Sands of time
10 September 2010
A little-known species of marine animal could provide a clue to the evolution of coral reefs over millions of years, says Associate Professor James Reimer of Japan's University of Ryukyus.
Professor Reimer is studying marine life on the CReefs field expedition to Lizard Island on the Great Barrier Reef this month.
His interest is Zoantharia, an order of colonial animals related to corals and sea anemones.
Zoanthids form colonies of polyps on the ocean floor or on other reef structures. Unlike hard corals, zoanthids do not grow skeletons, but many incorporate small pieces of sediment, sand and rock into the tissue of their body walls and the button-like, tentacled disc atop each polyp. There are some species that do not incorporate sand at all.
Professor Reimer is particularly looking for an unusual group of zoanthids, the genus Neozoanthus, which incorporate large, irregular particles of sand into their body walls, but do not have sand in the disc tissue.
Specimens of these zoanthids were found by scientists in Madagascar in 1973. Professor Reimer found similar species on the CReefs expedition to Heron Island last year, and more recently in Okinawa in Japan. He expects to find related species at Lizard Island.
The study of these species could provide insight into the evolution of coral reefs
"Evolution doesn't always take the simplest route," Professor Reimer says.
"This group of zoanthids looks like an intermediate stage between zoanthids that don't use sand and those that do, but our DNA analysis suggests that's not the case at all.
"It seems that most zoanthids had sand, then one group lost sand, and some species, the ones I'm looking for, partially regained it.
"This suggests that the zoanthids' use of sand might not be a particularly complex characteristic, but rather something they can turn on or off rather quickly, like a switch," he says.
Of course, this ‘quick switch' still takes place over several million years, but in the 600-million-year history of coral reefs, this is a relatively short time.
"My theory is that at one point in time, when corals couldn't build skeletons well, perhaps due to ocean acidification or other natural environmental disturbances, the zoanthids evolved. The sand provides structural strength to the zoanthids, an alternative to the way hard corals build skeletons, but with a similar outcome," Professor Reimer says.
"Zoanthids are found in a range of environments throughout the oceans, in Australia, Japan, Singapore, the Arctic, the Antarctic, Canada, and even at depths to 5600 metres – but the species that don't have sand are only found in coral reefs in shallow waters in warmer areas, and they all have symbiotic relationships with single-celled algae, so that may be significant," he says.
Professor Reimer will study the diversity of species of zoanthids found around Lizard Island, and compare this to Japan, Heron Island and Ningaloo Reef.