Marine organisms
Compare marine cell types, reproduction and classification, then identify representative marine organisms, adaptations and life cycles.
Section 3 · Marine organisms
Exam scope and assessment notes
This original lesson maps Cambridge IGCSE Marine Science 0697 syllabus sections 3.1, 3.2, 3.3, 3.4, 3.5, 3.6 for examinations in 2024–2029. These are original StudyForA explanations and practice, not Cambridge questions, mark schemes or official grades. Paper 2 assesses practical skills in both familiar and unfamiliar contexts. This course can rehearse planning, measurement, data handling and evaluation, but it cannot provide real laboratory or coastal-field experience, verify equipment use, certify safety, or award a practical mark. Follow teacher supervision, local safety rules and environmental permissions for all practical work.
Before you begin
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Before you start: Which feature distinguishes a bacterial cell from an animal cell at this syllabus level?
What you will learn
- Compare plant, animal and bacterial cells at the syllabus level
- Distinguish sexual and asexual reproduction
- Classify marine organisms using shared features and binomial names
- Identify representative animal, plant and protoctist features and interpret life cycles
Cells, reproduction and classification
A cell is the basic unit of life. Compare plant and animal cell structures such as cell membrane, cytoplasm and nucleus, noting plant-cell wall, chloroplasts and large vacuole. Bacterial cells have a cell membrane, cytoplasm and cell wall but no nucleus; use the exact syllabus level of detail.
Asexual reproduction uses one parent and produces genetically similar offspring; sexual reproduction involves male and female gametes and offspring with characteristics from both parents. Classification groups organisms by shared features. Scientific binomial names have genus and species; the three domains are Bacteria, Archaea and Eukarya.
Marine animal groups and practical identification
Identify marine vertebrates among mammals, birds, reptiles and fish using features such as hair, feathers, scales, fertilisation and gills or lungs. Fish features include the operculum, scales, fins, lateral line and nares. Invertebrate groups include crustaceans, cnidarians, echinoderms, molluscs and annelids; use observable features rather than habitat alone.
Observe specimens, images or slides carefully and draw only visible features. Magnification = image size ÷ actual size; keep units consistent. A dichotomous key splits choices using clear paired statements about visible features.
Marine plants, protoctists and life cycles
Seagrass is a plant with leaves, roots, rhizomes and flowers. Kelp is a macroalga in Protoctista with blades, a stipe and holdfast; some species have gas bladders. Dinoflagellates and diatoms are microscopic photosynthetic protoctists; diatoms have silica structures.
The 2024–2029 syllabus includes the leatherback turtle and coral-polyp life cycles. For turtles, nesting, incubation temperature, hatching and long maturation connect life stages to shore and ocean habitats. Coral polyps can bud asexually or release eggs and sperm; planktonic larvae disperse and later attach to hard surfaces.
Sources for this lesson
Pause and explain
A dichotomous key works best when each pair of statements uses what?
Worked example
A specimen image is 36 mm wide. The actual organism is 12 mm wide. Calculate magnification and show the unit treatment.
Show the worked solution
- Use magnification = image size ÷ actual size.
- Calculate 36 mm ÷ 12 mm.
- Cancel matching units; magnification has no unit.
Answer Magnification = 3. The millimetres cancel, so write ×3 or 3, not 3 mm. Measurements must be taken across the same feature in image and specimen.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
An image of a fish is 48 mm long; the actual fish is 160 mm long. Calculate magnification.
Give me a hint
Image size ÷ actual size; use the same units first.
Compare my reasoning
- Write 48 mm ÷ 160 mm.
- Cancel mm.
- Express the ratio as a decimal or multiplication factor.
48 ÷ 160 = 0.30, so the image is at ×0.30 magnification (reduced to 30% of actual size). A magnification below 1 is valid for a reduced image.
Look for these in your work
- Uses image size divided by actual size
- Cancels the units
- Interprets a value below 1 correctly
An unknown marine organism has a segmented soft body and visible setae. Which invertebrate group best fits, and which observation supports the choice?
Give me a hint
Compare the listed animal groups using distinctive external features.
Compare my reasoning
- Check the syllabus features for annelids.
- Compare segmentation, body texture and setae with the observation.
- State the group and cite the evidence without claiming traits not observed.
An annelid is the best fit because the syllabus lists a soft, segmented body with setae. If the image is unclear, record the classification as tentative and gather another view.
Look for these in your work
- Selects annelid
- Uses multiple diagnostic features
- Qualifies uncertain evidence
A coastal development reduces dark, sandy nesting areas used by leatherback turtles. Explain one way this could affect the life cycle and what evidence could help test it.
Give me a hint
Link nesting habitat to eggs, hatching and recruitment without assuming a population outcome from one beach.
Compare my reasoning
- Identify the nesting stage affected.
- Explain how fewer suitable nesting sites could affect egg laying or successful hatching.
- Compare repeated nest and hatchling observations over time with a suitable reference site.
Less suitable nesting area could reduce the number of nests or successful hatching locally. Repeated, ethically collected nest and hatchling data across impacted and comparison sites would help test the effect; one season cannot establish a long-term population trend.
Look for these in your work
- Connects habitat to a life-cycle stage
- Uses conditional reasoning
- Proposes repeated comparative evidence
Common mistakes
- Calling every marine photosynthetic organism a plant
- Classifying by location rather than diagnostic features
- Writing a binomial name with the species before the genus
- Leaving unmatched units in magnification
What can you explain now?
A dichotomous key works best when each pair of statements uses what?
Compare with the explanation
Clear alternative features that can be observed
Use the scientific relationship, relevant evidence and correct units. State the limit of any conclusion when the evidence does not support a universal claim.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Why is a dichotomous key based on visible features more reliable than one based only on where an organism was found?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.