Bottom locomotion
Small flatworms (Platyhelminthes) and some of the smaller molluscan species move along the bottom by ciliary activity. On their ventral (bottom) surface, a dense coat of cilia extends from head to tail. The direction of the ciliary beat is tailward, causing the animal to glide slowly forward. Generally, all animals that move by this type of ciliary activity secrete a copious stream of mucus over which the animal glides. The mucus not only attaches the animal to the surface but also raises its body so that the cilia can beat. Because ciliary forces are too weak for the movement of large flatworms, they must use muscular contraction for their propulsive force.
Aquatic invertebrates possess several other types of bottom locomotion. In species with well-developed legs, such as crabs and lobsters, bottom walking is common. Whereas the gaits in such cases are identical to those used on land, they tend to be slightly faster in water, because the buoyancy increases the animal’s stability. (See below Terrestrial locomotion.)
Another form of bottom locomotion is bottom creeping, which employs the contract–anchor–extend method of movement. Bottom creeping is best developed in leeches, which have two suckers, one at the anterior end and one at the posterior end. After the posterior sucker anchors the animal, it stretches its body forward and attaches the anterior one. It then releases the posterior sucker and contracts its body toward the anterior end. For effective contract–anchor–extend locomotion, the body musculature must consist of both circular and longitudinal muscles: the contraction of the circular muscles extends or elongates the body; the contraction of the longitudinal muscles flexes and shortens the body. Moreover, the skeleton should be hydrostatic; that is, a fluid skeleton that changes shape but not volume, thereby providing a firm but flexible base.
In pedal locomotion, which is a slow, continuous gliding that is superficially indistinguishable from ciliary locomotion, propulsion along the bottom is generated by the passage of contraction waves through the ventral musculature, which is in contact with the bottom surface. The pedal contraction waves are either direct (in the same direction as the movement) or retrograde (in the direction opposite to the movement). The direct waves produce locomotion in a manner analogous to that in which a caterpillar walks. When a direct wave reaches a muscle, the muscle contracts and lifts a small part of the body; the body is carried forward and set down anterior to its original position as the wave passes. With direct waves, the surfaces of the body touching the bottom surface are not the ones that contract; with retrograde waves, however, these are the surfaces that do contract. As the retrograde wave approaches, the body area immediately adjacent to it is extended upward. The body surface within the contraction area then anchors itself to the bottom surface, after which the body is pulled forward.
Large flatworms use pedal locomotion instead of or in alternation with ciliary activity. In the gastropod and amphineuran molluscans (e.g., snails and chitons, respectively), pedal locomotion is the primary locomotor mode and has become highly complex. The foot of these creeping animals is extremely muscular, penetrated by nerves, and capable of generating one, two, or four laterally adjacent contraction waves. If the foot generates a pair of waves, the lateral halves of the foot may alternate, thereby producing a shuffling movement, or they may be opposite. Generally, a foot can contain no more than one whole and two partial waves moving along a single axis.
Peristaltic locomotion is a common locomotor pattern in elongated, soft-bodied invertebrates, particularly in segmented worms, such as earthworms. It involves the alternation of circular- and longitudinal-muscle-contraction waves. Forward movement is produced by contraction of the circular muscles, which extends or elongates the body; contraction of the longitudinal muscles shortens and anchors the body (see below Fossorial locomotion).
Although peristaltic locomotion is frequently used by sea cucumbers, they and other echinoderms, such as sea urchins and starfishes, possess rows of tube feet that provide the main locomotor force. In starfishes, each arm bears hundreds of tube feet. Only one arm, however, becomes dominant in locomotion; while the tube feet on that arm move toward the tip of the arm, the tube feet of the other arms move in the same plane as those of the lead arm. Because there is no apparent metachronal wave of contraction within an arm, the movement of the tube feet is poorly coordinated, but small areas of the tube feet do move in synchrony. Each tube foot is a hollow elastic cylinder capped by a hollow muscular ampulla (a small, bladder-like enlargement). When the ampulla contracts, it forces fluid into the tube foot and extends it. Preferential contraction of muscles in the wall of the tube foot controls the direction of and the retraction of the tube foot. When the tube foot is fully contracted, fluid is withdrawn from it by relaxation of the ampulla, after which the muscles of the tube foot swing it forward in preparation for another step.
Swimming
Invertebrates have developed two distinct propulsive mechanisms for swimming: some use hydraulic propulsion; all others utilize undulations of all or parts of their bodies. The medusa (umbrella-shaped) body of coelenterates and ctenophores (e.g., jellyfish and comb jelly, respectively) is a flexible hemisphere with tentacles and sense organs suspended from the edge; a manubrium (handle-shaped structure) bearing the digestive system hangs from the internal tip of the hemisphere. Enclosed in the outer margin of the medusa is a wide muscular band; when this band contracts, the opening of the medusa narrows. Simultaneously, water is ejected from the medusa through the narrow opening, and the animal is propelled upward. Because the contractions tend to be regular but slow, locomotion is somewhat jerky.
Scallops are the best swimmers among bivalve molluscans that can swim. Locomotion is produced by rapid clapping movements of the two shells, creating a water jet that propels the scallop. The muscular mantle (a membranous fold beneath the shell) acts as a valve and controls the direction of flow of the ejected water, thereby controlling the direction of movement. Normally, the flow is directed downward on each side of the hinge that joins the two shells, and the resulting water jet lifts the scallop and moves it in the direction of the shell’s opening. If necessary, however, escape movement may occur in the opposite direction. The scallop is adapted to swim even though it is two or three times as dense as seawater. The hinge is elastic and opens the shell rapidly; this action, coupled with rapid and repeated contractions of the adductor muscle, which closes the shell, produces a powerful and nearly continuous water jet. Moreover, the body form of a closed scallop is an airfoil (like a wing, the curvature of its upper surface is greater than that of its lower surface); this shape, combined with the downward ejection of water, produces lift.
Cephalopods (e.g., squids, octopuses) are another group of mollusks that use hydraulic propulsion. Unlike the scallops, they have lost most of their heavy shell and have developed fusiform bodies. The mantle of cephalopods encloses a cavity in which are contained the gills and other internal organs. It also includes, on its ventral surface, a narrow, funnel-shaped opening (siphon) through which water can be forcibly ejected when all the circular muscles surrounding the mantle cavity contract rapidly and simultaneously. This water jet shoots the cephalopod in a direction opposite to that in which the siphon is pointed.
Many invertebrates, particularly elongated ones such as open-sea-dwelling annelids and mollusks, swim by undulatory movements produced by contraction waves that alternate on each side of the body. Although the arrangement of the musculature differs between invertebrates and vertebrates, the mechanics of undulatory swimming are the same in both and are described in the following section.
Fish and fishlike vertebrates
Undulatory swimming is roughly analogous to using one oar at the stern of a boat. The side-to-side movements of the oar force the water backward and the boat forward. The undulatory movement of a fish acts similarly, although the motions involved are much more complex.
Anguilliform locomotion
When an elongated fish such as an eel swims, its entire body, which is flexible throughout its complete length, moves in a series of sinuous waves passing from head to tail. In this type of movement, which is called anguilliform (eel-like) locomotion, the waves cause each segment of the body to oscillate laterally across the axis of movement. Unlike the simple side-to-side movement of the oar, however, each oscillating segment describes a figure-eight loop, the centre of which is along the axis of locomotion. It is these oscillations and the associated orientation of each body segment that produce the propulsive thrust.
The undulatory body waves are created by metachronal contraction waves alternating between the right and left axial musculature. During steady swimming, several contraction waves simultaneously pass down the body axis from head to tail; the resultant undulatory waves move backward along the body faster than the body moves forward. As the undulatory wave passes backward, its amplitude and speed increase, thereby producing the greatest propulsive thrust in the tail (caudal) region. Propulsion, however, is not limited to the caudal region, for all undulating segments contribute to the thrust. Because the speed, amplitude, and inclination of each body segment differ, the thrust of each differs. In all segments, the greatest thrust is obtained as the segment crosses the locomotor axis, for here it is travelling at its greatest speed and inclination.
Carangiform and ostraciiform locomotion
All undulatory swimming movements generate forward thrust in the manner described above. Not all swimming animals, however, possess the elongated shape of an eel; only those with a similar body form, in which the surface area of the head end is the same as that of the tail end, have anguilliform locomotion. Fish with fusiform bodies exhibit carangiform locomotion, in which only the posterior half of the body flexes with the passage of contraction waves. This arrangement of body form and locomotion apparently is the most efficient one, for it occurs in the most active and fastest of fish. The advantage of carangiform locomotion appears to be related to the effectiveness of the posterior half of the body as a propulsive unit and the fact that the shape of the body and its small lateral displacement create little water turbulence. In contrast to ostraciiform locomotion, in which only the caudal fin oscillates from side to side in a manner similar to moving a boat with one oar, the length of the propulsive unit of carangiform fish enables the unit to obtain maximum oscillatory speed and inclination.
Whales also use undulatory body waves, but unlike any of the fishes, the waves pass dorsoventrally (from top to bottom) and not from side to side. In fact, many mammals that swim mainly by limb movements tend to flex their body in a dorsoventral plane. Whereas the body musculature of fish and tail musculature of amphibians and reptiles is highly segmental—that is, a muscle segment alternates with each vertebra—an arrangement that permits the smooth passage of undulatory waves along the body, mammals are unable to produce lateral undulations because they do not have this arrangement. Nor does the muscle arrangement of mammals permit true dorsoventral undulations; however, with an elongated caudal region, as in whales, they can attain a form of carangiform locomotion as effective as that of any fish.
Stabilization and steering
To stabilize and steer, most aquatic vertebrates have, in addition to the caudal fin, a large dorsal fin and a pair of large anterolateral fins. Although they may possess other fins, these are of less importance. The balance of a swimming animal may be maintained in several ways. Rolling, or rotation, along the longitudinal axis of the body is reduced or controlled by any fins that extend at right angles to the body. Pitching, or dorsoventral seesawing, movements are counteracted by the anterolateral fins, which are also the major steering organs of fish, whales, and seals. Yawing, or lateral seesawing, is prevented by the dorsal fin and, if present, a ventral fin; for these fins to be effective, however, most of their exposed surface area should be behind the fish’s centre of gravity. Because fins of the above type are not common in most invertebrates that swim by undulation, their locomotion is less stable.
Tetrapodal vertebrates
Many of the various types of undulatory locomotion described above are also widely used by aquatic tetrapods (those with walking appendages). Larval frogs, crocodilians, aquatic salamanders, and lizards, for example, have long muscular tails that propel them by undulatory motion. Most aquatic tetrapods, however, move by appendicular locomotion, for which the major propulsive units are the hind legs. The exceptions are sea turtles, auks, penguins, and fur seals; in these, the hind feet are webbed and are used as rudders. For propulsion, these animals use their forelegs, which have become bladelike flippers in which the forearm and hand region are dorsoventrally compressed to form a single, inflexible unit. The movements of such flippers are analogous to the aerial flight of birds; by moving synchronously, they provide lift and thrust in the water. Unlike aerial flight, however, the upper arms do not produce lift or thrust; instead, they serve only as a pivotal or leverage point for driving the flippers.
Swimming movements in sea turtles, penguins, and auks are accomplished by the rotation of the flippers or wings through a figure-eight configuration. In the birds, however, the stroke is relatively faster than in sea turtles, because the entire cycle appears to be proportionately smaller in amplitude. Moreover, because the birds’ bodies are more streamlined, they can attain greater speeds than the turtles. Penguins may attain speeds of 40 kilometres (25 miles) per hour in water and have sufficient speed and thrust to enable them to leap two metres (six feet) or more above the water. The wings of penguins are so highly modified, however, that they have lost the ability to fly. The auks, on the other hand, are able to use their wings for both aerial and aquatic locomotion.
Some of the other aquatic birds, such as ducks and water ouzels, are said to propel themselves underwater through wing movements, but the evidence for such propulsion is incomplete and still open to question. The wing movements of ducks may be for steering and hydroplaning (skimming through the water) rather than for actual propulsion. The wings of the water ouzels, or dippers, were once thought to function as hydroplanes, but investigations have revealed that, although the wings are flapped underwater, the ability of dippers to bottom walk or fly underwater depends upon the velocity of the water flowing past the wings rather than the movement of the wings themselves.
Most aquatic birds are propelled by their webbed hind feet, which tend to move alternately in surface swimming and in unison when the bird is submerged. Of all the swimming birds that use their hind feet, the loons show the most extreme adaptations: the body, head, and neck are elongated and streamlined; the hind legs are at the very posterior end of the body; the lower legs are compressed and bladelike; and the feet are strongly webbed. The webbing increases the surface area exposed to the water during limb retraction and also permits the folding of the foot, thereby reducing water resistance during protraction.
In frogs and freshwater turtles, the hind legs are elongated and the feet enlarged and strongly webbed. But, whereas the hind legs of frogs move synchronously, except occasionally in slow swimming, when they alternate, the limb movements always alternate in freshwater turtles. Some aquatic turtles, however, such as snapping, mud, and musk turtles, are very poor swimmers and will swim only under extreme conditions. These turtles are bottom walkers, and their limb movements in water are identical to those on land except that they can move faster in water than on land.
The swimming movements of many mammals are also identical with their terrestrial limb movements. Hippopotamuses spend much of their time in the water, yet they bottom walk rather than swim. Most of the aquatic mammals—e.g., otters, hair seals, aquatic marsupials, insectivores, and rodents—use their hind legs and frequently their tails for swimming. The feet are webbed and usually move alternately; the tail tends to be flattened. Fur seals, polar bears, and platypuses swim mainly with forelimbs; only in the seals, however, are the movements of the forelimbs similar to those of sea turtles and penguins.
