Injury and regeneration of reef-crest corals at Lizard Island, Great Barrier Reef, Australia

Hall, Vicki R (1998) Injury and regeneration of reef-crest corals at Lizard Island, Great Barrier Reef, Australia. PhD thesis, James Cook University.

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Corals are frequently injured by natural processes and human activities. The response of scleractinian corals to damage is dependent on the nature and extent of damage, the characteristics of the injury, the life-histories of the coral, and the prevailing abiotic and biotic conditions. In this thesis I have examined several aspects of injury including (1) the nature and extent of natural injury, (2) the response of corals to injuries with different characteristics and (3) the influence of morphology and life-history in response to damage.

The spatial and temporal patterns of coral injury were recorded to determine the nature and extent of damage in common reef-crest corals at Lizard Island. The total amount of partial mortality on reef-crest corals was low (<2%) although there was a three-fold difference among sites. Sites with low partial mortality had reef-crest assemblages dominated (both numerically and in cover) by tabular and bushy corals. These corals have low levels of partial mortality, and on average, fewer small colonies with injuries. Conversely, the site where the partial mortality was three times higher had a lower abundance and cover of tabular corals, and an increase in the number and cover of massive and digitate corals. Massive and digitate corals, on average, have a higher amount of partial mortality and more small colonies with injuries.

The amount of injury present on a colony at a particular time is a balance between vulnerability (i.e. frequency of injury and resilience to damage) and recovery rate. An investigation into the patterns of injury over time showed that vulnerability to damage and recovery of injuries was species specific. In general Goniastrea retiformis had a high number of old injuries, a slow regeneration rate, and was injured infrequently, suggesting that injuries tended to accumulate on colonies over time. The addition of new injuries was also low for Acropora gemmifera, however colonies had few pre-existing injuries and faster recovery rates, reducing the accumulation of injuries on colonies. The injury dynamics for A. hyacinthus differed between censuses because of a change in injury regimes from routine to catastrophic, the latter regime caused by an outbreak of Acanthaster planci. Under routine conditions, there were few pre-existing injuries on colonies, a moderate addition of new injuries, and rapid regeneration, suggesting a fast turn-over of injuries. Under catastrophic conditions, there were many more pre-existing injuries, a high number of new injuries, and more injuries increasing in size than recovering, resulting in an accumulation of injuries.

The regeneration of injuries was influenced by the characteristics of the injury including initial size, type, and position within the colony The complete regeneration of an injury was more probable for small injuries (0 - 4 cm2) than larger injuries. However, recovery rates were also dependent on the type of injury as scraping injuries had a much faster regeneration rate than tissue mortality or breakage. Additionally, recovery was influenced by the position of injuries within colonies for one species Porites mayeri where the rate of regeneration of central injuries was greater than edge injuries. Conversely, the recovery of central and edge injuries was similar for A. robusta, A. hyacinthus, A. palifera, Pocillopora damicornis, and Porites lichen. Variations in levels of partial mortality, zones of tissue from which regeneration can take place, degrees of settlement by other organisms, intensities of damage, and amounts of resources available for regeneration all contributed to the differences in recovery rates found between injuries with varying characteristics.

The regeneration of injuries requires resources that are in limited supply. In this study, there was a marked effect of injury on reproduction for A. hyacinthus, A. gemmifera and G. retiformis, inferring a trade-off between reproduction and regeneration. Presumably the resources usually available for gamete production are being reallocated towards polyp regrowth and defence against fouling organisms. In contrast, injury had no effect on the survival or growth of colonies over nine months for the three species. This result suggests that future reproduction is being preserved through the iteration of new polyps but at the expense of current reproduction. It also suggests that these species are resistant to damage since their survival was unaltered by damage in the short-term.

Species resistant to damage have evolved two alternative, but not mutually exclusive, strategies in response to injury. Corals can invest resources in defensive mechanisms to avoid damage (avoidance strategies) or regrow lost parts after injury has occurred (tolerance strategies). Both strategies were utilised by corals in this study, although the amount of investment in either strategy varied. Generally, the longer-lived species, G. retiformis and A. gemmifera, seemed to invest more resources towards defence than the shorter-lived A. hyacinthus since the number of new injuries present on colonies was higher for the latter species. Conversely, the shorter-lived coral invested more in tolerance strategies by responding to infrequent damage events or minimal tissue losses with rapid regrowth. The cost of such a strategy is that shorter-lived species are more vulnerable to repetitive injury.

Experimental studies showed that branching species had more regrowth potential than massive and semi-massive species supporting the hypothesis by Jackson (1979) that morphology plays a role in the pattern of investment in regeneration and defence. The morphology of a coral influences its longevity, reproductive output, growth rate, and other life-history processes including regeneration. Consequently, the morphological strategy of an organism has evolved over time in response to a large number of biotic and abiotic processes including partial mortality.

In conclusion, this study on injury and regeneration of scleractinian corals has increased our knowledge of the underlying mechanisms that affect the recovery of corals from damage, and has provided a basis for understanding the consequences of different injury regimes on coral reefs. This is important because injury can adversely affect corals at the individual, population and community level and thus impact on the general ecology of coral reefs.

Item ID: 8
Item Type: Thesis (PhD)
Keywords: Reef-crest corals, Regeneration, Lizard Island, Great Barrier Reef
Date Deposited: 02 Nov 2006
FoR Codes: 06 BIOLOGICAL SCIENCES > 0602 Ecology > 060207 Population Ecology @ 50%
06 BIOLOGICAL SCIENCES > 0602 Ecology > 060205 Marine and Estuarine Ecology (incl Marine Ichthyology) @ 50%
SEO Codes: 96 ENVIRONMENT > 9608 Flora, Fauna and Biodiversity > 960808 Marine Flora, Fauna and Biodiversity @ 100%
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