Marine Organism Experiments

Why do experiments on sea life?

Sea life changes when the sea changes. An experiment is a test where you change one thing and watch what happens. It helps us know how fish, seaweed and plankton react to heat, light and food. We use this to farm fish, protect the sea and predict catches.

An organism is any living thing. Animals, plants and plankton are all organisms.

Marine animal experiments

A fish takes water in through its mouth and pushes it over its gills. Gills take oxygen from the water. Each push is a gill beat.

Test: keep a fish in a tank. Change the water temperature step by step (for example 10, 15, 20, 25, 30 degrees). At each step, wait a few minutes, then count the gill beats in one minute.

• Changed on purpose: temperature.
• Measured: gill beats per minute.
• Kept the same: tank size, fish, food, time of day.

Result (typical): warm water holds less oxygen and fish are more active, so the beats go up. Very hot water can harm the fish, so we stop before it is unsafe.

Other animal tests: how fast a shellfish closes when touched, how far a fish swims in a current, how fast a young fish grows with different food.

Marine plant experiments

Seaweed (algae) makes its own food using light. This is called photosynthesis. More light usually means more food and more growth, up to a limit.

Test: put equal pieces of seaweed in clear tanks of sea water. Give each tank a different light level (dark, weak, medium, strong). Measure the length or weight at the start and again after two weeks.

• Changed: light level.
• Measured: growth = final length minus starting length.
• Kept the same: water, salt, temperature, amount of seaweed.

Other plant tests: how salt level changes growth, or how nutrients (like fertiliser in the water) change growth.

Plankton experiments

Plankton are tiny living things that drift in water. Phytoplankton are plant-like and make food from light. Zooplankton are tiny animals that eat them. Fish feed on plankton, so plankton tell us how rich the water is.

Test: tow a fine net behind a boat or from a jetty for a fixed distance. Wash the catch into a jar. Count a small drop under a magnifier or microscope and scale up.

• Same net, same speed, same depth, same time of day for each tow.
• Count = number of plankton in the drop x (jar volume / drop volume).

You can compare two places (harbour and open sea) or two times (morning and evening).

Fair tests, control and care

A fair test changes one thing only. A control is a tank where nothing is changed, so you can compare. Repeat the test and take the mean, because one reading can be a fluke.

Care for animals: use the fewest animals you need, keep the water clean and cool enough, never hurt them, and return wild animals to where you found them after the test.

Record in a table, draw a graph, and say what the pattern is. Say what you are sure about and what you are not.

Try it

At home: grow two pots of cress or a few water plants, one near a window and one in a dark cupboard. Measure the height every two days for a week. In the 3D lesson: pick a test, guess the result, then move the slider to check.

Key formulas and definitions

• Growth = final length - starting length
• Mean = sum of readings / number of readings
• Plankton count = count in drop x (jar volume / drop volume)
• Fair test: change 1 thing, keep the rest the same, use a control

Worked examples

Common mistakes

• Changing two things at once (for example warmer water and a bigger tank), so you cannot tell the cause.
• Using only one reading instead of repeating the test and taking the mean.
• Skipping the control tank, so there is nothing to compare with.
• Forgetting the animals: not returning wild animals or leaving the water dirty.