Spawn and die happy

Dr Rob Adlard preparing to spearfish.
Image: Gary Cranitch.


5 September 2010
The diversity of parasite life can be a significant indicator in the overall health of an ecosystem – but parasites don't care about that, says Dr Rob Adlard of Queensland Museum.
Dr Adlard is one of a team of scientists studying parasites in fishes on the CReefs expedition to Lizard Island this month. His particular interest is in myxozoans of the genus Kudoa.
Myxozoans typically have two hosts within a life cycle. One life stage occurs in segmented worms (annelids), during which the parasite transmits spores into fish. A second developmental stage in the fish produces spores that are transmitted back into the annelid worms.
"If they can pass on their genetic material in this way, they have successfully completed their life cycle within that system," Dr Adlard says.
It's tempting to say that myxozoans procreate then die happy, but Dr Adlard warns against the tendency to anthropomorphise parasites.
He relates an anecdote in which he was speaking at conference and a participant demanded to know the purpose of such ‘disgusting' creatures as parasites.
As it happens, parasites play an important role in marine environments. Infected fish may be smaller or weaker than other fish, or may adopt erratic behaviour. This makes them easy targets for predators, and ultimately functions as a form of population control.
"Parasites have the ability to regulate host populations, and to change the inter-specific and intra-specific competition for resources," Dr Adlard explains.
For the parasites themselves, however, ‘the point' is simply existence, and any human perception of ‘purpose' is not their concern.
Parasites, by definition, are organisms that live in intimate association with their host and from which they derive benefit but also cause harm during the process.
This becomes of more importance to us when, for example, parasites hitch a ride on ships from around the world and are introduced into Australian waters.
In some environments, the absence of parasites may suggest high levels of pollution, that is, they are sensitive indicators. In others, the opposite may be the case: human interaction has introduced parasites or created conditions in which existing parasite populations have flourished, to the detriment of other marine life.
To understand these dynamics and put them to use is necessary to build a baseline against which we can compare – a central goal of the CReefs project.