The effects of partial mortality on corals in St. John, U.S. Virgin Islands and Moorea, French Polynesia.
Corals reefs are one of the most biologically diverse and productive ecosystems on earth, providing food and habitat for millions of organisms. However, these communities have undergone changes in structure and function due to natural and anthropogenic perturbations. Human-induced damage has been a recent focus in science; however, the damage caused by chronic, biotic disturbances such as predation, may also affect the fitness of corals. Predation is the interaction between a consumer and its prey species, and the direct consumption of live coral is known as corallivory. Until recently, the effects of vertebrate corallivores (fishes) were considered relatively unimportant in structuring coral reefs, even though corallivorous fishes are an abundant component in these communities. Corallivorous fishes can be classified into 3 groups based on their feeding strategies, with butterflyfish causing tissue damage, some parrotfish causing tissue and skeletal damage, and pufferfish causing more severe tissue and skeletal damage. Predation by these groups has been shown to negatively impact the fitness of some coral species by redirecting metabolic resources from growth to healing damage. In this study, I first explored the effects of multiple types of damage simulating predation by fish corallivores on the regeneration and growth of small colonies of massive Porites spp. and Pocillopora meandrina. Second, using two sets of experiments, I sought to determine whether there was a tradeoff between regeneration and growth of corals. This was done first by testing for the effects of 0, 1, 2, or 3 lesions on whole colony growth of massive Porites spp. Secondly, by comparing the regeneration of tissue and skeleton inside lesions simulating excavation damage to new skeletal growth elsewhere on the colony between 0, 1, 2, or 3 lesions. Linear extension was measured using a reference line that was deposited into the calcium carbonate skeleton of the corals in the beginning of the study as a way of measuring skeletal growth inside lesions (top of coral) and outside lesions (top and side of coral). Lastly, I explored the relationship of lesion area and number of lesions (i.e. partial mortality) among Porites astreoides and Siderastrea siderea at 5 and 10 m depths in St. John, US Virgin Islands. In Moorea, corals damaged to mimic excavating corallivorous fishes grew 27-37% slower than corals damaged to mimic browsing fishes, and 11-16% slower than corals damaged to mimic scrapers. The analysis of whole colony growth and linear extension outside the lesion (top and side) showed that lesion density had no effect on the growth or shape of the coral, although there was more new growth within lesions versus outside lesions on the top of damaged corals. For the St. John study, lesion area increased with colony area for both species at both depths. Furthermore, the lesion area per colony was larger at 10 m than at 5 m for both species. Porites astreoides had mostly basal lesions (categorized as type II lesions) at both depths; whereas Siderastrea siderea had slightly more lesions fully surrounded by living tissue (categorized as type I lesions) at 5 m depth and had more type II lesions at 10 m depth. The greater number of type II lesions at deeper sites could reveal the reduced capacity to regenerate lesions with reduced light and may also highlight the difference in the type of damage affecting corals at those depths. Together, the results from this study show that although excavation damage has the greatest affect on corals, browsing and scraping damage also affect the growth and regeneration of corals and thus, predation by these fishes should not be disregarded as a potential process that could alter reef ecosystems. Furthermore, the results of the linear extension study could provide evidence that considering growth and regeneration as two separate but related processes could aid in understanding how corals respond to certain disturbances. Lastly, using partial mortality to assess the ‘health’ of reefs may be helpful in explaining changes in the abundance of important reef species.
Thesis or Dissertation
California State University