The diverse life of (and inside) fish

Dr Terrence Miller spearfishing for target species off Mangrove Beach at Lizard Island.
Image: Gary Cranitch.


12 September 2010
"This fish," says Dr Terrence Miller, examining the body cavity of an emperor he has just opened, "has trypanorhynch cestodes."
He points out the numerous and relatively large cysts of the tapeworm larvae, which are extraordinarily concentrated around the throat region in this individual.
"Each fish may be host to a range of internal and external parasitic species – some individuals we examine host up to 10 or more different species of parasites – so we will systematically search through each of the tissues and organs of this fish to see what else is here," he says.
Dr Miller, a researcher with the University of Queensland and the Queensland Museum, is working with a team of parasitologists on the CReefs trip to Lizard Island. Other members of the team are focusing on cestodes (tapeworms), nematodes (roundworms), and microscopic parasites of the class Myxosporea, but Dr Miller's main focus on this trip are parasitic flatworms of the subclass Digenea, which belong to the phylum Platyhelminthes.
Digeneans are internal parasites commonly known as flukes or trematodes, which are primarily found in the intestines and stomach of their final fish host (where the worm develops into an adult), but may also be found in most tissues including the gills, body cavity, liver, spleen, and urinary bladder.
"The diversity of trematodes in marine fish is staggering," Dr Miler says, while picking out another unusual parasite species from the flesh of the emperor.
"On some of the previous CReefs trips we have observed fish species hosting up to 12 different trematode species. What is also interesting about this is that many of these parasites are very host specific, in that they only infect a single fish species or a small group of closely related hosts. And these are just the trematodes. When you include all of other internal metazoan and protozoan parasites, the diversity and richness of living creatures within a fish is quite fascinating. Lizard Island is home to fish that harbour a diverse range of Platyhelminthes, so every day we're discovering something new."
These CReefs expeditions have not only allowed Dr Miller to explore trematode species diversity in coral reef fishes, but are also helping to inform us of their life-cycles, evolutionary relationships and biogeography.
"While discovering and documenting new species of parasites is in itself really exciting, we are now starting to see some interesting patterns in the evolution and biogeography of some species. For example, some species of parasites have relatively restricted biogeographic distributions, while others are distributed almost Indo-Pacific wide. How these parasites have evolved and dispersed so widely in the various small reef-associated or larger pelagic reef-associated fish families is a key question these expeditions are allowing us to explore."