Rocky shores, sedimentary shores and mangrove forests

The zones of a rocky shore

A rocky shore is a hard, sloping surface between land and sea, and the tide climbs and falls across it roughly twice a day. Because the rock itself does not move, any one patch of shore is covered by water, then left dry, then covered again, and the length of time it spends under water is decided by one thing only: how high up the shore it is. That single fact produces the three zones you have to describe.

The supratidal zone lies above the high water mark. It is wetted only by spray thrown up by breaking waves and, occasionally, by the highest spring tides, so it is dry for most of the tidal cycle and can stay dry for days on a calm coast. The rock surface there gets far hotter than the sea in sunshine and far colder at night, and the only fresh water is rain. Anything living there must survive long periods without being submerged, so the zone is thinly populated: lichens and a few encrusting algae on the rock, periwinkles and small crabs sheltering in cracks, and often a band of salt-tolerant plants at its upper limit.

The intertidal zone is the part between the high and low water marks, covered and uncovered twice every day. It is the zone that makes this topic interesting, because the two tides set up a gradient: the upper intertidal is under water for the shortest time and exposed for the longest, the middle intertidal is roughly half and half, and the lower intertidal is covered for most of the cycle and exposed only briefly around low tide. Everything that lives here has to cope with both being submerged and being out of the water, which is why the adaptations in this topic are mostly about holding on and about not drying out.

The subtidal zone is below the low water mark, permanently submerged apart from the very lowest spring tides. Conditions there are far more stable: no desiccation, no baking rock surface, no sudden change in air temperature, and a continuous supply of oxygen and food carried by the water. The trade-offs are that there is less light with depth and that predators such as fish and larger crabs can reach the organisms all the time, so the subtidal zone is not automatically the safest place on the shore.

The zones are a description of the tide, not of the rock. Two shores of the same rock type will have their zones at different heights if one is exposed to big waves and the other is sheltered, because spray wets the exposed shore higher up and because wave action excludes the delicate species.

• Supratidal: above the high water mark, wetted by spray and the highest spring tides only; dry most of the time; extreme temperature and no water for much of the cycle.
• Intertidal: between the high and low water marks, covered and uncovered twice a day; divided into upper, middle and lower shore, with the exposure time falling as you go down.
• Subtidal: below the low water mark, permanently submerged; stable temperature, continuous oxygen and food supply, but less light and predators present all the time.
• The height of a zone above chart datum decides how long it is submerged, and that decides which organisms can live in it.

How abiotic and biotic factors control the distribution of organisms

The syllabus makes a single statement - environmental factors affect the distribution of organisms on a rocky shore - and then asks you to outline how. Split the factors into two groups. Abiotic factors are the physical and chemical conditions: exposure to air, wave action, oxygen, air temperature and water temperature. Biotic factors are the effects of other living things: predation and food availability. The abiotic factors set the limits of what can survive where; the biotic factors decide how much of that possible range a species actually occupies.

Exposure to air is the factor that matters most on a rocky shore. An organism high on the shore spends hours out of water, and water evaporates from any wet surface exposed to air and wind, so the danger is desiccation - drying out. Species differ in how long they can survive it, and that single difference is enough to arrange them in bands. It also explains why the upper shore is sparse: not because the rock is unsuitable, but because very few organisms can stay alive there for that long.

Wave action works in two directions. On an exposed coast the waves are powerful enough to tear weakly attached organisms off the rock and to batter them against it, so only low, streamlined or very firmly attached forms survive, and fragile, tall algae are absent. But waves also carry oxygen and food, and the spray from breaking waves keeps the rock wet well above the high water mark, so on a wave-beaten shore the same species are often found higher up than on a sheltered one. On a sheltered shore the water is calmer, so delicate, broad-fronded algae can grow, but there is less spray and the upper shore dries out more.

Oxygen is plentiful in the moving water of the open shore, so it is rarely limiting there. It becomes limiting in rock pools, where the water is still and trapped at low tide: at night, when there is no photosynthesis and every animal and alga in the pool is respiring, dissolved oxygen falls and carbon dioxide rises, and a pool can become dangerously short of oxygen by dawn. Air temperature sets another limit, because the rock surface in sunshine can be far hotter than the air or the sea, and in winter it can freeze; animals on the upper shore must tolerate both. Water temperature varies most in those same pools, which heat up in the sun at low tide and cool at night, and which can become saltier as water evaporates.

The biotic factors decide the lower limits. Predation is heaviest low on the shore and in the subtidal zone, because predators such as crabs, dogwhelks, starfish and fish can only reach the lower shore, and they can only feed there while it is submerged. Food availability works the same way: filter feeders need water moving past them carrying particles, grazers need a film of microalgae on the rock, and macroalgae need light as well as nutrients, so a species can only live where its food is delivered often enough.

Put the two groups together and you get the rule that answers most questions on this topic. The upper limit of a species on the shore is set by its tolerance of the physical conditions, especially desiccation and temperature - it cannot live any higher because it would dry out. Its lower limit is set by biotic factors - it does not live any lower because it is eaten or out-competed there. A limpet can survive high on the shore but is eaten lower down; Fucus can grow lower down but is shaded and grazed; a sea anemone cannot tolerate exposure at all, so it is confined to pools and to the low shore and the subtidal.

Adaptations of limpets, mussels, sea anemones and Fucus

Every adaptation on a rocky shore answers one of two problems: drying out at low tide, or being torn off and battered by waves. Learn each organism as a pair of problems with a solution, and say what the solution does, not just what it is called.

Limpet (a mollusc). A limpet has a hard, conical shell and a strong muscular foot. At low tide it clamps the foot down and pulls the shell tight against the rock, closing the shell to prevent water loss: the animal is sealed inside a small volume of water with its gills protected, and it can survive hours of exposure. The hard shell protects it against predators, so crabs and birds cannot break in easily, and the low conical shape sheds the force of the waves rather than catching them. A limpet that is prised off its rock is effectively dead, which is why the grip matters as much as the shell.

Mussel (a mollusc). A mussel has a hard shell of two valves that it can close tightly, which does two jobs at once: it stops water loss when the animal is exposed at low tide, and it shuts out predators. The shell is thick enough to resist the crushing and drilling of crabs and dogwhelks, and the animal lives packed in dense beds in the middle and lower shore where wave action and predation are both strong.

Sea anemone (a cnidarian). An anemone has no shell and no protection against drying out, so it uses behaviour instead. When the tide goes out it retracts its tentacles into its body column, which reduces the exposed surface to a small, wet dome and cuts evaporation to a minimum. It also lives in rock pools or below macroalgae, where water is trapped and the Fucus canopy shades the rock and holds moisture, so it is never fully exposed to sun and wind. That is why anemones are found low on the shore, in pools and under weed, and hardly ever on open rock high up.

Fucus (a macroalga, a brown wrack). Fucus has no roots and no water-conducting tissue, so it takes everything it needs from the water around it and has to survive being out of the water. Its thick, leathery fronds resist drying out and are tough enough to survive abrasion against rock and the impact of waves, and the fronds are flexible, so they stream out in the current instead of snapping. It is held in place by a holdfast, a disc that grips the rock and withstands currents and wave action. The holdfast only anchors the plant: it does not absorb water or nutrients, and it is not a root.

These four sets of adaptations explain the bands you see. Fucus dominates the middle shore where it can survive the exposure but still gets enough water; limpets graze the upper and middle shore and are limited downwards by predators; mussels occupy the middle and lower shore behind their shells; anemones keep to pools, the low shore and the subtidal, where they are never dried out.

The zones of sandy and muddy shores

A sedimentary shore is built of loose material - sand, mud or a mixture of the two - deposited by water. The same three zones apply, and for the same reason: the tide covers and uncovers them on a cycle, so height above the low water mark decides how long they are submerged. What changes is that there is no rock to attach to and the sediment itself is the habitat, so the animals live in the shore rather than on it.

The supratidal zone is above the high water mark. It is wetted by spray and by the highest spring tides, the sand or mud dries out and may develop a crust of salt, and the surface gets very hot in sun and very cold at night. Life is sparse and mostly hidden: sand hoppers and small crabs under stranded weed along the strandline, and insects and birds working the drier sand. On a muddy shore this zone often carries salt-tolerant plants at its upper edge, where the sediment stops being flooded every day.

The intertidal zone is the main habitat on a sedimentary shore and is where nearly all the syllabus examples live. Because the sediment holds water between its particles, the shore does not dry out as completely as rock, but the surface layers do drain at low tide, and the water table inside the sediment falls as the tide goes out. Animals survive by burrowing to the depth where it is still wet, which is why the visible life on a sandy beach at low tide is mostly a few holes, tubes, casts and trails rather than the animals themselves.

The subtidal zone is permanently submerged, below the low water mark. Sediment there is usually finer, because the finest particles only settle in still water, and it is less disturbed by waves, so it supports a richer and more stable community - more polychaete worms, more bivalves, and the fish and crabs that feed on them. Oxygen is the limiting factor, because the sediment is waterlogged and the deeper layers contain no oxygen at all, as the black colour and the smell of hydrogen sulfide show.

• Supratidal: above the high water mark; spray and spring tides only; dry, hot by day and cold at night, often with a salt crust; life sparse and hidden under weed.
• Intertidal: between the high and low water marks; the main habitat; the surface drains at low tide, so animals burrow to the depth where the sediment is still wet.
• Subtidal: below the low water mark; permanently submerged; finer sediment, less disturbance, richer community, but no oxygen in the deeper sediment.
• On a sedimentary shore there is nothing solid to attach to, so the animals burrow - the sediment is both the habitat and the shelter.

Comparing sandy and muddy shores

The syllabus gives you six features to compare, and they are not six separate facts: they form a chain. Sediment size decides how easily water moves through the sediment, and that decides the water movement, the oxygen availability, the stability and even the gradient. Learn the chain and you can reconstruct the whole comparison in an exam.

Sediment size. Sand particles are coarse enough to be seen, to feel gritty and to fall out of suspension quickly; mud is made of silt and clay particles so fine that they stay suspended in moving water for a long time. The consequence is permeability: water drains freely through sand, but cannot drain through mud, which is why a sandy beach feels firm underfoot at low tide and a mudflat feels soft and waterlogged.

Sedimentation rates compared with erosion rates. Because mud particles only settle out of still water, mud accumulates only where water movement is weak - in estuaries, behind spits, in sheltered bays - so on a muddy shore deposition exceeds erosion and the sediment builds up over time. On a sandy shore the waves both erode and deposit: an exposed beach loses sand in winter storms and gains it in calmer weather, so the beach profile changes through the year, and erosion can equal or exceed deposition. A mudflat therefore tends to grow upwards and outwards, while a sandy beach tends to shift.

Stability. Sand grains are loose and are moved by every wave and every gust of wind, so the surface is unstable: anything on it is scoured, rolled or buried, and nothing can attach to it. Mud particles stick to one another, so once deposited the surface is more stable in position, but it is soft, so animals sink into it and must burrow to move at all. This is why there are no holdfasts and no sessile animals on either shore: on sand the surface will not stay still, and in mud there is nothing solid to grip.

Gradient. Sandy shores are usually steeper, because the coarser sediment is thrown up into a beach face by the swash and because water drains through it quickly, so the beach is built from a narrow band of moving sediment. Muddy shores are almost flat, because the fine sediment settles out of slow water over a very wide area, producing a mudflat with a gradient so gentle that the tide can run out for hundreds of metres.

Water movement and oxygen availability. On a sandy shore, waves drive water into and out of the sediment with every swash, so the sand is well aerated and oxygen penetrates well below the surface; the sediment is pale because it is oxidised. On a muddy shore the water barely moves and the mud is impermeable, so the sediment is waterlogged and oxygen is used up within a few millimetres of the surface; below that the mud is black and anoxic, and decay there produces hydrogen sulfide, which is the smell of a mudflat at low tide. Oxygen availability is therefore the feature that most sharply separates the two shores, and it is the one that dictates the adaptations of the animals.

Life on a sandy shore: factors and the adaptations of clams and lugworms

On a sandy shore the abiotic factors are the unstable substrate, air temperature and exposure to air and wind; the biotic factors are predation and food availability. The unstable substrate is the one that shapes everything: because the sand moves, an animal cannot sit on the surface and cannot attach to it, so it either burrows or it is washed away. Air temperature and exposure matter because the surface of a beach at low tide is exposed to sun, wind and frost, and anything near the surface must tolerate that. Predation comes from wading birds and crabs at low tide and from fish and crabs at high tide, and it is heaviest where the animals are shallowest. Food is scarce in clean sand, so the animals feed on organic particles, on diatoms on the grain surfaces, and on the microorganisms living between the grains.

Clams are bivalve molluscs that solve the unstable substrate with a muscular foot. The clam pushes the foot down into the sand, swells the end of it to anchor, and then contracts the muscle to pull the shell down after it, repeating the process until it is buried. Burrowing does three jobs at once: it keeps the animal in wet sand when the tide is out, so it does not dry out; it puts a hard shell between the clam and the birds and crabs that hunt on the surface; and it keeps the clam in the layer where water still moves through the sand, so oxygen and food are available. Because the sand is unstable, a clam can be uncovered by a storm, which is why the burrow is deep and why the shell is thick and can be closed.

Lugworms live in U-shaped burrows in muddy sand. The worm keeps the burrow open, with its head at one end swallowing sediment and its tail at the other passing out the waste, which appears on the surface at low tide as a coiled cast. The burrow is a permanent refuge: it keeps the worm in water when the tide is out, protects it from predators and stops the sand collapsing on it, and it can be used for years.

The burrow water is a low-oxygen environment, because it is still and surrounded by waterlogged sediment in which decay is using up oxygen. A lugworm's blood therefore contains a specialised haemoglobin that binds oxygen strongly even when the concentration around it is very low, so the worm can extract enough oxygen from water that would not support most animals. Its other trick is its food: it ingests sand and digests the microorganisms within the sediment, so the sediment it lives in is also its meal. That is why lugworms are most abundant in the middle and lower shore of muddy sand, where the sediment is wet, rich in organic matter and still covered for most of the tidal cycle.

• Abiotic factors on a sandy shore: unstable substrate, air temperature, exposure to air and wind.
• Biotic factors: predation (birds and crabs at low tide, fish and crabs at high tide) and food availability (organic particles, diatoms, microorganisms between the grains).
• The unstable substrate is the master factor: nothing can attach to shifting sand, so animals burrow or are washed away.
• Clams burrow with a muscular foot - it keeps them moist, out of reach of predators and in the layer of sand where oxygen and food are still available.
• Lugworms live in U-shaped burrows, use a specialised haemoglobin to take up oxygen at very low concentrations, and ingest sand to digest the microorganisms in it.

The structure of a mangrove forest and the estuarine tidal cycle

A mangrove forest is a woodland growing in the intertidal zone of a tropical or subtropical coast, rooted in soft mud that is flooded and drained by the tide twice a day. Because it needs shelter from strong waves and a supply of fine sediment, it is most often estuarine - growing in an estuary, a delta or a sheltered lagoon where a river meets the sea - and because it occupies the strip between the sea and the land, it usually borders a terrestrial forest on its landward side. From the sea it looks like a wall of trees standing in the water on stilt roots; at low tide the same trees stand on open mud, surrounded by thousands of upright roots poking out of the surface.

The structure of the forest follows the tide. At the seaward edge are the youngest, most salt- and flood-tolerant trees, often growing singly and colonising new mud. Behind them is the main forest, with a closed canopy and a dense tangle of arching prop roots holding the trees above the mud. At the landward edge the mangroves become mixed with, and then replaced by, terrestrial forest species that cannot tolerate salt water or daily flooding. Underneath the canopy the ground is shaded mud, covered at low tide with pneumatophores - the aerial roots that stick up out of the sediment - and littered with fallen leaves and seedlings. The different tree species are arranged in bands across the forest according to how much salt and how much tidal flooding each can tolerate, so the forest has its own zonation, driven by the same tide that zones a rocky shore.

An estuary is where a river meets the sea, so its water is a mixture of fresh river water and salt sea water, and the mixture changes with the tide. That is why the abiotic factors in an estuary move through a cycle every tide, and why a mangrove forest growing in one has to cope with conditions that change hour by hour.

Salinity. At high tide the incoming sea water fills the estuary and salinity rises towards that of the open sea. At low tide the sea water drains out, the river's fresh water dominates, and salinity falls, sometimes to almost fresh in the upper estuary. In pools left behind on the mud, evaporation in the sun can push salinity back up again, so a mangrove root may be exposed to nearly fresh water and to very salty water within the same day.

Temperature. At high tide a large body of moving water buffers the temperature, so it stays close to that of the open sea. At low tide the water left in creeks and pools is shallow and still, and the exposed mud is heated directly by the sun, so temperature rises sharply by day and falls sharply at night. The daily range in temperature in an estuary is therefore much greater than in the open sea.

Dissolved oxygen. The incoming tide brings water that has been mixed with the air and is rich in oxygen, so dissolved oxygen is highest around high tide. At low tide the water is still and warm, warm water holds less oxygen, and the dense community of animals and the decaying organic matter in the mud consume oxygen faster than it is replaced, so dissolved oxygen falls - and it falls lowest at night, when there is no photosynthesis to replace it. These three changes together are what make an estuary a demanding place to live, and they are the reason the trees and animals of a mangrove forest have the adaptations they do.

How mangrove trees survive waterlogged soil, low oxygen and salt

A mangrove tree faces three problems at once. The mud it grows in is waterlogged, so there is no oxygen in it below the surface; it is soft, so there is little to hold a tree upright; and the water flooding it is salty, and salt drawn up into the leaves will poison the tissue if it accumulates. The syllabus limits the answer to three adaptations, and each one answers one of those problems.

Aerial roots (pneumatophores) for gas exchange. Because the mud contains no oxygen, an ordinary root system would suffocate. Mangroves send roots upwards, out of the waterlogged sediment and into the air, where they stand above the mud at low tide. The surface of these roots is covered in small pores, and oxygen diffuses through them into a spongy tissue that carries it down to the buried roots. The root system is shallow and spreads widely for the same reason: there is no point rooting deeply into mud that has no oxygen. This is why a mangrove forest at low tide looks as though it is standing on a field of spikes.

Prop roots for support. In soft mud an ordinary trunk would sink or be pushed over by the tide, so mangroves grow prop roots - roots that leave the trunk above the mud and arch down into the sediment like the legs of a tripod. They spread the load over a wide area, brace the tree against the push of tidal currents and waves, and hold it upright in a substrate that cannot grip a normal root. They also slow the water flowing past, so sediment settles among them and the shore builds up around the tree.

Leaves that can secrete salt. A mangrove takes up water from salty mud, and the salt comes with it. The tree cannot keep all of it out, so glands in the leaves secrete salt onto the leaf surface, where it is washed off by rain or shed with the leaf. That keeps the concentration inside the tissue below the level that would damage the cells, which is what allows the tree to grow in water that would kill an ordinary forest tree.

Read the three together and the logic is clear: the aerial roots deal with no oxygen, the prop roots deal with soft mud and moving water, and the salt glands deal with salt. Mangrove trees are not tolerant of the mangrove environment by accident - every one of their unusual features is a direct answer to one of its conditions.

Animals of the mangrove forest: the banded archerfish and the mudskipper

The mangrove forest is as demanding for animals as for trees. The water is warm, muddy and low in oxygen, especially at low tide; the mud is exposed twice a day; and there is no open water to escape into when a predator arrives. The two animals named in the syllabus show how far the adaptations go - one shoots its food down out of the air, and the other walks out of the water to feed.

Banded archerfish. The archerfish lives in mangrove creeks and estuaries and hunts insects sitting on leaves, roots and overhanging branches above the water. Its eyes are large, set forward and movable, so that the fields of view of the two eyes overlap and give it binocular vision - the depth perception it needs to judge how far away and how high a target is, which is essential because the light bends as it passes from air into water and the fish must aim accurately at something outside its own medium. Its mouth is specially shaped: a groove in the roof of the mouth, closed by the tongue, forms a narrow tube, and the fish forces water through it to spit a jet up to 150 cm and knock the insect into the water, where it is eaten. Its dark bands run vertically down the body and break up its outline against the shadowed roots and the stripes of light under the canopy, giving it camouflage so that fish-eating birds such as herons and kingfishers do not pick it out.

Mudskippers go further: at low tide they leave the water and move about on the exposed mud, feeding on algae and small animals on the surface. They can do this because they can breathe air. They absorb oxygen through the skin and the lining of the mouth and throat, as long as those surfaces stay moist - which is why they stay on wet mud and why they cannot survive in the sun for long. They also have enlarged gill chambers that trap a bubble of water, so they carry a reserve of water over their gills and can keep extracting oxygen from it while they are on land. Their pectoral and pelvic fins are shaped for movement on land, acting like small limbs that they use to prop themselves up and to pull and hop their bodies forward across the mud. Their eyes are set high on the head, so they can be kept above the body and used out of water, watching for predators and prey while the fish lies flat.

Both animals are tied to the rhythm of the tide. The archerfish exploits food above the water that no other fish can reach, and the mudskipper exploits the mudflat itself while the tide is out. Both depend on the mangrove forest for shelter and for the insects and algae they eat, which is why the adaptations of the trees and the adaptations of the animals are two halves of the same story.

• Banded archerfish: large, forward-set, movable eyes giving binocular vision to judge prey above the surface; a mouth shaped to form a tube that can spit a jet of water up to 150 cm; dark vertical bands as camouflage against birds.
• Mudskipper: absorbs oxygen through moist skin and the lining of the mouth and throat; enlarged gill chambers hold water so it can still respire on land; pectoral and pelvic fins shaped for movement on land; large eyes set high on the head for use out of water.
• Both are confined to the intertidal mangrove because their food and their shelter are there, and both must stay wet - the archerfish by staying in water, the mudskipper by staying on damp mud.