COASTS AND CORAL REEF
In the natural world, the boundaries between different environments are often rich habitats for wildlife. The world’s coasts are the ultimate example of this meeting of habitats because they bring together animals that live on the land and those that live in the sea. Coastal wildlife varies according to local conditions but, in rocky coasts as well as sandy ones, shore animals are associated with clear-cut zones that are usually determined by the tides.
Coral reefs are a special kind of coastal habitat. Famed for their spectacular shape and colour, they can grow to vast proportions and are unrivalled in the immense variety of aquatic life that they support. Reef-building corals require specific conditions, the most important being warmth and bright sunlight all year round. As a result, coral reefs are largely restricted to the tropics.
There are 2 types of corals: hard and soft. Coral reefs are formed by hard corals. The individual coral animals, called polyps, secrete external skeletons that persist after they die. Soft corals are found all over the world, but hard, reef-building corals grow in clear, nutrient-poor water, where their mutualistic algae can use the energy in sunlight.
Deep-water coral reefs do not rely on algae. Such is the richness of the habitat created by coral reefs that a huge number of animals are able to live side by side without, competing for the same food. There are 3 main types of reefs: fringing reefs, which grow close to the shore: barrier reefs which are separated from the coast by deep channels, sometimes over 100km (62miles) wide: and atolls, which are ring – or horseshoe-shaped reefs that grow around oceanic islands often where volcanoes have subsided into the sea.
Although every reef is unique, reefs share a common structure depth and exposure are 2 important factors that influence this because corals vary in their need for light and in their ability to withstand the force of the waves. The fastest-growing corals, which need lots of light and relatively calm water, typically form the central zone of the reef, projecting just above the surface at the lowest tides.
On its inshore edge, the central zone is often backed by lagoons – large pools of open water lying over coral sand. By contrast, the seaward edge of the reef often forms a submarine cliff that drops steeply into the depths. The corals in this zone are solid and resilient because they have to withstand heavy breakers rolling in from the open sea.
Some habitat looks much the same from one year to the next, but the seashore is always changing. Waves pound away at rocks undermining them and breaking them up while coastal currents reshape the shore in a less dramatic way by moving shingle and sand. Superimposed on this is the rhythmic movement of the tides — a twice-daily cycle that has a profound impact on seashore wildlife.
Tides vary, enormously in different parts of the world. Around islands in mid-oceans, the total rise and fall is often less than 30cm (12in) while in deep bays and inlets on continental coats, it can be over 10m (33 ft). Whatever it’s height range the tides divide the shore just beyond the reach of the highest tides.
Although this zone is never actually submerged it is affected by salt spray, which means animals that are sensitive to salt and also salt intolerant plants are rarely found in this zone. Below this is the eulittoral a zone that is regularly covered and then exposed as the tides floods and recedes.
The animals found here such as mussels and limpets, lead a double life in that they have to be able to survive both in water in air. The next zone or sublittoral, is always submerged, even during the lowest tides.
Most of the animals that live in this zone are fully marine, although a few leave the water to breed. Coastal wildlife is also affected by the geology of the shore. Many animals live on rocky coasts, while others specialize in living in sand or coastal mud. Compared with these, shingle is a difficult habitat for animals, although some shoreline waders use it as a place to nest.
LIFE IN COASTS AND CORAL REEF
Compared with the open sea, coasts and coral reefs are disproportionately rich in marine wildlife. Rocky coasts and mudflats abound with seabirds and invertebrates, while coral reefs probably contain at least a third of the world’s fish species. Conditions in coasts and reefs are variable, so the animals that use them are often extremely specialized.
Some roam over large areas of the shore, but for most, a distance of just a few meters makes the difference between an ideal habitat and one in which it is impossible to survive.
ADAPTING TO TIDES
For coastal animals life is governed by the rhythm of the tides. As well as adapting to the twice daily ebb and flow, they have to adapt to the drawn-out rhythm of extra- large spring tides. which are normally 14 days apart knowing where the tide stands is important: because animals caught unprepared run the risk of either drying out or drowning.
Some shore animals. such as barnacles, adjust their behavior according to whether or not they are submerged.
Most aquatic animals however, are much more sophisticated, reacting to their internal biological clocks, which march i step with the tides. Even if they are taken away from the shore and placed in a tank their tidal clocks continue to tick. This built-in ability to keep time enables animals to anticipate events.
For example, submerged limpets crawl over rocks to graze on microscopic algae, but since they are vulnerable away from their niche on the rock, they have to return home before the sea ebbs away. Their biological clocks tell them when to head back, ensuring that they are securely in place, and airtight, before the tide goes out.
Conversely, the fiddler crab’s biological clock prompts it to come out and feed at low tide and then return to its underground burrow before the incoming tide engulfs it.
FEEDING ON THE SHORE
Coasts provide a much greater variety of food than the open sea. High up the shore, an abundance of animal and plant remains left in strandlines by the retreating tide are consumed by beach-hoppers, springtails, and other scavengers, which are then preyed upon by gulls and waders.
In the intertidal zone, filter feeders are common. Unlike the giant filter feeders of the open sea coastal ones are generally small and often spend their entire adult lives fixed in one place.
Mussels and other bivalves which filter particles of food using modified gills are examples of these. Barnacles have a different filtering technique: despite their resemblance to molluscs, barnacles are crustaceans, with a set of feathery legs. At high tide, the legs protude from the barnacle’s case to collect food particles.
Rocky shores are home to some fast- moving swimmers as well as to many other animals that take a more leisurely approach to finding food. Starfish and sea urchins are among the slowest, crawling over rocks on hundreds of fluid-filled feet.
LIVING IN MUD AND SAND
Compared with rocky shores, coastal mud and sand seem to harbour only a limited amount of animal life, but appearances are deceptive. Mud in particular often teems with hidden life feeding on organic matter brought in by the tide. One of the advantages of living below ground is that despite the attacks of curlews and other long-billed birds, it provides good protection from predators.
The chief disadvantage is that the surface is constantly shifting, cutting of buried animals from the water above-and from oxygen and food. Some of these buried animals have specialize body parts that enable them to connect with the surface. Clams, for example, have leathery tubes or siphons. In any species, the siphon can be retracted inside the shell, but in some, they are too long to be stowed away.
Species that do not have such accessories often live in burrows. The lugworm is an example: it makes a U-shaped burrow, lining it with mucus to keep it intact so that sea water does not make it collapse.
Mutualism is a feature of life in all habitats, but it is particularly apparent on coasts. In some cases, the mutualistic partners are animals; but, in many others one is an animal and the other is microscopic algae that live inside its body. These, algae known as zooxanthellae, can be found in thousands of coastal animals, including corals, jellyfish, and giant clams.
Zooxanthellae live by photosynthesis – the same process by which plants grow. Through a complex series of chemical reactions, they harness the energy in sunlight and use it to build up organic matter.
The host animal provides the zooxanthellae with protection from the outside and, in return, the algae surrender some of their manufactured food. These partnerships are very important to ref-building corals because they allow them to live in places where the supply of food is otherwise low.
For the partnership between coral and algae to work, the corals have to encourage algae growth, which means they must live in bright sunshine near the water surface. However, this limits their upward growth because few corals can survive more than an hour of exposure at low tide.
Many land animals have adopted to life at sea. Some are now fully marine, but others such as turtles, some sea snakes, and seals and their relatives— must come ashore to breed.
These animals are often scattered over a wide area, so they tend to form colonies during the breeding season, congregating in the same place each year to maximize their chances of finding a mate. Many coastal invertebrates, on the other hand, spend their entire adult lives in one place. For them, reproduction is an opportunity not only to multiply but also to disperse.
Their eggs hatch into planktonic larvae, which may then drift long distances in coastal currents before eventually settling down. For some species. such as barnacles choosing a home is an irrevocable decision because a larva cannot detach itself once it has settled.
Chemical cues help it to “make up its mind” before it takes this momentous step. Some fish come inshore to breed because the shoreline offers plenty of hiding places for their eggs.
An extreme example of such a fish is the California grunion, which lays its eggs not in water but in damp sand on beaches. Grunions stage mass spawnings during high spring tides at night. At the next spring tide, the eggs hatch and the young are washed into the sea.