Basic
Classification
The fish an Eastern Cape angler calls kabeljou — Afrikaans, from the Dutch kabeljauw for cod — is dusky kob in English, daga salmon or just salmon to older KwaZulu-Natal fishermen (it is not remotely a salmon), and Argyrosomus japonicus everywhere a scientist writes about it. Common names change every few kilometres. Scientific names never change. This topic is the filing system that lets marine biologists in Cape Town, Tokyo and São Paulo talk about exactly the same species.
Imagine you're a marine biologist in Cape Town. You email a colleague in Argentina to say you've spotted "a Southern Right Whale" off Hermanus. He emails back saying he's spotted "ballena franca austral" off Patagonia. Are you both talking about the same animal? How can you be sure?
You use the scientific name: Eubalaena australis. That name is identical in every country, every language, every textbook on Earth. Common names change every time you cross a border (or just a few kilometres of coastline) — but every species has exactly one internationally agreed scientific name. This system is called binomial nomenclature ("two-name naming"), and it was set up in 1735 by a Swedish botanist called Carolus Linnaeus.
The system does two jobs at once. The first word (the genus) tells you a group of closely-related species. The second word (the species) narrows it to exactly one. And every species sits inside a nested hierarchy of bigger groups — seven levels deep — that tells you, at a glance, how it relates to every other living thing on Earth. That hierarchy is the framework for every marine biology topic that follows: viruses and bacteria, plankton, protists, seaweeds, vascular plants, sponges, jellyfish, and on up.
Why Classify? — The Filing System of Life
Classification gives every species a unique scientific address, organises millions of organisms into nested groups by shared features, and tells you instantly how closely two species are related.
Estimates of the number of species on Earth range from 8 to 11 million — most still undescribed. Without a system, the chaos would be impossible. Linnaeus' insight was that organisms can be grouped by shared features, and that those groups can be nested inside bigger groups. All cats fit inside a family Felidae; all mammals fit inside a class Mammalia; all vertebrates fit inside a phylum Chordata; all animals fit inside a kingdom Animalia. Each level is a wider container of similarity.
The modern system has 2 Domains at the top (Prokarya vs Eukarya — same divide you met in Topic 14), each split into Kingdoms (7 in total), and from there down through 5 more Linnaean levels to the individual species. It isn't perfect — every year, DNA evidence rearranges some branches — but it is the agreed working map of life, and you need it to navigate every biology topic from here on.
Two Domains, Seven Kingdoms
All known life sits inside the two-domain seven-kingdom system. Tap any kingdom to see what defines it and which marine organisms belong there.
Local connection — every SA marine species has a binomial address
The Two Oceans Aquarium in Cape Town has an interpretive sign next to every tank that gives you both the common name and the scientific name. Here's a quick tour of the South African coastline by kingdom:
- Animalia — Eubalaena australis (Southern Right Whale), Spheniscus demersus (African Penguin), Carcharodon carcharias (Great White Shark), Cyclograpsus punctatus (Two Oceans shore crab).
- Chromista — Ecklonia maxima (sea bamboo, the towering kelp of the Western Cape).
- Plantae — Ulva lactuca (sea lettuce, green algae on rocky shores).
- Bacteria — Prochlorococcus spp. (microscopic cyanobacteria; the most abundant photosynthesiser on Earth, dominating warm open ocean — though it is largely absent from the cold Southern Ocean, where diatoms take over).
Linnaeus' Filing Cabinet — Seven Levels Deep
Inside each kingdom, Linnaeus' system goes six more levels deeper. Each level narrows the group of organisms further, until only one species fits. Here's the address of the Southern Right Whale:
~ from "every animal on Earth" down to one species
One small CAPS note
For animals, the second level is called Phylum. For plants, the equivalent level is called Division. Same idea, different historical name. Everything below that is the same in both kingdoms.
Binomial Nomenclature — Writing the Two-Word Name
In day-to-day scientific writing, you don't write all seven Linnaean levels — you just write the last two: genus + species. That's why it's called binomial nomenclature ("two-name"). But there are strict conventions for how it must be written.
The four rules
- Genus first, then species. Always in that order.
- Genus starts with a capital letter; species is all lowercase.
- The two-word name is either italicised (when typed) or underlined (when handwritten). Both are acceptable; pick one.
- After first use, you can shorten the genus to a single capital letter — for example E. australis after introducing Eubalaena australis.
Wrong: eubalaena Australis Eubalaena Australis EUBALAENA AUSTRALIS
SA · The two names CAPS names — and one of them is ours
CAPS gives exactly two worked examples for binomial nomenclature, and it is worth knowing both by heart because they are the ones most likely to appear: Eubalaena australis (Southern Right whale), and Coracinus capensis — the galjoen, South Africa's national fish.
The galjoen is a good choice on CAPS's part. It is found almost nowhere else on Earth — essentially endemic to the South African coast from Namibia round to KwaZulu-Natal — it lives in the surf zone right where you would fish for it off Algoa Bay, and it is a species this country decided was worth naming as its own. Capensis in the species name literally means "of the Cape".
You may also see it written Dichistius capensis — taxonomists reassigned the genus. Both are correct; CAPS uses Coracinus capensis, so use that in an exam. It is also a neat illustration of the line in the same CAPS block: "as research continues, there is some movement and subdivision of these groupings." Even names change.
Worked examples — SA marine species
Galjoen — our national fish
Coracinus capensis
Southern Right Whale
Eubalaena australis
African Penguin
Spheniscus demersus
Great White Shark
Carcharodon carcharias
Sea Bamboo (Kelp)
Ecklonia maxima
Shore Crab (Cape rock-pool)
Cyclograpsus punctatus
Cape Rock Lobster
Jasus lalandii
Using a Key to Identify Something
Two of CAPS's three investigations for this block are key exercises — "choose kingdoms using a key" and "identify animals to different levels, using animal photos and keys". A classification system is only useful if you can actually work out what something is, and a key is the tool that lets you.
A dichotomous key is a series of paired either/or choices. Each answer sends you to the next pair — until you arrive at a name.
Dichotomous means "divided in two". At every step you get exactly two options, they must be mutually exclusive, and you can only take one. Follow the chain and you cannot help but arrive somewhere. The skill is not memorising organisms — it is following instructions precisely and observing carefully.
A key to the seven kingdoms
Work down it. At each step choose a or b, and go where it tells you.
🔑 Kingdom key
- a. Cell has no membrane-bound nucleus (prokaryotic) … go to 2
b. Cell has a true nucleus (eukaryotic) … go to 3 - a. Ordinary cell-wall chemistry; found in most everyday habitats … BACTERIA
b. Different cell-wall chemistry and ribosome shape; often an extremophile … ARCHAEA - a. Body is a network of filaments (hyphae); feeds saprophytically on dead matter … FUNGI
b. Not filamentous … go to 4 - a. Multicellular, heterotrophic, cells differentiated into tissues … ANIMALIA
b. Photosynthetic, or single-celled … go to 5 - a. Single-celled or colonial, never differentiated into true tissues … PROTISTA
b. Multicellular photosynthesiser with differentiated tissues … go to 6 - a. Brown pigment (fucoxanthin) masking the green; chloroplast of endosymbiotic origin behind four membranes … CHROMISTA
b. Green or red; chloroplast behind two membranes … PLANTAE
Try it — run three organisms through the key
Sea bamboo kelp (Ecklonia maxima). 1b (it has a nucleus) → 3b (not filamentous) → 4b (photosynthetic) → 5b (multicellular with differentiated tissue — holdfast, stipe, blade) → 6a (brown, fucoxanthin, endosymbiotic) → CHROMISTA. Note how many people would have said "Plantae" on sight.
A dinoflagellate causing a red tide. 1b → 3b → 4b (photosynthetic) → 5a (single-celled) → PROTISTA.
A galjoen. 1b → 3b → 4a (multicellular, heterotrophic, differentiated tissues) → ANIMALIA.
The rule that catches people out: you must answer the question the key actually asks, not the one you expect. Kelp looks like a plant. The key does not care what it looks like.
2. Use features you can actually see. "Lives in deep water" is useless when you are looking at a photograph; "has eight arms" is not.
3. Every path must end — either at a name, or at another numbered step. A key with a dead end is a broken key.
Evolutionary Trends — and Why Any of This Matters
The Depth column for this block asks for two things the kingdom list alone does not deliver: evolutionary trends, and the richness of marine biodiversity and why it must be conserved.
The seven kingdoms are a sequence, not a list
Read them in the order CAPS gives them and you are reading roughly the order in which life became more complex. Three trends run straight through:
1 · Prokaryote → eukaryote
Bacteria and Archaea have no nucleus. Everything after them does. That single step — a cell swallowing another and keeping it (endosymbiosis) — produced mitochondria, then chloroplasts, and made every other kingdom possible.
~ you met the evidence in Topic 14: mitochondria still carry their own DNA.2 · Single-celled → multicellular
Protista includes colonial forms — many cells living together but never differentiated. Chromista, Fungi, Plantae and Animalia crossed the line into true differentiated tissue, where cells specialise and depend on one another.
~ the colony rung from Topic 14's hierarchy is exactly this boundary.3 · Ways of feeding diverged
Early cells did a bit of everything. Later kingdoms specialised: Plantae and Chromista make their own food, Fungi absorb dead matter, Animalia eat other organisms. Specialising means depending on the others.
~ which is why removing one kingdom from an ecosystem breaks the rest.Why classification is a conservation tool, not just a filing system
It looks like bookkeeping. It is not. You cannot protect what you cannot name.
You cannot count it. Every conservation decision — a fishing quota, a Marine Protected Area boundary, a red-list assessment — starts with knowing which species is which. Two fish that look alike but are separate species can have completely different population sizes, and managing them as one will collapse the rarer one.
You cannot notice it disappearing. A species nobody has described cannot be recorded as declining. Most of the ocean's species have never been formally named, so most extinctions in the sea are invisible.
South Africa is a case in point. Our coastline runs from cold Atlantic upwelling to warm Indian Ocean reef, and it holds roughly 13 000 recorded marine species — of which around a third are found nowhere else on Earth. That endemism is why decisions taken here cannot be undone anywhere else. The galjoen in Section 4 is the emblem of exactly that: our national fish, essentially confined to our coast, and once so heavily fished that it now has a closed season and a bag limit.
In the Lab — the CAPS Investigations
All three of this block's investigations are key and photograph exercises — no apparatus, and all of them are genuinely good practice for the kind of question this topic produces in an exam.
🔬 The three investigations
- Choose kingdoms using a key. Run ten organisms — including the deliberately deceptive ones — through the kingdom key above. 🟢 desk
- Identify animals to different levels, using photographs with statistics and keys. A supplied photo-card set of SA marine animals with real measurements, worked down to class and order. 🟢 desk
- Write your own dichotomous key. Build one from scratch for eight shells or eight fish — the hardest and most revealing of the three, because a key only works if your observations are precise. 🟢 desk · or a beach walk
📄 Practical sheet: Open the Topic 15 practical →
Key Vocabulary
Tap each card to flip it. The terms added below carry the parts of CAPS block 15 that the kingdom list alone does not — keys, endosymbiosis, and the two worked binomials CAPS names by hand.
The scientific grouping of organisms into nested categories based on shared characteristics — giving every species a unique scientific name that is the same in every language and country.
The highest (largest) grouping in modern classification. Two domains: Prokarya (no nucleus) and Eukarya (membrane-bound nucleus). Some scientists argue for three (splitting Archaea off into its own domain).
The next level down from Domain. Seven in the modern system: Bacteria, Archaea (both Prokarya) plus Protista, Chromista, Fungi, Plantae, Animalia (all Eukarya).
The "two-name" naming system. Every species is identified by Genus + species. Genus is capitalised, species is lowercase, both italicised or underlined. Example: Eubalaena australis.
The second-last Linnaean level — a group of closely related species. Written with a capital first letter, italicised. The three right-whale species (Atlantic, Pacific, Southern) all share genus Eubalaena.
The smallest unit of classification — one specific type of organism whose members can interbreed and produce fertile offspring. Written all lowercase and italicised: australis, maxima, punctatus.
The second-largest Linnaean level (one below Kingdom). Animals with a backbone (or notochord) sit in phylum Chordata; insects, crabs and lobsters in Arthropoda. Plants use the term Division instead of Phylum.
The scientific name for multicellular animals with differentiated cells — i.e. every member of Kingdom Animalia. From the Greek meta- (after, beyond) + zoa (animals). Distinguishes true animals from the simpler colonial protists.
An identification tool made of paired either/or choices. At each step there are exactly two mutually exclusive options; the one you pick sends you to the next pair, until you reach a name. Dichotomous = "divided in two". Two of CAPS's three investigations for this block use one.
One cell engulfing another and keeping it alive inside itself, until the two become one organism. It produced mitochondria and chloroplasts. CAPS names an endosymbiotic origin as the defining feature of Chromista — which engulfed a whole red alga that already had a chloroplast, leaving its chloroplasts behind four membranes instead of two.
Cells that have become specialised for particular jobs, forming true tissues. It is the line CAPS draws around Protista: protists "can be colonial or multicellular, but never differentiated". Cross that line and you are in Plantae, Fungi or Animalia.
Feeding by secreting digestive enzymes onto dead organic matter and absorbing the products — digestion happens outside the body. CAPS gives this, together with filamentous bodies (hyphae), as the definition of Fungi. They are the ocean's recyclers.
Found naturally in one place and nowhere else on Earth. About a third of South Africa's ~13 000 recorded marine species are endemic — including the galjoen. Endemism is why conservation decisions taken here cannot be undone anywhere else: if we lose it, it is gone globally.
Quick Check
Question 1
Well done!
Next: Viruses & Bacteria — the first kingdom-by-kingdom tour begins with the tiniest residents of the ocean.