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CORAL REEF ADAPTATION Before we discuss the incredible diversity of species adaptation to coral reef life, let’s first review the different regions of the ocean. This will allow us to have a better understanding of the environmental conditions that lead to a species specialized characteristics. The ecological associations of the seabed and of the open sea can be subdivided vertically. The region of the sea which light penetrates--down to some two hundred meters (660 feet)-- is known as the epipelagic region. In the warm oceans, the temperature falls off to about 20°C (68°C) at a depth of two hundred meters. The epipelagic zone is followed by the mesopelagic zone, which goes down to one thousand meters (3,300 feet). At the lower level of this zone, the temperature measured in warm seas is 10°C (50°F). Below this level extends the bathypelagic zone of the deep sea. Here the temperature falls to 4°C (39°F), the lower boundary of this region being given as between 3,000 and 4,000 meters (some 10,000 to 13,000 feet). If we go still deeper into the abyssopelagic zone, the temperature drops to below 4°C. The region of the epipelagic zone that borders coastline is subdivided into a region called the littoral zone. This comprises the entire continental shelf down to a depth of 200 meters (660 feet) and ends as the continental slope begins. The coastal areas of the Earth cover some 28 million square kilometers (10.8 million square miles). To the marine biologist, the littoral is one of the most interesting regions, primarily because it requires of its inhabitants a series of extreme special adaptations. The upper-most section of this region, the tidal zone, is free of water once or twice a day. The creatures, which live there, are subjected to the constant impact of the waves--sometimes involving considerable forces--and this calls for special adaptive measures. The inhabitants of sandy water line regions must be able to protect themselves against the abrasive action of the sand which is stirred up in the surf. Creatures which adhere to the rocks develop heavy armor, which resists the impact of the waves and protects them against drying-out; they also develop firm adhesive areas with the capacity to anchor themselves mechanically to the rocks. Barnacles, limpets, winkles, edible mussels, oysters, the armored sea urchins, starfish, and the more agile crabs live here. Many of these species venture quite far inland, into the so-called spray zone, which is wetted only during storms or at the time of spring tides. Here we find barnacles, for instance, which can tolerate many hours of exposure to strong sunshine and can even resist weeks of dryness. In the tidal pool left behind by the ebbing water, we find fish--mainly gobies--which have developed an astounding capacity for learning in their adaptation to this region. If the Atlantic goby, Bathygobius soporator, is suddenly frightened, it springs from pool to pool until it can get back into the sea. These gobies can cover a distance of up to 10 meters (33 feet) and can make use of as many as eleven pools. These fish learn the location of the pools, and can recall this learned information over a period up to forty days. The sub-littoral zone finishes at 200 meters (660 feet). Following on this is the bathyalic zone, the lower boundary of which is given as varying between 3,000 and 4,000 meters (some 10,000 to 13,000 feet). The area covered is ninety million square kilometers (34.8 million square miles). Below this again is the abyssal zone, covering 240 million square kilometers (92.6 million square miles). The deep sea trenches are known as the hadal zone. These features contain interesting local associations of organisms, since the trenches are isolated from one another. Of all these different habitats, the coral reef stands out by reason of the richness of species which it houses and the multiplicity of forms which it presents. In the tropical oceans we can see forests of rock inhabited by shoals of jewel-like fish. Beautiful hedges of coral alternate with blocks of rock split by fissures inhabited by starfish and predators. The architects of this constantly changing landscape are the minute coral polyps. The best conditions for rock corals are those in the warm seas, and it is only here that their activities lead to reef formation. As the water temperature increases, so also does the number of species. For instance, on the Barrier Reef on latitude 35° south, there is only one genus of coral. The temperature in this region varies between 10° and 25°C (50° to 77°F). In latitude 20° south, with temperature of 20° to 30°C (68° to 86°F), we find forty genera and at 10° south, where the temperature is 24° to 32°C (75° to 90°F), there are sixty genera. In addition, the reef corals require sunlight, since their living tissues contain algae which live there symbiotically, using the sunlight to process carbon dioxide and the nitrogenous decomposition products of the polyps and produce oxygen. This varied coral landscape provides many organisms with the means of life. The coral, in particular, plays a decisive part in the life of these creatures, serving them as food, a place of refuge, a hiding-place and a home. Many of the creatures have become specialized in eating the coral polyps. The parrotfish bite off the shoots of the branching coral and gnaw at the coral blocks with their hard mandibles. Their function as sand producers along the reef is an important one. A whole series of butterflyfish can also eat the small coral polyps. Among the invertebrates, the crown of thorns (Acanthaster plancii) has attracted considerable attention as a destroyer of coral. This large starfish climbs into the stems of coral and engulfs them with its stomach. The coral polyps are digested and all that is left behind is the white calcareous skeleton. In recent years, the starfish have occurred in large numbers in certain parts of the Pacific, and have entirely laid waste to individual reefs; but at the same time they were destroying the very basic of their existence. Once the corals have been consumed, the crown of thorns starfish starve and disappear and the reef can go on growing.
As well as providing a source of food, the coral reefs offer hiding-places--and consequently protection from enemies--to a large number of sea-dwellers. The damselfishes prefer to live among the coral branches. They feed in open water, but if frightened immediately swim back to the refuges of the coral stems, and do not easily let themselves be driven out again. You can break the coral stem off and take it to the surface with the fish still in it. Even when the branch of coral is lifted out of the water, these small fish remain there. If we take a closer look at a stem of coral, we discover many hidden inhabitants; sponges grown on most parts of the coral which have died off; brittle-stars and shrimps lodge between the branches and small fish (Caracanthus) hide there too. Many creatures have drilled into the coral stems, such as bivalve mollusks and small crustaceans. There are specialized fish which with their long tweeze like snouts can even extract their food from these safe-seeming nooks. Checking over the population of a coral stem in this way soon reveals that it is a complete microcosm. This multiplicity of small inhabitants is the most striking characteristic of the coral reef. Each type of coral, each sponge and each small growth of algae contains its own little living society, each patch of sand has its own group of inhabitants which is typical of the particle size of the sand there. These living-spaces for small life-forms combined to form larger typical undersea landscapes. Precisely as on land there are various types of landscape--fields, woods or hedges--with their typical societies of living creatures. Also found below the sea forests and hedges of coral cliffs are patches of sand and fields of algae. And each of these landscapes is inhabited by a characteristic group of creatures. Enormous numbers of fish of the most varied forms and colors swim around the hedges and blocks of coral. There are rectangular boxfish, rotund globefish, slender wrasse and high-backed angelfish. Most of these fish which are typical of the living realm of the reef have on characteristics in common--they are exceedingly maneuverable. They can rapidly change direction, stop in one position, turn on the spot and skillfully steer between the branches of coral. In addition to this, many of them are astonishingly brightly colored.
These striking colorations have generally been developed as a means of identification of the species. Just as the flags flown by ships indicate that their nationality, the fish indicate the species to which they belong, and fishes of the same species recognize each other by their markings. However, not every one of these patterns--so striking to us--has this function. Many fish have a prominent dark stripe around their heads, passing over the eye. These are camouflage stripes which serve to hide the eye. Many of the sawtooth blennies attack the eyes of their prey. Many fish have also developed a distracting eye spot below the dorsal fin, and this false eye attracts the attacking predator. In addition to this, however surprising it may seem, there are bright color patterns which hide the fish. The jewelfish and the small grouper are covered all over with small spots. This spots are striking and it is difficult at first to discern the fish clearly because of the disruptive coloration. The shape of the predator is to some extent masked as well. Many fish can also adapt themselves by changing color and can even use this phenomenon as a means of 'communication.'
The sand patches house a very different association. At first sight these sandy areas seem very sparsely populated, and even desert-like. Only when we look more closely do we see depressions and holes in which a very varied range of creatures is hiding. Since the surface of the sand offers no protection, most of the sand-dwellers have developed the capability of digging into the sand as long as they live. Others again live in tubes. There are worms which extend from their dwelling-holes long, sticky tentacles which spread out in all directions over the sand surface. Particles of food remain adhering to these tentacles and from time to time the worm gathers its catch. One type of worm lives in a U-shaped tube and uses fin-like appendages to create a current of water in which it spreads a net of mucus, from which it periodically licks off the particles trapped.
On the surface of the sand we also find starfish, sea-cucumbers, prosobranch snails and a series of sand-colored fish. Many of these are flat like the soles and plaice. Others construct tubes which they line internally with tiny stones, like the fashion of a well. Whole regiment of individual corals about half an inch long, will advance across the surface of the sand with worms which pushed them forward, so preventing the tiny corals from being buried by the sand. In return each worm has a partner which protects it, since it lived in a tube in the limy skeleton of the coral. Sea-urchins are accompanied by cardinalfish. When danger threatened, the fish hide between the spines of the sea-urchins, and as payment for this protection--cleaned the urchins spines. Another remarkable sand adaptation is the garden eels. The small eels live in vertical holes in the sand supported by a thin mucus lining. The eel extends its head into the open water above, where it can catch its prey. When danger is present, they retreat backwards into the hole and disappear. They live in colonies that are often so numerous and thick that the seabed looks covered with grass from a distance. Collaboration between fish and shrimp
There are several species of shrimp which live, either individually or in pairs, with gobies in a common burrow. The fish keeps guard at the entrance to the hole, while the shrimp tirelessly shovels the sand away from the interior. If danger threatens, they both retreat inside.
In the Red Sea, shrimps' burrows are scattered at random over the sea-bed. Each tunnel contains two shrimps of roughly the same size, and each contains one goby. These burrow homes run under the sand parallel to the surface of the sea-bed and can be up to 70 centimeters (27 inches) long. The shrimps sift over the bottom of their tunnels to find small particles of food. During the daytime, they use their claws to remove a quantity of mud from the tunnel, so producing a clearly visible waste-tip in front of the entry. The entrance to the tunnel has to be shifted elsewhere, so that the position of the cavity constantly changes. Within a single day, the position of the burrow entrance can be moved quite a considerable distance. Meanwhile, the gobies lie against the slope of the small waste-heap and snap up small living organisms stirred up, and pass the whole day keeping watch, keeping their caudal fins as close to the tunnel entrance as possible or even inside it. If danger threatens, they flee head-first into the hole, warning the shrimp inside that danger is close. The shrimps are generally smaller than the gobies, but they dig burrows large enough for the fish to enter. The goby is constantly caressed by the long antennae of the shrimps and it would seem that the goby enjoys the contact stimuli, apparently not even being disturbed if occasionally one of the shrimps pushes underneath it or thrusts it aside. If the fish is on the watch outside, one of the two shrimps comes to the entrance at frequent intervals and extends its antenna towards the fish's caudal fin, as though wanting to reassure itself that the fish is still there. If the shrimp is at work outside the tunnel, then it maintains constant contact with the fish by means of at least one of its antennae. Movement typical of the normal long-term position of the fish and its slow movement from one place to another, with particularly gentle rippling of its dorsal and caudal fins, serve as signals to the shrimps that they can without danger go about their business outside the burrow. However, they disappear with lightning rapidity into the tunnel if the fish either darts off after a potential victim or is obliged by the approach of danger to prepare to take refuge in the hole, whether its movements in this case are rapid or slow. In every case, it gives advance warning to the shrimps with short, rapid beats of the tail fins. These movements of intention on the part of the fish are a signal to the shrimps to withdraw backwards immediately into the tunnel. The exchange of such tactile signals enables the half-blind shrimp to move about safely outside its burrow. Without the fish--and its function as an early warning system--the shrimp would be unprotected and at the mercy of its enemies on the sea-bed, which offers it no cover. When it leaves its subterranean living quarters for a short time, it can forage on the surface of the sea-bed, which has an abundance of food to offer. For its part, the fish derives from this partnership the advantage of having a secure sleeping-and hiding-tunnel which it does not have to dig itself.
The removal of parasites and diseased skin tissue from the body surface of fishes is one form of foraging practiced by many fish along the reef, several of which have become completely specialized in this function. Cleaning offers advantages for both participants. For the cleaner, the collection of parasites represents a supply of food, whereas the client is relieved of the troublesome organisms which inhabits its skin, and may even bring about its death.
The skin parasites of fish are principally crustaceans, which live on the surface of their bodies, under the scales or--a particular preference--between the gill-plates. Many parasites attach themselves firmly by suction to the gill-covers and can cause the death of the fish. The parasites probably produce an itch, since many fish are observed to scratch themselves on hard objects. Some, like the rainbow-runner (Elagatis) even make use of the rough skin of sharks as 'sandpaper.' They turn on their sides in front of a shark and rub their backs against it as they swim past. Most of the cleaners are attached to their localities. The fish clients recognize the spot and come even from far-distance areas of the reef to allow themselves to be cleaned. Frequently there is a waiting line, and the services of the cleaner shrimp are in demand for a considerable time. High-speed swimmers of the free water
In the free water above the reef, there are no hindrances such that exist in the confines of the reef. The fish can move freely and do not need to turn sharply as they hunt their prey. In these wide open regions, they can only catch their victims by moving at high speed. Certain predators achieve speeds of up to 80 km. an hour (50 m.p.h.), a velocity which is possible only with a slender, streamlined body. The main driving power comes from the upward-pointing caudal fin and the muscular root of the tail. Species of shark and mackerel belong to the high-speed swimmers of the reef. Their pectoral fins are the control surfaces, which act like the ailerons of an aircraft. One disadvantage of this design, however, is that the fish can make only minor directional changes by changing the position of their fins.
Many damselfishes and snappers wander in large groups through the reef or swim directly along the edge of the reef, searching for food. At the approach of an enemy, the rapidly hide in the fissures in the reef. Their manner of life requires not only that they should be long-distance swimmers in order to reach new sources of nourishment, but also that they should be capable of making a standing start, to escape from their enemies. These fish are highly maneuverable, and their equipment of fins enables them to swim rapidly and to accelerate quickly.
In the thickets of coral branches, there live the many high-backed species of fish, such as the brightly colored butterflyfishes or the Moorish idol. The surgeonfishes, which are specialized for grazing on the algae and microorganisms found in this highly structured region are flattened like a discus. This body form enables them to turn rapidly on the spot and at the same time assists them to maintain a steady course, in much the same way as a boat is steadied by its keel. These fish are not long-distance swimmers. They have large pectoral fins, with which they can row, and large dorsal and anal fins, which provide the necessary drive. In flight, many of them progress by undulatory movements, that is, they behave like fishes of 'normal' shape. Additionally, aids towards a greater degree of maneuverability in confined spaces have been provided by nature for the boxfish and globefish. These box-shaped or spherical fishes have four propellers--two pectoral fins, one dorsal fin and one anal fin--with which they can even turn on the spot. They make great efforts to accelerate, making powerful strokes with their rounded, supple tail-root, but even this does not enable them to move particularly quickly.
A specialist in movement of a particular kind is the sea-horse. Its dorsal, pectoral, and anal fins are arranged in different directions, so that it can move in any direction without changing the axis of its body. At the same time, it moves buoyantly towards its prey, which it swallows with its tube-like snout. The sea-horse is unable to make rapid forward movements because--like the boxfishes and globefishes--it does no possess a powerful drive. This also means that it is unable to flee rapidly. But nature has provided it with protection of another kind, since the sea-horse camouflages itself from its enemies by the color and texture of its indigestible, armored skin.
Groupers and scads lie in ambush for their victims, taking them by surprise with a rapid dash. Many other bottom-dwelling predators have developed similar hunting methods. The quick start they need for this purpose is achieved by using all their available fins, and in addition, they beat powerfully with the entire rear portion of their bodies. They are 'sprinters' and as such are unable to sustain this speed for any length of time. If they have to cover long distances, they repeat their acceleration movement and then glide on without moving their bodies. Since this type of lightning start, with all fins working, calls for a powerful muscular system, these fish are generally compact and powerful in build.
Moray eels can swim through the narrowest fissures and tunnels. Only a body built like a snake's could possibly negotiate such narrow spaces. The muraenas move by means of long narrow fringes of fin along the body, but also make use of the many obstacles in their habitat to help them to advance. In doing so, they can change their direction of movement at will and can even creep backwards. Nature has seen to it that all the projecting fin structures have been selected out in these creatures, because they would only be a nuisance in the confined world which they inhabit. Even these few examples will have shown clearly how the body shape and fin form of fishes have arisen in the course of adaptation to their manner of life and of progression. The fins are, moreover, not exclusively organs of locomotion, and can in fact perform other functions. For example, they can serve as signals to attract or impress their partners in the mating season, and as a threat signal in combat. Sometimes too, they are transformed into suckers, as with the suckerfish or remora.
One fish which lives in the coral reefs even uses its fins as a means of catching its food. In its effort to adapt to its special mode of life, this fish has developed a shape so bizarre that it has kept hardly anything of the classical fish appearance. This is the well-known firefish or long-spine turkeyfish (Pterois) whose ragged-edged winglike pectoral fins make it look like a moving, magnificently-colored but highly dangerous bush.
The firefish belongs to the poisonous scorpionfishe family, whose members have mostly adapted to bottom-living habits, which has led to atrophy of their swim-bladder, the flotation and equilibrium mechanism of free-swimming fishes. Scorpionfishes sit with the hard rays of their pectoral fins supporting them on the ground and patiently wait for a small fish to swim unsuspectingly past them. Since they are bad swimmers, they have adopted an efficient camouflage to help them in hunting their prey. Among this group of cunning predators which lie stationary in ambush on the sea-bottom, only the firefish has resumed the free-swimming habit. It has maintained the specialized features developed for bottom-living purposes, including the much reduced swim bladder. In order to be able, despite this, to float in the open water, it has converted its pectoral fins in buoyancy surfaces; these, however, at the same time rob it of the most important faculty of a free-swimming fish, namely rapid progression. The firefish is unable to pursue its prey, and would even be at the mercy of its enemies, had nature not conferred upon it strong poison spines, with which it is able to defend itself most efficiently in the event of danger. But how can this fish be a predator and catch prey at all, when it is one of the slowest-moving fish on the coral reef? They extend their front extremities wide, stretching them out rigid at right angles to the body and thus drive their victims into a corner. Namely, by extending the upper extremities and making compensatory movements to prevent every attempt made by the driven animal to escape to the right or left. In doing so, the fish has no need to advance as fast as some predatory animal which seeks to run down its prey in an open race, nor do they move as slowly as a predator which seeks to creep up on its victim unobserved. For this purpose, the firefish use the pectoral fins as a driving net. The cornered fish is presented with what looks like a way of escaping from his predicament--and this turns out to be a trap. The fact is that in the vicinity of the firefish's jaws are the pectoral fins, made wide by flaps of skin, which contain a transparent window which trap fish as they attempt to flee, but instead swim right into the firefish's mouth. Competitive adaptation between hunters and hunted
Every creature on the coral reef must each day fight for its existence, in a genuine life-and-death struggle. Even predators can become victims of other predators. Each living organism is faced with the same problem: how to obtain its daily nourishment and at the same time, to evade the attacks of its enemies, which threaten its very life. Predator and prey can engage in a continually inventive competition to find new ways of adaptation, in the course of which the potential victims must always be one step ahead of the predator to be able to survive. For instance, where flight is pointless because the predator can swim more rapidly, protective adaptations must be developed. One of the most vital discoveries to deceive the enemy is that of camouflage. But other methods have proved no less successful. In the event of danger, crabs slip into their holes or rapidly dig themselves in. The hermit crab withdraws into his snail-shell home, and the giant clam or thorny oyster snap their valves closed at the slightest disturbance. Most reef fish evade danger by retiring to hiding-places which they know with great accuracy, since they have an excellent locational sense. Garden eels disappear backwards into their tubes in the sand. Soles and flounders wiggle into the sand, while sand-eels dive headfirst into the sandy bottom. Even schooling behavior is an adaptation in the face of the enemy.
In a school of defenseless small fishes, the constituent members are all equal. There is no leader, there are no officers and no non-conformists. Each respects the other, none is privileged above another. In addition, they have no reciprocal relationships; they form a large crowd, in which the individual remains anonymous.
School adaptation appears to have arisen primarily under the selective pressure due to the 'enemy.' The prime means of achieving protection against this danger is the 'confusion effect'; this protection, in fact, relies in the last analysis on the incapacity of the predator to concentrate on one particular victim. Mathematically, the larger the school, the smaller the danger of being caught by a predator. The size of the swarm and the probability of being eaten by a predatory enemy are in approximately inverse proportion to one another. This is easily demonstrated from the example of a goldfish. If we put a few water-fleas into its bowl, it will eat them all, being able to concentrate on each in turn and catch them. One would assume that the goldfish would eat itself to death if faced with a host of waterfleas. But, in fact, the opposite occurs, and the number of creatures eaten remains constant, because the confusion effect is increased by the high density of the water-flea population. The position of the fish within the school must be subject to continual correction. Most fish have a fixed distance between individuals, which is strictly maintained. Optical stimuli play a very important part in the orientation of an individual fish to its neighbors in the same school. Several speculations have been put forward regarding the mechanisms which maintain the cohesion of the school, but no experimental proof has yet been provided. One thing is certain, however, namely that the fish possess a very strong schooling drive, a social instinct, and it is also known approximately in which part of the brain this is located. In a 1933 experiment, some minnows were operated on to remove the forebrain, and one operated minnow was returned to the group. The fish was able to eat normally and to swim normally, and its vision did not seem to be affected. But what turned out to be its fatal shortcoming was its behavior towards its schooling partners. In fact, the surgical operation had removed every part of the brain responsible for triggering its social drive, among other things. The fish had lost its 'social relationship' to the member of the group. It no longer felt itself linked to them and swam heedlessly away. For species which live in groups, the school is a living association in which it feels itself protected. So it is not surprising that creatures accustomed to living in groups should become sick and generally die when isolated. Sea-urchins as dwelling-places
In the coral reef, there is hardly a place that has not been adopted by one or another specialized species as a suitable living-place. Even the thick mantles of spines on the various kinds of sea-urchin are colonized by different organisms. But the sea-urchin itself tries to catch and eat them, and if it can reach a fish with its spines, it will stab it and carry it on the spines to its mouth, where it is slowly swallowed tail-first. The large diadem sea-urchins are by day colonized by numbers of cardinalfishes (Apogonidae), which settle in large groups in the forest of spines on the urchin's body. At night, when the sea-urchin goes foraging in the undersea meadows, the fish leave their prickly refuge. It is not only the cardinalfishes which inhabit the sea-urchins: the young of other species of crustaceans and even of cuttlefish take up at least temporary positions between the spines. The chance of survival of these creature would be lessened if they were not able to take cover in the sea-urchin's protective spines. The colonists recognize their prickly host by the dark circular body and by its spines.
The coral reef exhibits optimum living conditions, making it possible for many fish to find suitable places--so-called ecological niches--there. Some peck away at the soft parts of the coral polyps, others, like the parrotfishes, crunch bivalve shells or nibble at coral branches. the well-known cleaner wrasse (Labroides dimidiatus) and the cleaner or neon goby (Elacatinus) have specialized in collecting the 'parasites' from other fish. One angelfish from the Caribbean eats sponges containing pointed skeletal elements which are quite unacceptable to other fish. Some specialists, like the large blue triggerfish, use their mouth to blow water under sea-urchins to turn them over, exposing the soft underbody, then eats them.
One species of lipfish spends part of its time between the stinging nettle-like tentacles of sea-anemones, from the surface of which it picks off the particles of plankton which have become trapped. One goby (Cottogobius) steals the food laboriously gathered by its host, a type of whip coral. All these creatures have adapted themselves in body form and in behavior to their specialized method of feeding. This specialization, developed in the evolution of the species in question, opens up to them the utilization of sources of food which it is difficult for others to draw upon. Any creature which eats the same food as one of these specialists becomes a threat to it. It is thus not only the predators which menace the life of an individual, but even those of his own kind. The struggle for life unrolls itself in a no less dramatic way between closely related but competing species. If the random interaction of mutation confers upon a species some new feature, then even another creature which has efficiently adapted itself can be thrust aside. It is a common occurrence in nature that competing species should become distributed as widely as possible over the space available to them, to ensure maximum utilization of the sources of food and to moderate the acute intraspecific competition. This intraspecific aggression thrusts the individuals of a given species apart, so acting as a spacing mechanism and favoring the wider distribution of the species.
Along the coral reef, the expenditure of plant and animal life is on a prodigal scale. What then is the purpose of all this expenditure, when each living organism is doomed simply to be eaten by another? Plants and animals are interwoven in the eternal cyclical exchange of energy and mineral substances; in a word, they are all elements in the food pyramid. The broad base of the food chain is formed by the masses of microscopically small creatures and plants, the plankton organisms--hardly visible with the naked eye--which can be present in a single drop of seawater in thousands and, as the ladder of interlinked food chains progresses, are eaten by organisms somewhat larger than themselves. If we move up towards the apex of the pyramid the number of consumers continues to fall off, while the body size of the creatures increases. At the other end of the chain are the giants of the sea, whose existence would be quite impossible without the presence of all the smaller species.
In a tropical ocean the life of the animal world--just as on land-- would be inconceivable without plants, since these are the only organisms capable of converting solar energy to sugar, starch and protein. The marine plants, however, must not be imagined to be like land plants, with roots, branches, flowers and leaves. The majority of marine vegetation consists of speck-sized single-celled organisms, the algae. Diatoms form by far the greater part of the whole. They are surrounded by vitreous capsules consisting of mineral matter precipitated from the sea-water. Nature has given the fullest rein to fantasy in the development of the--frequently grotesque--forms of these casings. The algae form the basic nourishment for the animal plankton carried along in the sea with them, and consisting of an inconceivable variety of living organisms, representing almost every species of animal kingdom. Minute medusae, sponge larvae, bryozoans, hydrozoans, larvae of coral, polyps of echinoderms, of snails, of bivalves, of polychaetes (bristleworms), and tiny predatory arrow-worms which hunt crustaceans--all these form part of the plankton 'soup.’ The copepods are particularly numerous in the plankton and are of considerable importance as nourishment for many reef-dwellers. The existence of all these planktonic creatures would be threatened if the algae which float with them were absent. There are almost as many kinds of trap developed by the larger creatures to catch the plankton as there are forms of plankton itself--and they are very numerous. Even the 'plankton corpses,' which rains down onto the sea-bottom, are eaten. The plankton-trappers have developed a varied range of methods and have adapted themselves in behavior and body structure to their particular pattern of feeding. Frequently, they no longer need to move, but pick out of the undersea currents the plankton carried along by them or trap the particles slowly sinking towards the bottom. Many of these creatures therefore sit tightly attached to the seabed and so closely resemble plants that one can hardly imagine that they are, in fact, animals.
One plankton trap is found everywhere on the reef: stinging, poisonous tentacles which shoot tiny 'harpoons' from specialized cells, and hold the plankton tightly. This multi-purpose device can also be used for defensive purposes. They are also used by sea-anemones, and medusae. This type of plankton-catching system is a monopoly of the jellyfish, corals and anemones. The tentacles of the medusae are particularly impressive, and include huge curtain-like hanging processes which trail freely in the water. The poison of the sea-wasp, a medusa found along the Barrier Reef, and which is only an inch or two across, is particularly feared. It is reported that contact with this jellyfish has caused human death within periods varying from thirty seconds to three hours. The Portuguese man-of-war consists of a large number of social polyps exhibiting division of labor. The tentacles, which can be up to 50 meters (160 feet) in length, belong to specialized polyps which occupy themselves only with trapping food. The tentacles are retracted spirally at regular intervals, forming small clumps, so making the prey accessible to the polyps specialized in digestion and assimilation of the food. Despite the highly poisonous nature of the tentacles, one species of fish uses them as a refuge. This is the man-of-war fish Nomeus, which is apparently unaffected by the poison. A water turtle will even eat an entire Portuguese man-of-war, tentacles and all. An even stranger phenomenon has been recorded: young octopuses of species Tremoctopus violaceus collect portions of the tentacles and use them as a weapons of defense or offense.
The sponges which are found on the reef exhibit a multiplicity of forms. They produce magnificently colored carpets across dead corals or stand clear of the ground in striking trumpet-forms. Some are of fine filigree structure, like coral. These are sponge animals that suck in water through numerous pores. The interior of their bodies are divided into many small chambers, the walls of which are clothed with flagellae, which maintain the water in the cavities in movement and screen out the particles of food. In this way, the sponges uninterruptedly draw in eddy currents of water containing nourishment and eject the filtered water. In shallow bays, the surface of the water can seethe under the whirlpool-creating action of the sponges. Other organisms, like the tube-living worms, stretch out around them beautifully colored crowns of tentacles, bearing a large number of cilia. The tentacles form a funnel leading to the creature's mouth; the cilia feed a whirling current of water into the funnel, and special grooves carry the sieved-out food into the mouth.
Larger plankton-feeders, particularly the copepods, are delicacies for many fish which hunt them in the water above the reef. If these fish suddenly shoot steeply upwards, making a snapping movement of their jaws, this is an indication that they have just had a bite. Many damselfish hang in dense swarms over the reef. In their hunt for plankton, they move in a spellbinding dance, their shimmering, brightly-colored bodies dipping and rising playfully. But this picture of innocent busyness entirely masks the fact that the fishes must daily go hunting uninterruptedly to fill their bellies with their minute prey. The garden eels have fixed feeding times which vary with current movements and availability of food, such as plankton and pteropot snails. Shortly before sunrise, they put their heads out of their underground homes and start feeding intensively. By late morning, their first meal is ended. They disappear again below the sand, and then come out again late in the afternoon. During the bright light of a full moon, they also feed. This mode of life in semi-sessile colonies, unique among vertebrates, could presumably only have been developed because the eels find themselves in a sort of nutritive soup which is constantly flowing past them. Without any great demand for movement, they are thus able to nourish themselves from the prey passing within their reach.
It strikes one as paradoxical that it should be the giants of the tropical oceans (and of the North Sea) which are plankton-eaters. Less selective in their manner of eating than the garden eels, which clearly take aim at their prey and then catch and eat it, the plankton-sieving creatures simply swim through the water with their mouths wide open and leave it to chance what they pick up. Once the water has had all the plankton screened out of it, it is discharged via the gill-slits. In the whale sharks, the jaws are more than 2 meters (6-1/2 feet) wide.
The huge manta rays and devilfish sieve the plankton as they swim with widely opened jaws, generally along the steep reef walls. But it is not only the giants of the fish world which sieve the plankton. In the Red Sea, generally towards evening, swarms of mackerel (Sarda), some only 20 centimeters (8 inches) long, patrol up and down the reef edge in a tight group, with their jaws wide open, eating plankton. Mackerel differ from their relatives--most of whom are predators--in feeding on marine plankton, and by utilizing this source of nourishment, they avoid competition with other species.
A good way off from the reef, the pelagic associations, the life associations of the open sea, begin; these are the creatures which are independent of the sea-bottom and live their whole lives out in the endless blue expanse of the ocean. To prevent them from sinking into the depths, many have developed feather-like body processes which can keep them floating freely. Others store in their bodies drops of an oil which is lighter than water. The strange medusae are almost 99 per cent water, and maintain themselves in a free-floating state by pumping, pulsing movements of their 'umbrella.' Around the edge of the umbrella, they have simple orientation organs. Since they do not have to perform any difficult movements, their sense organs are minimal. Some medusae possess gas-filled floats, by which they maintain themselves on the surface. These creatures trap plankton with their exceedingly long tentacles which can extent to as much as 50 meters (160 feet), plankton being a staple diet of most of the creatures that inhabit the open sea; even the largest fish alive today, the whale shark (15-30 meters, or 50-65 feet, in length) or the manta-ray--which can reach a 'wing-span' of up to 8 meters (25 feet)--live on plankton. The smaller fishes generally collect in schools, since the individual is safer in a group of this type. The large predators of the high seas, such as the tiger shark, blue shark and white-tip shark, have greater difficulties in obtaining their food. They have to maintain a continuous hunting patrol in order to satisfy their hunger. The high seas they inhabit do not offer them much abundance of food as is available to their reef-hunting relatives who live, comparatively, in abundance. |
