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Which parasite has the weirdest way of life?
From the body-snatching barnacle to the mind-controlling fungus check out the amazing survival strategies of four parasites from the Museum collections.
Which one do you think has the strangest lifestyle?
The body-snatching barnacle
Sacculina carcini, the parasitic barnacle, on a female swimming crab © Hans Hillewaert. Licensed by CC BY-SA 4.0.
A barnacle turned puppet master: this parasite chokes the bodies of crabs, and even has the power to change their sex.
The Sacculina carcini parasite holds so much power over its crustacean hosts, it even tricks them into caring for it as lovingly as they would their own young.
Found wherever there are large crab populations, the female parasite larva seeks out an unwitting host and sheds its own hard shell. Burrowing its way into the crab's body through soft joint openings, it takes hold underneath the belly.
As soon as the Sacculina is in place, the crab can no longer grow, digest food or reproduce. Every ounce of energy is redirected into nurturing its swelling, slug-like guest.
The parasite spreads through the host's nerve centre, stomach and genital area, sucking the life away.
It positions itself on the abdomen where a female crab would normally shelter her eggs. The crab then nurtures the parasitic sac with as much enthusiasm as if it were carrying real young.
Sacculina reproduces when tiny male parasites enter the sac the female has created. They live within the female - which in turn lives within the crab - and fertilise her eggs.
Not content with colonising female crabs, if the barnacle finds itself inside a male crab, it has the power to make its host infertile. Male crabs undergo endocrine changes and start to act like females, nurturing their parasite in exactly the same way.
The crab limps along looking for food for the rest of its life, while all nutrients it ingests go straight to the belly of its jelly-like controller.
Parasitic worm Schistosoma mansoni grows in the blood vessels and causes schistosomiasis
The parasite that causes schistosomiasis, 'snail fever', can live covertly in humans for years, causing devastating damage to internal organs.
Hundreds of thousands of people die every year after catching the parasite from contaminated water.
The larvae first develop inside snails, and then float freely in fresh water. Humans become infected when the larvae break into the skin. Once inside a human, the parasite develops into a worm and grows in the blood vessels.
They are masters at disguising themselves from our immune systems, and are able to thrive in the blood. Females release eggs, some of which are passed out of the body to infect more people. Other eggs nestle within body tissue, causing major organ damage.
The worms are unique in their class of flatworms. All other species are hermaphroditic, but schistosomes have evolved separate sexes.
Schistosomiasis is widely regarded as second only to malaria as the most catastrophic parasitic disease in tropical countries. Those most at risk are rural, agricultural communities.
The body's reaction to parasitic eggs can cause anaemia and malnutrition in children, and the eggs themselves can damage the bladder and kidneys.
Stomach pain, fever, bleeding, genital lesions and infertility can also follow if the parasite is left untreated. A short course of medicine can kill the worms inside the body, but rural communities in developing countries are often without access to the necessary healthcare.
Three female Stylops melittae piggy-backing on a female bee © Aiwok. Licensed by CC BY-SA 3.0.
Wedged tightly into the backs of mining bees, female Stylops melittae parasites spend their adult lives with their rear ends out, waiting for a male to come along.
Despite being fully grown, these females lack any functional eyes, antennae or legs, and look more like larvae than adult insects. But they produce thousands of eggs, and give out a pheromone that draws in males, in order to get those eggs fertilised.
The males, unlike the females, have wings, so they spend their short adult lives (just a few hours) flying around, looking for a female to mate with. Shortly after they do, they die.
The newly-fertilised eggs soon hatch - inside the female - into thousands of microscopic larvae. Less than 0.03 centimetres long and with poorly-developed eyes, these larvae crawl their way out of their mother, searching for a new host to parasitise.
Often they do this by hitching a ride on bees foraging for pollen who then unwittingly take the parasites back to their nest. There, the larvae quickly find an immature bee with a less-developed exoskeleton protecting it and latch on to it.
Once they've burrowed inside, the larvae take in nutrients from their host as they grow and develop into their adult forms.
There are consequences for the bee, which grows to a smaller than average size, and sustains damage to its internal organs and exoskeleton.
Many bees become infertile in the process, and some (primarily female) bees even change gender. This gives the parasite more time to mate and spread its larvae, since male mining bees leave the nest earlier than females.
Close up view of a dead ant attached to a leaf vein, showing the fungus emerging from the back of the ant's head. © David P. Hughes, Maj-Britt Pontoppidan. Licensed by CC BY 2.5.
This fungus has a handy way to find its ideal home: it makes ants do all the leg-work for it.
Native to tropical forests in Asia and South America, the Ophiocordyceps unilateralis fungus has a very specific set of temperature and humidity requirements it needs to thrive and reproduce.
Rather than leave it to chance, the fungus recruits nearby carpenter ants to scout out the perfect position.
Using its spores, the fungus infects the ants and dramatically changes their behaviour. Instead of working normally alongside their colony, these ants leave their nest, high in the trees, and make their way down to the lower leaves.
Once there, the ant clamps down hard on the underside of a leaf in a so-called 'death grip'. The fungus, having got what it needs, kills the ant.
This system is remarkably effective for the fungus. The ants almost always find a leaf 25 centimetres above ground, on the north side of the plant, in a location where humidity is 90-95%, and the temperature is 20-30°C. This environment creates the perfect spot for O. unilateralis.
As a result, the fungus soon begins to grow. Emerging from the head of the dead ant, it reaches a length of 10 millimetres in a week, making it twice as long as the original insect.
The fungus then showers its spores over the forest floor beneath it, creating a 'killing field', around one square metre in size, that infects any ants unfortunate enough to cross it - and producing another generation of zombie ants, ready to carry the fungus to its next location.
