Marine Science (0697) | Classification

Welcome to Chapter 3.3: The Science of Classification!

Hello future Marine Scientists! This chapter is all about organizing the incredible diversity of life found in the oceans. Imagine trying to find a specific book in a library where everything is piled up randomly—it would be impossible! Classification is the biological filing system that helps us organize, name, and study marine organisms around the world.

Don't worry if the names seem complicated at first. We will break down the rules and focus only on the marine examples required by your IGCSE syllabus. Let's dive in!

1. Why Do We Classify Marine Organisms?

Classification is essential because it allows scientists everywhere to use a single, internationally agreed system to name organisms, avoiding confusion caused by different common names.

1.1 Classification by Shared Features

The basic principle is simple:
Marine organisms are classified into groups by the features that they share.

Example: All marine mammals (like whales) share common features (like breathing air and giving live birth), grouping them together, separate from fish (which breathe through gills).

2. The Universal Naming System: Binomial Nomenclature

If you travel the world, a dolphin might be called bottlenose dolphin, tonina, or tursiops. To solve this, all organisms get one unique scientific name using the binomial system.

2.1 Describing the Binomial System (3.3.2)

• It is an internationally agreed system developed by Carl Linnaeus.
• The scientific name is always made up of two parts: the Genus and the species.
• The name is always written in italics.
• The first part, the Genus, starts with a Capital letter (e.g., Orcinus).
• The second part, the species, is written entirely in lowercase (e.g., orca).

The Scientific Name Example: The Killer Whale is named Orcinus orca.

Key Takeaway: The binomial name provides a unique, universal identifier for every organism, consisting of its Genus and species.

3. The Three Domains of Life (3.3.3)

Classification starts at the very top with the broadest groups called Domains. All life on Earth is sorted into three domains:

• Bacteria: Simple, single-celled organisms without a true nucleus (prokaryotes). Includes many marine bacteria and cyanobacteria.
• Archaea: Also simple, single-celled organisms (prokaryotes), often found in extreme environments like deep-sea hydrothermal vents.
• Eukarya: Organisms whose cells have a true nucleus and specialized organelles (eukaryotes). This includes all plants, animals, fungi, and protoctists.

4. The Main Kingdoms of Eukarya in Marine Habitats (3.3.4)

Within the Domain Eukarya, we study four main kingdoms, three of which are vital in marine environments:

Key Takeaway: The three major domains are Bacteria, Archaea, and Eukarya. Within Eukarya, the marine kingdoms we focus on are Animals, Plants, and Protoctists.

5. The Animal Kingdom: Vertebrates and Invertebrates

The Animal Kingdom is split into two major groups: Vertebrates (animals with a backbone, like fish and whales) and Invertebrates (animals without a backbone, like crabs and sea stars).

5.1 Marine Vertebrate Classification (3.4.1 & 3.4.2)

Vertebrates are grouped based on key characteristics like skin covering, reproductive method, and gas exchange organs.

Group	Skin Covering	Reproductive Method	Gas Exchange	Marine Examples
Mammals	Hair/Fur (blubber for insulation)	Internal fertilisation (live young, milk)	Lungs (must surface to breathe)	Cetaceans (whales, dolphins), Pinnipeds (seals, sea lions), Sirenians (manatees)
Birds	Feathers	Internal fertilisation (hard-shelled eggs)	Lungs	Pelagic birds (albatross), Shorebirds (gulls, sandpipers)
Reptiles	Scales or bony plates	Internal fertilisation (soft-shelled eggs laid on land)	Lungs	Sea Snakes, Marine Turtles
Fish	Scales or smooth skin	Usually External fertilisation (spawn eggs in water)	Gills	Cartilaginous fish (sharks, rays), Bony fish (tuna, cod)

5.2 External Features of Fish (3.4.3)

Fish have specialized external features adapted for movement and sensing in water:

• Operculum: A hard, bony flap that covers and protects the gills.
• Scales: Overlap and cover the body, providing protection and reducing drag.
• Fins (Dorsal, Pelvic, Pectoral, Anal, Caudal):
  • Used for control of pitching, rolling, and yawing (maintaining balance and stability).
  • The caudal (tail) fin provides forward thrust.
• Lateral Line: A system of sensory organs running along the side of the fish, used to detect vibrations and movement in the water.
• Nares: Small openings (nostrils) used to detect chemicals (smell) in the water.

5.3 Marine Invertebrate Classification (3.4.4)

Invertebrates are organisms without a backbone. In Marine Science, you need to know the features of five major invertebrate groups (Phyla):

(a) Crustaceans

These are often found crawling on the seabed or filtering plankton.

• Bilateral symmetry (can be divided into two mirror-image halves).
• Have a tough outer covering called an exoskeleton.
• Possess compound eyes (eyes made of many small lenses).
• Have two pairs of antennae.
• Body has abdominal segments with jointed legs.

Examples: Crabs, Shrimp, Lobsters, Barnacles.

(b) Cnidaria (Nid-air-ee-ah)

These are stinging animals, named for their stinging cells (cnidocytes).

• Radial symmetry (body parts arranged around a central point, like a bicycle wheel).
• Possess tentacles with stinging cells.

Examples: Jellyfish, Sea Anemones, Corals.

(c) Echinoderms (Eck-eye-no-derms)

The name means "spiny skin".

• Pentaradial symmetry (five-part radial symmetry, most obvious in starfish).
• Have spiny skin (often hardened by calcium plates).
• Use tube feet for movement, feeding, and gripping the substrate.

Examples: Starfish (Sea Stars), Sea Urchins, Sea Cucumbers.

(d) Molluscs

A very diverse group including soft-bodied animals often protected by a shell.

• Bilateral symmetry.
• Have an unsegmented body (unlike annelids or crustaceans).
• Possess an internal or external shell (some have lost it entirely).

Examples: Clams, Mussels, Oysters, Squid, Octopus.

(e) Annelids

These are the segmented worms of the ocean.

• Bilateral symmetry.
• Segmented body (body divided into repeating sections).
• Have a soft body.
• Often have bristles called setae for movement or anchoring.

Examples: Marine ragworms, Lugworms.

6. Plant and Protoctist Kingdoms in Marine Habitats

Not all marine producers are the same! They belong to different kingdoms and have different structures.

6.1 Seagrasses (Kingdom Plantae) (3.5.1)

Seagrasses are true flowering plants (angiosperms) that evolved to live entirely underwater.

• Leaves: The main organs of photosynthesis.
• Roots: Secure the plant into the sediment (substrate) and absorb minerals.
• Rhizomes: Underground stems that connect individual seagrass plants, used for asexual reproduction.
• Flowers: Used for sexual reproduction.

Did you know? Seagrass beds are incredibly important nursery habitats for many fish and invertebrate species.

6.2 Kelp (Macroalgae) (Kingdom Protoctista) (3.5.2)

Kelp is a giant seaweed. It is *not* a true plant; it belongs to the Protoctist kingdom.

The key features of Kelp show that it does not have true roots, stems, or leaves like a plant:

• Flat blades: Used for photosynthesis (like leaves).
• Stipe: Acts like a stem, developing the blades.
• Holdfast: Anchors the kelp to the hard substrate (rock). It does not absorb nutrients like a root.
• Gas bladders: Air-filled structures in some species that help float the blades towards the sunlight.
• Chloroplasts: Found within the cells for photosynthesis.

6.3 Marine Microalgae (Kingdom Protoctista)

These microscopic producers form the base of many marine food webs.

Dinoflagellates (3.5.3)

• Microscopic, single-celled organisms.
• Presence of chloroplasts for photosynthesis.
• Presence of two flagella (whip-like tails) for movement.

Diatoms (3.5.4)

• Microscopic, single-celled organisms.
• Presence of chloroplasts for photosynthesis.
• Presence of a rigid, ornate silica skeleton (made of glass-like material).