The ocean is divided into zones based on depth and light penetration:

Special Adaptations by Zone

• Bioluminescence (mesopelagic+): Organisms produce their own light for hunting, communication, and camouflage
• Vertical migration: Many zooplankton rise to surface at night to feed, descend by day to avoid predators
• Deep-sea adaptations: Large mouths, expandable stomachs, slow metabolism, no gas-filled organs

Coral reefs are built by tiny coral polyps that secrete calcium carbonate skeletons. Often called the "rainforests of the sea" because they support ~25% of all marine species.

Conditions Required

• Warm water: 23-29°C
• Clear, shallow water (sunlight for zooxanthellae)
• Stable salinity (~35 ppt)
• Hard substrate for attachment

Coral and Zooxanthellae (Mutualism)

Zooxanthellae are photosynthetic algae living inside coral tissue. They provide up to 90% of the coral's energy through photosynthesis and give corals their colour. The coral provides shelter, CO₂, and nutrients.

Coral Bleaching

When corals are stressed (usually by rising temperatures), they expel their zooxanthellae. Without algae, corals turn white (bleaching) and lose their energy source. Prolonged bleaching causes coral death.

Mangroves are salt-tolerant trees (halophytes) that grow in tropical/subtropical intertidal zones.

Adaptations

• Aerial/prop roots: Above waterline for oxygen uptake in waterlogged soil
• Salt excretion: Glands in leaves excrete salt, or roots filter it out
• Viviparous seeds: Seeds germinate on the parent tree before dropping into water

Importance

• Nursery habitat: Tangled roots shelter juvenile fish and invertebrates
• Coastal protection: Reduce erosion, buffer against storms and tsunamis
• Carbon storage: "Blue carbon" — store large amounts of CO₂
• Water filtration: Trap sediment and pollutants, protecting coral reefs

Kelp forests are underwater ecosystems dominated by large brown algae (kelp) that can grow up to 60m tall.

Conditions

• Cool, nutrient-rich waters (10-20°C)
• Rocky substrate for attachment (holdfasts)
• Clear water with good light

Importance

• Provide habitat and food for hundreds of species (fish, sea otters, sea urchins)
• Absorb CO₂ and produce oxygen through photosynthesis
• Protect coastlines from wave energy

Sea urchin threat: If sea otter populations decline (from hunting), sea urchin numbers explode and they overgraze kelp, destroying the forest — a trophic cascade.

Hydrothermal vents are openings in the ocean floor near mid-ocean ridges that release superheated, mineral-rich water.

Conditions

• Depth: ~2,000-4,000m (complete darkness)
• Temperature: 2-4°C in surrounding water; up to 400°C at vent
• Extreme pressure (~250-400 atm)
• Rich in chemicals: hydrogen sulfide (H₂S), methane, minerals

Food Chain Based on Chemosynthesis

Chemosynthetic bacteria → giant tube worms → vent crabs → vent octopus

The bacteria use H₂S as an energy source (not sunlight). Some bacteria live inside tube worms in a mutualistic relationship.

The intertidal zone is the area between high and low tide marks — one of the most challenging marine environments.

Challenges

Adaptations

• Limpets: Strong muscular foot clamps to rock
• Barnacles: Hard calcium carbonate plates seal in moisture
• Seaweed: Flexible to bend with waves; mucus to prevent drying
• Anemones: Retract tentacles and close body when exposed

Symbiosis means "living together" — a close, long-term relationship between two species.

Ecological Succession

Succession is the gradual change in community structure over time:

1. Pioneer species colonise bare rock/substrate (e.g., bacteria, algae)
2. They modify the environment, making it suitable for other species
3. More complex species move in and may outcompete pioneers
4. Eventually, a stable climax community develops

Example in the ocean: bare rock → algae → barnacles → mussels → diverse reef community

Biodiversity

Biodiversity = the variety of different species in an ecosystem. High biodiversity makes ecosystems more resilient (able to recover from disturbance).

• Highest marine biodiversity: Coral reefs, mangroves, kelp forests
• Lowest marine biodiversity: Deep ocean, polar seas, polluted areas