Plenty of fish in the sea

Coral reef off Heron Island.
Image: Gary Cranitch.


16 November 2010
For University of Queensland Associate Professor Thomas Cribb, one of the attractions of studying parasites in marine fishes is that there are so many to find.
There are more than 27,000 fish species identified worldwide, and almost every fish carries several types of parasite. For Tom, who is collecting parasitic flatworms of the class Trematoda during the CReefs field expedition to Heron Island, this means there is a great diversity to be studied.
"There are thousands of species of parasites in coral reef fishes. The diversity of parasites is certainly richer than that of their fish hosts," he says.
Tom is working with a team of parasitologists,on the CReefs trip to Heron Island, including a PhD and several honours students whom he is supervising.. In addition to trematodes, other members of the team are focusing on external parasitic flatworms of the class Monogenea and microscopic parasites of the class Myxosporea. The group is interested in a wide variety of host fishes including rabbit fish, tangs, triggerfish, cods, blennies and mullet.
The team's research on this trip will contribute to science's understanding of the diversity, abundance and distribution of parasites on coral reefs in Australia.
The findings will add to and refine parasite taxonomy: that is, the researchers will describe and name new species and place them within existing hierarchies, taking into account the evolutionary relationships between species.
Tom estimates his work has contributed to the identification of 300 trematode species on the Great Barrier Reef so far.
His research highlights the diversity of parasite species within the Great Barrier Reef. Although many of the same fish species are found around Heron Island, at the southern end of the Great Barrier Reef, and around Lizard Island, at the northern end, some of the parasites found in the same species of fish,are different.
"This means you can't understand coral reef fish parasites by just looking at them in one place. It's important to study them in different locations," Tom explains.
The team is also interested in increased understanding of parasite life cycles. Most trematodes have complex, multi-host life cycles, with larvae passing through several hosts, including molluscs and other invertebrates, before the adult develops, usually in the intestines and stomach of their final fish host. 
"Parasites are an important part of the system that we think is worth understanding," Tom explains.
"Sometimes the parasites can cause disease that can a have a significant effect on other animals – but that's not my starting point. I'm not trying to save the fish from these ‘hideous' parasites; I'm interested in parasites for their own sake," he says.