Practical 28 Marine Biology CAPS block 16 · Term 2 · 5 hours

Evolutionary Trends, Body Plans & Symmetry

Build a tree of life from six animals' bodies. Build four body plans out of coloured dough. Watch vinegar creep into jelly cubes to see why a large animal cannot rely on diffusion. Then sort the whole zoo that lives inside a single washed-up kelp holdfast.

What CAPS asks for

Investigations column, block 16: "Study cladograms and phylogenetic trees of the 7 kingdoms and major animal phyla, to see how they work" — that is Investigation 1 — and "Study photos of marine heterotrophs; name and identify them", with "preserved animals or a kelp holdfast" listed as resources — that is Investigation 4.

CAPS also says the body cavities must be explained "with the aid of diagrams": Investigation 2. And life pattern 5 links "diffusion and body plan": Investigation 3 puts numbers on it.

What you need

🟢 kitchen Modelling dough in three or four colours — or make it: 2 cups flour, 1 cup salt, 1 cup water, 2 tablespoons oil, food colouring
🟢 kitchen A quarter of a red cabbage, unflavoured gelatine, white vinegar, a flat dish, a sharp knife, a ruler with millimetres
🟢 kitchen A tray or basin of seawater, a teaspoon, a phone camera, paper and coloured pencils
🌊 shore A kelp holdfast washed up on the beach after a storm — the root-like base of a kelp plant
🟡 optional A hand lens or magnifying glass, for the holdfast animals

Missing something? Everything still works.

Investigation 1 is a desk task. Investigation 2 can be drawn straight onto paper with coloured pencils. Investigation 3 has a full supplied data set. Investigation 4 has a described holdfast to identify if you cannot find a real one.

Safety Boiling water and a sharp knife for the cabbage, the gelatine and the cubes — have an adult with you, cut on a board, and cut away from your hand. On the shore, holdfasts wash up after big seas: look along the high-tide line, not on wave-washed rocks. Return every animal from the holdfast to a rock pool alive when you have finished.

1 Build a Cladogram CAPS Investigation: "study cladograms and phylogenetic trees … to see how they work"

Aim

To build a cladogram from body characters, and to use it and the booklet's phylogenetic tree (Fig 28.1) to explain how such trees work.

The data

Six marine animals and six characters. A 1 means the animal has the character; a 0 means it does not.

Data set 1.1 — character table
Animal	A · true tissues	B · bilateral symmetry	C · three tissue layers	D · coelom	E · jointed exoskeleton	F · backbone
Sponge	0	0	0	0	0	0
Sea anemone	1	0	0	0	0	0
Marine flatworm	1	1	1	0	0	0
Bristleworm	1	1	1	1	0	0
Crab	1	1	1	1	1	0
Klipfish	1	1	1	1	0	1

Method — how to build it

Count the 1s for each animal. The animal with none of the characters is the outgroup: it branches off first.
Draw a long diagonal line — the main trunk. The outgroup branches off at the bottom.
Mark character A on the trunk just above that branch: every animal above that point has it.
Branch off the next animal, then mark the next character, and so on up the trunk.
If two or more animals share all the same characters up to a point, they branch from the same point. Mark any character only one animal has on its own branch.

Your cladogram

Analysis

Count the characters each animal has. Which animal is the outgroup? 3
Draw the cladogram, with each of the six characters marked where it first appears. 8
Name the character that separates the sea anemone from the sponge, and the two that separate the flatworm from the anemone. 2
Your cladogram cannot decide the order in which the bristleworm, crab and klipfish branched. Explain why, and suggest one extra character that would help. 3
Use the booklet's phylogenetic tree (Fig 28.1). Which phylum appears to have branched off first? When does the Cambrian "explosion" begin? What do the question marks on some branches mean? Which two vertebrate classes branch off most recently? 4
DNA studies place flatworms on the same branch as bristleworms and molluscs, rather than branching off earlier. Flatworms have no coelom. Suggest how both facts can be true. 3
What is the difference between a cladogram like yours and a phylogenetic tree like Fig 28.1? 2

2 Body Plans in Modelling Dough CAPS: "explain these terms with the aid of diagrams"

Aim

To build, draw and label cross-sections of the four body plans: diploblastic, acoelomate, pseudocoelomate and coelomate.

Method

Choose one colour for each layer and keep to it: blue = ectoderm, red = mesoderm, yellow = endoderm, and white or purple for the mesogloea.
On a plate, build each cross-section flat, like a target: roll each layer into a thin sausage and curl it into a ring. Start from the gut in the middle and work outwards.
Where a body plan has a cavity, leave a gap of about 1 cm between rings — that empty ring is the cavity.
For the coelomate, remember that the cavity lies inside the mesoderm: a red ring on both sides of the gap.
Photograph all four from above. Then draw and label each one.

No dough? Draw the four cross-sections directly with coloured pencils, using the same colour code. The marks are for the drawings, not the dough.

Your four diagrams

Analysis

Draw and label the four cross-sections. For each: the layers in the right order (2) and every layer and cavity labelled (2). 16
Complete a table with one row for each body plan and three columns: number of tissue layers; type of cavity (none, pseudocoelom or coelom); an example phylum. 8
Explain the difference between a pseudocoelom and a coelom. 2
Give three advantages that a coelom gives an animal. 3
The booklet's definition list gives the ectoderm as "the inner layer of body tissue". What is wrong with this, and what is the correct definition? 2

3 Why Big Animals Need Gills and Blood life patterns 5 and 6 · "diffusion and body plan"

Aim

To investigate how the size of a block affects how much of it a substance can reach by diffusion in a fixed time — and to explain what that means for animal body plans.

Method

Chop the red cabbage, cover it with boiling water, leave it for 15 minutes and strain off the purple juice. Red cabbage juice is an indicator: it turns pink in acid.
Dissolve the gelatine in the hot cabbage juice (follow the packet amounts, using a little less water so it sets firm). Pour it into a flat dish to a depth of at least 3 cm and let it set in the fridge.
Cut three cubes of each size: 1 cm, 2 cm and 3 cm along each edge.
Put all nine cubes into a bowl of vinegar at the same time. Leave them for exactly 10 minutes.
Lift them out, rinse them, and cut each one in half through the middle.
Measure the depth of the pink layer from the surface inwards, in millimetres.

Supplied results — nine cubes, 10 minutes in vinegar

Data set 3.1 — depth of the pink layer (mm)
Cube edge	Cube 1	Cube 2	Cube 3
1 cm	2.4	2.6	2.5
2 cm	2.5	2.6	2.4
3 cm	2.6	2.5	2.4

The 1 cm cubes were pink almost all the way through, with a small purple core. The 3 cm cubes were mostly purple, with a thin pink skin.

Results

Table 3.1 — your results and calculations
Cube edge	Surface area (cm²)	Volume (cm³)	SA : V	Mean depth (mm)	% of cube turned pink
1 cm
2 cm
3 cm

Analysis

Calculate the surface area, the volume and the surface-area-to-volume ratio (SA : V) of each cube size. 6
Calculate the mean depth of the pink layer for each cube size. 3
The unchanged purple core is a smaller cube. Its edge = cube edge − 2 × depth. Calculate the volume of the core and the percentage of each cube that turned pink. Show full working for one size. 6
Draw a line graph of the percentage turned pink (y-axis) against SA : V (x-axis). Marks: title (1), labelled axes (2), scale (1), plotted points (1), line (1). 6
State the relationship your graph shows. 2
Use your results to explain why a flatworm can be flat and live without blood or gills, while a large fish needs gills, a gut and a circulatory system. 4
The pink layer was about the same depth in all three sizes. What does that tell you about diffusion? 2

4 The Holdfast Zoo CAPS Investigation: "study photos of marine heterotrophs; name and identify them" · resource: a kelp holdfast

Aim

To find, identify and classify the animals living in a kelp holdfast by phylum and symmetry, and to link each one's body to its way of life.

Method

After a storm, search the high-tide line for a kelp holdfast. Choose one that is still wet and fresh.
Put it straight into a basin of seawater. Over the next half hour, animals will crawl out of the tangle; gently tease the branches apart to find the rest.
Move each kind of animal into a teaspoon of water and look at it closely, with a hand lens if you have one. Photograph it.
For each, record its shape, its symmetry, how it moves and how it seems to feed.
When you have finished, return every animal to a rock pool.

Supplied data — a described holdfast, twelve kinds of animal

Read this as a composite of animals commonly found in kelp holdfasts on our coast. The animals are described, not named — identifying them is the task.

A An orange crust spreading over part of one branch, dotted with tiny holes and a few larger ones. No definite shape. Does not move.

B A soft pink cylinder about 1 cm tall, attached at its base, with a ring of tentacles around a central mouth on top.

C A small central disc with five long, thin, snake-like arms that wriggle rapidly.

D A green ball about 1 cm across, covered in spines, moving slowly on tiny sucker-tipped tube feet.

E A pinkish worm about 4 cm long, made of many similar segments, each with a pair of bristly paddles down its sides.

F A paper-thin oval creature that glides smoothly over the surface, with two tiny dark eye-spots at its front end.

G Dozens of tiny shrimp-like animals about 5 mm long, flattened from side to side, that jump and skitter.

H A small crab about 1 cm across, with jointed legs, that scuttles sideways.

I An oval animal clamped tightly to a branch, with eight overlapping shell plates along its back.

J A small coiled shell; the soft animal inside pulls back in and closes a little door when touched.

K A translucent, jelly-like blob attached at one end, with two openings on top; it squirts water when touched.

L A mottled brown fish about 3 cm long, with fins, hiding among the branches.

Results

Table 4.1 — classify each animal
Animal	Phylum	Symmetry	Moves or fixed?	How it feeds
A
B
C
D
E
F
G
H
I
J
K
L

Analysis

Complete the phylum and symmetry columns of Table 4.1 for all twelve animals. (One mark for each animal with both correct.) 12
How many phyla are represented in this one holdfast? Which kind of symmetry is the most common, and why might that be? 3
Two of the twelve are chordates, the phylum that includes humans. Name them, and explain why one of them is a surprise. 3
Animals A, B and K stay in one place. Explain how their symmetry and feeding suit a fixed life, compared with an active animal like E or H. 3
Suggest three reasons why a kelp holdfast is such a rich habitat for small animals. 3
Why should every animal be returned alive? Give one other rule for collecting on the shore responsibly. 2

Memo — worked answers and mark allocation (does not print)

Total: 111 marks — Investigation 1: 25 · Investigation 2: 31 · Investigation 3: 29 · Investigation 4: 26.

Investigation 1 — build a cladogram

Q1 (3): Sponge 0 · anemone 1 · flatworm 3 · bristleworm 4 · crab 5 · klipfish 5 (2). The outgroup is the sponge (1).
Q2 (8): Sponge branches first (1); A marked above it (1); anemone next (1); B and C marked above it (1); flatworm next (1); D marked above it (1); bristleworm, crab and klipfish branching from one point, or in any defensible order (1); E on the crab's own branch and F on the klipfish's (1).
Q3 (2): True tissues (A) separates the anemone from the sponge (1). Bilateral symmetry (B) and three tissue layers (C) separate the flatworm from the anemone (1).
Q4 (3): All three share A–D, and E and F are each found in only one animal, so no character is shared by just two of them (2). A useful extra character: segmentation (bristleworm and crab), a dorsal nerve cord / notochord (klipfish), or moulting (crab) (1). Credit a learner who notes that segmentation would group the worm with the crab — and that DNA suggests segmentation evolved twice.
Q5 (4): Porifera (1); about 540 million years ago (1); the phylum's exact position is uncertain because of unusual features or a poor fossil record (1); birds and mammals (1).
Q6 (3): Flatworms are probably descended from ancestors that had a coelom (1) and have since lost it (1); characters can be lost as well as gained, so body characters alone can mislead, while DNA reflects ancestry more directly (1).
Q7 (2): A cladogram shows only the order of branching — shared characters (1); a phylogenetic tree like Fig 28.1 also places the groups on a timeline of when they appeared (1).

Investigation 2 — body plans

Q8 (16): Four marks per diagram. Diploblastic: ectoderm – mesogloea – endoderm – gut cavity (2), all labelled (2). Acoelomate: ectoderm – solid mesoderm – endoderm – gut (2), labelled (2). Pseudocoelomate: ectoderm – mesoderm – cavity – endoderm – gut, with no mesoderm around the gut (2), labelled (2). Coelomate: ectoderm – mesoderm – coelom – mesoderm – endoderm – gut, with mesoderm on both sides of the cavity (2), labelled (2).
Q9 (8): Two marks per row. Diploblastic: 2 layers · no true cavity (only the gut cavity) · Cnidaria. Acoelomate: 3 · none · Platyhelminthes. Pseudocoelomate: 3 · pseudocoelom · Nematoda. Coelomate: 3 · coelom · Annelida, Mollusca, Arthropoda, Echinodermata or Chordata.
Q10 (2): A pseudocoelom lies between the mesoderm and the endoderm, so mesoderm lines only its outer side (1). A coelom lies within a split mesoderm, so it is lined by mesoderm on both sides (1).
Q11 (3): Any three: the gut and organs can move independently of the body wall; there is room for larger, more complex organs; the fluid helps transport materials; the fluid can act as a hydrostatic skeleton.
Q12 (2): It gives the same definition as the endoderm — the ectoderm is the outer layer, not the inner (1). Correct: the outer layer of body tissue, essentially the skin of the animal (1).

Investigation 3 — diffusion and size

Q13 (6): 1 cm: SA 6 cm², V 1 cm³, SA:V 6. 2 cm: SA 24, V 8, SA:V 3. 3 cm: SA 54, V 27, SA:V 2. (2 per size.)
Q14 (3): 1 cm: (2.4 + 2.6 + 2.5) ÷ 3 = 2.5 mm; 2 cm: 2.5 mm; 3 cm: 2.5 mm. (1 each.)
Q15 (6): 1 cm cube (10 mm): core edge = 10 − 2 × 2.5 = 5 mm; core volume = 125 mm³ of 1 000 mm³ → 87.5% pink. 2 cm: core 15 mm → 3 375 of 8 000 mm³ → 42.2% purple → 57.8% pink. 3 cm: core 25 mm → 15 625 of 27 000 mm³ → 57.9% purple → 42.1% pink. (Two marks per size — core volume (1), percentage (1) — with the full working shown at least once.)
Q16 (6): Mark against the listed criteria. Points: (2, 42), (3, 58), (6, 88).
Q17 (2): The higher the surface-area-to-volume ratio, the greater the percentage of the block reached by diffusion in the same time (2).
Q18 (4): A flatworm is thin and flat, so it has a very high SA:V and every cell is close to the surface — diffusion alone is enough (2). A large fish has a low SA:V: diffusion through its skin could never reach its inner cells in time, so it needs gills (a large exchange surface), a gut, and blood pumped around a circulatory system to carry materials to every cell (2).
Q19 (2): Diffusion moves material the same distance in the same time whatever the size of the block (1) — so it is effective only over very short distances, and larger bodies need other transport (1).

Investigation 4 — the holdfast zoo

Q20 (12): A sponge — Porifera, asymmetrical · B sea anemone — Cnidaria, radial · C brittle star — Echinodermata, pentaradial · D sea urchin — Echinodermata, pentaradial · E bristleworm — Annelida, bilateral · F flatworm — Platyhelminthes, bilateral · G amphipods — Arthropoda, bilateral · H crab — Arthropoda, bilateral · I chiton — Mollusca, bilateral · J sea snail (top shell or whelk) — Mollusca, bilateral (accept "bilateral, but coiled") · K sea squirt — Chordata, bilateral as a larva (accept "no clear symmetry as an adult") · L klipfish — Chordata, bilateral. One mark per animal with both correct.
Q21 (3): Eight phyla — Porifera, Cnidaria, Echinodermata, Annelida, Platyhelminthes, Arthropoda, Mollusca and Chordata (1). Bilateral is most common (1) — most of these animals move around the holdfast to find food and shelter (1).
Q22 (3): The klipfish (L) and the sea squirt (K) (1). The sea squirt is the surprise: as an adult it is a sessile, filter-feeding blob with no backbone (1), but its swimming larva has a nerve cord along its back and a stiffening rod (notochord), which makes it a chordate — closer to us than any other invertebrate in the holdfast (1).
Q23 (3): Fixed animals cannot chase food, so they filter it from the water (A, K) or catch what touches them (B) (1). Food and danger can arrive from any side, so a body that is asymmetrical or radial — the same all round — suits them (1). Active animals meet the world front-first, so bilateral symmetry with a head end suits them (1).
Q24 (3): Any three: shelter from waves and predators in its tangle; a firm surface to attach to; trapped sediment and detritus as food; plankton carried in by water flowing through it; a stable, moist place to hide when exposed.
Q25 (2): They are living animals, and removing them depletes the shore's populations (1). Any one rule: take only what you need; do not collect in an MPA without permission; turn rocks back over; follow permit rules (1).

Marking note. Q6 is the hardest question on the sheet and the most important: it is where a learner discovers that a tree is a hypothesis built from evidence, and that different evidence can redraw it. Reward any answer that reaches "the character was lost" or "the character evolved more than once", however it is worded.