Explain how the adaptations of deep-sea fish allow them to survive at high hydrostatic pressures.

IB DP Biology Standard Level (2023 syllabus) — B4.1 Adaptation to environment · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Deep-sea fish live at depths exceeding 1000 m, where hydrostatic pressure can be more than 100 times greater than at the ocean surface. Many species lack a swim bladder and have unusually flexible, low-density body structures.

Model answer (4 marks)

1. The absence of a gas‑filled swim bladder means the fish have no compressible cavity that would collapse under high pressure.
2. Their cell membranes are rich in unsaturated and polyunsaturated fatty acids, which keep the bilayer fluid and functional despite the pressure and low temperature.
3. Proteins and enzymes have adapted tertiary structures – either more flexible or stabilised by pressure‑resistant amino‑acid sequences – so they retain activity at high pressure.
4. The body tissues are soft, gelatinous and highly hydrated; this equalises internal and external pressure, preventing crushing of the body.

Examiner tips

  • Use the exact phrases from the mark scheme (e.g. ‘absence of a gas‑filled swim bladder’),
  • Show each point clearly and link it to the pressure problem.
  • Keep the answer concise – one sentence per point is enough for 4 marks.

Common mistakes

  • Writing about gills or oxygen supply instead of the swim bladder
  • Using vague terms like ‘flexible proteins’ without mentioning pressure‑resistant sequences
  • Including unnecessary detail such as temperature effects beyond membrane fluidity

Mark scheme (4 marks)

  1. Absence of a gas-filled swim bladder prevents compression/collapse of the body cavity at high pressure
  2. Cell membranes contain a high proportion of unsaturated/polyunsaturated fatty acids, maintaining membrane fluidity at high pressure / low temperature
  3. Proteins/enzymes are adapted (e.g. more flexible tertiary structures / stabilised by pressure-resistant amino acid sequences) to function at high pressure
  4. Soft, gelatinous/flexible body tissues with high water content equalise internal and external pressure, preventing crushing

Key terms in this question

hydrostatic pressure

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