Practical 15 — Basic Classification
CAPS sets three investigations for this block and all three are key-and-photograph exercises. No apparatus, and no shortcut either: a key only works if you read the question it actually asks and observe precisely. That is the whole skill, and it is exactly what gets examined.
And from the Depth column, the two things the kingdom list alone will not give you: "Evolutionary trends" and "the richness of marine biodiversity and why it must be conserved."
A pencil. Everything here is desk work. Investigation 3 is better with a handful of real shells — a five-minute beach walk at Kings Beach or Schoenmakerskop will get you eight different ones — but a set of eight described specimens is supplied if you would rather work from the page.
Aim
To assign organisms to one of the seven kingdoms using a dichotomous key, and to explain why appearance alone is unreliable.
🔑 Kingdom key — the same one as Section 5 of the topic page
- a. No membrane-bound nucleus (prokaryotic) → 2 · b. True nucleus (eukaryotic) → 3
- a. Ordinary cell-wall chemistry, everyday habitats → BACTERIA · b. Different cell-wall chemistry and ribosome shape, often extremophile → ARCHAEA
- a. Body a network of filaments (hyphae), saprophytic → FUNGI · b. Not filamentous → 4
- a. Multicellular, heterotrophic, differentiated tissues → ANIMALIA · b. Photosynthetic, or single-celled → 5
- a. Single-celled or colonial, never differentiated → PROTISTA · b. Multicellular photosynthesiser with differentiated tissues → 6
- a. Brown fucoxanthin masking the green, chloroplast of endosymbiotic origin behind four membranes → CHROMISTA · b. Green or red, chloroplast behind two membranes → PLANTAE
The ten organisms
Work each one down the key. Write the route you took, not just the answer — the route is where the marks are, and it is what proves you used the key rather than guessed.
| Organism | Route (e.g. 1b → 3b → 4a) | Kingdom |
|---|---|---|
| a) Sea bamboo kelp, Ecklonia maxima — multicellular, brown, holdfast + stipe + blade | ||
| b) Galjoen, Coracinus capensis | ||
| c) A dinoflagellate causing a red tide — single cell, photosynthetic | ||
| d) A methanogen from a hydrothermal vent — single cell, no nucleus, unusual cell-wall chemistry | ||
| e) Sea lettuce, Ulva — multicellular, bright green, differentiated | ||
| f) A marine fungus decomposing driftwood — network of hyphae | ||
| g) Prochlorococcus — single cell, no nucleus, photosynthetic, ordinary cell wall | ||
| h) A sponge — multicellular, heterotrophic, a few specialised cell types, no true tissues | ||
| i) A diatom — single cell, photosynthetic, silica wall, brown pigments | ||
| j) Eelgrass, Zostera capensis — flowering plant, roots, green |
Analysis
- 20Complete the route and kingdom for all ten organisms. (1 for the route, 1 for the kingdom.)
- 3Organisms (a) and (e) are both large multicellular photosynthesisers that most people would call "seaweed". Explain why the key separates them, and name the feature that decides it.
- 3(g) is photosynthetic but the key sends it to a kingdom most people associate with disease. Explain why, and state which step of the key made the decision.
- 3(h), the sponge, is the hardest one. Explain the difficulty, and say what extra information you would need at step 4 to be confident.
- 2Explain in one sentence why the key never asks "does it look like a plant?"
Aim
To classify South African marine animals to phylum, class and order, and to write correct binomials for them.
The photo-card set
Six SA marine animals, described as a photo-card with its statistics — exactly the format CAPS asks for.
Card 1 — Galjoen
Length 55 cm · mass 3.5 kg · surf zone, 0–10 m · endemic to SA
Deep-bodied fish, bony skeleton, single dorsal fin with spines, paired fins, bony gill cover, scales.
Card 2 — Great white shark
Length 4.6 m · mass 1 100 kg · False Bay & Algoa Bay · coastal to 250 m
Torpedo-shaped fish, cartilage skeleton, five exposed gill slits, no gill cover, rough skin of tooth-like denticles.
Card 3 — Southern Right whale
Length 15 m · mass 47 000 kg · Hermanus & Algoa Bay, Jun–Nov
Warm-blooded, breathes air through a blowhole, gives birth to live young and suckles them, no dorsal fin, baleen instead of teeth.
Card 4 — African penguin
Height 60 cm · mass 3.1 kg · Bird Island, Algoa Bay · Endangered
Warm-blooded, feathers, lays eggs in a nest, wings modified into flippers, beak.
Card 5 — Cape rock lobster
Carapace 12 cm · mass 0.9 kg · West Coast rocky reef, 0–40 m
Hard jointed external skeleton, segmented body, ten legs, two pairs of antennae, moults to grow.
Card 6 — Common octopus
Arm span 90 cm · mass 3 kg · Algoa Bay rocky reef
Soft unsegmented body, eight arms with suckers, beak, well-developed eyes, no skeleton at all, changes colour.
| Card | Phylum | Class | Order or group | Binomial (correctly written) |
|---|---|---|---|---|
| 1 Galjoen | ||||
| 2 Great white | ||||
| 3 Right whale | ||||
| 4 Penguin | ||||
| 5 Rock lobster | ||||
| 6 Octopus |
Analysis
- 18Complete phylum, class and order for all six cards. (1 each.)
- 6Write each binomial correctly, applying all the publishing conventions. (1 each — capitals, lowercase and italics/underlining all count.)
- 4Cards 1 and 2 are both called "fish". Give the two features from the cards that place them in different classes, and name both classes.
- 3Cards 3 and 4 are both warm-blooded and both live in the sea. Explain what places them in different classes, using evidence from the cards.
- 3Cards 5 and 6 are both invertebrates. State the phylum of each and the single feature that separates them.
- 3Card 3 is 15 m long and card 1 is 55 cm. Explain why the statistics on these cards are useful for identification but are never the basis of classification.
Aim
To construct a working dichotomous key for eight specimens, and to test it on someone else.
This is the hardest of the three, and the most revealing. Using a key tests whether you can follow instructions; writing one tests whether you can observe.
Your specimens
Best option: collect eight different empty shells from the beach — twenty minutes at Kings Beach or Schoenmakerskop is plenty. 🌊 shore Take only empty shells; check nothing is living inside, including a hermit crab.
Or use the eight described specimens below. ✏️ desk
📊 The eight supplied specimens
| # | Specimen | Observable features |
|---|---|---|
| 1 | Limpet | Single cone-shaped shell, no coiling, no operculum, opening covers the whole underside |
| 2 | Periwinkle | Single coiled shell, spiral, under 2 cm, has an operculum (a "trapdoor") |
| 3 | Turban shell | Single coiled shell, spiral, over 4 cm, thick chalky operculum |
| 4 | Black mussel | Two matching halves hinged together, elongated, dark, byssal threads attached |
| 5 | Venus clam | Two matching halves hinged together, rounded, pale, concentric ridges |
| 6 | Pansy shell (urchin test) | Flat rigid disc, five-part radial pattern, no hinge, no coil |
| 7 | Barnacle plate | Several small plates forming a cone, cemented to rock, not coiled |
| 8 | Cuttlebone | Light chalky internal plate, oval, chambered, no opening at all |
Write your key here
Remember the three rules: exactly two options per step, observable features only, and every path must end — at a name or at another numbered step.
Test it
Hand your key and the specimens to someone else — a parent, a sibling, anyone. Ask them to identify three specimens using only your key. Record where they get stuck. A key that only its author can use is a failed key.
| Specimen given | What they identified it as | Correct? | Where they hesitated or went wrong |
|---|---|---|---|
Analysis
- 12Submit your key. (8 — every specimen reachable and uniquely identified · 2 — every step has exactly two mutually exclusive options · 2 — all features are observable.)
- 3Record your tester's three attempts and note every point of hesitation.
- 4Rewrite the one step that caused the most trouble, and explain what was wrong with your original wording.
- 3Explain why "is it big or small?" is a poor first step, and suggest a better one for this set.
- 3Your key sorts by shell shape. A biologist's key would sort by features that reflect ancestry. Explain the difference, and why the biologist's version is more useful.
- 3Specimen 8, the cuttlebone, is an internal shell from an animal in the same phylum as the octopus. Explain what that tells you about relying on shells alone to classify animals.
✅ Memo & marking guide — try it first
Total: 96 marks — Investigation 1: 31 · Investigation 2: 37 · Investigation 3: 28.
This memo does not print with the worksheet.
Investigation 1 — the kingdom key
- Q1 (20): a) 1b→3b→4b→5b→6a = CHROMISTA · b) 1b→3b→4a = ANIMALIA · c) 1b→3b→4b→5a = PROTISTA · d) 1a→2b = ARCHAEA · e) 1b→3b→4b→5b→6b = PLANTAE · f) 1b→3a = FUNGI · g) 1a→2a = BACTERIA · h) 1b→3b→4a = ANIMALIA · i) 1b→3b→4b→5a = PROTISTA (accept CHROMISTA with justification — a diatom carries the same brown fucoxanthin and four-membrane chloroplast as kelp, and many schemes place diatoms in Chromista; this key sends it to PROTISTA at step 5 because it is single-celled) · j) 1b→3b→4b→5b→6b = PLANTAE.
- Q2 (3): Both are multicellular photosynthesisers, so the key separates them only at step 6 — on pigment and chloroplast origin. Kelp is brown (fucoxanthin) with an endosymbiotic chloroplast behind four membranes → Chromista. Ulva is green with a chloroplast behind two → Plantae.
- Q3 (3): Prochlorococcus is a cyanobacterium — photosynthetic, but with no nucleus. Step 1 decides it: 1a sends every prokaryote to step 2 before photosynthesis is ever considered. Being able to photosynthesise does not make something a plant.
- Q4 (3): A sponge is multicellular and heterotrophic, but it has specialised cell types without true tissues — so step 4a's wording ("differentiated tissues") is not cleanly satisfied, while 4b is wrong too since it is not photosynthetic or single-celled. You would need step 4 to distinguish specialised cells from true tissues. (Sponges are Animalia — you meet them in Topic 29.)
- Q5 (2): Because appearance reflects adaptation to a habitat, not ancestry — unrelated organisms living the same way come to look alike. Kelp looks like a plant and is not one.
Investigation 2 — the photo-cards
- Q6 (18): 1 Galjoen — Chordata · Actinopterygii (bony fish) · Perciformes. 2 Great white — Chordata · Chondrichthyes (cartilaginous fish) · Lamniformes. 3 Right whale — Chordata · Mammalia · Cetacea. 4 Penguin — Chordata · Aves · Sphenisciformes. 5 Rock lobster — Arthropoda · Malacostraca · Decapoda. 6 Octopus — Mollusca · Cephalopoda · Octopoda.
- Q7 (6): Coracinus capensis · Carcharodon carcharias · Eubalaena australis · Spheniscus demersus · Jasus lalandii · Octopus vulgaris. Full marks require capital genus, lowercase species, italicised or underlined.
- Q8 (4): The galjoen has a bony skeleton and a bony gill cover (operculum); the great white has a cartilage skeleton and five exposed gill slits with no cover (2). Classes: Actinopterygii and Chondrichthyes (2).
- Q9 (3): The whale gives birth to live young and suckles them → Mammalia. The penguin lays eggs and has feathers → Aves. Both being warm-blooded and marine is convergence, not relatedness.
- Q10 (3): Lobster = Arthropoda, octopus = Mollusca (2). The separating feature: the lobster has a hard jointed exoskeleton and segmented body; the octopus has a soft unsegmented body with no skeleton (1).
- Q11 (3): Statistics help you recognise a specimen and rule options out in the field. But size varies with age, sex, season and individual, and unrelated animals can be the same size — so classification must rest on structural features that reflect ancestry, which do not change with how big the individual happens to be.
Investigation 3 — write your own key
- Q12 (12): A workable first split is "one piece / more than one piece", or "coiled / not coiled". A model route: 1 two hinged halves (4, 5) vs not → then elongated with byssal threads (4) vs rounded with concentric ridges (5); coiled (2, 3) → under 2 cm (2) vs over 4 cm with chalky operculum (3); remaining — cone-shaped single shell (1), flat five-part disc (6), several plates cemented to rock (7), internal chambered plate with no opening (8). Marks: all eight reachable and unique (8) · exactly two mutually exclusive options per step (2) · features observable (2).
- Q13 (3): Three attempts recorded with the points of hesitation noted. Credit honest reporting of failures — a learner who records that their key broke has done the investigation properly.
- Q14 (4): The rewritten step must remove the ambiguity, and the explanation must identify what caused it — usually a vague comparative ("large/small", "rough/smooth") with no threshold, or two options that are not mutually exclusive.
- Q15 (3): "Big or small" is relative and undefined — the tester has nothing to compare against, and specimens of the same type vary in size. A better first step uses a presence/absence feature: "shell in two hinged halves / shell in one piece". Accept any absolute, observable split.
- Q16 (3): A shape-based key is artificial — it sorts by convenience and may group unrelated organisms that happen to look alike. A biologist's key is based on shared ancestry, so the groups it produces also tell you about evolutionary relationships and let you predict other features an organism will have.
- Q17 (3): The cuttlebone is an internal shell, and cuttlefish share a phylum (Mollusca) and a class (Cephalopoda) with the octopus, which has no shell at all. So the presence, absence or shape of a shell is a poor guide to relatedness — closely related animals can differ completely in it, which is exactly why classification uses many features rather than one obvious one.