Coral reefs comprise one of the most diverse habitats in the ocean, hosting a large variety of colourful animals. These reefs form intricate three-dimensional structures built by stony corals whose hard, calcareous skeletons create refuges for thousands of species.
There are many different coral types each with numerous species all belonging to the phylum Cnidaria. The scleractinians or stony corals (hexacorals) are the main builders of the reef, but there are also the blue corals (helioporids, octocorals), the fire corals (milleporids, hydroids) and the lace corals or stylasterids (hydroids).
Unlike fire corals and some other corals, stylasterids have spectacular bright colours, are attractive for the aquarium hobbyist and when polished are utilised for jewellery or as souvenirs. Due to over exploitation through these economic uses stylasterids are considered threatened with extinction, so trade is strictly controlled under the Convention for International Trade in Endangered Species (CITES). Stylasterids differ from the other reef building corals in that they do not host symbiotic algae (zooxanthellae). Therefore, they are not susceptible to ‘bleaching’, and are able to live in deep water, where they originated, and this is the environment in which stylasterid corals are most diverse and abundant.
While the majority of stylasterids occur in the deep sea, shallow coral reefs also host many species. Globally there are about 330 species belonging to 29 genera with the most biodiverse area for these corals located in the deep sea of the New Caledonian region. However, only scattered information is available about the diversity, biology and ecology of stylasterids found in shallow tropical areas such as the Great Barrier Reef and Australian waters generally. Only eight shallow water species have previously been identified around Australia: three Distichoporaand five Stylaster, these are also the genera most frequently recorded in the shallow Indo-Pacific region. Moreover, the Australian Museum collection holds no colonies from Queensland, including Lizard Island although they do occur there as indicated in the Lizard Island field guide.
To address this knowledge gap, I received a John and Laurine Proud Fellowship. The main goal of my research was to improve understanding of the taxonomy, ecology and distribution of the stylasterid corals from Lizard Island and nearby reefs.
During late 2018 I spent 25 days at the Lizard Island Research Station studying stylasterids of the area with the help of Dr. Penny Berents as dive buddy and assistant in laboratory work. In total we made 33 dives investigating 29 sites, 24 around Lizard Island and five along the outer reef (Fig. 1A).
Stylasterid corals were absent at only four sites and otherwise spread over a wide area. At Lizard Island, the diversity and the abundance of colonies was typically less in the southwest and northeast areas of the island. On the southwest side of the island there is a lagoon where the depth and the current are lower, and this could explain why diversity and density is reduced. Most colonies occur where there are well developed healthy reefs with many small caves and crevices available, the perfect habitat for stylasterids (Fig. 2A, B).
Preliminary identifications indicate the presence of up to 11 species, an increase of 38% from this relatively small area when compared to previous records for all of Australia! This includes five Stylasterand six Distichopora species (Fig. 3A, B, C).
In the laboratory the specimens were analysed for the identification. I also had the opportunity to see them alive and collect information about their soft tissue, polyp morphology, stinging cells (nematocysts) and reproductive aspects (Fig. 4A, B). Identification of stylasterids is currently based mainly on the study of the skeletal morphology, and information about the soft tissue is very limited but I have found that stinging cells are useful to discriminate different species.
Additionally, at Lizard Island many stylasterid colonies were found to have associated with them other animals such as snails, boring algae and barnacles (Fig 5A, B).
A total of 149 specimens were collected and deposited in the Australian Museum in Sydney including alcohol preserved samples for genetic analysis. Further analysis is underway to determine if they include any new species (i.e. any not previously “described” taxonomically). Genetic studies are also planned in a separate study to understand the relationships among the shallow water tropical species and validate the currently reported wider distribution of some species. This checking is necessary because the larval dispersion of stylasterids is more typically very low.
I sincerely thank the John and Laurine Foundation for supporting this research. This Fellowship has provided the opportunity to collect information about the diversity, distribution and relationships of tropical shallow water stylasterids from the northern part of the Great Barrier Reef with specimens for future reference being deposited in the Australian Museum. This also adds important new records to the list of the Australian marine fauna. Additionally, the opportunity to study live specimens while at Lizard Island Research Station has allowed me to document the morphology of the soft tissue which is an important step in understanding stylasterid biology. Overall, information obtained from this study represents an important milestone for increasing knowledge about the biogeography, ecology and taxonomy of shallow water stylasterids, an otherwise poorly known but significant and threatened component of coral reef ecosystems.
Dr Daniela Pica
Post-Doc at Laboratory of Zoology at Università Politecnica delle Marche (Italy), with assistance from
Dr Stephen Keable
Collection Manager, Marine Invertebrates, Australian Museum.