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The ecology and conservation of the endangered soft coral Dendronephthya australis
Thesis   Open access

The ecology and conservation of the endangered soft coral Dendronephthya australis

Meryl Frances Larkin
Southern Cross University
Doctor of Philosophy (PhD), Southern Cross University
2024
DOI:
https://doi.org/10.25918/thesis.404
Appears in  Recent Southern Cross PhD Theses
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Larkin MF 2024 PhD7.07 MBDownloadView
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Abstract

Dendronephthya australis Biology Ecology Restoration Transplant programs Conservation
Habitat-forming benthic organisms are key components of marine ecosystems in both tropical and temperate reefs. When foundational benthic species rapidly decline, populations of reliant species are also impacted. Research is vital to not only establish rates and causes of declines in foundation species, but also to understand the biology of these species to better inform conservation strategies and potential avenues for restoration. Dendronephthya australis (Family Nephtheidae) is an Endangered soft coral that provides habitat for unique marine communities and is an important contributor to the marine ecosystem in Port Stephens, New South Wales, Australia, where the largest known populations of this soft coral have historically been found. Prior to this study, a rapid decline in D. australis populations had been observed, though not quantified. As little was known about its biology and ecology, insufficient data were available to establish viable restoration approaches. This study explored the spatial ecology and reproductive biology of D. australis to facilitate future restoration and conservation measures. Changes in D. australis distribution and abundance relative to historical baselines were assessed using towed video in 2019. A Forest-based Species Distribution Model was used to determine the variables most likely correlated with the presence of soft coral colonies and General Linear Models (GLMs) were used to determine which of these was likely associated with coral decline. Further mapping was completed after unprecedented La Niña-related flood events in 2021-2022, to document further impacts on D. australis populations. Mapping in 2019 revealed that in the eight years since the previous targeted mapping study (2011), using the same methods, the areal extent of soft coral aggregations in Port Stephens declined by 67.5%, from 28,600 m2 to 9,300 m2. These data contributed to D. australis being listed as an Endangered species in NSW. GLMs identified that coral losses were strongly correlated with sediment mobilisation, distance from shore, change in depth, and change in current velocity. Continued monitoring after the flood events in 2021-2022 revealed the disappearance of all remaining D. australis aggregations in the Port Stephens estuary and highlighted the urgent need for restoration. Aquarium-rearing methods were explored as a potential option for using soft coral cuttings in transplant programs. These investigations demonstrated that D. australis fragments in captivity could attach to a moveable substrate, providing a potential method for species restoration via transplantation. The first observations of reproductive processes were made whilst cuttings were in captivity, expanding options for aquarium-based rearing methods. Observations in aquaria and historical images identified that D. australis is a gonochoric broadcast spawner, with reproductive activity visible throughout the Austral summer months and spawning occurring during neap tides. Larvae were propagated in aquaria using in vitro fertilisation techniques, which then settled and metamorphosed to polyps. Juveniles transplanted in the field grew to a maximum 435 polyps in size after six months, thus demonstrating another viable approach for restoration of this species. In summary, this study quantified the decline of D. australis aggregations within the Port Stephens estuary and provided vital data for the ongoing management of the species. Effective methods for species restoration were established, utilising both sexual and asexual-based strategies. This work substantially advanced understanding about the ecology of D. australis and identified promising avenues for restoration to aid its recovery.

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