START WITH A QUESTION
How can tiny plankton be connected to a large fish?
LOOK CLOSELYStart with the tiny organisms and follow one arrow at a time. Here, arrows go from food to the animal eating it.
A simplified example of feeding relationships. Organisms are not drawn to scale. Diets and connections vary with species and life stage.
Small producers powered by sunlight
NOTICE THISWhere does the food at the start of the chain come from?
From light to a feeding relationship
WHAT THIS TELLS USPhytoplankton use light to make the organic material they grow from.
Near the sea’s surface, phytoplankton use light energy in photosynthesis to make organic matter. They are the starting point for many marine food chains.
Small, easily overlooked organisms have a large part to play in the ocean’s connections. When learning about marine life, size is only one thing to notice.
Go deeper: is sunlight all they need?
Photosynthesis involves light, carbon dioxide and water. Growth also needs nutrients, including sources of nitrogen and phosphorus. Think about both the light available and the materials needed. Phytoplankton do not grow equally well everywhere in the ocean.
Follow the food, not the swimming direction
NOTICE THISDoes the arrow show a chase, or the transfer of food?
Read the arrow as a sentence
WHAT THIS TELLS USIn this diagram, the arrow goes from the food to the organism that eats it.
Our arrows point from the organism being eaten towards the eater. A feeding connection from phytoplankton to zooplankton and then to fish is an example of a food chain.
Some large fish eat plankton. There is no universal rule that a large fish eats only smaller fish. The dotted line shows that other feeding links are possible.
Go deeper: turn an arrow into a sentence
Read “zooplankton → small fish” as “the small fish eats zooplankton”. Some large fish, such as whale sharks, filter-feed on small animals including krill. The dotted arrow in the opening diagram represents an alternative link from zooplankton to a large fish.
From a chain to a web
NOTICE THISIf one fish declines, must another decline by the same amount?
WHAT THIS TELLS USFood relationships do not form just one straight chain.
In the real ocean, one organism may eat several kinds of food and be eaten by several predators. These overlapping relationships form a food web.
Changes in numbers or habitats can affect other organisms. To understand what changes, and by how much, researchers need evidence about the particular species and sea area.
Go deeper: what the diagram can and cannot tell you
An arrow tells you that a feeding relationship exists. It does not tell you how much is eaten, whether another food can replace it, or how populations will change. Species, life stage, place and season also matter.
CONNECT THE PICTURE AND THE WORD
Words to remember
- Phytoplankton
- Small drifting organisms that photosynthesise.
- Food chain
- One pathway of feeding relationships.
- Food web
- Many feeding relationships connected into a network.
CAN YOU EXPLAIN IT IN YOUR OWN WORDS?
Check your understanding
Think first, then open each question to read the explanation. You can look back at the diagrams.
Q1A small fish eats zooplankton. Which way should the arrow point here?
Explanation: From the zooplankton to the fish: the direction in which food is transferred.
Q2If two arrows lead to a fish, does it eat equal amounts of both foods?
Explanation: The number of arrows does not show amounts. You need additional observations or research to find out how much of each food it eats.
LET’S THINK
A question to explore further
If you added people or seabirds to the diagram, where might the arrows start? Find evidence for the links you draw.
Show a hint and explanation
There is more than one possible answer. Look up examples of what a person or a particular seabird eats, and draw arrows from the food towards the eater. Keep evidence-based links separate from guesses.
Explore together, at home or in class
Write organisms on cards and connect them with feeding arrows. You could choose a fish familiar from meals and investigate what it eats.
Sources & further reading
We developed our own explanations, diagrams and learning questions using these sources. Inclusion does not imply review or endorsement by the source organisation. Links marked “Japanese” lead to Japanese-language material.
Sources for the added diagrams and explanations
Additional sources checked: 2026-09-21. Numerical diagrams use invented teaching values.
- [G1] NOAA: What are phytoplankton? ↗Light, nutrients and phytoplankton as a food-web foundation.
- [G2] JAMSTEC: research explained (Japanese) ↗Photosynthesis, nutrients and the basic feeding chain.
- [G3] NOAA Fisheries: whale shark research and feeding ↗An example of a large fish that filter-feeds on plankton and other prey.
Original lesson sources
- [1] JAMSTEC: research explained (Japanese) ↗Marine food chains beginning with phytoplankton.
- [2] JAMSTEC: ocean STEAM learning library (Japanese) ↗Further learning about marine life and the environment.
First published: 2026-09-20 · Updated: 2026-09-21 · Original sources checked: 2026-09-20 · Editorial policy