Why did I lose the mark?

The answer that scoresThe mantle is about 2 900 km thick and is made of solid rock rich in silicate minerals. It is hot, but the pressure keeps it mostly solid; where the temperature is high enough or the pressure falls, patches of it melt to form magma, a viscous liquid that is less dense than the rock around it and so rises. The tectonic plates are rigid slabs of crust that float on this hot, slowly deforming mantle. Heat from the core and from radioactive decay sets up convection currents in the mantle: rock at depth expands, becomes less dense and rises, then cools, becomes denser and sinks. The moving rock drags the base of the plates, so the plates move a few centimetres a year.

The answer that scoresA tsunami is a series of waves caused by a sudden displacement of a very large volume of sea water, most often by a shallow-focus earthquake under the sea at a convergent boundary. When the locked plates finally slip, the sea floor jerks up or down by several metres over a long distance, pushing the water above it out of place. In the open ocean the wave has a wavelength of hundreds of kilometres and a height of less than a metre, but it travels at up to about 800 km/h because the whole depth of water is moving. As it reaches shallow coastal water it slows, the wavelength is compressed and the wave grows in height, sometimes to more than 10 metres, and it runs inland as a flow of water and debris. Several waves follow, and the later ones can be larger than the first.

The answer that scoresAt a transform boundary two plates slide past each other horizontally, either in opposite directions or in the same direction at different speeds. No crust is created and none is destroyed, so nothing is subducted, no water is carried down to lower the melting point of the mantle, and no magma forms - which is why there are no volcanoes. Instead the rough edges of the plates lock together, the rock on either side bends and stores strain, and when it finally breaks the two sides jerk past each other and release the energy as a shallow earthquake. The San Andreas Fault in California is the standard example.

The answer that scoresOceanic crust is made of dense basalt and continental crust of lighter granite, so at a convergent boundary the denser oceanic plate is always the one that sinks. As it bends down it forms a deep ocean trench and carries water and sediment into the mantle; the water lowers the melting point of the mantle rock above, so magma rises and builds a line of explosive volcanoes or island arcs behind the trench. The lighter continental plate is crumpled and thickened at its edge to form fold mountains, as the Andes have been built above the subducting Nazca plate.

The answer that scoresThe Earth's magnetic field is created by the iron core, and specifically by the liquid iron of the outer core. As that molten metal flows and churns it carries electric currents, and those currents generate a magnetic field that extends far into space as the magnetosphere, deflecting the solar wind. A compass needle aligns with it, and it has reversed many times in Earth history, leaving magnetic stripes in the rocks of the ocean floor. The plates are not moved by magnetism: they float on the mantle and are dragged along by convection currents, in which hot rock rises, cools and sinks.

The answer that scoresCoordinates are always written latitude first, then longitude, so the position is 20°S, 80°E: twenty degrees south of the equator and eighty degrees east of the prime meridian. That places the point in the Indian Ocean, in the tropical belt, since 20°S is between the equator and the Tropic of Capricorn at about 23.5°S. A latitude of 80°E would be impossible, because latitude runs only from 0° at the equator to 90° at the poles.

The answer that scoresThe Moon's gravity pulls on the Earth's water, and because gravity weakens with distance the pull is strongest on the water nearest the Moon, which is drawn out into a bulge - a high tide. On the opposite side of the Earth the Moon pulls the solid Earth towards it more strongly than it pulls the water on the far side, so the Earth is pulled away from that water and a second bulge forms facing directly away from the Moon. Between the two bulges water is drawn away towards them, so sea level falls and a low tide occurs. As the Earth rotates once a day, each coast passes through both bulges and both low areas, giving two high tides and two low tides about six hours apart.

The answer that scoresSpring tides occur at new Moon and at full Moon, when the Sun, the Moon and the Earth lie in a straight line. The gravitational pulls of the Sun and Moon then act along the same line, so their effects add, the bulges are at their largest and the tidal range is greatest. Neap tides occur at the first and third quarters, when the Sun and Moon are at right angles as seen from the Earth, so the Sun's pull partly cancels the Moon's and the range is at its least. Both occur about twice a month, whatever the season; the word spring refers to the tide springing up, not to the time of year.

The answer that scoresA rip current forms when water pushed shorewards by waves piles up in the surf zone and returns seaward through the easiest route, usually a gap in a sand bar or a break in the breaking waves, as a narrow, fast flow running straight out from the beach. It carries a swimmer away from the shore into deeper water, and the danger is exhaustion and panic rather than being held under. A swimmer caught in one should stay calm and swim parallel to the beach until out of the narrow channel, then return to shore with the waves, or float and signal for help. Risk is reduced by swimming between the flags on a lifeguarded beach, because rips are often where the surface looks calmest.

The answer that scoresMoving out from the coast, the first feature is the continental shelf: shallow, gently sloping, and running from the shoreline to the shelf break at about 200 m. Sunlight reaches its bed, so it is the most productive part of the ocean. At the shelf break the floor steepens into the continental slope, which drops to the deep ocean floor and is cut by submarine canyons. Beyond the foot of the slope lies the abyssal plain, which is flat rather than sloping, at roughly 3 000 to 6 000 m and covered in fine sediment. An ocean trench is a separate feature, a long, narrow, very deep depression formed where one plate is subducted beneath another, as at the Mariana Trench.

The answer that scoresSalinity is the concentration of dissolved salts, measured in parts per thousand (ppt), which means grams of dissolved salts per 1 000 g of sea water. The open ocean averages about 35 ppt - 35 g of salts in every kilogram of sea water, which is the same as 3.5%. To calculate it, evaporate a known mass of sea water to dryness, weigh the solid salts left behind and divide by the mass of water, then multiply by 1 000: a 500 g sample that leaves 17.5 g of salts has a salinity of 17.5 / 500 x 1 000 = 35 ppt.

The answer that scoresIce that is already floating - sea ice, icebergs and floating ice shelves - displaces its own weight of water. When it melts, the meltwater fills exactly the space the ice had been occupying, so the water level does not change; you can confirm this by floating an ice cube in a beaker of water, marking the level and finding it unchanged after the cube has melted. Ice that rests on land, such as a glacier or an ice sheet, is not displacing any sea water, so when it melts the water runs into the sea and the level rises. In both cases the meltwater is fresh, so it dilutes the surface layer and lowers its salinity.

The answer that scoresAn increase in temperature reduces the solubility of both oxygen and carbon dioxide in water. At a higher temperature the dissolved gas particles have more kinetic energy, so more of them have enough energy to escape from the surface back into the atmosphere, and the water cannot hold as much gas. That is why cold polar water is rich in dissolved oxygen while warm tropical surface water holds much less, and why a warm bottle of drink goes flat faster than a cold one.

The answer that scoresSea water is a mixture of water, dissolved salts and dissolved gases, not a compound. A compound is two or more elements chemically combined in fixed proportions, such as calcium carbonate, CaCO3, whose properties are different from those of calcium, carbon and oxygen. In sea water the sodium chloride and the water are not chemically combined, they can be present in any proportions, and they can be separated by a physical method - evaporating the water leaves the salt behind - so sea water is a mixture, and because the salt is dissolved in the water it is a solution.

The answer that scoresEvaporation takes place at the surface of a liquid at any temperature, because the particles in a liquid have a range of energies and the fastest ones at the surface can escape into the air. It is faster when the water is warmer, when the surface area is larger, when the wind is stronger and when the air is drier, because each of these lets more particles escape or keeps the air above the surface dry. Boiling is different: it happens throughout the whole liquid, only at the boiling point, and produces bubbles of vapour inside the liquid. Evaporation from the ocean therefore happens continuously, which is why it concentrates the remaining sea water and leaves the salts behind in every sea, not only in the hottest ones.

The answer that scoresDensity is mass per unit volume, calculated as density = mass / volume. Sea water is denser than fresh water at the same temperature because the dissolved salts add mass while adding almost no volume: the ions fit into the spaces between the water molecules, so the same volume of sea water contains more mass. Fresh water has a density of about 1 000 kg/m3 and sea water of ordinary salinity about 1 025 kg/m3. The correct comparison is between equal volumes: 1.0 m3 of fresh water has a mass of about 1 000 kg, while 1.0 m3 of sea water has a mass of about 1 025 kg.

The answer that scoresWhen water is heated, its particles gain kinetic energy and move faster, so the average distance between them increases. The water therefore occupies a larger volume. The mass is unchanged, because no particles have been added or removed, and the same mass spread through a larger volume means a lower density. Cooling reverses the process: the particles lose kinetic energy, the attractions between them pull them closer together, the volume falls and the density rises. In the sea this is why cold water is denser than warm water at the same salinity, and why it sinks beneath it.

The answer that scoresPressure is force per unit area, and in the sea it is caused by the weight of the water above the point, plus the pressure of the atmosphere at the surface. Water is almost incompressible, so its density barely changes with depth and each extra metre adds about the same amount of pressure: roughly 1 atmosphere (about 1.0 x 10^5 Pa) for every 10 m. The total at 30 m is therefore about 4 atmospheres - 3 from the water and 1 from the air - which is about 4 x 10^5 Pa, not 3 Pa. Pressure increases steadily and in proportion to depth, and the shape of the container and the temperature of the water make almost no difference.

The answer that scoresUpwelling is caused by wind. Where a steady wind blows along a coast or offshore, it drags the surface water away from the coast, and water from below rises to replace it. That water is cold, because it has come from beneath the sunlit, wind-mixed layer and has not been warmed by the sun, and it is rich in nutrients such as nitrate and phosphate, which are released at depth by the decay of dead organisms. In the sunlit surface layer those nutrients are quickly used up by phytoplankton, so the surface is normally nutrient-poor; upwelling carries the deep store of nutrients up into the light, where phytoplankton can use them. That is why upwelling regions such as the Peruvian coast and the Benguela Current are so productive.

The answer that scoresIn an El Nino the trade winds across the Pacific weaken or reverse. Warm surface water spreads eastwards instead of piling up in the west, so the surface water of the eastern Pacific becomes warmer than usual, and because the wind has weakened, upwelling off South America is reduced. Less nutrient-rich water reaches the surface, so phytoplankton production falls, the anchoveta and other fish decline, and the fishery suffers. The warm eastern Pacific drives heavy convection and floods in Peru and Ecuador, while the western Pacific has descending air, bringing drought to Indonesia, Papua New Guinea and eastern Australia.

The answer that scoresBoth plant and animal cells are surrounded by a partially permeable cell membrane, which controls what enters and leaves the cell. A plant cell also has a cell wall, a rigid layer outside the membrane made of cellulose, which supports the cell, gives it shape and stops it bursting when water enters. An animal cell has no wall, so its membrane is its only outer boundary, which is why animal cells are more variable in shape.

The answer that scoresA bacterial cell has only a cell wall, a cell membrane and cytoplasm. It has no nucleus, so its genetic material is a single loop of DNA lying free in the cytoplasm, and it has no membrane-bound structures such as chloroplasts or mitochondria. Respiration still takes place: the enzymes needed are in the cytoplasm and on the cell membrane, and some bacteria carry out photosynthesis on folded membranes in the cytoplasm rather than in chloroplasts.

The answer that scoresAsexual reproduction is reproduction in which a single organism produces exact copies of itself. There is one parent, no sex cells are produced and no fertilisation takes place, so the offspring are genetically identical to the parent. In bacteria and diatoms the cell divides in two by binary fission; in corals a new polyp grows out of the parent by budding; in seagrass new shoots arise from rhizomes. Because the offspring are clones, a change in conditions that harms one harms them all.

The answer that scoresIn sexual reproduction organisms produce male and female sex cells, and these fuse at fertilisation to produce offspring with characteristics from both parents. The sperm contributes genetic material from the father and the egg from the mother, so each offspring is a mixture and is not identical to either parent or to its siblings. In most fish the eggs and sperm are released into the water and fertilisation happens externally, and the offspring that survive show a wide range of variation.

The answer that scoresA chloroplast contains chlorophyll, which absorbs light energy and uses it to build glucose from carbon dioxide and water in photosynthesis, so the energy is transferred into a chemical store. Mitochondria are the site of aerobic respiration, where glucose is broken down and the energy released is made available for the cell's activities. That is why a photosynthesising leaf cell has many chloroplasts and an active animal cell such as a gill cell has many mitochondria, and why neither structure can be said to 'make' energy.

The answer that scoresKelp is placed in the protoctist kingdom as a macroalga. It does photosynthesise and it does have chloroplasts, but its body is a thallus rather than a plant body: the holdfast only anchors it to the substrate and does not absorb minerals, the stipe supports the blades without any vascular tissue, and the blades are flat photosynthetic surfaces rather than leaves. It has no true roots, stems or leaves, no vascular tissue and no flowers or seeds, so it cannot be a flowering plant. It is therefore grouped with the other algae in the protoctists, alongside the dinoflagellates and diatoms.

The answer that scoresA whale is a mammal, and the three syllabus features show it. It has hair, at least as bristles around the blowhole, where a fish has scales. It breathes air with lungs through a blowhole, where a fish takes oxygen from water with gills. It fertilises internally and gives birth to a live calf that is fed on milk. It is a cetacean, one of the three groups of marine mammals alongside the pinnipeds and the sirenians. Its flippers are modified forelimbs with bones inside, and its tail is a horizontal fluke that moves up and down, not a vertical fish tail that moves side to side.

The answer that scoresA jellyfish has radial symmetry: the body is built around a central axis, with the tentacles arranged in a ring around the single opening, so any plane through that axis divides it into similar halves and there is no left or right. A starfish has pentaradial symmetry: five similar arms arranged around a central disc, a five-part form of radial symmetry. That is the adult form only - the larva of an echinoderm is bilaterally symmetrical. Bilateral symmetry, with one plane giving mirror-image halves, is found in the crustaceans, molluscs and annelids.

The answer that scoresThe features listed belong to bony fish such as cod, herring and tuna: a bony skeleton, thin overlapping scales, gills covered by an operculum, and external fertilisation with eggs and sperm shed into the water. Cartilaginous fish such as sharks, rays and skates have a skeleton of cartilage, small tooth-like scales, and five to seven exposed gill slits on each side with no operculum at all. They also mate, so fertilisation is internal, and many give birth to live young. Both are still fish, and both have scales and gills, so the operculum and the method of fertilisation are the two features that separate them.

The answer that scoresA dinoflagellate is a protoctist. It is a microscopic single-celled organism with chloroplasts, so most species photosynthesise, and it has two flagella - one in a groove around the cell that spins it and one trailing behind that steers - which is how it swims towards light and nutrients. Being single-celled and motile does not make it an animal, because animals are multicellular and obtain their food by feeding on other organisms. Some dinoflagellates are not photosynthetic and live as parasites, and some photosynthetic species multiply into red tides that release toxins. Diatoms are the other microscopic protoctists, and they are identified by a silica skeleton in two halves.

The answer that scoresA female leatherback reproduces every 2 to 5 years. In a nesting year she returns to the sandy shore where she hatched, excavates a nest in the sand with her hind flippers, lays her eggs, buries them and departs, leaving no parent to guard the nest. She repeats this egg-laying several times at 10-day intervals during the same season, so one female may lay several clutches on the same stretch of coast before she returns to the sea for another few years.

The answer that scoresThe eggs incubate in the sand for 55 to 60 days before hatching. The hatchlings leave the nest, usually at night, and go to sea, where they feed and grow. Reaching maturity takes 15 to 25 years, and males spend all of that time at sea and never return to land. A mature female then returns to the shore where she hatched and begins nesting every 2 to 5 years.

The answer that scoresPolyps reproduce both asexually and sexually. Asexual reproduction is budding, in which a parent polyp produces a new polyp next to itself, genetically identical and attached, so repeated budding builds the colony. Sexual reproduction is the synchronised release of a huge number of eggs and sperm into the water; the fertilised eggs float to the surface and become planktonic larvae, which swim to the light, feed, grow and are moved by currents. Each larva eventually descends and attaches to a hard surface such as bare rock or dead coral skeleton, and there it grows into a polyp and begins to bud.

The answer that scoresPlankton, fish, squid and shrimp rise from the twilight zone into the sunlight zone at dusk and sink back at dawn. They rise because the food is at the surface: phytoplankton, and the zooplankton that graze on them, are concentrated in the sunlit layer where photosynthesis can happen. They do it at night because the predators that hunt them rely on sight, so the dark surface water hides them, and by day they retreat into the dim twilight zone where their silhouettes are much harder to see. The energy cost of the swim is repaid by feeding in the richest water while avoiding being eaten.

The answer that scoresMigration of marine organisms occurs in a three-dimensional environment, so it can be horizontal - movement across the surface between one region or habitat and another, as when tuna, turtles and whales cross ocean basins - or vertical, as when plankton, fish, squid and shrimp move up and down the water column between the twilight zone and the sunlight zone. Distance varies from a few metres, as when reef fish move from the reef flat to the reef slope to spawn, to thousands of kilometres across an ocean. The reasons are to find food, to find mates, to reach a different habitat in which to reproduce, and to avoid predators.

The answer that scoresRespiration is the chemical reactions in cells that break down nutrient molecules such as glucose and release usable energy, summarised as oxygen + glucose -> carbon dioxide + water. It happens inside every living cell, all the time, because every cell needs a continuous supply of usable energy for growth, repair, movement and the other processes that keep it alive. Gas exchange is a different process: it is the diffusion of oxygen and carbon dioxide across a surface, at the gills of a fish or the lungs of a whale, and its job is only to move the gases in and out of the body. A photosynthetic organism respires as well as photosynthesises, in the same cells and at the same time; at night photosynthesis stops because there is no light, but respiration continues, which is why the carbon dioxide in the water around a plant rises overnight.

The answer that scoresThe test for a reducing sugar such as glucose uses Benedict's solution, which is blue; the sample is heated with it in a water bath for a few minutes, and if a reducing sugar is present the solution turns green, yellow, orange or brick-red with a precipitate, the colour depending on how much sugar there is. The test for protein uses biuret solution at room temperature, and a positive result is a change from blue to purple or lilac. The test for starch uses iodine solution at room temperature, which changes from orange-brown to blue-black. The test for lipid is the emulsion test: the sample is shaken with ethanol and the liquid is poured into water, giving a cloudy, milky white emulsion if a fat or oil is present. Only Benedict's needs heating.

The answer that scoresA nutrient is a substance required by an organism for growth, repair and to provide chemical energy. In the sea, nutrients occur in three forms: gases such as carbon dioxide, which supplies the carbon used to make organic compounds; dissolved salts such as nitrate, phosphate, calcium and magnesium, which supply essential elements; and organic compounds, which are the carbohydrates (starch and sugars such as glucose), the lipids (fats and oils) and the proteins. A food, by contrast, is a material an organism takes in, and it contains nutrients rather than being one.

The answer that scoresProductivity is the rate at which producers transfer energy into carbohydrates, so it is measured per unit area and per unit time, for example as the mass of carbon fixed per square metre per year. Biomass is the total mass of living producer material present at one moment, which is a different quantity: a small standing crop of phytoplankton that is dividing quickly can have very high productivity, because it is turning over rapidly, while a large, slow-growing bed of kelp can have lower productivity despite holding far more material. Productivity also depends on the supply of light and nutrients at the same time, which is why upwelling areas, where nutrients reach the sunlit zone, are so productive.

The answer that scoresNutrients are cycled rather than lost. Remains of dead organisms and faecal pellets clump together and sink through the water column as marine snow, carrying carbon and nutrients into deep water, where bacteria decompose them and release dissolved mineral salts and carbon dioxide. That deep water is nutrient-rich, and where winds and currents drive surface water away from a coast, upwelling brings it back to the surface. In the sunlit zone light is available again, so producers take up the nutrients and photosynthesis and productivity recover. The cycle is completed by the bacteria that return nutrients from dead tissue to the water in the first place; without them, material would stay locked in organic form and the surface would indeed become impoverished.

The answer that scoresDiatom -> copepod -> sandeel -> puffin, with each arrow read as 'energy passes to'. The diatom is the producer and the first trophic level; the copepod is a primary consumer and a herbivore; the sandeel is a secondary consumer; the puffin is a tertiary consumer. Energy travels from the diatom to the copepod when the copepod eats it, and so on up the chain, with most of the energy lost as heat at each transfer.

The answer that scoresA decomposer is an organism that gets its energy by breaking down dead or waste organic material; in the sea these are mainly bacteria and fungi, which digest the material externally with enzymes and absorb the soluble products. A sea cucumber is a detritivore: an animal that gets its energy from eating dead or waste organic material, digesting it inside its body. Both feed on dead matter, but only the bacterium or fungus is a decomposer.

The answer that scoresEnergy flows through a food chain in one direction and is gradually lost. At each level some energy is released by respiration and leaves as heat, some is used in movement and leaves as heat, some is lost in excreted waste products, and some leaves the chain when organisms are removed or harvested. Because heat cannot be reused, each level receives only about a tenth of the energy of the level below, which is why chains are short. Nutrients such as nitrate and phosphate are recycled by decomposers, but energy is not.

The answer that scoresA pyramid of numbers shows the number of organisms at each trophic level and has no unit of energy, so it cannot be used to compare the energy at each level. A pyramid of energy shows the energy passing through each trophic level per square metre per year, and it is always widest at the producer level and narrows upwards, because energy is lost at every transfer. The phytoplankton may be the most numerous and still hold the most energy, but that conclusion comes from the energy pyramid, not from counting cells.

The answer that scoresIt is the pyramid of biomass that can appear inverted in open water: phytoplankton divide and are grazed so quickly that the standing crop at any moment is smaller than the biomass of the zooplankton feeding on it, even though the total production over a year is much greater. The pyramid of energy for the same water is always upright, because it measures the energy passing through each level over time, and about 90 per cent of the energy at each level is lost - mainly as heat from respiration - before the next level can use it.

The answer that scoresA population is a group of organisms of one species, living in the same area, at the same time - for example all the limpets on a rocky shore. A community is all of the populations of different species in an area, so it includes the limpets, the barnacles, the seaweeds and the crabs together. If the non-living surroundings are included as well - the water, light, temperature and rock - then the unit is an ecosystem, which is the community plus its environment interacting together.

The answer that scoresPlankton is a collection of organisms that drift in water currents: they may swim weakly but they cannot move against a current. It contains both producers and consumers. The phytoplankton are microscopic producers - diatoms, dinoflagellates and cyanobacteria - which photosynthesise in the sunlit surface water. The zooplankton are consumers, and they include the larvae of fish and invertebrates, which can be microscopic, as well as larger animals such as jellyfish. So plankton is a way of life rather than a size class, and it is not made up only of plants.

The answer that scoresCoastal ecosystems generally have a higher productivity than the open-ocean ecosystem, partly because run-off from the land carries nutrients such as nitrates and phosphates into the shallow coastal water. Light also reaches the seabed there, so rooted seagrasses and attached seaweeds can photosynthesise, and nutrients released by decay in the sediment are recycled straight back into the sunlit water. The open ocean has no land to supply nutrients, and dead organisms sink out of the sunlit layer before their nutrients can be recycled there, so its surface water is nutrient-poor and production is limited by nutrients rather than by light. The open ocean is vast, but its rate of production per square metre is low.

The answer that scoresCountershading - dark above and pale below - is an adaptation of fish in the sunlight and twilight zones, where there is enough light for a predator to see a silhouette from above or below. In the midnight zone there is no sunlight at all, so deep-sea species are dark brown or black, which makes them hard to see in the light produced by other organisms' bioluminescence. They move slowly and live long lives because dissolved oxygen is low and food is scarce, so a fast, high-energy lifestyle is impossible. Their other adaptations follow the same conditions: bioluminescence to attract mates or prey or to illuminate predators, gelatinous bodies to withstand very high pressure, and large backward-facing teeth so that captured prey cannot escape.

The answer that scoresRandom sampling means that every point in the area has an equal chance of being sampled. Lay two tape measures at right angles along the edges of the shore to make a grid, then use a random number table or generator to choose a pair of coordinates, walk to that point and place the quadrat. Repeat for at least ten quadrats, without choosing the positions. Count the individuals of each species in every quadrat, or estimate percentage cover using the subdivisions, and list every species present to find species richness. Then calculate population size as the mean number per quadrat multiplied by the number of quadrats that would fit into the whole area, and increase the number of samples until the running mean stops changing.

The answer that scoresThe supratidal zone is above the high water mark, wetted only by spray and by the highest spring tides, so it is dry for most of the tidal cycle. The intertidal zone is between the high and low water marks and is covered and uncovered twice a day; it is divided into upper, middle and lower shore, with the upper shore exposed for longest. The subtidal zone is below the low water mark and is permanently submerged apart from the very lowest spring tides. Getting the order right matters because the whole explanation of zonation depends on exposure time falling as you go down the shore.

The answer that scoresThe holdfast is a disc that grips the rock and withstands currents and wave action, holding the alga in place; it does not absorb water or nutrients, because a macroalga takes those from the water through its fronds and has no root system. A mangrove's aerial roots, the pneumatophores, grow upwards out of the waterlogged mud into the air so that oxygen can diffuse through pores in their surface into the tissue and down to the buried roots - they are for gas exchange, because the mud itself contains no oxygen. The prop roots provide support in soft mud, and the leaves secrete salt.

The answer that scoresThe lugworm lives in a U-shaped burrow in waterlogged sediment, where the water is very low in oxygen, so its blood contains a specialised haemoglobin that binds oxygen strongly even at very low concentrations and allows it to extract enough oxygen to respire. It does not filter food from the water: it swallows sand at the bottom of its burrow and digests the microorganisms living on and between the grains, which is deposit feeding. The U-shaped burrow itself keeps the worm in water at low tide, protects it from predators and stops the sediment collapsing on it.

The answer that scoresSand has much coarser particles than mud, so water drains through it freely, and on an open beach the waves both erode and deposit it, which makes the surface unstable and the gradient steeper. Mud is made of fine particles that settle only in still water, so deposition exceeds erosion, the shore is almost flat, and the sediment is impermeable and waterlogged. As a result, oxygen penetrates deeply into sand but is used up within a few millimetres of the surface of mud, where the sediment below is black and anoxic. The animals respond to that, not by attaching - nothing can attach to a shifting or soft substrate - but by burrowing: clams use a muscular foot, and lugworms live in U-shaped burrows and use a specialised haemoglobin to obtain oxygen at very low concentrations.

The answer that scoresThe banded archerfish has large, forward-set, movable eyes that give it binocular vision, so it can judge the position of an insect on a leaf or root above the surface; its mouth is shaped to form a narrow tube, and it spits a jet of water up to 150 cm to knock the insect into the water, where it is eaten. Its dark bands camouflage it against the roots and the striped light of the canopy, protecting it from birds. The mudskipper leaves the water at low tide and can do so because it absorbs oxygen through its skin and the lining of its mouth and throat while they are moist, and because its enlarged gill chambers trap water so that it can still obtain oxygen on land; its pectoral and pelvic fins are shaped for movement on land, and its eyes are set high on the head for use out of water.

The answer that scoresCorals are animals, related to sea anemones and jellyfish, which live in colonies of many small polyps joined by living tissue. Each polyp has a mouth surrounded by tentacles with stinging cells, a stomach, and a skeleton of calcium carbonate that it secretes itself. Most reef-building corals also have single-celled photosynthetic algae, the zooxanthellae, living inside their tissues. The algae photosynthesise and supply the polyp with oxygen and food, but the coral is still an animal: it can also catch plankton with its tentacles, and it builds the reef from its own skeleton.

The answer that scoresThe relationship is mutualistic because both organisms benefit. The zooxanthellae live protected inside the cells of the polyp, where they are carried into the light and supplied with carbon dioxide from the coral's respiration and with nitrogen- and phosphorus-containing waste, which they need to grow. In return the algae give the coral oxygen for respiration and sugars and other organic compounds made in photosynthesis, which are the coral's main food supply, and they take up the coral's waste and help it deposit its calcium carbonate skeleton. Because the algae photosynthesise, reef-building corals can only live in shallow, clear water where light reaches them.

The answer that scoresThe water around a coral reef is clear because it contains very little dissolved nutrient such as nitrate and phosphate, so there is little phytoplankton to cloud it. The nutrients that exist are held within the organisms living on the reef rather than in the water: the zooxanthellae take up the coral's waste directly, and anything released is taken up quickly by sponges, algae and bacteria or eaten by grazers. The reef therefore recycles its nutrients tightly instead of letting them accumulate. This is essential for the reef, because clear water lets light reach the zooxanthellae, and it is easily upset: if sewage or fertiliser adds nutrients, algae grow over the coral and shade it.

The answer that scoresIn the Darwin-Dana-Daly theory the volcanic island subsides, not the reef. Coral larvae settle on the hard shoreline of a volcanic island and grow outwards as a fringing reef attached to the land. As the volcano becomes extinct and the island slowly subsides and is eroded, the coral keeps growing upward at a rate that keeps pace with the sinking, so the reef surface stays in shallow water where there is light. A widening lagoon separates the reef from the shore, making it a barrier reef, as in the Great Barrier Reef. When subsidence carries the last of the island below sea level, the upward-growing coral is left as a ring enclosing a central lagoon - an atoll, as in the Maldives. Daly added that falls and rises in sea level during the ice ages, and wave erosion of the exposed reef top, flattened the reef and helped form the lagoon.

The answer that scoresParrot fish are mainly grazers. Using their beak of fused teeth they scrape algae off the coral rock and the dead skeleton, which stops the algae from growing over the living coral and competing with it for space and light; on reefs where parrot fish have been removed by overfishing, fleshy algae have taken over and coral cover has fallen. They do also eat coral, grinding it with specialised teeth in the throat and passing the limestone as sand, which causes some local damage but creates bare space for new coral larvae to settle and produces much of the reef's sand. The effect of the population therefore depends on its size: moderate grazing keeps the reef coral-dominated, while very heavy grazing, or the loss of the grazers altogether, tips the balance towards algae.

The answer that scoresEcotourism is ecologically sustainable tourism with a focus on experiencing natural areas that encourages environmental and cultural understanding, appreciation and conservation. It is small in scale and low in impact: small groups, local guides, simple accommodation, minimal waste and transport, and part of the fee paid into managing and protecting the area. The purpose is not only to see the wildlife but to teach the visitor about it, so that they value it and support its conservation, and to give local people an income from keeping the ecosystem intact. Diving from a large resort that anchors on the reef, feeds the fish and takes no part in management is nature tourism, not ecotourism.

The answer that scoresTourism brings both benefits and costs, and management decides which dominates. On the benefit side it provides employment and income, builds infrastructure such as roads, water supply and sewage treatment that local people also use, and creates a constituency that wants the reef protected: visitors who have seen it support conservation, visitor fees pay for MPAs and rangers, and conservation organisations and volunteers are drawn to the area. On the cost side it competes for land, water and fish, adds sewage, litter and fuel, breaks coral with anchors and fins, removes organisms as souvenirs and changes the behaviour of wildlife that is fed or chased. The answer is not a ban but zoning, permits, carrying capacity, moorings, education and legislation, and places that apply them keep the income and the reef.

The answer that scoresAquaculture can reduce pressure on fisheries because farmed fish, shrimp, molluscs and seaweed supply food without taking that species from the wild, and because a reliable farmed supply lowers demand and price pressure on wild stocks. But a farm changes the environment it sits in. Uneaten feed and faeces enrich the water and the seabed, causing algal blooms and low oxygen; parasites such as sea lice and diseases spread from cages to wild fish; farmed fish escape and interbreed or compete with wild populations; antibiotics and treatments enter the ecosystem; and pelleted feed made from wild fish means a carnivorous farm can consume more fish protein than it produces. Warm-water shrimp farms have also been built by clearing mangrove forest, which destroys the nursery the wild fishery depends on.

The answer that scoresDispersants are chemicals sprayed onto a slick to break it into fine droplets that mix into the water column instead of floating. That protects the surface and the shore, so seabirds, mammals and beaches are spared, and it can be done quickly in weather that would defeat a boom. The cost is that the oil is dispersed through the water, where it exposes plankton, fish eggs and larvae, filter feeders and the seabed community to toxic hydrocarbons, and where it cannot be recovered. Booms contain oil in calm water and protect a particular inlet, and skimmers recover it, but both fail in waves; burning removes oil but kills surface life and produces smoke. The choice depends on the sea state, the distance to the coast and which ecosystem is most at risk, and no method recovers more than a fraction, so prevention through double hulls, inspection and port reception facilities matters most.

The answer that scoresA fish aggregating device is a floating object, anchored or drifting, that pelagic fish gather around for shelter, shade and the food that grows on it. Its principle is attraction, not production: the fish are drawn from a wide area into one place, so a boat with GPS can return and set a purse seine around the whole school. For fishers the benefits are less search time, less fuel and more reliable catches, which matters most for small-scale boats. The ecological impacts are that the net takes the whole mixed community, including juvenile tuna, sharks and turtles, that entanglement in the FAD netting and ropes kills wildlife, that drifting FADs become marine plastic and ghost-fish for years, and that concentrating fish makes local depletion faster and can change natural migration. Managing them means limits on numbers, banning netting on the device, and requiring recovery of lost FADs.

The answer that scoresPlastics are very slow to break down. Most are not biodegradable, because micro-organisms cannot break the strong carbon-carbon bonds in the polymer, so a bag or bottle that reaches the sea persists for decades or centuries. They do break down over very long time periods, but by physical and chemical processes: ultraviolet light makes the plastic brittle, and waves, sand and abrasion grind it into smaller and smaller fragments called microplastics, below about 5 mm across. That fragmentation releases toxins, because additives such as plasticisers leach out of the polymer and because pollutants such as PCBs adsorbed on the surface are concentrated there. So the plastic does not disappear; it spreads, becoming harder to remove and easier for plankton and filter feeders to take up.

The answer that scoresThe Great Pacific Garbage Patch is a region of the North Pacific gyre, between Japan and the west coast of North America, where plastic fragments are much more concentrated than in the surrounding ocean. It forms because the gyre is a circular system of surface currents driven by the prevailing winds and deflected by the Coriolis effect, and the water converges on its slow centre, so floating plastic is carried in and trapped rather than dispersed. It is not a solid island: most of the material is small fragments and weathered fishing gear suspended in the upper few metres, spread over a very large area, which is why it is so difficult and expensive to remove and why the plastic cannot be collected before it fragments further.

The answer that scoresFertilisers are water soluble, so nitrates and phosphates are leached and washed off farmland into rivers and then into the sea, and untreated sewage adds the same nutrients. The added nitrogen and phosphorus removes the limit on the growth of producers, so phytoplankton and macroalgae grow rapidly into an algal bloom. The bloom then dies - the nutrients are used up and the producers shade one another - and the dead material sinks, where decomposers break it down by aerobic respiration. That respiration uses dissolved oxygen faster than it is replaced, so dissolved oxygen in the water falls, and fish, crustaceans and molluscs that need dissolved oxygen suffocate or leave. The fish are not poisoned by the fertiliser; they are killed by the lack of oxygen that follows the bloom.

The answer that scoresSea level rises for two reasons. Thermal expansion: the ocean has absorbed about 90 per cent of the extra heat trapped by the enhanced greenhouse effect, sea water expands as it warms, and the same mass of water therefore occupies a larger volume, so sea level rises without any water being added. Melting land-based ice: glaciers and the Greenland and Antarctic ice sheets lose water that flows into the sea, which adds water to the ocean, and this is the greater long-term risk because of the volume stored. Melting floating sea ice does not raise sea level, because floating ice already displaces its own weight of water. Thermal expansion is slow to reverse because deep water warms and cools over centuries, which is why sea level keeps rising after temperatures stabilise.

The answer that scoresCoral bleaching is caused by warming water. Corals live with single-celled algae called zooxanthellae in their tissues, which give them their colour and supply most of their food by photosynthesis. When the water becomes too warm, for example during a marine heatwave, the coral expels the algae, the white skeleton shows through, and the coral is bleached. It is still alive but starving, and it dies if the heat stress lasts for weeks. Ocean acidification is a different effect of the same carbon dioxide: the gas dissolves to form carbonic acid, which releases hydrogen ions and lowers pH, and the lower pH makes it harder for the coral to build its calcium carbonate skeleton. Warming causes bleaching; acidification slows growth and weakens the skeleton.

The answer that scoresA marine protected area can restore species richness, because it protects whole habitats rather than single species and allows populations to recover and spill over into the surrounding fished water. But it works only under conditions: the reserve must be large enough and connected to larval sources, its habitats must be intact, the rules must be enforced so that poaching and illegal fishing are controlled, and the pressures outside it must not be overwhelming. Because warming, acidification and nutrient pollution cross the boundary, an MPA cannot protect a reef from bleaching or a bay from eutrophication. That is why MPAs are most effective as part of a package with sustainable harvesting, controls on pollution and action on greenhouse gas emissions.