Module 6 — Biological Adaptations of Organisms in Aquatic Environments

• In the oceans and lakes, living things meet conditions that are harsh in different ways: temperature changes, high or low salinity (and therefore high or low osmotic pressure — Module 4), little oxygen in some layers, darkness in the depths (Module 7), and enormous pressure in the deep sea (Module 7).
• Each organism survives and reproduces because it has a set of adaptations.

Adaptation = a behavioural or physical trait of a living organism that helps it to survive better in its ecosystem.

• An adaptation arises over many generations through natural selection: individuals with useful traits survive and reproduce more, so the trait becomes common in the population.
• An individual does not "choose" to adapt; the species adapts.
• (Short-term changes in one individual, such as getting a suntan, are called acclimatisation.)

The book classifies adaptations into three types:

1. Structural (anatomical) adaptations — the structure and shape of body parts.
2. Behavioural adaptations — actions, inherited or learned.
3. Physiological (functional) adaptations — how the body performs its vital functions.

These are changes in the structure and shape of parts of the organism that help it survive.

• Note: sharks and rays are cartilaginous fish; most sharks have no swim bladder (they use an oil-rich liver and constant swimming to avoid sinking).
• Bony fish have the swim bladder.

• Very large eyes that collect the little available light, enabling vision in the dark deep water (or, in some species, special light-producing organs).
• Compressed (flattened) bodies that can withstand the extremely high pressure in deep water. Example in the book: the icefish, which lives in the cold southern oceans at depths reaching up to about 2000 metres.
• Flexible skeleton, as in the viper fish (section 3).

• Dolphins are mammals: they are warm-blooded, give birth to live young, suckle their babies and breathe air with lungs.
• But they differ structurally from land mammals:

• a streamlined shape;
• fins (flippers) instead of legs, for swimming;
• a breathing opening (blowhole) on the top of the head instead of a nose at the front, so the animal can breathe atmospheric air by lifting only the top of its head above the water.

• Countershading (dark back, light belly) camouflages a fish from predators above and below.
• Webbed feet and waterproof feathers in ducks; thick blubber (fat) in whales and seals for insulation and buoyancy.
• Hard shells and spines in crabs and sea urchins for protection.

These include certain behaviours or actions, inherited or learned, that help organisms face harsh conditions or use resources better.

Other examples (extension): schooling of fish (safety in numbers), hiding in coral crevices, diel vertical migration of plankton (rising to the surface at night to feed and sinking by day to avoid predators), and the building of nests by some fish.

These are modifications in the way the organism performs its vital functions that allow survival in its habitat.

1. Coping with oxygen deficiency. Some deep-sea fish can regulate respiration under low-oxygen conditions by slowing their metabolic rate to reduce their oxygen needs. (Lower metabolism means a lower need for food too, which is useful because food is scarce in the deep ocean.)
2. Coping with pressure. They can regulate blood pressure so that it is proportional to the high external pressure, and they have strong, durable arteries and veins that resist the high pressure. (A structural feature that supports a physiological one: strong vessel walls are required to make the high blood pressure possible. This answers the exercise "physiological adaptations require structural adaptations — give one example".)
3. The viper fish (found in the tropical regions of the major oceans) shows both kinds: flexible skeletal structures (structural) to withstand high pressure, and a high concentration of haemoglobin in the blood (physiological) to compensate for the decreased oxygen in the depths.

• Recall (Module 4) that water moves by osmosis from the solution of lower concentration to that of higher concentration.
• A solution of higher concentration has a higher osmotic pressure.
• The osmotic pressure of the body fluids of aquatic organisms differs from that of the surrounding water, whether fresh or salty, so they must adapt.

Large amounts of fresh water pass into the bodies of the organisms; the body must get rid of the excess water to avoid bursting.

• Unicellular organisms (amoeba, paramecium, euglena) have a contractile vacuole: a cell structure that collects the excess water, then pushes it toward the cell membrane and discharges it outside the cell (it fills and empties rhythmically, every few seconds to a minute).
• Multicellular organisms such as freshwater fish eliminate the excess water through the kidneys as large volumes of dilute urine. They hardly drink, and the gills actively absorb salts from the water to replace those lost (an adaptation of the gill cells).

• Most marine bony fish lose water through their bodies by osmosis. To compensate they swallow large amounts of sea water; the gut absorbs water, then the excess salts are excreted through the kidneys (producing small volumes of concentrated urine) and by specialised cells in the gills (chloride cells) which pump salt out against the concentration gradient.
• Sharks use a different strategy: they retain a high concentration of urea (a nitrogenous waste of many animals, excreted in urine) in the blood (together with another compound, TMAO, that protects proteins from urea). This raises the osmotic pressure of their body fluids until it is close to that of the surrounding sea water, so there is hardly any net loss of water by osmosis.

• Problem. A paramecium in pond water gains water by osmosis at a rate of 4.0 pL (picolitres) per minute.
• Its contractile vacuole expels 0.5 pL each time it contracts.
• (a) How many contractions per minute are needed to keep the cell volume constant?
• (b) How long is the interval between contractions?

Solution. (a) 4.0 ÷ 0.5 = 8 contractions per minute. (b) 60 s ÷ 8 = 7.5 s. If the pond water is made saltier, the water entry slows and the contraction rate falls; in sea water the vacuole stops pulsing. This is a practical way to verify that the vacuole is an osmoregulation organelle.

• Problem. A marine bony fish has body fluid with a total dissolved-particle concentration of 0.35 osmol/L.
• Sea water is 1.0 osmol/L and river water is 0.002 osmol/L.
• (a) In which direction does water move in each case?
• (b) What would happen to a marine fish moved to river water?
• (c) How does salmon cope?

• Solution. (a) In sea water (1.0 > 0.35) water moves out of the fish; in river water (0.002 < 0.35) water moves into the fish.
• (b) A strictly marine fish placed in river water would take in water and lose salts and die (it cannot produce large volumes of dilute urine).
• (c) Salmon change their physiology: in the sea they drink and excrete salt through the gills; when they enter the river their gills take up salt and their kidneys produce large amounts of dilute urine.
• The change is controlled by hormones and takes some days, so they stay in the estuary for some time while adjusting.

• Problem. A shark's blood contains 0.40 osmol/L of salts, 0.35 osmol/L of urea and 0.25 osmol/L of TMAO.
• Sea water is 1.00 osmol/L.
• Is the shark's body fluid isotonic with sea water?
• Why does this avoid water loss?

Solution. Total = 0.40 + 0.35 + 0.25 = 1.00 osmol/L = same as sea water, so the blood is (almost) isotonic, and there is no net osmotic water loss. If the shark had only its 0.40 osmol/L of salts it would lose water like a bony fish.

• A fish keeps its depth when its average density equals the density of the surrounding water.
• The swim bladder is an adjustable gas volume: when the fish dives, the water pressure compresses the gas (Boyle's law: P × V constant at constant temperature); so the fish becomes denser and sinks faster unless it adds gas to the bladder; when it rises, the gas expands and the fish must release gas.
• (Module 7 gives the pressure values.)

• Some deep-sea fish (like rays and sharks) have no gas bladder.
• Instead, they rely on an oil-rich liver (oil is less dense than water) or a liquid-filled bladder to give buoyancy.
• A liquid is incompressible, so these fish do not suffer from the pressure changes (Module 7).

Activity: analysis of the relationship between biological adaptations and the aquatic environment — the lionfish and the colourful (mimic/blue-ringed) octopus.

Aim. To search for adaptations of the lionfish and the colourful octopus and to classify them as structural, behavioural or physiological, linking each to a feature of the environment.

Method. Use books or reliable websites. Make a three-column table (adaptation, type, benefit) for each animal.

What you should find (summary).

• Lionfish (coral reef predator): showy fan-like pectoral fins and long venomous spines (structural: defence and warning colouring — red/white stripes tell predators it is dangerous); it herds small fish into corners using its fins and swallows them in one gulp (behavioural); its venom and its ability to hunt at night at different depths (physiological).
• Colourful octopus (e.g. mimic octopus or blue-ringed octopus): a soft body without bones that can squeeze through very small gaps (structural); chromatophores in the skin that change colour and texture to camouflage it or to warn predators (physiological/structural); ink released as a smoke screen and mimicry of dangerous animals (behavioural); some are venomous.

Expected conclusion. Features such as camouflage, venom and flexible bodies match the coral reef environment where there is rich predation and many hiding places.

• Bioluminescence: many deep-sea fish, squids and jellyfish make light by a chemical reaction (luciferin + oxygen, catalysed by the enzyme luciferase). They use it to attract prey (the anglerfish's lure), to find mates, or to confuse predators. This is a physiological adaptation that depends on a structural one (light organs).
• Counter-illumination and countershading: surface fish are dark on the back and light on the belly so that they are hard to see from above and from below.
• Red colour of many deep animals: red light does not reach the depths (Module 7), so red animals look black and are hidden.

Whales, dolphins and seals must hold their breath during a dive. Adaptations:

• large amounts of myoglobin in muscles (stores oxygen) and a large blood volume with much haemoglobin;
• slowing of the heart (bradycardia) and narrowing of blood vessels to non-essential organs during a dive, so oxygen is kept for the heart and brain;
• flexible rib cage and lungs that collapse under pressure so that nitrogen is not forced into the blood (this avoids "the bends");
• a thick layer of blubber for insulation and buoyancy, and a high metabolic efficiency.

• Freshwater plants: floating leaves with stomata on the upper surface (water lily); air spaces (aerenchyma) that give buoyancy and carry oxygen to the roots; thin finely divided submerged leaves with a very thin cuticle to absorb gases and minerals directly from the water; long flexible stems to bend in currents.
• Salt-tolerant plants (halophytes and mangroves): roots that exclude salt; glands that excrete salt on the leaves; thick succulent leaves storing water; breathing roots (pneumatophores) rising above the mud in mangrove swamps (as on Egypt's Red Sea coast) so that roots in oxygen-poor mud can get air. These solve the same osmotic problem as sea-water fish: the soil water has a higher osmotic pressure than the root cells.

• Symbiosis: clown fish live among sea-anemone tentacles and are protected from stings by a coat of mucus, while they clean the anemone and defend it; coral and zooxanthellae (Module 7).
• Defence: spines and venom (lionfish, stone fish), ink (octopus, squid), electric shock (electric catfish of the Nile), armour (crabs, turtles), and camouflage (flatfish, leaf-like sea dragons).
• Reproduction in water: huge numbers of eggs in the plankton, parental care in some fish (tilapia mouth-brooding), and spawning at seasons when food is plentiful.
• Temperature: fish of cold seas have antifreeze proteins (Module 11); desert-pool fish and killifish can tolerate high temperature and low oxygen.

Ask: Is it a part of the body (structure)? → structural. Is it something the animal does (action)? → behavioural. Is it how a body system or cell works (function, chemistry)? → physiological. Many real adaptations are a combination.

• Mixing the types: gills and fins are structural; migration is behavioural; slowing metabolism and osmotic regulation by kidneys are physiological.
• Saying that a fish "decides" to develop an adaptation. Adaptations arise over generations.
• Forgetting that dolphins are mammals (they breathe air).
• Saying that freshwater fish lose water. They gain water.
• The icefish of the book (deep, cold southern ocean) should not be confused with the "ice fish" of Module 11, which is the same animal group (white-blooded icefish, with antifreeze proteins and no haemoglobin).