Although many organisms are carried passively by currents, the ability to swim allows movement towards or away from stimuli.

Swimming organisms displace water to move directionally. Two major approaches to the displacement of water have evolved: waves (either whole body or of body parts) and power strokes.

Each wave displaces water at an angle, with successive waves being propagated. The angular displacement can be resolved into a lateral and a rearward component so that a complete wave generates opposite lateral displacements and two rearward components. As the opposite lateral displacements cancel out, the result is forward locomotion resulting from the rearward displacement of water. Movement is smooth if waves are generated continuously.

Power strokes are generated by extensions of the body, with water being displaced rearwards followed by a recovery stroke to bring the body extension back to its starting position. The forward movement of water in the recovery stroke would result in stasis if it were of equal dimension to the power stroke. To produce locomotion, the displacement of water in the recovery stroke must always be less than that from the power stroke and this is achieved by folding of the body extension, either by flexibility or by joints. A smaller area is thus exposed to the water and the recovery stroke is often slower than the power stroke so that acceleration of water is also reduced. If all body extensions beat in unison (synchrony), movement is jerky, but smooth movement results where power strokes of a series of extensions are in sequence (metachrony).

An early example of the use of waves in evolution is in the flagella [9.1.] of some protists, with power generated along their length, or part of their length, from a chemical reaction between internal components. Whole-flagellum waves are either generated from the base of the flagellum (pushing the body forward) or from the tip of the flagellum (pulling the body through the water). In some organisms, the flagellum beats in a helical fashion that induces spin in the body and thus some directional stability. Among invertebrates and vertebrates we see the use of whole body waves, or waves along parts of the body [9.2.]. These are produced by contraction and relaxation of muscles or bands of muscle fibres. Waves along the body are produced either by alternate lateral muscular contractions as in fish (piscine waves) or from dorso-ventral muscular contractions as in whales (cetacean waves).

Swimming leeches flatten their body and use a vigorous cetacean wave propagated from the head along the whole body [9.3.]. Flattening of the body allows the generation of lift and thus upward movement, as the leech swims at an angle towards the water surface to feed from a potential host. In contrast, many insect larvae use vigorous piscine waves along the whole body to provide directional movement. If the body is rounded as in midge larvae [9.4.], displacement of water is not efficient and much energy appears to be expended in "thrashing" through the water and moving a small distance. As these animals live primarily on/in the substratum, swimming is used mainly after displacement into the water column or to escape from predators.

Propulsion in fish involves whole body waves of almost equal amplitude (anguilliform [9.5.]), or waves of increasing amplitude (carangiform, thunniform). Tuna [9.6.] are typical fast-swimming fish with a powerful musculature that generates side-to-side movement of the posterior vertebral column and flexible tail (thunniform locomotion). Only a small proportion of the musculature is used in cruising, and these muscles are adapted physiologically for continuous activity. The majority of the musculature (most of that we eat) is used for bursts of high-speed swimming and these muscles tire quickly so that fast swimming is confined to chasing prey and avoiding predators. Once the fish has accelerated, momentum is maintained by small amplitude, powerful lateral movements of the posterior tip of the body and of the caudal fin. The hydrodynamics of propulsion with this side-to-side movement are complex, but involve the generation of vortices within the wake through which water is propelled, the tail also generating lift. The same principle is employed by whales in their up-and-down movement of the flukes (and by human swimmers using the crawl leg kick and the "dolphin" leg kick of butterfly stroke).

Waves along part of the body are seen in many animals. Cuttlefish [9.7.] have lateral fins on the sides of the body and waves move either toward the anterior or posterior, effecting movement in either direction. Similar movements along fins are used by some rigid-bodied fish [9.8.], and power and recovery strokes (see below) are also employed by fish. Other fish like the skates and rays "fly" through the water using movements of the "wings", rather like the underwater flight of penguins and diving birds.

The fins of fish provide a means of controlling pitch, yaw and roll [9.9.] and the use of the pectoral and pelvic fins in concert affords effective braking. Alternate use of fins by fish (or flippers by aquatic mammals) also allows excellent turning ability and manoeuvrability. For optimal efficiency, fins must operate in clean (non-turbulent) water so they must be held away from the body. Fast-swimming fish hold the fins (other than the caudal fin) close to the body during bursts of speed to reduce drag. The caudal fins of fast-swimming fish (and the tail flukes of whales and dolphins) move in highly turbulent water. This partly explains the evolution of the shape of caudal fins and flukes that often have a notch where water is most turbulent (immediately behind the posterior body of the swimming animal). The outer margins and tips are long, narrow and rigid to provide optimum hydrodynamic performance in clean water and thus the greatest propulsive force.

Cilia [9.10.] are the most primitive body extensions used for locomotion and are structurally very similar to flagella. They use the same method of chemical power generation but beat with a characteristic power and recovery stroke. Cilia are used for locomotion by many protists and swimming invertebrates, among which are rotifers and dispersing larvae of many benthic animals.

Powered limbs came with the development of muscles and their ability to produce longitudinal contractions. Swimming polychaete worms use a body wave but this moves from the posterior to the anterior so should propel the animal backwards. However, there are lateral extensions of the body, the parapodia [9.11.], which beat strongly on each wave crest. The power strokes of the parapodia on these crests have an optimal effect on rearward displacement of water, driving the animal forwards. Little water is displaced forwards by the parapodia as the recovery stroke is made within the trough of each wave as it passes along the body.

Among the most effective users of power strokes are the arthropods that have a jointed exoskeleton and internal muscles. Crustaceans are very common planktonic animals and the most characteristic are copepods [9.12.] and cladocerans [9.13.]. Both use modified antennae for swimming, the power stroke using a rapid downward movement of the extended antennae and the recovery stroke having the antennae partially folded at the joints. Setae on the antennae further add to the displacement of water and these give maximum resistance on the power stroke and minimum resistance on recovery. As displacement of water is principally downwards, copepods and cladocerans use locomotion to maintain their position in the water column or to move towards the surface. The movement of these animals is characteristically jerky, each rapid upward movement resulting from the power stroke. Other crustaceans swim using power and recovery strokes of legs.

Among the aquatic insects, water boatmen [9.14.] row through he water with elongated hind legs. These legs bear setae that are fully extended by water resistance during the power stroke but which bend back on the recovery stroke when the hind legs are also partially folded. The result is fast, jerky locomotion.

The use of waves and power strokes is not the only means of locomotion in aquatic animals, as some move by jet propulsion. Many jellyfish move by pulsing contractions of muscle fibres within the body causing displacement of water from the umbrella [9.15.]. The supportive jelly found throughout the body has elastic properties that allow a return to the relaxed state. This is analogous to power and recovery strokes but pulsing is not efficient and jellyfish mainly drift on currents, with the jelly providing near neutral buoyancy for the animal.

Modified respiratory currents are used in jet propulsion by both dragonfly larvae and cuttlefish (Sepia). Dragonfly larvae [9.16.] respire across lightly strengthened parts of the body surface and also have respiratory surfaces within the rectum. Water is passed into and from the rectum for respiration and a sudden contraction of rectal muscles expels a jet of water from the anus to propel the dragonfly larva forwards. Clearly this can be used only to the capacity of the rectum but it does allow escape from predators, rapid attacks on prey, and a means of re-locating. A similar use of jet propulsion occurs in cuttlefish [9.17.]. These molluscs draw water into the mantle cavity and expel exhalant water through a siphon during respiration. When the mantle cavity is filled with water, a sudden contraction of the muscles of the mantle wall accelerates water through the narrow siphon with resultant rapid movement. As the siphon is flexible, cuttlefish can move both forwards and backwards using jet propulsion.

A wide array of mechanisms has evolved for movement over the substratum and only the principal methods are mentioned here. The earliest forms of locomotion are found in bacteria, cyanobacteria, and protists that use cilia, flagella or amoeboid movement. Primitive invertebrates like flatworms [9.18.] and the most primitive snails have retained cilia for locomotion, progress of the animals being lubricated by the mucus they secrete. The majority of snails move by pedal locomotion with waves of muscle contraction or stretching passing along the foot. Some snails use either direct waves (moving in the same direction as the animal) or retrograde waves (moving in the opposite direction to the animal) and these are seen clearly if snails are observed moving on sheets of glass [9.19.]. Direct waves involve contraction of the muscles, with small steps being propagated from the posterior of the foot that is lifted. Muscles contract to shorten the foot and the posterior tip is placed further forward on the substratum. In other snails, the anterior of the foot is lifted and stretched forward to make a step, a series of waves of stretching passing along the foot to afford locomotion. Mucus is secreted both for attachment and to create a surface over which the snails glide effectively. Other aquatic invertebrates make steps using the whole body e.g., leeches [9.20.] and blackfly larvae [9.21.] and this requires attachment at each end. Leeches have suckers and blackfly larvae use silk pads to which they are anchored by hooks on the posterior of the abdomen and on a proleg near the head.

Many aquatic animals use limbs to move over the substratum. Among the commonest are arthropods that have exoskeletons and varying numbers of legs that support the body clear of the bed. This conserves energy as the body has momentum once in motion and only the legs are accelerated and decelerated with each step.

The tube feet of echinoderms are another type of limb that has evolved. Anyone picking up a stranded starfish from the sea shore is familiar with the movement of the many tube feet on the ventral surface of the arms [9.22.]. The tube feet are connected to the water vascular system of the echinoderm so that they operate hydraulically. Muscles in the wall of the tube foot allow it to be moved in any direction and the result is a series of steps. The extended tube foot gives a power stroke and the contracted tube foot a recovery stroke, returning the foot to the starting position. Brittle stars [9.23.] are close relatives of starfish but they use their tube feet primarily for suspension feeding, sinuous movements of the arms being used for locomotion.

Some animals that move over the substratum seek shelter in cracks and fissures within rocks and others burrow into soft substrata. Several fish swim into the poorly consolidated sediment of the sea bed and many other animals disturb the sediments that then settle to cover them. Burrowing is often carried out using limbs and/or mouthparts and many aquatic animals live permanently in burrows and show special adaptations to burrowing. This may be in the form of body shape or the presence of secretory cells that allow tube construction. Among the most effective burrowing animals are some worms and bivalve molluscs that use alternate muscle contraction and relaxation to create anchors [9.24.]. Terminal anchors, produced by lateral expansion, allow the body to be pulled into the substratum while penetration anchors brace the animal for the extension of the burrowing device. In worms the whole body is used in burrowing, with alternate contraction of circular muscles to narrow and extend the body for penetration, while using contraction of longitudinal muscles to shorten and broaden the body to form anchors. Bivalves use the muscular foot for penetration, its tip being expanded to give a terminal anchor with the shell valves being closed to give a cutting edge as the bivalve is pulled downwards. Relaxation of the muscles closing the valves then allows the elasticity of the hinge to force the shell valves apart and this forms a penetration anchor to allow the foot to be extended downwards. Some bivalves also use their shells as a boring tool and burrow into hard substrata like wood, coral or rocks [9.25.].

Not all bivalves burrow into the substratum. The common blue mussel barely moves, anchoring to the substratum of rocky shores by means of proteinaceous byssus threads [9.26.]. Other common shoreline inhabitants that attach are acorn barnacles and various kinds of snails. Adult acorn barnacles are unable to move as they are attached by cement and their calcareous body plates. Snails move to feed when covered with water but attach firmly to rocks when the tide is out. Many other types of invertebrate anchor by means of direct attachment of the body, or by constructing tubes in which to live. Many familiar aquatic animals have a sessile or sedentary mode of life and these include corals, hydroids and sea stars. Seaweeds also need to be attached to the substratum. This is effected by a holdfast with the sole purpose of preventing dislodgement as it is not used for the uptake of nutrients. In contrast, aquatic macrophytes use roots both for uptake and for anchorage.

The most primitive form of reproduction is asexual, by binary fission of single-celled organisms that split into two. Some multicellular organisms also reproduce asexually to form colonies and often cover parts of the substratum. However, the offspring of sexually reproducing organisms usually disperse from the parents to avoid competition for space and food. This is also to the advantage of the parents. Some organisms e.g., aquatic insects, have adult stages that disperse.

In aquatic habitats, external fertilisation is the commonest strategy, with gametes passed to the water. Some animals use internal fertilisation and some also invest time and energy in rearing their offspring, whether they result from internal or external fertilisation. Most offspring are not reared by parents and are free-living from the time of fertilisation, often being found in huge numbers and with very high mortality in the immature stages.

We recognise the spores of bacteria, seeds of plants and eggs of some aquatic animals but the immature stages of many aquatic animals are not familiar to us. Hydroids, flatworms, worms, molluscs, echinoderms, and several other major groups of aquatic animals all have larval forms that are planktonic [9.27.] and that disperse away from the parents. These larval forms are characteristically small and quite unlike the parents in shape. Most move through the water using bands of cilia for locomotion and feeding, although crustacean larvae swim and feed using limbs. The larval forms are suspension-feeding collectors and are found living among the permanent members of the planktonic community of lakes and the sea. The permanent plankton are often defined as holoplankton [9.28.] and the dispersing larval forms as meroplankton. Meroplankton are of greatest significance in oceans and can be swept long distances by ocean currents.

Meroplanktonic larvae [9.29.] are classified into two groups: those that have an initial food reserve (lecithotrophic) and those that have no food reserve (planktotrophic). The latter are dependent on suspension feeding at the beginning of their lives, whereas lecithotrophic larvae use reserves donated by parents before needing to feed. In addition, the lecithotrophic larvae of some species are brooded by a parent, some have protective coatings of jelly, and some larvae are retained within the body of the female parent to be born at a more advanced stage than usual (viviparity).

If a finite amount of energy is available for reproduction, it follows that there are fewer lecithotrophic larvae per total unit of energy than planktotrophic larvae. For example, two species of starfish (Asterias) show contrasting strategies, with a planktotrophic species laying 2,500,000 eggs per season and a lecithotrophic/brooder species only 100. These numbers indicate the likely level of mortality resulting from the two strategies and it is clear that planktotrophic larvae are very abundant indeed during the breeding season. Many fish also have pelagic larvae that result from external fertilisation of staggering numbers of eggs (millions per individual fish in some cases). Add to this the dispersal stages of benthic algae and one appreciates the number of minute living organisms that are added to the water column each year and the number that die or are consumed by predators.

Many meroplankton are carried offshore by local or large-scale currents and never reach coastal or sedimenting regions suitable for adult life. Along with the victims of predation, they do not survive to breed. Those that do survive must have a mechanism that enables them to return to the substratum . Intertidal animals are carried ashore by tides and they settle readily on any exposed surface. New piers and pilings develop a concretion of barnacles and other invertebrates with calcareous coverings. High on the shore are found barnacles that settled as larvae during a very high spring tide, or that were washed ashore in sea spray.

Oceanic islands are colonised by algae and invertebrates that are familiar from mainland coasts perhaps thousands of miles away. They can only have reached the islands after being carried on oceanic currents as spores or larvae. This shows that chance plays a large part in dispersal and colonisation, but larvae also react to physico-chemical factors. Positive phototaxis ensures that larvae remain in surface waters where food is available and where dispersal is likely to be enhanced. When they have grown, larvae show a behavioural switch to negative phototaxis and swim down in the water column to increase their chances of locating a suitable substratum (except for those carried in the surface waters on to shores). Other factors affecting the settlement of larvae include depth, pressure and chemical signals, including pheromones. It is in oceans that one appreciates the scale of dispersal, the chance occurrence of settlement on a suitable substratum, and the very small percentage of propagules that must survive. In fresh waters there is a similar dispersal but the scale is less impressive than that in oceans, even in large lakes.

Animals are carried by water. In streams and rivers, the drift of organisms (downstream displacement within the water column [9.30.]) often has serious consequences, as most of the water body is moving and there is a risk of organisms being swept far away. This explains why planktonic communities are found only in dead-water zones in streams and rivers, and in regions where there is relatively slow current velocity. This lack of plankton distinguishes flowing fresh waters from other water bodies. Large-scale drift of animals in streams and rivers has two causes: catastrophic events or behavioural processes.

Examples of catastrophic events are sudden spates, ice formation, or drying of streams. Spates sweep up animals from the substratum and turn over stones under which animals shelter. If the onset of a spate is sudden, animals are not able to move into safer locations, as they would do when the start of flooding is more gradual. Animals in high latitude and/or high altitude streams are affected by anchor ice during some periods of the year. Although water continues to flow in the stream, the ice sheet over the substratum traps animals and the onset of anchor ice promotes an increase in drifting downstream. Some streams have intermittent flow and some dry out for weeks or months at a time. Animals living in these streams must seek refuge from the threat of desiccation, become dried, or carried in declining discharge to continue life wherever they settle.

Several causes of behavioural drift have been identified. When population density is high there is an increased risk of interference between individuals and therefore the likelihood that they are dislodged into the current. Dislodgement also results when invertebrates move on to the surface of stones and other parts of the substratum at night. This is a common strategy of scrapers that move upwards at dusk to feed on attached algae, detritus, and biofilm, the animals hiding from visual predators during the day. As this diel (every 24 hours) movement is under the control of light, animals displaced are considered part of the behavioural drift.

The consequences of drift are not always negative as some animals use drift as a means of dispersal. Many animals that live in streams have mechanisms that aid their return to the substratum. Among the mechanisms used are strong swimming (e.g., in freshwater shrimps and some mayfly larvae), the use of grasping appendages (e.g., in some caddisfly larvae), or the use of sticky silk threads (e.g., in blackfly larvae). Once animals have returned to the substratum they often begin to move upstream and this compensatory movement ensures that upstream reaches are not denuded of animals. The dispersal phase of many stream organisms is aerial (e.g., insect flight, movement of algal and bacterial spores) and there is some evidence that many stream insects fly upstream to oviposit. This may complete a "colonisation cycle" to replenish upstream stocks after reduction by drifting.

Sonar recordings from the surface waters of oceans commonly show a dense layer of sonar-opaque material several hundred metres below the surface. This material rises at least 100 to 200 metres at dusk [9.31.] and returns to its previous level at dawn. A similar pattern is found in sonar traces from lakes, although the scale of movement is usually smaller.

Sampling this layer [9.32.] reveals it to be composed of crustacean zooplankton which undergo vertical migrations each day. Not all follow the usual pattern of ascent at dusk and descent at dawn, some showing the opposite pattern. Various theories have been put forward to explain the vertical movement of zooplankton. (i) It allows avoidance of visual predators. (ii) It allows horizontal movement, as different layers of water move laterally at different rates, so that the zooplankton do not deplete living and dead POM. (iii) Food quality is different at night, as excess carbohydrate is produced by algae during the day and protein synthesis occurs at night, thus making algae a better-quality food. There is probably no single explanation but it is clear that such a large vertical movement of small organisms must take a long time. It might also be supposed that a large energy expenditure is required for the upward migration. However, if changes in buoyancy are effected by changes in the composition of the body, the upward and downward migrations are achieved with little outlay of energy for locomotion.

Vertical migrations are not confined to the pelagic. As we have seen, animals are swept into the water of streams and rivers while moving up to the surface of stones to feed. The diel migration of these animals is directly analogous to the vertical movement of zooplankton, as higher quality food is likely to be harvested by scrapers from the surface of stones at night. Some algal cells migrate in the opposite direction, moving down into the substratum to become charged with nutrients at night before returning to the surface of stones to photosynthesise during the day.

Surface ocean currents transport communities of aquatic organisms of many sizes passively over long distances [9.33.]. Terrestrial organisms or propagules are also carried on ocean currents and this represents one means of transmission to new colonisation sites [9.34.]. Coconuts provide an excellent example of how effective this transmission can be. Surface ocean currents are not only a means of passively displacing organisms but they also serve to transport those that move actively, as we have seen from the discussion on vertical movements of zooplankton.

The largest aquatic animals to use locomotion for long-range migration are whales [9.35.]. Some species migrate over more than 60^o latitude each year, spending summer feeding in rich polar oceans before migrating to warm sub-tropical waters to calve. In contrast to most animal offspring, whale calves have a very heavy maternal investment as they are suckled on rich milk. The warmer waters of the temperate ocean result in less energy being needed for generation of body heat in the calves and they grow rapidly and develop an insulating layer of blubber. On return to the Antarctic, the cows replenish their body reserves on the abundant food found there and the calves are weaned.

The migratory routes of whales were exploited by whalers and migration routes of fish are known to fisherman, coastal waters having areas to which adult fish migrate to breed. In facing the problems of overfishing it is thus clear that stocks can be decimated if fishing occurs in breeding areas at the time of reproduction (the same reason why there is a close season for many sport fish in inland waters). Young fish leave the breeding sites and some have nursery areas in sheltered waters that provide good conditions for the growth of the immatures which then grow large enough to migrate back to the regions normally occupied by adult fish.

Some oceanic fish like tuna undergo large-scale migrations that allow breeding in warm waters and subsequent migration of the stock to productive temperate waters or regions of upwelling [9.36.]. Again, those fishing for tuna are aware of these migrations and know where to go in search of fish. Like several other food fish and whales, tuna are powerful swimmers and migration is certainly active, rather than the passive drifting on ocean currents seen among smaller animals. Such migrations require navigation and an ability to recognise cues, something that is highly developed in fish that migrate between the sea and fresh water.

European eels [9.37.] live in the beds of rivers and some lakes and move to the sea before swimming across the Atlantic Ocean to spawn. Huge numbers congregate in the warm waters of the tropical west Atlantic that contain masses of a floating seaweed that form an ideal breeding ground. Young eels begin a migration that takes them back across the Atlantic on the North Atlantic Drift, swimming to maintain their position in the surface waters. As breeding is seasonal, there is a mass influx of elvers in some rivers and elver fisheries have resulted in many large European rivers draining into the Atlantic.

In contrast, salmon breed in fresh waters and spend their adult life in the sea where they inhabit the same waters as truly oceanic fish [9.38.]. When the adults achieve a certain age they migrate back towards the rivers where they were spawned and begin to swim upstream. Spawning occurs in fast-flowing, oxygen-rich water where the substratum consists of small stones and gravel [9.39.]. Once they have bred it is usual for the adult salmon to die and this results in a mass input of protein-rich detritus. This is broken down by the heterotrophic community of micro-organisms and invertebrates on which the larval salmon feed, directly or indirectly. The death of the adult salmon is therefore analogous to the examples of parental investment in lecithotrophy and milk-production. Depending on the species, the immature salmon spend some time in the river and then migrate to the sea and swim far away from land to commence marine life.

Both eels and salmon require long-distance navigation to locate their breeding sites and this probably involves tissues containing metallic granules that are affected by changes in gravity. When close to the shore, salmon use olfactory cues to recognise the river in which they spent larval life. For both types of fish, the movement between fresh and salt water requires their ability to cope with osmotic stress as fresh water is very dilute compared to the body tissues.

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