PROTECTIVE COLOURATION

Monarch butterfly

DEFINITION: (or colouring) natural colouration of certain organisms allowing them to blend in with their normal environment and escape detection by enemies.

As animals evolved, most of them developed body colours and markings that improved their chances of surviving. This adaptive mechanism, known as protective colouration, may serve any number of functions. Colouring can help protect an animal by making it hard to see. For an animal that spends much of its life trying to avoid dangerous enemies, this is the most useful function. Thus protective colouration is often found among the most helpless creatures those who have little or no other means of defence A white snow hare, for example, blends into its white surroundings and so becomes less visible to predators.

Conversely, colour can help an organism by making it more conspicuous the bright colours of a poisonous snake may warn off intruders, for example. In general, the purpose of protective colouration is to decrease an organism's visibility or to alter its appearance to other organisms. Sometimes several forms of protective colouration are superimposed on one animal.

Types of Protective Colouration

There are a variety of protective colouration schemes. Each works in a slightly different manner.

Cryptic colouration helps disguise an animal so that it is less visible to predators or prey. One of the most common types of cryptic colouration is background matching, which may take various forms. Many helpless animals have developed colours and markings that match their surroundings in order to hide from predators. Fish eggs and microscopic zoo plankton, for example, are transparent and nearly invisible as they drift in the upper layers of oceans and freshwater lakes. A fawn's spotted coat camouflages the animal against the speckled forest floor. Some animals attempt to camouflage themselves physically. The decorator crab, for example, cements bits of algae, seaweed, and other ocean debris onto its shell so that it resembles the ocean floor.

Grasshoppers and other insects that live among green plants are often green, and insects that live in the soil, such as ants, are often earth-coloured. The pepper moth has coloured patches that camouflage it against the tree on which it lives. The Sargasso sea dragon lives amid masses of floating algae. The fish is not only coloured to match the plants, but its fins and scales are even shaped like algae. The oriental leaf butterfly, which lives on leaf-littered forest floors, is so intricately and completely camouflaged that its markings include leaf veins and a stem.

Sometimes it is the predator that is camouflaged. Certain predatory fish, for example, blend in with harmless schooling fish and then prey on members of the school. Some species of groupers are camouflaged against the ocean floor as they lie motionless, waiting for prey to swim by.

Certain animals can change their colour in response to different environments or situations. Certain lizards are well known for their ability to match their colour to their surroundings.

Varying hares change colours with the season: through the winter their fur is white, and as the snow disappears, their fur turns brown. Thus they remain camouflaged throughout the year.

Another form of cryptic colouration is called disruptive colouration, a scheme in which spots, stripes, or other colour patterns visually break up an animal's outline. Such patterns may mask the animal's true shape or make it difficult for a predator to visually resolve it from a colourful or similarly disruptive background. Predators, such as the cheetah, tiger, and leopard, may use their disruptive colouration to avoid being seen. The spots or stripes on their fur allow them to get close to their prey before being observed, improving their chances of getting food. Many fishes and certain birds exhibit disruptive colouration, as do some snakes. The boa constrictor, a tree dweller that grows to several feet in length, is marked with a complex pattern of spots and stripes so complete that a stripe even extends across its eyes. Some patterns of disruptive colouration operate on the same principle to conceal movement. Snakes that are concentrically banded, for example, are difficult to detect when they move between long blades of grass.

A third form of cryptic colouration is counter shading, designed to mask an organism's three-dimensional form. Many animals, particularly vertebrates, are counter shaded, or shaded lighter on their lower surfaces and darker on their upper surfaces. This colouration counteracts the effects of overhead light, which accentuates an animal's three-dimensional form by lightening the animal's upper body and casting its lower body into shadow.

Counter shading gives the body a more uniform darkness and less depth relief so that the animal is less conspicuous.

Many marine animals are counter shaded so that they will not appear as silhouettes when seen from below. A silhouetted organism would be conspicuous and thus attract predators. When viewed from above, counter shaded marine animals blend into the darkness of the sea bottom; when viewed from below, their light lower bodies match the appearance of the water's surface.

Alluring colouration Some animals are coloured so that a predator's attention is drawn to a non-vital part of the animal's body. The lizard known as the blue-tailed skink has a bright blue tail that the animal can shed at will with no harm to itself. Potential predators are attracted to the tail; if they attack the tail, the skink sheds it and darts away unharmed.

Monarch butterfly, insect (Danaus plexippus) of the order Lepidoptera, family Danaidae; breeds on milkweeds.

Warning colouration is intended not to camouflage an organism but to make it more noticeable. Such colouration is found among animals that have natural defences that they use to deter or fend off predators. These defences can take many forms. An animal may simply cause a disagreeable smell (such as a skunk's odour), or it may actually cause pain (as from bee's sting) or even death (as from snake's venom). Many of these animals are brightly coloured, presumably as a warning to potential aggressors. The monarch butterfly, for example which bears a conspicuous pattern of bright orange and black has such a disagreeable taste that a bird will often regurgitate after eating it. Behavioural biologists believe that predatory animals learn to associate such brightly coloured animals with unpleasant or painful experiences and therefore are likely to pass them up as potential prey in favour of a more drab animal. Common warning colours are red, black, and yellow.

Dewlap, in reptile anatomy, a hanging fold of skin under the neck.

Some organisms can change their colour from drab to bright when threatened. The octopus, for example, turns white when agitated and red when it is suddenly frightened. Certain chameleons, usually camouflaged, display a brightly coloured throat sac, or dewlap, as a warning signal to invaders. Furthermore, when a male chameleon enters another's territory, the dewlap display of the territory's "owner" serves as a warning to keep out.

Fin, in zoology, external membrane used for propulsion in water.

Other forms of protective colouration Some animals are coloured in such a way that they draw attention to themselves only when they are in motion. Certain birds have light-coloured feathers that are visible only during flight. When the bird comes to rest, these feathers are tucked under darker feathers, so that the bird is once again inconspicuous.

Similarly, many fishes have colourful dorsal fins that are extended while the fish is swimming then folded down when the fish is at rest.

In both cases, the animal can use its colouration to perform a sort of disappearing act. It can draw a predator away from a certain area, perhaps a nest of vulnerable offspring, by catching the predator's attention and moving to another location. If the predator pursues the decoy, the bird or fish can disappear by coming to rest.

Some organisms imitate the protective colouration of others. This phenomenon is known as mimicry. A harmless animal may display the same warning colouration as a dangerous or inedible one in order to deceive predators into reacting as though the benign animal had the same defences as its model. In other cases, several noxious species will share a similar warning colouration so that potential predators will generalize and avoid all species with such colouring

Evolution of Protective Colouration

The intricate schemes of protective colouration are the results of long-term evolution. Through aeons of adaptive changes, certain organisms have acquired patterns of colouration that have helped them survive and reproduce.

Effective forms of protective colouration have been passed on to following generations. The processes of mutation, natural selection, and reproduction have combined to produce many organisms with colourations that are fine-tuned to their individual environments and their individual protective needs.

Assisted by Elliot Mitchell, science teacher, Latin School of Chicago.

MOLLUSCS

Common snail

The large group of animals called molluscs live on land and in both fresh and salt water. They constitute the phylum Mollusca, a major group of animals known to have as many as 100,000 living species and more than 50,000 fossil forms. Most molluscs, including snails, clams, oysters, and mussels, have shells. A major group called the cephalopods, however, including octopuses, squids, and cuttlefish, have shells that are either greatly diminished or absent. One of the most distinctive anatomical features characteristic of molluscs is a true coelom, a body cavity that contains most of the vital organs. The digestive tract, heart, liver, and reproductive organs are all housed inside the coelom. Many of the lower invertebrates, in contrast, have no coelom.

Mantle, in biology, outer fold of tissue that envelops the body and lines the shell of a mollusc; the shell is produced by secreting glands in the mantle.

Two other features that are characteristic of the molluscs and absent in most other groups of animals are the visceral, or buccal, mass and the mantle. The visceral mass is the main body of the animal and contains all the vital organs. The name mollusc, which is derived from a Latin word meaning "soft," refers to this large, soft body mass. The mantle is a thick covering of tissue that surrounds the visceral mass and has glands that secrete the shell, if the animal has one. Many of the aquatic molluscs also have another distinctive feature gills that are enclosed within a cavity formed by the mantle.

Giant squid (also called devil fish), a mollusc of the class Cephalopoda and the genus Architeuthis.

Most molluscs are only a few inches in length and weigh less than a quarter of an ounce (7 grams). However, they can vary considerably in size: some are minute and others enormous. The giant clam (genus Tridacna) may reach 3 feet (1 meter) in length and weigh more than 440 pounds (200 kilograms). Giant squids (genus Architeuthis) found in the North Atlantic off the coast of Newfoundland have been reported to be longer than 50 feet (15 meters) and weigh more than 4,400 pounds (2,000 kilograms).

Habitat and Locomotion

Most molluscs are marine animals; some are found in shallow coastal areas and others live in the deepest parts of the ocean. Most live in the bottom sediments, though the cephalopods are primarily free-swimming species. Some molluscs have been found at depths of 2,200 feet (670 meters) or more, in regions where molten volcanic sediments come in contact with the cold ocean waters. Molluscs also inhabit most freshwater and terrestrial habitats on Earth. The majority are free-living species that feed on algae that they scrape from underwater surfaces, food particles collected from the water, or larger prey that they have captured. Some species are parasitic.

Shelled molluscs move around by means of a foot that they extend from the shell to contact the substrate, or the surface on which they live. The foot is a muscular extension of the ventral, or lower, side of the animal, and in many species it can be completely withdrawn into the shell. Octopuses and squids, on the other hand, are open-water molluscs that propel themselves by forcing water out of a funnel-shaped apparatus located at the front end of the body. The mantle of squids is flattened into fins that are used to propel the animal slowly backward or forward.

Reproduction

Trochophore, free swimming ciliate larvae typical of molluscs and worms.

Mollusc reproduction is highly varied and can be very complex. Some species lay eggs, whereas others are live-bearers. Some of the marine species produce enormous numbers of eggs that develop into larvae known as trochophores. Most terrestrial and freshwater snails hatch from eggs as tiny replicas of the adult, without passing through a free-swimming stage. Some molluscs are parthenogenetic; that is, the female produces eggs or young directly without the need for the eggs to be fertilized by a male. Thus, in some species, males are unknown. Parental care has been observed in some groups of molluscs, such as the whelks, which deposit their eggs in grooves on the shells of both the female and male so that the eggs are protected from predators.

Body Structures and Functions

The basic body structure, consisting of a foot, visceral mass, and mantle, is similar in most molluscs The foot is located below the visceral mass. The external shell consists of three layers. The thin outer layer, called the periostracum, is made of a tough hornlike material and serves to protect the lower layers. The thicker underlying layer, called the prismatic layer, is composed of calcium carbonate. In some species the inner surface of the shell, called the nacreous layer, is composed of very thin, alternate layers of calcium carbonate and hornlike material. Its translucent surface in some species is referred to as mother-of-pearl.

The organ systems of molluscs are highly developed and include a complex nervous system. Their circulatory system is open; that is, blood is pumped from the heart into large sinuses, where it supplies oxygen to the various tissues. From the sinuses the blood passes to the gills or pulmonary tissues to be resupplied with oxygen, then returns to the heart. The heart consists of a ventricle, which pumps blood to the body, and auricles, which receive the blood before it returns to the ventricle. The digestive system consists of a mouth, oesophagus, stomach, intestines, and anus. A large gland connected to the stomach contains digestive enzymes and functions like a liver. Most molluscs have a mouth structure called a radula. The radula has numerous tiny teeth along the sides and can be extended from the mouth to scrape algae from rocks. Excretion of nitrogenous waste materials is accomplished by means of organs called nephridia or similar structures that function as kidneys.

Classes of Molluscs

The phylum Mollusca is divided into six living classes. These classes are defined primarily on the basis of body plan, type of shell, and modifications of the internal organs. The following classes are discussed in order from simplest to most complex.

Monoplacophorans. The class Monoplacophora was first known from fossil records, and all species were assumed to have been extinct for the past 350 million years. In 1952, however, marine biologists discovered ten live specimens at a depth of almost 13,000 feet (4,000 meters) in the Pacific Ocean off Costa Rica. These animals, placed by zoologists in the genus Neopilina, are segmented, suggesting that the molluscs may be closely related to the segmented annelid worms.

Amphineurans. The class Amphineura comprises about 850 species of marine organisms including the chitins, or "coat-of-mail shells." Chitins have a slender, ovoid shell consisting of eight plates arranged in a series. They have bilateral symmetry divided longitudinally, the two halves of the body are similar. The mouth is located at one end and the anus at the other. Chitins are usually found in shallow coastal waters, though some species live at great depths in the ocean. The foot is located on the lower surface and is used to carry the animal over rocks in search of algae, a primary source of food. The foot is also used as a suction device to help prevent predators from dislodging the animal from the surface of a rock. Included in this class of molluscs are the worm like solenogasters (subclass Aplacophora). These are among the most primitive molluscs and are the only ones that completely lack a mantle, head, foot, and nephridia. Chitins and other amphineurans are of little commercial value because of their small size and relative scarcity in most regions.

Gastropods, molluscs of the class Gastropoda, including snails and slugs.

Gastropods. The snails and slugs belong to the largest class of molluscs, Gastropoda, which contains more than 40,000 species. Most gastropods have a single shell that covers the viscera and is coiled in some manner. Some gastropod shells are extraordinarily elaborate and ornamental. Other members of the class, however, such as the terrestrial slugs and the strikingly coloured sea slugs, are without a shell entirely. Although during their larval stages the gastropods are bilaterally symmetrical, they normally become spiral-shaped in their later development. Gastropods have a complete digestive system, and the head and sense organs are well developed.

Most gastropods live in the ocean and are a major source of many of the seashells collected on coastal shores. In addition, freshwater snails are found worldwide, and an array of terrestrial species can be found throughout the world. Some of the terrestrial forms are arboreal, or tree climbing. Many gastropods are parasites that attach themselves to other animals, such as fish.

Some land snails have completely lost their gills, and the mantle cavity serves to exchange oxygen and carbon dioxide, similar to a lung. These air-breathing forms are known as pulmonate snails. Land snails and slugs have a slime gland in the forward end of the foot that secretes a mucus over which the animal glides as it moves. A silvery trail of dried mucus marks the progress of these creatures. Most pulmonates have a spiral shell that may be attenuated or flattened. Also included in the class Gastropoda are the whelks, conchs, periwinkles, and abalones.

Members of the gastropod class are of extensive economic value. Seashells from the shores and coastal waters of most oceans of the world are prized by collectors and are sold commercially in many regions. Some snails, particularly those in Europe, are gourmet delicacies known as escargots.

Unfortunately many of the freshwater species of snails act as intermediate hosts for parasites that infect vertebrates, including humans. The disease schistosomiasis is caused by a parasitic flatworm that spends a portion of its life cycle in a snail. The larvae of the flatworm leave the snail and enter the water. They then penetrate the skin of any mammal, including humans, that they encounter. Attempts to control schistosomiasis in tropical regions are often directed toward eliminating the freshwater snails.

Scaphopods. Members of the class Scaphopoda are called tusk shells because their soft bodies are enclosed in slender, tapering shells. The shell is open at both ends, but because one end is larger than the other, the shell has the appearance of a long tooth. Tusk shells live in the ocean sediments of both coastal and deep water areas. They move around very little. There are only about 350 species of tusk shells.

Pelecypods. There are more than 7,000 species of bivalve molluscs in the class Pelecypoda. These animals have two shells, or valves, that are usually similar in shape and size and are hinged along one side. The two shell halves can close up completely as a means of protection. Two large muscles called adductors connect the shells and make it difficult to pry the shells open. In some species, when the shells are open, the foot can be extended so that the animal can move slowly over the rocks or sediments. Many species, however, are quite sedentary and move little during their adult lives. Pelecypods are the only major group of molluscs that do not have a radula for scraping algae from rocks. Instead, they gather food particles from the water by straining the water across their gills. In their adult stage the pelecypods also lack a head, though one may be present in earlier stages.

Cockle, bivalved mollusc, especially common in tropical waters; the European cockle (Cardium edule) is a valuable shellfish; related to the oyster.

Scallops and cockles belong to this class, as do clams, oysters, and mussels, which make up a major portion of the world's seafood. Some pelecypods, particularly oysters, occur in very high concentrations, and millions are harvested annually in many parts of the world. The crushed shells of oysters and clams are the primary road-building material in many coastal regions. Oysters are also of value in the production of pearls, and freshwater mussels were once a major source of material for making buttons.

These creatures are not available in unlimited supply, however. Oyster drills, small gastropods that burrow into live oyster shells and kill the oyster, pose a serious threat to the oyster industry in some areas. In addition several species of freshwater clams in the United States are endangered and can be found only in certain stream and river systems.

Cuttlebone, dietary supplement for caged birds providing calcium; comes from the internal shell of the cuttlefish.

Cephalopods. The approximately 1,000 species in the class Cephalopoda differ in general appearance and behaviour from most other molluscs Generally the shell is lacking or greatly diminished. In many species a small remnant of the shell is embedded in the mantle. This internal shell is called the pen in squids and the cuttle in cuttlefish. Cephalopods are highly mobile marine animals that actively seek large prey. Some cephalopods rank as the most active and largest invertebrate animals in existence. Various fishes and crustaceans are their chief sources of food.

Sepia ink, drawing ink obtained from the ink sac of cuttlefish and squid.

Because cephalopods lack a protective shell, the visceral mass of the smaller animals is quite vulnerable to attack. For protection, they rely on speed and mobility to help them flee from their predators. In addition some cephalopods produce an inky substance called sepia that they squirt into the surrounding water when confronted by an enemy. The clouded water serves as a distraction or cover that allows the cephalopod to escape. At other times many cephalopods may rapidly change colour in order to camouflage themselves.

Cephalopods have a distinctive head region usually surrounded by eight or ten large tentacles. The mouth is located in the centre of the disk from which the tentacles radiate. Octopus tentacles bear large suckers that are used to grasp prey or to attach the animal firmly to a rock. (The popular tales of giant cephalopods that use their tentacles to prey on humans is a gross exaggeration.) The internal anatomy contains cartilaginous support structures, including a brain case, or skull. They also have large brains and complex nervous systems.

Octopuses and squids are major commercial food molluscs in many parts of the world. Small squids are popular as fish bait. The calcareous cuttle of cuttlefish is placed in cages of pet birds as a dietary supplement and bill sharpener. The cuttle can also be ground into a powder that is used as a polishing agent. In addition cuttlefish ink has been used by artists to produce the brownish pigment known as sepia.

Nautilus, cephalopod mollusc of genus Nautilus; adult lives in innermost chamber of approximately 36-chambered, coiled shell, 10 in. (25 cm) in diameter; found in seas surrounding East Indies and Fiji Islands; last surviving genus of ancient order Nautiloidea.

The chambered nautilus is unusual among the living species of cephalopods because it has a shell, though a number of extinct species are known to have had shells as well. The shell of the nautilus is divided into compartments. As the animal grows, a new compartment is formed and the older, smaller chamber is sealed off. The nautilus has a large number of tentacles extending from the head.

Assisted by J. Whitfield Gibbons, Senior Research Ecologist and Professor of Zoology, Savannah River Ecology Laboratory, University of Georgia.