Discoveries

Rolloliths of Adele Island

New record

Dr. Zoe Richards, Australian Museum

An interesting finding at Adele Island was the discovery of a unique rollolith habitat on the SW side of the island. Rollolith is the common name given to benthic organisms that grow as unattached free-living spherical colonies enabling live tissue to cover the entire colony surface. Rolloliths can be made of coral (corallith); red crustose coralline algae (rhodolith); bryozoans (bryoliths); barnacles (balanuliths) or vermetulid worms (vermetuliths). At Adele Island we found a mixed corallith and rhodolith assemblage. Among the coralliths we identified four different genera of coral (Pavona, Psammocora, Cyphastrea and Milleporina). We found Milleporinid coralliths and rhodoliths dominated the Adele rollolith habitat, however numerous colonies of the other 3 scleractinian corals were located indicating that they are by no means rare.

rhodolith
Rhodolith
Photo by Clay Bryce
Image copyright of WA Museum

The existence of free-living spheroidal growths of corals and calcareous red algae in shallow tropical reef environments was first noted at Vanuatu in 1885 and subsequently during the Siboga expedition to Indonesia in 1904. Recent paleontological studies have dated fossil coralliths as far back as the Eocene (approx 65 mya). Throughout the 70s, various papers arose about coralliths and rhodoliths conferring that they occur right across the globe in locations such as Bermuda, Florida, Brazil, Panama and Madagasgar. More recently, rhodoliths and coralliths have been recorded from the Cook Islands, Barbados and in the NW Indian Ocean. In Brazil, rhodoliths are so abundant that they are commercially mined.

milleporina
Milleporina
Photo by Roger Springthorpe
Image copyright of Australian Museum

The spherical growth form of coralliths is hypothesized to result from physical transport by wave action and currents, which are strong in the Kimberley region due to the large tidal exchange. It has also been hypothesized that frequent disturbance of bottom sediments by browsing fishes and gastropods prevents attachment to the substrate. The relatively uniform radial growth resulting from periodic movement inhibits prolonged growth on any particular side and it is even considered that tidal pull on individual grains of sediment at the coral interface is enough to maintain the constant movement of coralliths.

Pavona
Pavona
Photo by Roger Springthorpe
Image copyright of Australian Museum

In Australia, coralliths do not appear, on present records, to be particularly common however, they have been described as abundant on the reef flat at Heron Island and have been anecdotally recorded to occur at several Great Barrier Reef islands. One species of Cyphastrea is suggested to form coralliths at Cocos Islands and rhodolith beds are reported to occur off Western Australia. On current records, coral reef habitat dominated by a mixed assemblage of rolloliths appears to be relatively rare and such unique habitat has never before been recorded from the Kimberley Region. We will continue to search for coralliths and rhodoliths on future Kimberley expeditions to clarify the regional significance of the Adele Island rollolith habitat.

Psammocora
Psammocora
Photo by Roger Springthorpe
Image copyright of Australian Museum