Biology • Year 12 • Module 5 • Lesson 2
Reproduction in Animals
Apply external and internal fertilisation to real animal mechanisms, real spawning data, and a real-world scenario.
1. Sequence the steps — broadcast spawning on the Great Barrier Reef
The events below describe how a hard coral (e.g. Acropora) achieves external fertilisation during the annual mass spawning event. They are shuffled. Write the correct order (1–8) in the box beside each event. 8 marks
| Order | Event |
|---|---|
| Buoyant gamete bundles rise from the polyp to the sea surface. | |
| Sperm and eggs separate at the surface and disperse into the water column. | |
| A haploid sperm meets a haploid egg in seawater and the two membranes fuse. | |
| The diploid zygote begins cleavage, developing into a free-swimming planula larva. | |
| Environmental cues (rising sea-surface temperature, lunar cycle, dusk) synchronise polyps across the reef. | |
| The planula larva eventually settles on a hard substrate and metamorphoses into a new polyp. | |
| Polyps simultaneously release packaged bundles of sperm and eggs into the water. | |
| Cross-fertilisation between gametes of different colonies maximises genetic variation. |
2. Cause–effect chain — why internal fertilisation supports terrestrial life
The cause boxes (left column) are given. In the effect boxes (right column) write what each cause directly leads to in a terrestrial animal such as a reptile. Then in the final "so…" line write the overall biological outcome. 5 marks
Overall outcome (so…):
3. Interpret real data — coral fertilisation success vs sperm concentration
The figure below is adapted from research on broadcast-spawning corals (Oliver & Babcock, 1992, Biological Bulletin 183: 409–417; subsequently confirmed by Levitan & Petersen, 1995, Trends in Ecology & Evolution). It shows percent fertilisation of eggs as a function of sperm concentration in seawater.
Figure adapted from Oliver & Babcock (1992), Biological Bulletin 183, 409–417 — coral broadcast-spawning fertilisation kinetics.
3.1 Describe the trend between sperm concentration and percent fertilisation shown in the graph. 2 marks
3.2 Estimate the percent fertilisation when sperm concentration is approximately 10³ sperm mL⁻¹, and again at 10⁵ sperm mL⁻¹. 2 marks
3.3 What does this graph suggest about why mass-spawning corals must release enormous numbers of gametes synchronously rather than spawning alone or in small numbers? 3 marks
4. Case study — bleaching delays the spawning window
In November 2016 a severe mass bleaching event on the northern Great Barrier Reef coincided with the annual coral spawning window. Surveys reported that bleached Acropora colonies released far fewer gamete bundles than unbleached colonies, and that the timing of release across the reef was less tightly synchronised. Even in unbleached colonies, sperm concentrations downstream of bleached patches were significantly lower than in pre-bleaching years.
In 4–6 sentences, explain — using the graph in Section 3 and what you know from the lesson about external fertilisation — why these observations are concerning for fertilisation success and for the continuity of the species. 5 marks
Q1 — Correct order of coral broadcast spawning
- Environmental cues (rising sea-surface temperature, lunar cycle, dusk) synchronise polyps across the reef.
- Polyps simultaneously release packaged bundles of sperm and eggs into the water.
- Buoyant gamete bundles rise from the polyp to the sea surface.
- Sperm and eggs separate at the surface and disperse into the water column.
- A haploid sperm meets a haploid egg in seawater and the two membranes fuse.
- Cross-fertilisation between gametes of different colonies maximises genetic variation.
- The diploid zygote begins cleavage, developing into a free-swimming planula larva.
- The planula larva eventually settles on a hard substrate and metamorphoses into a new polyp.
Marking note: award 1 mark per correctly placed event. Award full marks even if the student swaps steps 5 and 6 (fertilisation and cross-fertilisation overlap in time across the population), provided cleavage (step 7) follows.
Q2 — Cause–effect chain
Effect 1: Gametes are protected from dilution, predation, currents and desiccation.
Effect 2: Sperm can move directly toward the egg without depending on open water for transport.
Effect 3: Far more fertilisations succeed per million gametes than under external release.
Effect 4: Energy can be redirected from producing huge numbers of gametes to investing in fewer, better-protected offspring (higher parental investment per offspring).
Effect 5: Reproduction can take place fully on land, freeing the animal from dependence on aquatic habitats.
Overall outcome: Internal fertilisation lets reptiles, birds and mammals reproduce successfully on land using fewer but better-protected gametes, with higher per-offspring survival — a key adaptation for fully terrestrial life.
Marking note: 1 mark per biologically valid effect statement; full marks require the overall outcome to link the chain back to terrestrial reproduction.
Q3.1 — Trend description
Percent fertilisation increases as sperm concentration increases, but the relationship is sigmoidal (S-shaped), not linear. Below about 10³ sperm mL⁻¹ fertilisation is near zero, then rises steeply between 10³ and 10⁵ sperm mL⁻¹, and finally plateaus near 95–100% at high sperm concentrations.
Q3.2 — Read-off values
At 10³ sperm mL⁻¹ ≈ 5% (very low). At 10⁵ sperm mL⁻¹ ≈ 90–95% (near plateau). Accept ±5 percentage points either side.
Q3.3 — Interpretation: why mass synchrony matters
Because the curve is nearly flat at low sperm concentrations, isolated or unsynchronised release would produce almost no successful fertilisations — most gametes would simply be diluted below the threshold needed for sperm and egg to collide. By spawning simultaneously across the whole reef, corals push the local sperm concentration up into the steep middle portion of the curve, where percent fertilisation rises rapidly. This is why large numbers and synchronised timing are both essential features of external fertilisation in corals.
Q4 — Case study (sample top-band response)
Bleached colonies release fewer gamete bundles, so the total number of sperm and eggs put into the water is reduced. The fertilisation-vs-sperm-concentration curve is sigmoidal, so a drop in local sperm concentration from ~10⁵ to ~10³ sperm mL⁻¹ would push fertilisation success from ~90% down toward ~5% — a much larger biological effect than the small drop in gamete number suggests. The loss of tight synchrony makes this worse, because gametes released at slightly different times have less opportunity to meet before being diluted. With external fertilisation depending entirely on large numbers and synchronised timing, bleaching simultaneously attacks both. The result is far fewer zygotes and planula larvae, threatening recruitment of new polyps and therefore continuity of the species on the reef.
Marking note: 1 mark for identifying reduced gamete numbers, 1 mark for identifying the loss of synchrony, 1 mark for explicitly using the sigmoidal graph to argue that a small absolute drop causes a large fertilisation drop, 1 mark for linking back to continuity of species, 1 mark for biological precision (uses terms like fertilisation success, sperm concentration, planula).