Effects of coral-dwelling damselfishes' abundances and diversity on host coral dynamics

Chase, Tory John (2019) Effects of coral-dwelling damselfishes' abundances and diversity on host coral dynamics. PhD thesis, James Cook University.

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View at Publisher Website: https://doi.org/10.25903/yns5-yq13


Interspecific interactions, particularly positive interactions, between organisms and their physical environment are important forces in shaping ecological diversity and ecosystem structure. In coral reef ecosystems, the associations between habitat-forming corals and coral reef fishes have critical implications for the structure and function of coral reef ecosystems. Coral-dwelling damselfishes rely on branching corals for shelter and confer benefits to their host corals that promote enhanced performance, growth, and colony health. However, there is variability (in strength and symmetry) in both fish-derived and coral responses to abiotic factors and partner ecology. Global environmental change is likely to considerably disrupt fish-coral interactions on reefs through reef degradation, coastal sedimentation, and severe widespread bleaching events. Prior to collapse, however, fish-coral interactions have the potential to act as stabilizing forces on reefs, promoting coexistence, and enhancing the coral holobiont during accelerated environmental change. Thus, the overall focus of this thesis was to understand the relationship between coral-dwelling damselfishes and their small branching coral colony hosts. Specifically, I investigated the prevalence of these fish-coral associations across space, the variations in the intensity of damselfish-coral interactions over time and evaluated the influence of these coral-dwelling damselfishes on the health of their host corals under two types of environmental stress.

In Chapter 2, I addressed one of the critical first steps to understanding the magnitude by which coral-dwelling damselfishes impact coral health by establishing the abundance and prevalence of select, fish-coral interaction pairs across different coral colonies, habitats, and seascapes. Through a series of underwater surveys at locations spanning > 1700 km of the Great Barrier Reef, I aimed to: (a) determine if suitable coral habitat governs patterns in damselfishes' distributions and abundances, and (b) quantify variations in damselfish species-specific biomass among coral colonies species. The abundance of fish-coral associations varied with respect to exposure level and habitat with an overall average prevalence of ~30% occupancy, with biomass hotspots confined to sheltered lagoon sand patch and reef slope habitats. Further analysis of colony microstructure traits revealed that isolation from adjacent colonies, branch spacing patterns, and colony orientation governed fine-scale usage. The research presented in Chapter 2 illustrates that coral occupancy (coral-dwelling or sole habitat use) varies significantly by damselfish species, and subsequent fish-derived services are confined to specific reef habitats.

While coral-dwelling damselfishes are intimately associated with branching corals, interspecific behavioural variation can alter the nature and strength of these interactions with corals, thereby altering the dynamics of small-scale coral association networks and benefits conferred to host corals. Chapter 3 used in-situ observations to explore interspecific differences in diurnal and nocturnal behaviour among five coral-dwelling damselfish species. Resident damselfishes displayed marked differences in colony interaction and usage, with Dasycllus species exhibiting frequent and sustained interactions with host corals. Pomacentrus species displayed weaker associations and behaviours consistent with commensalistic interactions. Host coral bleaching status altered damselfishes' interactions with colonies, forecasting shifting interchanges between fishes and corals under future stress conditions. Quantifying these focal fishes' behaviours through small-scale observations is relevant to interspecific interactions and coral holobiont persistence, as environmental stressors alter the prevalence of coral-damselfish interactions and the intensity of associated mutualistic services.

Global environmental change, particularly in the forms of amplified sedimentation and elevated sea-surface temperatures, could pose to significantly alter how these fish-coral interactions function in isolation and as components of the entire coral holobiont. Many species interactions, mutualistic ones in particular, arise from the ability of species to modify local conditions and diminish stress for their own benefit as well as for their partners, thereby conferring resilience. In Chapters 4 and 5, I evaluated the influence of coral-dwelling fishes (Dascyllus aruanus and Pomacentrus moluccensis, selected due to their abundance and behaviours exhibited in Chapters 2 and 3) on the health of their host corals (Pocillopora damicornis) under two types of environmental stress. When exposing the fish-coral system to daily manipulated severe sedimentation stress in Chapter 4, damselfishes were able to significantly reduce sediment accumulation and sediment-induced partial mortality on coral hosts, 2-10-fold more, compared with fish-vacant colonies. Colonies with Dascyllus aruanus exhibited up to two-fold higher chlorophyll and protein concentrations under sediment conditions compared with other treatments, reinforcing the positive nature and benefits connected with a frequent and sustained (strong) interaction with host colonies. Further linking these results to the behaviour of the damselfish species (Chapter 3), diurnal and nocturnal position of D. aruanus and P. moluccensis in aquaria, helped explain the species-specific services rendered. In Chapter 4, I demonstrated that fish mutualisms may be critical for maintaining coral health and resilience under chronic and severe sediment stress and indicated that some mutualistic or facilitative interactions may become more important for species persistence as stress levels increase.

Many studies have independently investigated the effects of increased sea-surface temperatures on fishes and coral bleaching, but little is known about the impacts of coral-dwelling damselfishes on the health of their coral hosts, during and after a thermal-bleaching event. With many services that damselfishes provide to their host colonies, especially those that mimic natural mechanisms mitigating external stress, in Chapter 5, I hypothesized that colonies with symbiont damselfishes would bleach less and recover more quickly during thermal bleaching events, compared to vacant corals, due to key services of enhanced water flow and nutrients. During a natural thermal anomaly, it is evident that P. damicornis with damselfish that are subjected to temperature stress have higher Symbiodinium (+25%), chlorophyll (+30%), and tissue proteins (+57%). These results were reflected in a manipulated thermal bleaching experiment in aquaria, where corals with damselfish subjected to temperature stress again had significantly more Symbiodinium (five-fold), chlorophyll (nine-fold), and tissue biomass (three-fold) compared with vacant colonies during the recovery period. Tissue component differences translated into considerably higher photosynthetic rates in P. damicornis colonies with fish, compared with non-damselfish colonies. However, from the in-situ results from the 2016 bleaching event, it is evident that this fish influence on colony susceptibility/resilience and recovery operates only under moderate level stressors, as severe bleaching events overwhelm the coral holobiont, rendering fish-services insufficient to maintain coral health.

This thesis reveals the importance of resident fishes as a fundamental aspect of the dynamic interface between corals and the abiotic environment. Although limited spatially across reef seascapes, and heavily dependent upon the species-specific behaviour of fish partners, these findings suggest that certain coral-dwelling damselfishes have the ability to mediate the impacts of environmental change with regards to coral colony stress susceptibility and survival.

Item ID: 64231
Item Type: Thesis (PhD)
Keywords: behavioral trade-offs, biological interactions, colony usage, coral reefs, coral−fish associations, damselfish, fish behavior, habitat structure, interspecific variation, prevalence, species coexistence, symbiosis
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Copyright Information: Copyright © 2019 Tory John Chase.
Additional Information:

Four publications arising from this thesis are stored in ResearchOnline@JCU, at the time of processing. Please see the Related URLs. The publications are:

Chapter 2: Chase, Tory J., and Hoogenboom, Mia O. (2019) Differential occupation of available coral hosts by coral-dwelling damselfish (Pomacentridae) on Australia's Great Barrier Reef. Diversity, 11 (11). 219.

Chapter 3: Chase, T. J., Pratchett, M. S., and Hoogenboom, M. O. (2020) Behavioral trade-offs and habitat associations of coral-dwelling damselfishes (family Pomacentridae). Marine Ecology Progress Series, 633. pp. 141-156.

Chapter 4: Chase, T. J., Pratchett, M. S., McWilliam, M. J., Hein, M. Y., Tebbett, S. B., and Hoogenboom, M. O. (2020) Damselfishes alleviate the impacts of sediments on host corals. Royal Society Open Science, 7. 192074.

Chapter 5: Chase, T.J., Pratchett, M.S., Frank, G.E., and Hoogenboom, M.O. (2018) Coral-dwelling fish moderate bleaching susceptibility of coral hosts. PLoS One, 13 (12). e0208545.

Date Deposited: 01 Sep 2020 22:37
FoR Codes: 06 BIOLOGICAL SCIENCES > 0602 Ecology > 060205 Marine and Estuarine Ecology (incl Marine Ichthyology) @ 35%
06 BIOLOGICAL SCIENCES > 0602 Ecology > 060201 Behavioural Ecology @ 35%
05 ENVIRONMENTAL SCIENCES > 0502 Environmental Science and Management > 050202 Conservation and Biodiversity @ 30%
SEO Codes: 96 ENVIRONMENT > 9608 Flora, Fauna and Biodiversity > 960808 Marine Flora, Fauna and Biodiversity @ 35%
96 ENVIRONMENT > 9605 Ecosystem Assessment and Management > 960507 Ecosystem Assessment and Management of Marine Environments @ 30%
97 EXPANDING KNOWLEDGE > 970106 Expanding Knowledge in the Biological Sciences @ 35%
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