I’m in Exeter Cathedral and I’m looking up at Antarctica. This is Gaia by artist Luke Jerram: a 7m-diameter model of Earth suspended above the nave, created from detailed NASA satellite imagery of the planet’s surface.
The monumental installation is designed to summon ‘the overview effect’. Commonly reported by astronauts, this describes a cognitive shift towards feelings of awe and a renewed urge to protect the planet.
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Inspired by the Gaia hypothesis – the hive-mind between chemist James Lovelock and biologist Lynn Margulis in the mid-1970s – Gaia reintroduces Earth as one self-regulating, interconnected system.
These teachings reveal the planet’s vitality and vulnerability, its strength and interconnection – a world teeming with relationships – driven by the environment and change.
As I orbited our planet that night, symbiosis was on my mind – the captivating situation where different species have evolved to live together to survive.
Lynn Margulis championed symbiosis as the essential state of interbeing. Species join forces, share resources, operate within ancient evolutionary pacts of give and take – shaping ecosystems, binding pivotal connections and building resilience.
Perhaps you know symbiosis as synonymous with mutualism – a cosy narrative promoting equal shares in the relationship. But modern scientific understanding leads us towards an enigmatic version distinguished by raw inequality and reciprocal exploitation so precisely executed, it upholds ecosystem stability.
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Symbiosis demands dark and dangerous lifestyles. Parasitism is its dominant form, where the suffering of one species enables the survival of another. More than half of all animal life exists in a parasitic relationship, and all life lives in symbiosis.
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But as the planet simmers under heatwaves, wildfires, flooding, drought – when global populations of mammals, birds, amphibians, reptiles and fish have plummeted by nearly 70 per cent since 1970 and ocean carbon dioxide concentrations are higher than ever – the Gaian message of connection has never been more critical.
Seeking counsel from the wild, I made a series of low-carbon journeys in 2024 across the British Isles, and this is just a snapshot of that adventure. I hoped to glimpse how nature thrives in symbiosis – and maybe learn some relationship hacks.
If symbiosis has been the fabric of nature for millions of years, the species involved must know something about longevity. It’s no coincidence that the relationships I highlight here are centred around water. Far from casual intimacy, symbiosis surges and sustains life like this, behaving in slippery, volatile, animate ways.
Late September 2024. On the ferry from Poole Harbour, I bounce across the Channel towards Jersey and discover ‘how-to’ lessons in symbiotic resilience.
A photosynthesising animal
Descending in their millions to safety is but a day in the office for mint-sauce worms. As is tolerating hours of UV radiation to near-frozen water. Even now, barely 3mm long, this species shatters my definition of what it means to be ‘animal’.
Animal by trade, plant by nature, I am soon sharing a tidal pool with a biological miracle: a photosynthesising, marine flatworm.
A species informing stem cell research with brain regenerating powers, inspiring cancer research with minute control of cell proliferation, and even informing sustainable space travel with its efficiency of waste recycling.
Also known as Roscoff worm, mint-sauce worm, plant-animal worm and (my favourite) ‘animalgae’, this oxymoronic animal-plus-plant relationship accounts for around half of photosynthesis in marine environments.
Though found across the Atlantic coast, the Channel Islands are a favoured site and in Jersey, animalgae faces a hugely volatile tide, averaging a 12-metre range. Its needs are simple: clear, shallow pools to sunbathe in; its days structured by tide and sunlight.
Animalgae appear dolloped across the sand, much like the minty condiment. But with the help of a hand-lens and an upturned shell, into which I scoop a sample, I see individual worms moving as a vast collective, eddying like a hurricane on a weather map.
Except they haven’t always moved like this, and they certainly haven’t always been green. By evolving to accommodate free-living algae from seawater into its body tissues, this flatworm has flourished into a solar factory.
It is a secret garden, thriving on a burgeoning algal population’s produce: glucose, oxygen, starch and lipids. To maximise photosynthesis, animalgae performs a spellbinding ‘circular milling’ motion – famous in shoals of fishes and starling murmurations – but rarely seen in worms.
At low tide, spinning flotillas of animalgae form into verdant beach biofilms. Ebbing and flowing with the tide, ‘social flocculation’ is animalgae’s mesmerising vertical movement.
Descending to safety with the incoming tide, animalgae grouped in hydrodynamic ‘flocs’ fall 50 per cent faster than individual worms. Once safety is reached, groups disperse underground, before ascending to sunbathe again at low tide.
With tide their clock, gravity their compass and sunlight their fuel, animalgae is a deal-breaking bond. Having shed major body parts save for the chloroplast, algae live and grow, die and reproduce semi-embedded in the worm’s muscle fibres – absorbing sunlight through their hosts’ epidermis.
They may be protected, but they cannot return to seawater and are unable to contribute to the gene pool of the population beyond the worm. Are the algae prisoners at the worm’s bidding? Or is there a hidden payoff for both partners?
Research suggests the worm gains more from the relationship, controlling algal photosynthetic activity and nutrient flow to an almost abusive level. Curiously, it seems animalgae finds longevity in this inequality.
The ‘bodysnatching’ parasite
At Bracelet Bay on Wales’s Gower Peninsula, I’m joining a research group from Swansea University with a bodysnatcher on my mind. Sacculina carcini (Sacculina, for short) is a marine parasite reminding me more of fungus than shellfish, in the way it stealth-raids the body of its shore crab host.
Sacculina has chosen well. Shore crabs shape coastal areas as omnivorous hunters and opportunistic travellers. Their serrated carapaces are tough and accommodating – good news for a hopeful Sacculina larvae looking to engage in bodily espionage.
Sacculina can infect up to 80 per cent of a crab population and has been considered a biological control option for areas in the USA where crabs are outcompeting rarer species.
Sacculina mystifies biologists, with the look of a mollusc but the genetics of a crustacean. Using specialised sensory organs, female ‘cyprid’ larvae detect a freshly moulted shore crab by smell.
Should the crab be male, Sacculina deploys a secret weapon of temporary castration – ‘feminising’ the crab – exercising a brutal yet prevalent utensil in the parasitic toolbox, forcing a host to allocate all available energy to the parasite.
Travelling via the bloodstream, Sacculina grows via rootlets and tendrils, creeping and crawling, nourished by the bodily bounty. Sacculina’s goal is to hack and re-programme the crab into a barnacle-making machine.
Where one animal ends and the other begins feels impossible to determine. The intimacy has such depth that studies show how an infected crab can adopt the parasite genotype, hybridising blood, brain and body to a genetic level.
They still look like a crab – but do they feel like one? Disguised as the crab’s own egg sac, adult Sacculina emerges in external form, fooling the crab into the ultimate act of servitude: parental care.
From here, male Sacculina cyprids swoop in and fertilise. With the next generation secured, Sacculina departs, leaving nothing but a scar on the crab’s abdomen. The entire ordeal feels violent, almost deadly.
But like all parasites, Sacculina must keep its host alive. Sacculina taps resources with measured greed and cunning precision, making for a fleeting encounter, and completing their life-cycle.
Remarkably, shore crabs can survive and even regain their ability to moult, grow and reproduce while hosting Sacculina. Studies have found that castrated males can even regenerate their testes.
Brain-deep manipulation
Parasitism has evolved at least 223 times and continues to test our acceptance of nature’s ferocity. Heading upstream, I encounter another species whose symbiotic relationships aren’t so much rare as (deliberately?) unnoticed.
Hairworms – also known as horsehair worms or cabbage worms – resemble brownish strands of hair up to 20cm long. In water, they writhe and thrash.
Hairworms are infamous for brain-deep manipulation of their invertebrate hosts, pushing crickets, grasshoppers, beetles, crabs and others to perform strange, disturbing acts.
Globally, more than 350 species of hairworm have been identified as specialised parasites of freshwater and marine invertebrates.
Yet this undersells their biodiversity – perhaps describing just 18 per cent of the possible species. The British Isles has five known species of hairworm, but there could be more.
Freshwater hairworms spend at least one year submerged in rivers, streams and puddles. Here, they undergo five life-cycle stages in various identities between land and water: as free-living adults and larvae, and as parasitic cysts and juvenile larvae
Unusually, these latter two parasitic stages can only live inside a host – thriving incognito, like the Sacculina barnacle. To reach their host(s), hairworm larvae must be eaten.
First, by a transport host, such as an aquatic snail, which hopefully is gobbled by a larger invertebrate – a cricket or beetle – becoming a hairworm’s terrestrial ticket. Once inside this final host, the hairworm transforms from ball-like cyst into its next guise.
Growing into juvenile larvae is arguably the riskiest stage for the hairworm. It needs time and invisibility, evading the host’s defences and devouring nutrients from within. Studies find that in certain cricket hosts, hairworms can suppress their iconic chirping – an operation of clandestine proportions.
It could be months before the hairworm, now occupying the body cavity and head, neurologically instructs their host to jump into freshwater – returning the hairworm to its watery world. This so-called ‘suicidal leap’ seems to occur at night where celestial light on water bodies acts as a compass to the water’s edge.
Once in water, the adult hairworm spills from the host’s head, coiling, writhing, spooling – free to find a mate. Whether this ordeal proves fatal for the host remains a mystery. It’s the kind of unhinged relationship we sensationalise – a parasite zombifying another also holds us in a stasis of both captivation and revulsion.
The beauty and brutality of symbiosis
But hairworms signal water health. Like rockpool species they face extremes in temperature and oxygenation to lay eggs and find mates. They need unpolluted, moonlit skies to potentially navigate.
Their life-cycle stages shower organic matter – body parts, corpses, fallout from up to 27 million eggs, even novel combinations of genes – supplying other species with resources and food. Hairworm-infested insects have been deemed more favourable prey versus insects without hairworms.
Those streams also experienced more nutrient cycling. But their world – shared by animalgae, shore crabs, Sacculina and millions of others – is warming, acidifying, drowning and drying, becoming noisier and brighter.
For all their resilience, nature’s hidden relationships are delicate. Research finds a low tolerance to climate change in many specialist parasites, disrupting interaction – even escalating relationship separation – prompting an urgency to research the limits of parasites and determine their relationship security.
Whether you see animalgae as an algal-hostage-taker, Sacculina as a thief-on-the-run, or the hairworm as toxic-manipulator, there is beauty in the brutality of symbiosis. Our own bodies are relied upon by up to 400 different species of parasite. These are necessary battles fighting for a greater ecological good – a story we are part of.
Symbiosis invites an overview effect, offering an arresting reminder: nothing exists in isolation. We are never alone.
