Almost all marine animals have other animals living on or inside them. Some form loose associations, like cleaner fishes and coral trout, while others are more intimately connected, such as the algal cells that live within corals and giant clams. Many parasites, for example the protozoan that causes 'white spot' disease in fish, are often clearly pathogenic to the host. The feeding mechanisms of these organisms, their methods of transfer between different host species, and their effect on the hosts have fascinated biologists for many years.
With the development of mariculture, study of marine diseases and parasites has become particularly important. Diseases and parasites flourish in the intensive culture of single species of fish and shellfish, especially if water exchange in the culture system is limited. The skin and gills of fish raised in sea cages can be attacked by bacteria, amoebae and small crustaceans, while prawns in hatcheries are weakened by viruses and killed by bacteria. Oysters die in thousands each year from the protozoans that cause 'winter mortality' and 'QX disease'.
Marine biologists with skills in microbiology and parasitology determine the sources of infection and explore methods of control. Veterinarians with an interest in marine pathology play a valuable role in diagnosing infection, prescribing the necessary antibiotics to control bacterial disease, and helping to develop vaccines. Microbiologists and parasitologists detect and describe the new diseases that arise as mariculture develops and also investigate the diseases of marine fish in aquaria and oceanaria. Real and imaginary diseases sometimes delay or prevent sale of Australian seafood products overseas. Understanding the diseases which exist in Australia circumvents potential problems quickly and helps boost our exports.
Marine biologists and veterinarians investigate marine parasites and diseases for other reasons. Their work helps to determine the cause of strandings of turtles and whales, unravels links between skin diseases in fish and water quality, clarifies migrations of fish and prawns when conventional tagging methods are not feasible, and seeks biological control agents for pests.
The more we manage marine organisms, the more we will be able to manage their diseases. A career in this area has several possible starting points: as a veterinarian, microbiologist, parasitologist, or marine biologist. For many, it provides the opportunity to combine a scientific career with recreational interests such as fishing or diving. Several universities offer courses and specialist workshops to allow scientists to become proficient in the identification and control of parasites and diseases in marine organisms.
Photos: (Top) A researcher removing parasites from fish. (Bottom) A didymozoid trematode in the gill of a skipjack tuna. This and other parasites are valuable natural markers to investigate migrations of tuna.