Difficulty: Introductory | Prerequisites: Basic biology vocabulary (cell, organism, photosynthesis)
This is the opening section of Chapter 4 in the Cambridge International AS & A Level Marine Science coursebook. It covers how marine biologists organise the enormous variety of ocean life into a structured hierarchy, name organisms using an internationally agreed system, and identify unknown specimens with dichotomous keys. It then introduces plankton, the microscopic foundation of virtually every marine food web. If you have missed earlier chapters, you should be comfortable with the idea that oceans cover roughly 70% of the Earth's surface, that life first evolved in water, and that energy flows through ecosystems via producers and consumers.
Marine organisms are classified into groups (domain through species) based on shared features and evolutionary relationships, and every species gets a unique two-part Latin name. Plankton, split into photosynthesising phytoplankton and consumer zooplankton, form the energy base and the critical link between sunlight and the rest of the marine food web.
Taxonomic hierarchy
The classification of species by arranging them into a nested series of groups: domain, kingdom, phylum, class, order, family, genus, species. Think of it as a set of folders inside folders, each level narrowing down until you reach a single species.
Binomial nomenclature
The two-part Latin naming system (genus + species) given to every organism, first formalised by Carolus Linnaeus in 1736. In simple terms, it is the organism's official scientific name, always italicised in print and underlined when handwritten (e.g. Prionace glauca for the blue shark).
Dichotomous key
An identification tool that presents a series of paired choices based on observable characteristics, directing you step by step until the species is identified. Think of it as a "choose your own adventure" for specimen ID.
Plankton
Microscopic, free-floating marine organisms with limited mobility that drift with ocean currents. The word comes from the Greek for "wandering."
Phytoplankton
Plankton that are producers: they obtain energy through photosynthesis. In simple terms, they are the tiny floating plants of the ocean.
Zooplankton
Planktonic consumers that are either floating or weakly swimming animals. They feed on phytoplankton or other zooplankton.
Keystone species
An organism that plays a unique, disproportionately large role in how an ecosystem functions. Without keystone species, the ecosystem would be dramatically different or could collapse entirely.
Diatoms
Unicellular phytoplankton with intricate silica cell walls, found in surface water. They reproduce rapidly and form blooms in spring when light and nutrients peak.
Dinoflagellates
Unicellular protoctists (phytoplankton) that lack a silica cell wall. Some species produce toxins that cause harmful algal blooms (HABs), also called red tides.
Copepods
Small crustaceans that make up the most abundant and diverse group of zooplankton. They have a body divided into head, thorax and abdomen, with a tough calcium carbonate exoskeleton.
Larvae
A planktonic developmental stage occurring between the egg and the juvenile form, found in nearly all species of fish and invertebrates.
Harmful algal blooms (HABs)
Blooms of toxin-producing dinoflagellates (red tides) that can poison fish, contaminate shellfish, and cause illness in humans who consume them.
One way to make sense of the biotic variety in the oceans is a taxonomic hierarchy, which classifies organisms into nested groups based on shared features, relationships and evolutionary pathways.
The hierarchy runs: domain > kingdom > phylum > class > order > family > genus > species.
Similar species are grouped into a genus (plural: genera), genera into families, families into orders, orders into classes, classes into phyla (singular: phylum), phyla into kingdoms, and kingdoms into domains.
Example: the blue shark (Prionace glauca) sits in domain Eukarya, kingdom Animalia, phylum Chordata, class Chondrichthyes, order Carcharhiniformes, family Carcharinidae, genus Prionace, species glauca.
Every marine organism receives a Latin name composed of two parts: the genus (capitalised) followed by the species (lowercase).
The system was first formalised by Carolus Linnaeus in 1736.
In print, the binomial name is always italicised. When writing by hand, underline it (e.g. Spheniscus mendiculus for the Galapagos penguin).
The genus is shared by closely related species; the species name is unique within that genus.
A dichotomous key is an identification tool built from a series of paired statements about observable characteristics.
At each step you choose the statement that matches your specimen, which either identifies the organism or directs you to the next pair.
Example: a shark dichotomous key might start with "kite-like body shape" versus "non-kite-like body shape," then narrow via features such as horn-like appendages on the snout, number of gill slits, presence of dorsal spines, snout shape and eye position.
Marine scientists produce biological drawings to record specimens accurately.
Key rules for a good biological drawing:
Use a sharp HB pencil; no shading, only smooth continuous lines.
The drawing should fill at least half the page, with room for labels on each side.
Labels should be printed clearly in ink, with horizontal label lines drawn with a ruler that touch the feature being identified.
Proportions and scale must be accurate (e.g. tail length relative to body length).
In annotated diagrams, each label should explain how that feature is an adaptation for life in a marine ecosystem (e.g. "The operculum is essential for ventilation as it is a bony flap that covers and protects the gills").
A keystone species is a consumer that affects biodiversity to a greater extent than its population numbers alone would suggest.
Keystone species control other species through grazing, predation and competition. Some also engineer the habitat physically.
Without them, the ecosystem would look dramatically different or could cease to exist.
Conservation programmes increasingly focus resources on maintaining keystone species rather than trying to protect every endangered species across a whole habitat.
Plankton are microscopic organisms with limited mobility that float with the current. The word derives from Greek, meaning "wandering."
They divide into two functional groups:
Phytoplankton (producers): obtain energy via photosynthesis.
Zooplankton (consumers): gain nutrition from food energy, feeding on phytoplankton or other consumers.
Free-floating, found in surface water where light intensity is highest. Carried by ocean currents with no need to attach to a substrate.
Grouped by size:
Picoplankton: 0.2 to 2 µm
Nanoplankton: 2 to 20 µm
Microplankton: 20 to 200 µm
Diatoms
Unicellular phytoplankton with intricate silica cell walls.
More than 100 genera; reproduce very rapidly.
Blooms appear in spring when light intensity and temperature are optimal, then disappear quickly as primary consumers (planktonic crustaceans and krill) graze them and mineral ions are depleted.
Phytoplankton are critical for removing CO₂ from the atmosphere and form the base of many marine food webs.
Dinoflagellates
Also unicellular protoctists, but lack a silica cell wall.
Live in upper surface waters; undergo rapid reproduction when conditions are right.
Some species produce toxins that poison fish and accumulate in shellfish, causing illness in humans (shellfish poisoning).
Toxin-producing blooms are called harmful algal blooms (HABs), including red tides, which turn ocean water red.
Agricultural fertiliser run-off is a major source of the nutrients that fuel HABs.
Some dinoflagellates bioluminesce at night as a defence mechanism, attracting larger predators that consume whatever is eating the dinoflagellates.
Important consumers that include larvae, copepods and larger animals such as jellyfish.
Vertical migration: zooplankton migrate up the water column daily to feed on phytoplankton in the photic surface layer.
They provide a critical link in the food chain between primary producers and larger consumers (shellfish, fish, whales).
Sensitive to environmental change: pollution, microplastics, ocean acidification and rising water temperatures can kill them.
Larvae
A planktonic stage of development between egg and juvenile, found in nearly all fish and invertebrate species (e.g. seastar larvae).
Adapted to floating life: some fish larvae have oil globules for added buoyancy.
Copepods
Small crustaceans; the most abundant and diverse zooplankton group.
Herbivores that feed on diatoms.
Body in three sections: head, thorax, abdomen. Two antennae for swimming, two pairs of appendages from the thorax.
Tough calcium carbonate exoskeleton with spikes for protection and improved surface-area-to-volume ratio for buoyancy.
Jellyfish
Belong to the cnidarian group (alongside corals and sea anemones).
Soft-bodied, with two main parts: a transparent bell (medusa) with dangling tentacles. The medusa can be up to 1 m across, tentacles up to 65 m.
Unlike larvae, jellyfish remain planktonic throughout their lives; the pulsing medusa gives limited mobility.
Defend against predators with stinging cells called nematocysts on their tentacles.
Krill
Shrimp-like carnivores that feed on other zooplankton and phytoplankton.
Vital food source for birds, fish, seals and baleen whales.
About 80 species. Antarctic krill (Euphausia superba) are among the most important species in the Southern Ocean.
Females lay up to 10,000 eggs; swarms can reach 30,000 individuals per cubic metre.
Krill abundance depends on phytoplankton blooms triggered by ice melt in spring and summer, which releases nutrients and sunlight.
Global warming threatens krill populations by altering ocean temperatures and currents, retreating Antarctic sea ice.
Annual global krill harvest is up to 20 million kg. Over-harvesting reduces krill grazing on phytoplankton, potentially leading to larger, toxic phytoplankton blooms.
Krill biomass in the Southern Ocean: approximately 110 billion kg.
Phytoplankton are responsible for roughly half of global oxygen production and are a major carbon sink, drawing CO₂ from the atmosphere. Understanding plankton dynamics is central to climate science, fisheries management and predicting harmful algal blooms that close shellfish harvests and threaten public health.
Dichotomous keys are used daily by marine ecologists doing field surveys, customs officers checking imported seafood species, and conservation biologists identifying bycatch.
Students often think "plankton" means only tiny plants. It does not. Zooplankton are animals (or animal-like consumers), and jellyfish are plankton too, despite some being over a metre across.
Students sometimes write binomial names with both parts capitalised (e.g. Prionace Glauca). The species name is always lowercase: Prionace glauca.
Students confuse the terms "species richness" and "species diversity." Richness is just the number of species present; diversity also accounts for how evenly individuals are spread among those species.
Students may assume diatoms and dinoflagellates are the same thing. Diatoms have silica cell walls; dinoflagellates do not. Dinoflagellates are the ones that produce toxins and cause red tides.
⚠️ Be able to write the eight levels of the taxonomic hierarchy in order (domain to species) and slot a named organism into each level.
⚠️ Know the rules for writing binomial names: genus capitalised, species lowercase, italicised in print, underlined by hand.
⚠️ Expect a question asking you to use or construct a simple dichotomous key from a set of specimens.
⚠️ Be ready to explain the difference between phytoplankton and zooplankton and give named examples of each.
⚠️ Understand why phytoplankton blooms form and disappear (light, nutrients, grazing), and how HABs are linked to agricultural run-off.
⚠️ Know the role of krill as a keystone link in the Antarctic food web, including the effects of climate change and over-harvesting on krill populations.
True or false: In binomial nomenclature, both the genus and species names are capitalised.
Fill in the blank: The eight levels of the taxonomic hierarchy, from broadest to narrowest, are domain, kingdom, ______, class, order, family, genus, species.
True or false: Diatoms produce toxins that cause harmful algal blooms.
Fill in the blank: Zooplankton migrate ______ in the water column each day to feed on phytoplankton in the photic zone.
True or false: Copepods are the most abundant group of zooplankton.
Q: State the eight levels of the taxonomic hierarchy in order from broadest to most specific.
A: Domain, kingdom, phylum, class, order, family, genus, species.
Q: Explain the difference between phytoplankton and zooplankton, giving one named example of each.
A: Phytoplankton are producers that obtain energy through photosynthesis (e.g. diatoms). Zooplankton are consumers that gain nutrition by feeding on other organisms (e.g. copepods).
Q: Describe two rules that must be followed when writing a binomial name.
A: The genus name must begin with a capital letter while the species name is lowercase. The name must be italicised in print or underlined when handwritten.
Q: Explain how harmful algal blooms (HABs) form and why they are dangerous.
A: Certain dinoflagellate species undergo rapid reproduction when conditions are favourable (e.g. nutrient-rich water from agricultural run-off). These blooms produce toxins that poison fish and accumulate in shellfish. Humans who eat contaminated shellfish can become seriously ill.
Q: Using the Lincoln index formula N = (n₁ × n₂) / m₂, a biologist captures and marks 20 turtles, then returns two weeks later and captures 25 turtles, 5 of which are marked. Estimate the population size.
A: N = (20 × 25) / 5 = 100 turtles.
This material connects directly to Chapter 5 (marine ecosystems), where you will explore how the organisms classified here interact in coral reefs, mangroves and the open ocean. The concept of keystone species reappears in discussions of trophic cascades (e.g. the sea otter, kelp and sea urchin relationship). Plankton dynamics also link to Chapter 9 on human impacts, particularly eutrophication and climate change.
Taxonomy, classification, Linnaeus, binomial nomenclature, scientific naming, dichotomous key, identification key, plankton, phytoplankton, zooplankton, diatoms, dinoflagellates, copepods, jellyfish, krill, larvae, harmful algal blooms, HABs, red tides, keystone species, food web base, marine producers, marine consumers, biological drawings, taxonomic hierarchy, domain kingdom phylum class order family genus species