Reef Structures and Zones
What is a coral cay made from?
Accumulated reef-derived sand and rubble forming a low island.
What is a corallite?
The skeletal cup occupied by an individual hard-coral polyp.
According to the syllabus, how long ago did corals first appear in the geological record?
More than 250 million years ago.
How are neighbouring polyps in a coral colony connected?
Living tissue and connected gastrovascular canals allow materials and nutrients to be shared.
How does abundance differ from species richness?
Abundance counts individuals; richness counts species.
How does a ribbon reef differ from a platform reef?
Ribbon reefs are long and narrow, often near a shelf edge; platform reefs are broad, relatively flat reef structures.
What is the coelenteron of a coral polyp?
The gastrovascular cavity where digestion occurs and materials are distributed.
According to the syllabus, when did corals appear in Australian waters?
Approximately 500000 years ago.
How can polyp detachment result in asexual reproduction?
A detached viable polyp can settle and grow into a new colony without fertilisation.
What does the horizontal axis of a rank-abundance plot represent?
Species rank, ordered from most abundant to least abundant.
Which reef zone is usually most exposed to breaking waves: lagoon, crest or lower slope?
The reef crest.
Which coral group is Alcyonacea?
Soft corals. Adapted from QCAA 2025 Paper 1 Q2, viewed on Exam Insights.
Why can very low salinity after flooding reduce coral survival?
It can exceed corals’ salinity tolerance and disrupt osmotic balance, causing stress.
Illustrative counts: 400 polyps extend at night and 80 by day. What feeding pattern is supported?
Greater night feeding: extended tentacles capture prey, and five times as many polyps are extended at night. Adapted skill: QCAA 2024 Paper 1 Q24.
What do a longer curve and a flatter curve respectively suggest on a rank-abundance plot?
A longer curve suggests more species; a flatter curve suggests greater evenness.
Where are the reef slope and lagoon located relative to the reef crest?
The slope descends seaward; the lagoon or back reef lies landward of the crest. Adapted skill: QCAA 2024 Paper 1 Q21.
How do hermatypic and ahermatypic hard corals differ?
Hermatypic corals contribute to reef formation; ahermatypic corals do not build extensive reef frameworks.
Why can the outer shelf support different coral communities from inshore reefs?
Distance from land influences sediment, nutrients, salinity fluctuations, light and exposure, favouring different species.
How can predation and competition reduce recruitment after coral larvae settle?
Predators remove recruits; competing organisms occupy space or overgrow them, lowering survival.
Illustrative reef data change from 12 species and coral dominance to 7 species and algal dominance. Identify two changes.
Species richness decreases by five; the dominant organism group shifts from coral to algae. Adapted skill: QCAA 2025 Paper 1 Q26.
Why can coral growth forms differ between a wave-exposed crest and a sheltered lagoon?
Different wave energy, light and sediment conditions favour different tolerances and growth forms; robust forms can better withstand strong waves.
Which ions combine to build a coral calcium carbonate skeleton? Give their charges.
Calcium ions, Ca2+, and carbonate ions, CO3 2-. Adapted skill: QCAA 2025 Paper 1 Q22.
Illustrative data: at depths 2, 6 and 12 m, oxygen is 6, 5 and 3 mg/L; living coral occurs only at 2 and 6 m. What can be inferred?
In these observations, living coral is associated with higher oxygen. The data suggest an oxygen limitation but cannot establish a universal threshold or exclude other depth-related factors. Adapted skill: 2025 Paper 1 Q24.
A supplied practice key says: branching with an axial corallite = Acropora; branching without axial corallites = Pocillopora. A specimen has axial corallites. What genus is indicated, and why use a key?
Acropora under this supplied key. A key uses diagnostic features rather than assuming all branching corals have the same genus. This is a simplified practice key, not a complete field key.
Why can nutrient runoff be associated with crown-of-thorns outbreaks and lower coral cover?
Nutrient enrichment can increase phytoplankton food for crown-of-thorns larvae, supporting larval survival; more adult coral predators can reduce coral cover. The association does not prove nutrients are the only cause.