. They actually form two types of pivot points - each individual parapodium forms its own pivot point, which it presses against as a result of local contraction of longitudinal muscles, and the outer curves of the body, which assumes a sinusoidal shape when swimming, also form pivot points. When crawling, the individual parapodia press against the substrate, and the worm can also twist its body, moving the parapodia rapidly.
Another type of locomotion frequently performed by organisms with hydrostatic skeletons is a looping motion. Perhaps the most familiar example of this is the motion of the inchworm (which has an exoskeleton); looping motions are also performed by Hydra (no skeleton) and starfish (endoskeleton), as well as cheerleaders (endoskeleton, usually double-jointed). For our purposes though, the leech is most instructive. The leech has taken a different approach to locomotion than its kin the earthworms and polychaetes. Leeches have abandoned segmentation and gone back to a more "primitive" body arrangement with a single internal cavity. They have then taken this simple body plan and combined it with suckers on either end of the body so that when it plants a pivot, it really plants a pivot. Leeches can also swim (not very efficiently) by waving their flattened body and forming pivots on the outer edges of the curves against the water. We will come back to the concept of undulating a flattened body to propel oneself later.
A final type of propulsion, before we leave hydrostatic skeletons, is jet propulsion. It operates by actively pushing water from inside to the outside in a narrow stream or jet. Since the water has mass, the body will acquire a momentum equal to that of the mass of the water times its velocity. It should be obvious that even a massive organism can achieve jet propulsion with a relatively small amount of water if it can accelerate that water to high velocity; you already know that the effort required to hold a hose nozzle is proportional to both the velocity and volume of water that is exiting the nozzle. Keep in mind also that as an organism squirts out water it is decreasing its own mass, making such a simple system a rather complicated one to model mathematically. Jet propulsion is found among animals with hydrostatic skeletons, internal skeletons (squids), external skeletons (dragonfly larvae, scallops), and no skeletons (jellyfish).
Before discussing propulsion in organisms with exoskeletons and endoskeletons, we should go to the old chore, repeated in every biology course that deals with animals (and even, by habit, in some botany courses) of explaining the differences between the two. For the millionth time, exoskeletons have a mechanical advantage only for smaller organisms, where they can serve as both support and protection. Because the muscles are on the inside, they are limited as to the amount of space they can occupy, and they cannot gain leverage in their attachments. To increase the size of the muscles, you must increase the size of the tube (skeleton) which holds them, but as you increase its size you must also increase its thickness to support its own weight. The increased weight of the skeleton offsets any additional power obtained from the larger muscles. It's really a variation of our old friend surface/volume ratios in a slightly different form. Internal skeletons place no constraint on the size of the muscles, which can grow as large as necessary and which also may find more advantageous positions to attach; positions that increase leverage and thus amplify the power of the muscle. The practical limitations of each type of skeleton are obvious to us; the largest practical size for an external skeleton is about the size of the largest invertebrates in the water (30-kg lobster today, larger fossils in the past). The water supports some of the weight of the exoskeleton; larger organisms on land all have endoskeletons.
The basics of locomotion are the same for organisms with exoskeletons and endoskeletons, and we will consider them both together. In general, these can be divided up into movements of the whole body, such as pivot-crawling or pivot swimming, looping, and jet propulsion, which we have discussed above, and movements performed by the limbs which skeletons make possible such as rowing (a type of swimming), walking, and swimming by lateral undulation, which differs (somewhat) from pivot swimming.
Walking is only done by the densest aquatic organisms. It's not that they're stupid, it's just that walking requires pivot point maintained by friction, and friction requires weight (in a current, some organisms substitute drag for weight, see McShaffrey and McCafferty 1987). Walking involves a complex set of movements that must be carried out in sequence by several different sets of antagonistic muscles. First comes elevation, where the leg is lifted; next is protraction, moving the limb forward; followed by depression, lowering the leg to the substrate and establishing a pivot; and finally there is retraction, where the body is swung around the limb with the new, forward pivot point, thus moving the body forward. The coordination comes in keeping the legs from tangling with each other and making sure that retraction only occurs on those limbs that have just placed forward pivot points. Aquatic insects, for example, lift three legs at once, two on one side of the body, and one on the other. The other three legs thus form a stable tripod for the insect to rest on while the three lifted legs are protracted and depressed. With all six legs back on the substrate, the insect then lifts the three legs that hadn't been in the air, and while they are lifted, the other three legs are retracted, allowing the body to move forward. This forward movement also insures that the legs in the air will establish new pivot points even further forward.
Rowing is a very common form of propulsion among aquatic organisms with limbs, including insects, crustaceans, fish (using the lateral fins), amphibians, reptiles, birds, and mammals. Laypeople refer to it as swimming, but it is very distinct from the swimming we will take up next. Rowing is much like walking, the limb is moved forward, places a pivot, and is moved backward. The big difference is that using water as a pivot point means that rather than the limb being planted firmly, it will move backward, carrying the water along with it. It is the momentum of this moving water that constitutes the Newtonian action; the reaction is the forward momentum of the organism.
For rowing to be efficient, the backward motion of the limb (power stroke) must move more water than the forward motion (recovery stroke). For most organisms this all occurs at high Re, and here the answer is obvious to us. The limbs of rowing organisms are usually paddle-like, wide in one dimension, narrow (streamlined) in the other. On the power stroke the wide dimension is pressed against the water; the extreme drag of this configuration causes lots of water to move along to the rear with the limb; therefore moving a large mass of water rearward with the equal forward force applied to the mass of the organism. On the recovery stroke the paddle is rotated or feathered (for you rowing fans) and the streamlined edge moves easily forward through the water, bringing little water along with it and thus minimizing the rearward force applied to the organism. Examine rowing organisms closely and you will note flattened hind legs on beetles, collapsible fins on turtles, frogs, fish and fowl (not to mention platypuses), or even the streamlined shape of bird wings (penguins fly, or row, underwater with their wings).
At low Re, the same basic concepts apply, but streamlining is no longer effective. Small organisms typically use hairs rather than solid structures to make up their paddles; the hairs can be folded to reduce drag on the recovery stroke and do not create as much friction drag on the power stroke as a solid structure would. The physics of flow at low Re suggests that as far as rowing is concerned, a series of hairs moves water backwards as effectively as a flat plate would anyway, since water does not tend to flow between the hairs at low Re. This explains the hairs on the back legs of beetles (which are small enough to be in the transitional zone), and on the legs, antennae, mouthparts, etc. of small aquatic crustaceans.
Undulatory swimming, as exhibited by fish, salamanders lizards, snakes, cetaceans, and some worms, is a refinement of pivot swimming. In undulatory swimming, thrust, or a forward force, is generated by pushing against pivot points in the water in a highly coordinated, stereotypic pattern. The body of the organism is bent to form moving waves, and the length of the body involved has an impact on the swimming motion that will result. Long flexible fish like eels, or aquatic snakes, involve most of their bodies in the undulations and their swimming style is referred to as anguilliform; some fish keep most of their body rigid and flex only the tail region in what is known as carangiform swimming; and others flex only the tail itself to perform ostraciiform swimming movements (Fig. 1). As a general rule, the less of the body involved in the swimming stroke the more effective it will be; therefore the carangiform and ostraciiform movements are more efficient because less of the body is involved (and creating drag on the recovery stroke), also, in carangiform and ostraciiform swimming certain hydrodynamic factors such as angle of attack can be optimized.
The fastest fish are those which have powerful muscles in the body coupled through a small peduncle (the base of the tail) to the tail fin, which provides most of the propulsion. This is the case for such fish as the tuna and swordfish, and also for dolphins and whales. Remember that the undulatory movements of fish are lateral and the movements of marine mammals are vertical; both are about equally efficient; the vertical movements of the marine mammal's tail are a result of the tetrapod ancestry, not some obscure hydrodynamic principle. In most fish, the caudal, or tail fin, provides most of the thrust, while the dorsal and ventral fins provide control over side-to-side motion or yaw; the lateral fins (corresponding to the appendages of a tetrapod, the pectoral and pelvic fins) provide control over rolling movements as well as the attitude of the body (head up or down, pitch). In most fish, rowing movements by the lateral fins provides slow-speed movement with great control, and in some fish undulations of the dorsal and/or ventral fins also provides slow speed movements, again with great control. Some fish use these other fins, either lateral, dorsal or ventral, to the exclusion of the caudal fins; examples of this include the rays and the knifefish.
Figure 1. Above:Five fish showing body forms adapted for certain styles of swimming. A. Trunkfish showing ostraciiform swimming. Note the boxy body, the small peduncle, and relatively small tail. B. Wrasse showing labriform swimming. Labriform swimming is rowing using the fins; note the well-developed pectoral fins. Of course, the wrasse can move using the tail in a form of carangiform swimming. C. and D. Triggerfish illustrating balistiform swimming. Here the dorsal and anal fins move the fish with short-wavelength oscillations. E. Rajiform swimming as demonstrated by the knifefish. Movement occurs in response to long-wavelength undulations of the anal fin. Not illustrated: Carangiform swimming of many types shown by many fish, and anguilliform swimming as performed by eels, dogfish, lampreys, snakes, etc. Below: Images of a spotted eagle ray (left), a stingray (middle), and a flying grunard (bottom). The rays swim by rajiform movements of the modified pectoral fins. The flying grunard uses its pectoral fins as "wings" which allow it to leap from the water and glide for some distance.
Finally, one last form of locomotion should be mentioned (we'll still end up skipping a few). Snails and other gastropods often move by means of complex waves propagated across the interface between the foot and the substrate. The waves are caused by muscles, and, just like waves in the water, can cause motion. Crucial to this method of movement is the ability of the mucus secreted by the gastropod to bind tightly to the substrate at some points (pivot points), and flow freely at others, all as a result of changes in pressure.
Resident Function Groups
It is possible (and even useful) to group organisms according to how they move (or stay put) in the environment. In aquatic systems, for instance, we make distinctions between nekton, organisms which can swim, even against some currents, and plankton, organisms which, although they can move, are really at the mercy of the currents (plankton also tend to be microscopic). We also refer to benthic organisms, organisms that live on the bottom. There are many subdivisions of these groups. We have already discussed the various forms of swimming; let's move to some of the distinctions among benthic organisms (Fig. 2).
Figure 2. Resident function groups of aquatic organisms. The diagram shows the three main habitats for aquatic organisms, what resident groups are found in each, and what types of currents to expect in those habitats. As with many other classification schemes, these are only approximations and many exceptions occur in nature.
First, remember that benthic, as used here, includes not only the actual bottom, but any substrate in contact with the bottom, such as aquatic plants. Organisms that use claws or other means of attachment to hold onto the substrate are called clingers; if they hold onto plants we call them climbers. Both climbers and clingers are often distinguished by strong claws, and long legs; others have specialized means of attachment. For instance, rheophilous (current-loving) organisms like blackfly larvae can use silk to hold themselves in place; other organisms may have suction disks. In a sense, organisms like plants, which use roots to grip the substrate; and sessile animals such as crinoids or sea fans, which use root-like holdfasts to stay in place; are examples of clingers.
Sprawlers are organisms that live on the bottom (or in crevices within the substrate). They tend to be more mobile than clingers are, and lack specialized, permanent attachment structures. Many sprawlers are heavily ballasted with structures such as cases or shells that help them stay on the bottom.
Burrowers are organisms that live in the substrate. Some actively excavate the substrate to create a burrow; others simply move into crevices or burrows created by other organisms. If the organisms are so small that they can freely move between the naturally occurring pores in the substrate, they are termed interstitial. In many natural substrates, such as gravel or sand beds, the size of the interstitial pores are large enough that organisms such as tardigrades and rotifers predominate. These organisms are larger than most other truly microscopic forms and are often referred to as the meiofauna. Many burrowers line their burrows with silk or other materials to keep the burrow from collapsing. Burrows may be constructed to take advantage of natural currents to induce a water flow through the burrow for feeding or respiratory purposes; other organisms actively pump water through their burrows.
At the other end of the water column is the neuston, consisting of organisms that exist in association with the surface film. Floaters remain at least partially submerged, held at the surface due to buoyancy or attachment to the surface film (such as in mosquito larvae). Skaters glide over the surface of the water and include water striders (Fig. 3, right). Walkers move more slowly over the surface; some spiders can walk on the water's surface. Jumpers leap from the water surface; springtails and pygmy mole crickets are examples of jumpers.
As with any other classification system, there are problems with the above categories. Most organisms are capable of more than one of the above roles, at least at some part in their lives. Many aquatic organisms start out life as plankton, for instance, and as they grow, they are able to specialize. Others may change resident function group swiftly. For instance, how do you place a mayfly larva which crawls out from under a rock, scampers across its surface, drifts downstream with the current, then swims to another rock which it promptly burrows under?
RIBERA, I, GN FOSTER. 1997. Functional types of diving beetle
(Coleoptera:Hygrobiidae and Dytiscidae), as identified by comparative swimming