The role of macroalgae and the corallivorous fireworm Hermodice carunculata on coral reef resilience in the Caribbean

Abstract

This thesis is composed of five chapters representing studies that investigate the impact of two important stressors strongly influencing coral reef resilience in the Caribbean: macroalgal competition with corals and corallivory by fireworms. Macroalgae are superior competitors for space in coral reefs due to their 1) high ability to adapt to changing environmental conditions such as elevated nutrient availabilities and sea surface temperatures, 2) optimized reproductive strategies and life cycles, and 3) potent allelochemical characteristics during interaction processes. Invertebrate corallivory is an important stressor of present-day reefs, in which its impact can lead to fundamental changes in reef state. During a preliminary visit to Curaçao, Netherlands Antilles, the designated study area for this thesis, we observed high abundances of macroalgae and an invertebrate corallivore, the bearded fireworm Hermodice carunculata, particularly in the presence of macroalgae. Together with gastropods, fireworms constitute potentially the most influential corallivorous organisms in Caribbean reefs, and population outbreaks have been associated with impaired coral recovery after disturbances. Recent work showed that this invertebrate corallivore may also act as a vector for coral bleaching pathogens in the Mediterranean, but its importance with regard to coral reef resilience has not been addressed since the late 1960s. Reciprocally, impaired coral reef resilience will inevitably lead to 1) declining reproductive output and recruitment, 2) higher susceptibility to and longer regeneration times after disturbances such as mass coral bleaching and disease events, and 3) the loss of diversity and structural integrity. Alternate and stable state macroalgae-dominated reefs deprived of original functionality are of major concern and have been observed throughout the Caribbean, especially under declining environmental conditions, which hold true for most Caribbean reef systems. The fringing reefs of Curaçao, Southern Caribbean, are considered relatively intact for Caribbean standards, i.e. still in a coral-dominated state, and therefore represent suitable study sites to carry out experimental work. Although detailed studies exist with regard to macroalgal characteristics, very little is known about the corallivore H. carunculata, and no information exists with regard to their specific effects on scleractinian corals, particularly during coral-algal interactions, and their ecological role during impaired coral reef resilience scenarios. The objectives of my thesis thus were to investigate separate and synergistic effects of these two prominent stressors on Caribbean reef systems. As a fundament, we provide the first comprehensive ecological study on principle characteristics of the corallivore H. carunculata. Using multiple approaches including in situ experiments, quantitative analytical tools and molecular techniques, we investigated its ecological role within coral reefs and determined its origin and distribution based on its phylogeny. Then, we explored the specific effects of macroalgal competition and corallivory on the major reef building coral in Curaçaoan reefs. Supplementary in situ and laboratory experimental studies provided detailed information on the kind, diversity and severity of their detrimental impacts for scleractinian coral communities. Ultimately, potentially synergistic effects of both stressors for Curaçao s major reef building coral were identified under natural conditions and discerned using specific laboratory experiments.