The Hidden World of Parasites: unseen stagehands in the theatre of life

The base of this barnacle has been transformed into a network of roots which extend deep into the shark’s body.

Dr Tommy Leung
School of Environmental and Rural Science.

To most people the word “parasite” evokes disgust and contempt, and is often used as a term of insult. Beyond that, parasitic organisms themselves are what a lot of people associate with disease and bodily invasions, so it is no surprise that many science fiction horror stories feature parasitic organisms as their monsters. When it comes to parasites in real life, most people think of creatures like tapeworms, hookworms, fleas, and ticks, but there is a wider world of parasites out there. The vast majority of them do not infect humans nor their domesticated beasts or crops; they are just quietly living out their lives and, by doing so, providing vital services for various ecosystems.

The etymology of the word “parasite” is derived from the Greek word parasitos, which means “eating at another’s table” – and this is a lifestyle that has been adopted by many species from all branches of life. It has been estimated that at least half of all animal life on this planet is parasitic, and parasitism is a lifestyle that is also found in many lineages of plants, fungi and microbes. Even groups of animals that people are familiar with, such as crustaceans (which include crabs and lobsters) and molluscs (which include snails and clams), include species which have evolved to have parasitic lifestyles. One example of that is Anelasma squalicola, a peculiar barnacle that has evolved to infect deep sea sharks. Unlike other barnacles which simply attach themselves to some kind of hard substrate like a rock or the hull of a ship, the base of this barnacle has been transformed into a network of roots which extend deep into the shark’s body and allows it to draw its nutrients directly from the host.

While most people see parasites as the agents of diseases and maladies, on an ecological level, the presence of parasites is an indication of a vibrant and healthy ecosystem. The work of UNE researcher, Dr Tommy Leung, focuses on the factors that determine parasite diversity in different groups of host animals, basically asking the question: “Why do some hosts get infected by more parasites than others?”

From the parasite’s perspective, not all host species are equal. Some might provide more resources, others might have a stronger immune system that makes them difficult to infect. Some host animals are more likely to encounter parasites because of what they eat and where they live. By looking across different host-parasite combinations and working out the common patterns between them, we can begin to understand the process that drives parasite diversity.

Many parasites have complex life-cycles that require different types of host animals to complete. Some of these parasites actually help their hosts by making it easier for them to obtain a meal. An example of that is Polypipapiliotrema – a type of parasitic fluke that infects coral polyps and causes them to swell up, making them easier for coral-eating fish such as butterfly fish to prey on them.

Meanwhile, the absences of such parasites can indicate environmental disturbance or the population decline for any one of those host animals. For example, a study in the UK on parasitic flukes that infect the common whelk found that whelk populations closer to a sewage dump site actually had a lower level of infections than those that were further away and not as exposed to pollutants.

Additionally, some parasites can drastically alter various aspects of their host physiology and behaviours, and their presence can actually increase biodiversity and shape habitats through the effects they have on their hosts, making them what ecologists call “ecosystem engineers”. For example, on the coast of New Zealand, there is a species of fluke that infects the foot of cockles, and degrades the cockle’s ability to dig itself back into the sand. The side effect of this is that the fluke ends up creating additional habitats for organisms that can make use of the cockle’s exposed shell.

Since about half of all life forms on this planet are parasitic and their actions significantly impact their hosts, understanding how parasite diversity has come to be, is, in a way, understanding how life on this planet works.

[Top banner image: drawing copyright Dr Tommy Leung]
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