Melbourne's Parasites: Biodiversity Unseen

This is a guest post by zoology researcher Mackenzie Kwak.

The word ‘biodiversity’ is a new one. Coined in the late 20th Century, it is now a household word, but what precisely does it mean? For most people, the word summons up images of large, majestic animals like elephants and whales or heard of hooved animals marching across the planes of Africa. Others think more of delicate orchids or colourful fungi and ferns. In reality, biodiversity is everything living; it is the elephants, the orchids and even some of those living things we are less fond of - the parasites.

Parasites are an extremely diverse group, not linked by a shared ancestry but instead by a shared means of survival: feeding off of others. The group includes various insects, arachnids, crustaceans, worms and even fish and birds. Parasites include a huge proportion of the Earth’s biodiversity, with some categorising more than half of the world’s animal species as parasites. This is an aspect of biodiversity rarely seen and even more rarely thought about. However, this group includes some of the most fascinating animals found in the natural world. Parasites are all around us and many call Melbourne home.

Lampreys are perhaps one of Melbourne's more alien-like parasites. 

Lampreys are perhaps one of Melbourne's more alien-like parasites. 

Lampreys – Vampires in the Yarra

Just below the surface of the Yarra River lurk vampires: long streamlined creatures, swimming silently through the dark water in search of a host. They are ancient, having existed for millions of years. These fascinating vampires are the lampreys: elongated eel-like fish with a row of round gill openings along their sides and a sucking disk shaped mouth adorned with many circular rows of sharp teeth, reaching sizes of 30 to 70 cm. The Yarra is home to two species: the short-headed lamprey (Mordacia mordax) and the pouched lamprey (Geotria australis). The pouched lamprey spends most of its life in the ocean, only returning to the river to spawn. However, the short-headed lamprey spends much of its life in the Yarra, particularly near the salty mouth of the river where it meets the sea. In the case of the Yarra, this is where it runs through the city. While these species are rarely encountered by fishermen, they are widely distributed and can be found through many of the river systems in southern Australia.

Cuckoos – Nest Thieves

Darting though the treetops in the parklands and botanical gardens of Melbourne, a thief scours the canopy for an unguarded nest of eggs. Cuckoos are some of the most unexpected parasites in the natural world. It seems against the natural order that a bird should act as a parasite. Cuckoos are parasites in a less than conventional sense; they do not suck blood like the lamprey. Instead they are nest parasites, laying their eggs in the nests of unsuspecting host birds and fooling them into raising the cuckoo’s young as their own. Not only are adults duplicitous, but the young are also malevolent and are known to push eggs and live chicks out of the nests of host birds so that they will receive more food from their ‘adopted parents’.

However, this has triggered an arms race between cuckoos and their hosts, leading to increased discrimination by host birds regarding egg colour and proportions. There are also increasingly more well-disguised eggs produced by the cuckoos that mimic the eggs of their intended host species. Amongst the treetops of Melbourne many species of cuckoo can be found, including the spectacular looking fan-tailed cuckoo (Cacomantis flabelliformis), pallid cuckoo (Cacomantis pallidus) and Horsfield’s bronze-cuckoo (Chalcites basilis).

Helminths – Worms Within

Just under the skin of a possum foraging for food in any of Melbourne’s parks, a slender worm slowly moves. Some of the most common parasites that spring to mind when one hears the word ‘parasite’ are the worms, or helminths as parasitologists call them. They can be found in many different regions within the bodies of many different species. From the heart, to the eyes, the lungs, intestines or body cavity, worms are everywhere! In fact, it is likely that you yourself have worms. Some species can be many metres long in the case of tapeworms (Cestodes) or as thick as your little finger in the case of the giant human roundworm (Ascaris lumbricoides). However, although from the outset worms often make one’s skin crawl, they are not all bad.

Recently, research has suggested that an infection of worms actually helps our immune systems practice when it comes to fighting off diseases. It is likely that infections with helminths may do the same in other species, including out native wildlife. Helminths have also been used in recent years as biological tags with which to track individual animals over large distances. The worms within the scats of rare and endangered animals have also been used to make predictions about the level of inbreeding and the genetic stability in the populations of their hosts, as inbred animals often also have inbred parasites. There are hundreds of species of helminths living in Melbourne, from those in the fish of our rivers to those in our birds and mammals. They are a diverse group with incredibly complex life cycles and ecologies.

Image: Mackenzie Kwak

Image: Mackenzie Kwak

Batflies – From Flies to Walks

Every night, as the sun sets, black wings beat through the skies of Melbourne and loud cries signal the arrival of the fruit bats. However, along with the fruit bats come tiny, wingless passengers holding on for the ride. These are the batflies, and although they are true flies, they are wingless owing to their close association with their bat hosts. Batflies like lampreys are vampires and spend their lives wading through the forest of hair that covering their hosts. These parasites are a great example of evolution, descending from winged ancestors that visited bats to feed on their blood. However, over time their ancestors spent more time on these hosts, eventually never leaving them.

As wings are costly for an animal to produce, eventually they were lost as there is little need for wings on a fly that rarely, well, flies. These flies could now more accurately be termed ‘walks’. Another interesting trait of batflies is their dedicated parental behaviour, somewhat akin to that of humans. Female batflies will nurse their young on a diet of milk which they produce from internal milk glands. The legless, grub-like larvae will live with their mother, growing on a diet of rich milk until they are ready to pupate and begin metamorphosis into an adult. At this point, the mother fly will leave the host and carefully select a place for her larvae to be deposited. They really are some of the most doting mothers in the world of insects.

Although lampreys, batflies, cuckoos and helminths may be considered somewhat creepy, even sinister creatures that lack the cuddly appeal of koalas and kangaroos, that does not make them any less valuable. Parasites contribute to the assemblage of species which makes up our biodiverse city. Not only do they have intrinsic value as members of our urban ecosystems, but with a little understanding, one soon realises how fascinating these creatures truly are. Biodiversity is not just the pretty butterflies and flowers; it’s also these fascinating parasites and everything in between.