Coral reproduction Flashcards
(25 cards)
What is asexual reproduction?
It involves pieces breaking off from a colony and potentially reattaching to a substrate to grow into new, mature colonies. This can occur via fragmentation.
These new colonies are genetically identical to the original colony.
What is a genet?
A group of genetically identical individuals that originate from a single ancestor through asexual reproduction.
What are the advantages of asexual reproduction for corals?
- Fragments have the potential to survive independently and reproduce sexually, benefiting the genet
- The mortality risk is spread across numerous fragments, reducing overall genet loss
- Bypasses/postpones size-limits in tree-like colonies
- Can extend local distribution and abundance, allowing live fragments to move passively up to tens of meters.
- Allows colonisation of habitats where larvae are unable to settle, such as sandy areas.
- Larger fragments are more likely to tolerate unstable sediments better than larvae due to size and elevation
What are the disadvantages of asexual reproduction for corals?
- Tissue loss during fragmentation
- High initial mortality; many fragments die or are injured.
- Fragments can be transported to unfavourable habitats.
- Survivorship is strongly size-dependent, -> small fragments have a higher rate of mortality.
- Temporary (until damaged tissue is regenerated) or permanent infertility of fragments can suppress reproductive output of the genet -> this is a substantial cost
- Fragments are vulnerable to damage by severe hurricanes, from which they recover very slowly.
- Damaged tissues are vulnerable to disease, and exposed skeleton is often colonized by fouling organisms and bioeroders.
- High mortality rates can occur due to abrasion, collisions, polyp disorientation, or inversion during storms.
What are the advantages of asexual reproduction for the reef?
- Fragments can be successfully seeded onto degraded reefs to promote small-scale recovery.
- Fragments can be important geologically in promoting reef growth and extension.
What are the disadvantages of asexual reproduction for the reef?
- The magnitude of rubble piles, particularly at the base of reefs and intertidally, points to a low rate of survival of fragments following cyclones, when breakage and transportation are extreme.
- High mortality of fragments adds to reef rubble rather than contributing to live coral cover – excess rubble accumulates
What are the three key factors that influence survivorship of coral fragments after asexual reproduction?
- Species
- Fragment size
- Reef habitat
What can vary between species that increases the survivorship of coral fragments after asexual reproduction?
Morphological differences.
3D species survive better than more 2D species due to increased light and food access, whilst reducing abrasion and sedimentation near the substrate
Are large or small fragments better at increasing the survivorship of coral fragments after asexual reproduction?
Larger fragments survive better - smaller fragments incur greater damage in proportion to their size
What reef habitat is best for increasing the survivorship of coral fragments after asexual reproduction? What is the worst?
The reef flat is best (harder substrate and reduced shading from tabulate corals).
The reef slope has the lowest survivorship due to unfavourable micro-habitats and lower light levels.
What is the significance of reduced fecundity of asexual fragments?
Only species with higher fragment survivorship and associated higher likelihood of resuming sexual reproduction are likely to benefit overall from fragmentation as an adaptive advantage.
Otherwise, fragmentation may represent a substantial loss of tissue and reproductive potential, rather than a successful adaptive strategy.
What is sexual reproduction in corals?
A strategy that involves gametogenesis and fertilisation, resulting in genetically unique (planktonic) larvae that must eventually settle and metamorphose to develop a primary polyp.
In colonial corals, this primary polyp will subsequently divide to form a (clonal) coral colony.
What are the two types of coral sexuality?
- Hermaphroditic
- Gonochoric
What are hermaphroditic corals and what are the different types?
Polyps within a colony have both female and male functions.
- Simultaneous hermaphrodites (most common) - release egg and sperm in a single breeding cycle
- Protandrous simultaneous hermaphrodites (only male until a certain size is reached, probably because the female function is more costly)
- Sequential protandrous hermaphrodites - start as male, then become female
What are gonochoric corals?
Polyps within a colony only express one sex throughout their life, so are either male or female.
What is a mixed breeding system example?
Where some polyps are male, some are female, within the same colony.
What are the two reproductive modes in corals (in terms of larvae dispersal)?
- Brooding
- Broadcasting
What is brooding as a reproductive mode?
Involves internal fertilisation, where the embryo develops within the polyp. The developing coral is then released as motile planula larva.
What is broadcasting as a reproductive mode?
This involves external fertilisation, where the embryo develops in the water column after gametes (eggs and sperm) are released into the water. This is the most common mode in Scleractinians.
What are the positives of gonochoric and hermaphroditic spawning?
- Produce large numbers of sexual propagules with wide dispersal potential
- Tend to produce greater numbers of asexual fragments compared to brooding species (increasing resilience)
- May return to former abundance levels faster than brooding species
What are the negatives of gonochoric and hermaphroditic spawning?
- Synchronous spawning is important – fertilisation success is greatly diminished with low colony numbers due to colony dilution
- Smaller larvae typically without zooxanthellae (see above) -> therefore rely on maternal energy reserves and as a result have limited dispersal
What are the positives of gonochoric and hermaphroditic brooding?
- Greater capacity to recruit and survive in marginal conditions
- Potentially produce sexual recruits and asexual fragments with a greater survival capacity
- Typically larger and symbiotic larvae (autotrophic) -> greater potential for long-distance dispersal
Increased probability of survival of sexual recruits and asexual fragments observed in warming disturbances
What are the negatives of gonochoric and hermaphroditic brooding?
- Took longer to return to former abundance levels after bleaching compared to broadcasting species
- Tend to produce fewer asexual fragments compared to broadcasting species
What does synchronous spawning mean?
Synchronous spawning – the coordinated release of gametes (sperm and egg) by corals.
This requires precise timing, often controlled by environmental cues, and is a crucial adaptive advantage for broadcast spawners to increase likelihood of fertilisation.
