When we think about living things and where we find them, we tend to think at large scale. The word that comes to mind is ‘habitat’, which conjures up images of forests, deserts, oceans, swamps and the many other diverse places on Earth where life can be found. Of course, we might also be thinking at a smaller scale, focusing perhaps on our gardens, ponds or perhaps the local hedgerows.
If you think at the right scale, though, pretty much any physical space can be habitat to something. Anyone who has looked closely at mites living on a leaf, or at tiny springtails under the decaying bark of a dead tree, will have marvelled at the wonder of microhabitats. But when you start thinking at a really small scale, when a microscope is needed to see what’s going on, then habitat can become very interesting indeed – because once you think small enough, there’s no better habitat than that offered by other organisms.
Living in or on other organisms offers many advantages – if you can pull it off. Your host may not be entirely amenable to you taking up residence, but they can provide you with all the shelter, resources and travel you might require. You may even provide your host with some advantages in return: many of the bacteria that live within our guts, for example, help us with aspects of digestion. Such mutually beneficial relationships are known as mutualisms, and are quite common.
But not every organism that lives in or on another organism brings benefits – sometimes, these passengers can cause harm. When an organism is harmful to its host, we call it a parasite.
Interactions between hosts and their parasites can be some of the most fascinating, and sometimes disturbing, in the entire natural world. Many of the more bizarre interactions have evolved because it’s advantageous for the parasite to control its host, causing it to behave in odd ways that benefit the parasite. In some cases, parasites can literally take over their host.
The images in this feature reveal the rather grim aftermath of successful parasite takeovers. Some have reduced their hosts to a ‘zombie’ state, while other hosts face a quicker and simpler fate: simply being overwhelmed by their alien invaders.
Fungal invasion
Insects, spiders and other invertebrates are affected by a whole range of parasites. The organisms that you can see growing out of the bodies of the unfortunate creatures here are parasitic fungi. In all cases, the fungus has infected, and then taken over, the body of the host.
The initial stages of infection are straightforward. Spores from a parent fungus are tiny and easily transmitted through the air or on surfaces. Insects are very good at grooming themselves and, in the case of social insects, such as ants, each other. Many insects have special adaptations to their legs or mouthparts that make grooming more effective, enabling them to remove spores and other undesirables to prevent infection. But no matter how scrupulous insects are with their hygiene, sometimes they miss something.
Fungal spores left unchecked by grooming can start to develop, or germinate, on the surface of an insect. The first challenge facing the fungus is finding a way to drill down through the tough chitin that forms the insect’s exoskeleton. To do this, the spore forms a sticky ‘holdfast’ structure that firmly attaches it to its host and provides a stable drilling platform. Once stuck in place, this structure produces a cocktail of enzymes that allow the developing fungal strands, or hyphae, to punch their way through the exoskeleton into the nutrient-rich environment of the insect’s insides.
Once through the exoskeleton, the fungus is able to feed on the living structures of the host. Fungi can grow rapidly and when the invading parasite has become sufficiently large, two things occur.
The first is that the insect dies: there is, after all, only so much insect biomass that can be converted into fungal biomass before the insect ceases to function. The second thing that occurs is that the fungus reproduces. The structures you see coming out of the bodies of the insects and spiders in the photographs here are the fruiting bodies of the infecting fungi. Akin to mushrooms, these structures produce the spores that can go on to infect future hosts.
Spreading spores
Natural selection has favoured fungi that are better able to infect the next generation of unfortunate hosts. One way to do this is to produce many spores. There is a limit to how many spores can be produced, though, because the fungus only has the nutrients supplied by a single, generally quite small host. Another option is to find ways to spread spores more effectively. A good way to do that is to get the spores up high, into a breeze that can carry them further afield, but for a fungus inside an insect host on the forest floor, that’s not so easy. Fruiting bodies can be made longer, an adaptation you can see in many of these images, but there’s a limit to that due to the trade-off between making more spores and making longer structures from which to broadcast them.
Fungi have other problems, too. Sometimes the environment in which insects live is not favourable to fungal growth. Ant nests, for example, may not have the right temperature or humidity.
If only there was a way of moving to a better location – perhaps getting higher and saving resources for more spore production.
Some fungi, once they have made it into the body of their host, are able to manipulate its behaviour to get them to exactly where conditions are right for the fungus. There are at least two groups of fungi that can turn their hosts into such fungal-controlled ‘zombies’: Cordyceps and Ophiocordyceps.
I saw my first zombie ant when I was doing field work in Panama. Once I got my eye in, they become a common sight because all the infected ants had the same odd appearance. They were all firmly
clamped onto plant stems with their jaws, a short distance above the ground. In most cases, but I suspect not all, the ants were dead. The ant bodies stuck out horizontally from the plant stem, with fungal growth and fruiting bodies clearly visible. Looking like little flags, perhaps a better way to describe them would be ‘spore salt-shakers’, since the fruiting bodies being held aloft were broadcasting spores across the forest.
The reason these ants climbed to their final resting place to clamp onto a stem and await death is that the fungus was able to exert chemical control over their nervous systems. The infected species of ants usually live in the forest canopy, but when infected they start to convulse, causing them to fall to the forest floor. Once there, the host then finds a plant stem and begins to climb to a height of 25-30cm. It’s there that the final flourish of behavioural control occurs – the ant clamps its jaws onto the stem. The insect dies and the fungus reproduces, sending out spores to start a new generation.
Natural born killers
There are many other species of fungi that are ‘entomopathogenic’ – that is, they cause serious disease and often death in insects. One common type is Metarhizium, a group consisting of more than 50 identified species that can infect a wide range of insect hosts. We can make use of these fungi’s incredible efficiency in pest control. Some species are now used to control insect pests, giving us the option for more environmentally sympathetic biocontrol compared to using chemical pesticides.
Fungi tend to be overlooked when we think about biodiversity, but we really should give them more attention and pay them far more respect. They are crucial to ecosystem functioning and, as decomposers, they are the interface between life and death. Whether recycling waste, enhancing plant growth, controlling potential pests or hijacking insect nervous systems, there’s much we can learn from them.
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