Curvy in all the right places

Phil Bock and Kevin Tilbrook search for bryozoans.
Image: Gary Cranitch.


22 November 2010
Up to 40 new bryozoan species may be recorded just from this one CReefs field trip to Heron Island, according to the Museum of Tropical Queensland Curatorial Fellow Kevin Tilbrook; half of these will probably be described as new to science, while others are new records for Australian waters.
Dr Tilbrook is a taxonomist specialising in bryozoans – tiny invertebrate organisms that create often quite substantial colonies on rocks or dead coral in coral reefs.
"Bryozoans are phenomenally diverse in all marine habitats; they seem to be particularly diverse on coral reefs," Dr Tilbrook says. "And they play a major role."
"Most of them produce calcareous skeletons, which form part of the coral reef framework; some act like nets to trap sediment, accumulate it and become an integral part of the reef matrix. Ecologically, bryozoans are food and hosts for other animals, and they are a substantial part of the biodiversity of the reefs," he explains.
"But historically, because they're small and they sit on the undersides of coral, they've been relatively overlooked. Approximately 125 species of bryozoans have previously been described from Heron Island – but those specimens were collected only from snorkelling and intertidal reef walks. We know that the diversity of bryozoans appears to increase substantially below ocean depths of 10 metres, so my interest on this trip is to SCUBA dive in order to access a whole different suite of the fauna that is potentially in this area," he says.
Dr Tilbrook says that while he does not expect to collect every species of bryozoan living on the reefs, the fieldwork will allow him to make an informed estimate of the total number of species in the area.
The likely total number of species can be calculated using a scientific model known as the collectors' curve. This approach weighs the effort involved, such as the number of specimens picked up or the search time while diving, against the number of new species or records identified from each dive.
"You might go for one dive and find 10 species. On the second dive and you might find 20 species, but 10 of those you had found already, a net gain of 10; and on the third dive you might find 25 species, but 20 of those you found already, a net gain of five. There comes a point where there are diminishing returns from the collecting effort; so for X amount more dives, you would only potentially get two or three more species recorded," Dr Tilbrook explains.
At this point, scientists assume that their collection of species recorded is approaching the true total diversity.
"We plot our collection data to see how it peters out, and extrapolate that to get a good estimate of the total fauna. After this field trip, having sampled for two weeks at depths that have not been studied here for bryozoans previously, we might have reached the plateau of our collectors' curve – and with some confidence estimate that there are in the region of 200 species of bryozoans just for this island," Dr Tilbrook explains.
"And this island isn't necessarily representative of the Great Barrier Reef as a whole, because diversity appears to change latitudinally across its length. So while we've found a substantial number of species here, I don't think it is even close to the total bryozoan fauna off the tropical Queensland coast. The total bryozoan fauna from the Great Barrier Reef is probably in the order of 700 species," he says.
Dr Tilbrook has very recently transferred from the Natural History Museum, London to take up a three-year Curatorial Fellowship at the Museum of Tropical Queensland, in Townsville. The position has been created and funded equally through CReefs Australia and the Queensland Museum Foundation as part of CReefs' commitment to capacity building in the field of taxonomy in Australia.