Topic 3: Marine Organisms
The biology, classification, and adaptations of life in the sea.
Topic Summary: This topic covers the biology of marine life — from cell structure (animal vs plant cells) to the classification system (Kingdom to Species) and major marine groups (Cnidaria, Mollusca, Echinodermata, Chordata). You must understand key adaptations like osmoregulation, buoyancy control, streamlining, and gills, plus the difference between phytoplankton and zooplankton.
🗺️ 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