Spatial extent, population size and demographics of Cycloseris distorta and C. fragilis within a large mesophotic mushroom coral bed off Ningaloo Reef, north-western Australia

Abstract Mesophotic mushroom coral (Fungiidae) beds form distinctive off-reef habitats in Indo-Pacific coral reef ecosystems. We mapped a large mushroom coral bed at Ningaloo Reef off north-western Australia. The 82-hectare (2700 m by 680 m) bed was comprised of Cycloseris distorta and C. fragilis , with densities at some sites more than 1100 m −2 (mean 774 m −2 ) and an estimated population of 653 million individuals making it the largest such bed recorded. The dense Cycloseris bed was seaward of, and adjacent to, the steep slope drop-off from a shallower (ca. 25 m deep) reef bank, which itself was offshore of the main reef slope. The bed occupied a narrow depth range (35.5–40.7 m), suggesting it is located where conditions are optimal and/or where competition from other sessile benthos is minimal. Both Cycloseris species are known to reproduce predominantly asexually with lobes breaking off along suture lines and becoming clones, which regrow more lobes. The size of Cycloseris in our study varied from 10.8 to 64.9 mm, with each coral comprising between one and 13 lobes (median 4 lobes). We also examined the death assemblage in the Cycloseris bed, with dead corals comprising 58% by number and 34% by weight of all live and dead corals. Although the Cycloseris bed lies within the Ningaloo Marine Park, it falls outside any fully protected or sanctuary zoned areas and given its size and unique features it warrants greater protection than is the case at present.