Habitat: Marine Habitat – SS2 Biology Lesson Note (Third Term)

A habitat is the place where organisms are naturally found. A marine habitat is the salty water of the oceans and seas, the largest habitat on earth, covering about 70% of its area. It has high salinity, high density and stable conditions, and its plants and animals have features that help them survive there.

Behavioural objectives

By the end of the lesson, students should be able to:

• name the types of habitat and the three kinds of aquatic habitat;
• state the characteristics of marine habitats;
• describe the horizontal and vertical zones of the marine habitat;
• list organisms found in the different marine zones;
• describe how some marine animals and plants are adapted to their habitat.

What is a habitat?

A habitat is a place where organisms (plants, microorganisms and animals) are naturally found, e.g. the habitat of tadpoles is the bottom of freshwater ponds or streams. There are three main types of habitat: aquatic habitat (in or around water), terrestrial habitat (in or on land) and arboreal habitat (in or on trees).

What are the kinds of aquatic habitat?

1. Marine (saltwater) habitat, e.g. oceans and seas.
2. Brackish water habitat (where salt water and freshwater mix), e.g. delta, lagoon and bay.
3. Freshwater habitat (containing little or no salt), e.g. lakes, rivers and streams.

What are the characteristics of marine habitats?

1. Marine habitats are the largest habitat in the biosphere (70% of the earth’s area).
2. They do not undergo sudden or rapid changes in physical factors such as temperature, pH and specific gravity. Hence they show the greatest stability of all habitats.
3. Chemical composition: marine water contains many kinds of dissolved ions, including Na⁺, K⁺, Mg²⁺, Ca²⁺, PO₄^3-, I^– and NO₃^–.
4. Hydrogen ion (H⁺) concentration (pH): salt water is alkaline, with a pH of about 8.0 to 9.0 near the surface.
5. Salinity (salt concentration of water): seawater has a high salinity. The average salinity of seawater is 35 parts per thousand.
6. The density of marine water is high. It is about 1.028, while that of freshwater is 1.0. This allows many organisms to float in it.
7. The temperature of the sea changes less quickly than that of the land. However, the temperature falls as the depth of the sea increases.
8. Oxygen concentration is highest at the surface, where atmospheric oxygen dissolves in the water. It decreases with depth.
9. Waves are the temporary movement of the surface water of the sea, which can occur in any direction. They are caused by wind blowing against the surface of the water. They also mix the seawater. Waves can beat against the shore and sometimes erode it.

What are tides?

Tides are the alternate rise and fall of the surface of the sea at least twice daily. They are caused by the movement of water resulting from the combined gravitational pull of the sun and the moon on the earth.

How is the marine habitat zoned?

Horizontal zonation

The marine habitat is made up of the seashore and the open sea. The major zones are generally as follows:

1. Supratidal or splash zone: the exposed zone with occasional moisture, where water splashes when waves break at the shore.
2. Intertidal or neritic zone: the planktonic zone, which is exposed at low tide and covered by water at high tide. This zone has high photosynthetic activity because of abundant sunshine. The water temperature fluctuates.
3. Subtidal or littoral zone: about 200 m deep and constantly underwater, with abundant sunlight and nutrients.
4. Benthic zone: about 500 m deep, with low light penetration and low nutrients. The water is dark, cold and has little oxygen. Hence, it is unfavourable for life.
5. Abyssal or pelagic zone: about 7000 m deep, with low light penetration, low temperature and high pressure. The low light leads to low photosynthetic activity. Hence food production is mainly by chemosynthesis.
6. Hadal or aphotic zone: the deepest zone, over 7000 m deep. It forms the floor or bed of the ocean. There is no light penetration and no photosynthesis.

Vertical zonation

Based on light penetration or depth, the marine habitat can be divided into three zones:

1. Euphotic zone: the area in direct contact with sunlight. There is enough light penetration for photosynthesis. Therefore producers, consumers and decomposers are all present.
2. Disphotic zone: a region of dim light. Light penetrates the water with low intensity for photosynthesis to take place. Consumers and decomposers are found in this zone.
3. Aphotic zone: the bottom or bed of the seas and oceans. It has cold, dark water without light penetration and very few living organisms.

How are organisms distributed in the marine habitat?

1. Organisms of the splash zone include periwinkles, crustaceans (e.g. ghost crabs), seaweeds and sargassum (algae).
2. Organisms of the intertidal zone include starfish, sea anemones, sponges, sea urchins, annelids, Mollusca and barnacles.
3. In the subtidal zone are snails, crabs, lobsters and crayfish.
4. The benthic zone is unfavourable for life. Producers are absent, and only a few saprophytic animals are present.
5. The neritic zone houses plankton (microscopic floating organisms, e.g. diatoms, algae, protozoa, crustaceans and worms) and nekton (e.g. fishes, crabs, prawns and whales).
6. Oceanic water houses sharks, croakers, sea catfish, mackerel, bonga fish, etc.

How are animals adapted to the marine habitat?

Animals such as barnacles, fishes and crustaceans survive in marine habitats because of the following adaptive features.

Barnacles have:

• a protective mantle for attachment to the rocky shore and for water retention;
• cilia for feeding;
• a shell that prevents desiccation (drying up).

Fish possess:

• reduced or no kidney to retain urea in their bodies to cope with high salinity, e.g. cartilaginous fishes like shark and dogfish;
• salt-secreting glands in their gills or eyes for maintaining osmoregulation (salt balance), e.g. bony fishes like tilapia and herring.

Starfish possess:

• tube feet, which enable them to hold on to rock shores, and hard shells to prevent desiccation.

Whales have:

• fins for stability in water;
• an organ in front of the nostril for detecting pressure changes in water;
• a thick layer of dermal fat for insulation and as a food reserve.

Shrimps possess:

• powerful claws for holding food or prey.

Periwinkles possess:

• lungs for breathing and a foot for attachment.

Crabs:

• burrow fast into the mud to protect themselves against predators and strong waves, or to hide.

How are plants adapted to the marine habitat?

Plants such as seaweeds, algae, Vesuvius and diatoms are naturally found in marine habitats. They have the following adaptive features.

Seaweeds have:

• holdfasts for attachment;
• a mucilaginous cover to prevent desiccation;
• divided leaves or floating devices for buoyancy.

Algae (e.g. sargassum) have:

• chlorophyll for photosynthesis;
• small size or large surface area for floating in water.

Plankton (e.g. diatoms) possess:

• air spaces in their tissues;
• rhizoids for attachment to rocks;
• air bladders for buoyancy (floating).

What are examples of food chains in a marine habitat?

• Diatom → crabs → tilapia
• Diatom → zooplankton → tilapia → shark

Evaluation

1. Define a habitat and name the three kinds of aquatic habitat.
2. State five characteristics of marine habitats.
3. Name the three vertical zones of the marine habitat and state the organisms found in each.
4. Describe two adaptive features of barnacles.
5. Explain how seaweeds are adapted to the marine habitat.

Assignment

1. Explain how waves and tides are caused.
2. List the organisms found in the splash zone, the intertidal zone and the subtidal zone.
3. Describe how cartilaginous fishes and bony fishes cope with the high salinity of seawater.