Topic 3: Marine Organisms
The biology, classification, and adaptations of life in the sea.
🗺️ Topic 3 Mind Map
3.1 Cell Structure in Marine Organisms
All marine organisms are made of cells — the basic unit of life. There are key differences between animal and plant/algae cells.
| Structure | Animal Cell | Plant/Algae Cell | Function |
|---|---|---|---|
| Cell membrane | Yes | Yes | Controls what enters/leaves the cell |
| Cytoplasm | Yes | Yes | Where chemical reactions occur |
| Nucleus | Yes | Yes | Contains DNA; controls cell activities |
| Mitochondria | Yes | Yes | Site of aerobic respiration (releases energy) |
| Cell wall | No | Yes (cellulose) | Structural support and protection |
| Chloroplasts | No | Yes | Site of photosynthesis |
| Large vacuole | No (small ones) | Yes (permanent) | Stores cell sap; maintains turgor |
Diagram: Animal Cell vs Plant Cell
3.2 Classification
Classification is the grouping of organisms based on shared characteristics. Scientists use a hierarchical system with 7 levels:
| Level | Example (Bottlenose Dolphin) | Memory Aid |
|---|---|---|
| Kingdom | Animalia | King |
| Phylum | Chordata | Philip |
| Class | Mammalia | Came |
| Order | Cetacea | Over |
| Family | Delphinidae | For |
| Genus | Tursiops | Good |
| Species | truncatus | Spaghetti |
Binomial Nomenclature
Binomial nomenclature is the two-part naming system using the Genus and species names in Latin/Greek. Rules:
- Always written in italics (or underlined when handwritten)
- Genus has a capital letter; species is lowercase
- Example: Tursiops truncatus (bottlenose dolphin)
Remember the mnemonic: King Philip Came Over For Good Spaghetti — Kingdom, Phylum, Class, Order, Family, Genus, Species!
3.3 Major Marine Groups
Key Marine Animal Phyla
| Phylum | Key Features | Marine Examples |
|---|---|---|
| Porifera | Simple, no organs, filter feeders, porous body | Sponges |
| Cnidaria | Stinging cells (cnidocytes), radial symmetry | Jellyfish, corals, anemones |
| Mollusca | Soft body, often with shell, muscular foot | Octopus, squid, clams, snails |
| Arthropoda | Exoskeleton, jointed limbs, segmented body | Crabs, lobsters, shrimp, barnacles |
| Echinodermata | Spiny skin, 5-fold symmetry, water vascular system | Starfish, sea urchins, sea cucumbers |
| Chordata | Backbone (vertebrates), notochord at some stage | Fish, whales, dolphins, turtles, seabirds |
Fish: Cartilaginous vs Bony
| Feature | Cartilaginous Fish | Bony Fish |
|---|---|---|
| Skeleton | Cartilage (flexible) | Bone (hard) |
| Swim bladder | No — use oily liver | Yes — gas-filled sac |
| Gill cover | No (gill slits visible) | Yes (operculum) |
| Scales | Placoid (tooth-like, rough) | Smooth, overlapping |
| Examples | Sharks, rays, skates | Tuna, cod, clownfish, salmon |
3.4 Adaptations to Marine Life
Marine organisms have evolved adaptations — special features that help them survive in their environment.
Key Adaptations
| Challenge | Adaptation | How It Works |
|---|---|---|
| Water/salt balance | Osmoregulation | Marine fish drink seawater, excrete salt via gill chloride cells |
| Staying afloat | Buoyancy control | Swim bladder (bony fish) or oily liver (sharks) |
| Moving efficiently | Streamlining | Torpedo-shaped body reduces water resistance/drag |
| Breathing underwater | Gills | Extract dissolved O₂ from water; counter-current exchange maximises uptake |
| Detecting prey | Lateral line | Senses vibrations and pressure changes in water |
| Deep-sea darkness | Bioluminescence | Organisms produce their own light for communication, luring prey, camouflage |
| High pressure | Flexible bodies | Deep-sea organisms often lack gas-filled spaces; have flexible membranes |
Diagram: External Features of a Bony Fish
- Osmoregulation = controlling water and salt balance
- Marine fish drink seawater and excrete salt through gills
- Freshwater fish do the opposite — excrete water, absorb salts
- Counter-current exchange in gills maximises oxygen absorption
3.5 Phytoplankton and Zooplankton
The word plankton comes from the Greek word "planktos" meaning "wanderer" — organisms that drift with ocean currents.
| Phytoplankton | Zooplankton | |
|---|---|---|
| Type | Plant-like (autotrophs/producers) | Animal-like (heterotrophs/consumers) |
| Nutrition | Photosynthesis | Eat phytoplankton or other zooplankton |
| Contains chlorophyll? | Yes | No |
| Size | Mostly microscopic | Microscopic to a few cm |
| Examples | Diatoms, dinoflagellates, coccolithophores | Krill, copepods, larval fish, jellyfish |
| Where found | Photic zone (needs light) | Throughout water column |
- Phytoplankton produce over 50% of Earth's oxygen
- They are the base of most marine food chains
- Phytoplankton need sunlight + nutrients + CO₂ to grow
- Phyto = plant, Zoo = animal, Plankton = drifter
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