From the moon jellyfish to the humble garden snail, invertebrates play a central – and often invisible – role in shaping our world. Numbering in their millions, species of insects, arachnids, snails, crustaceans, corals, jellyfish, sponges and echinoderms are among the least understood animals on Earth, often overshadowed by their vertebrate cousins.

We asked scientists to tell us about how invertebrates shape our world and structure its ecosystems – and the unforeseen consequences of their disappearance.

‘Nobody likes to step in poo’: decomposing world waste

Without shrimp, dung beetles and thousands of fly species, vast amounts of organic matter would not break down and the nutrients would not be recycled through ecosystems. Many invertebrate species feed and breed in the waste of plants and animals, and play a vital role in their healthy functioning.

“Nobody likes to step in poo when out for a walk and we often complain there is too much about – but things would be far worse without dung beetles and green bottle flies, both of which consume and break down animal poo removing the odour and creating fertiliser for our soils,” says Paul Hetherington, director of Buglife.

In the oceans, mussels, clams and lobsters are all important decomposers, while species such as sea cucumbers also play a role akin to earthworms.

“Burrowing sea cucumber that move through the sediments as part of their daily activities help to oxygenate them, which is important for numerous other processes,” said Annie Mercier, of Memorial University of Newfoundland, and co-chair of the International Union for the Conservation of Nature (IUCN) species survival commission sea cucumber specialist group. “Sea cucumbers are not only active recyclers, they are prey to many other animals, including crabs, fishes, turtles, sea otters, pinnipeds, eider ducks and more.”

Caffeine-hit blossoms and shrinking pansies: invertebrates shaping the plant world

Bees are famously crucial pollinators of human staples. But beetles, flies and other invertebrates are also essential for helping plants fruit and reproduce. One in every three mouthfuls that humans eat is the result of pollination, researchers estimate. The role of invertebrates shapes the way plants behave and evolve – such as coffee and citrus trees, that give insects a reward in return when they visit their flowers.

“Coffee and citrus flowers produce caffeinated nectar, which has a pharmacological effect on honeybees and bumblebees – it enhances their memory for the unique smells emanating from flowers and so helps the bees find these important food sources in complex floral landscapes,” says Prof Phil Stevenson, head of trait diversity and function at Kew Gardens. “In doing so, it helps the flowers get pollinated as the bees are more likely to return,” he says.

Many plants have evolved to attract particular species of invertebrates, such as prosopanche plants that are native to South America. The group produce heat to woo small nitidulidae beetles to spend the night inside them. When pollinators disappear, it can also change plants – a study in December found French wild pansies were producing smaller flowers and less nectar as pollinator numbers declined, effectively giving up on scarce insects and evolving to self-pollinate, scientists said.

Dave Goulson, a professor of biology at the University of Sussex who specialises in bee ecology, said the disappearance of pollinators was already having dramatic consequences in some parts of the planet.

“In parts of south-western China there are almost no pollinators left, and farmers are forced to hand-pollinate their apples and pears, as otherwise their crops would fail. In Bengal I have seen farmers hand-pollinating squash plants, and reports are coming in of farmers in parts of Brazil resorting to hand-pollinating passion fruits,” he wrote in his recent book Silent Earth.

Breaking down plastic waste

Invertebrates could help break down some of the billions of tonnes of plastic waste that humans produce every year. In 2022, researchers found that the larvae of the Zophobas morio, a beetle species, were capable of digesting polystyrene and successfully completed their lifecycle.

“Within 48 hours … the faeces they produce turn from their usual brown – when they eat bran – to white,” said Dr Chris Rinke of the University of Queensland, a co-author of the study.

Architects of the coral reefs

More than half a billion people depend on reefs around the world for food, protection and their livelihoods. They are natural barriers to storms, flooding and erosion, safeguarding human settlements, while also providing a home for thousands of fish species. Invertebrates are crucial reef builders, but they are threatened by the climate crisis.

“Reef-building hard corals are the architects of coral reefs – they create the physical structure of a reef as new corals grow on the skeletons of dead corals. On a diverse reef, the growing and eroding matrix of old and new skeletons results in complex structures and spaces, creating three-dimensional habitat for the myriad species living on a reef,” says David Obura, founding director of Cordio east Africa and head of the Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services (IPBES).

Corals are not the only reef-building animals, says Julia Sigwart, a member of the IUCN species survival commission mollusc specialist group, who warns that marine invertebrates risk being overlooked.

“Marine invertebrate animals comprise the vast majority of the species diversity in the ocean, including many species that are not yet described or named. Because they are not so familiar to us humans, we often lump groups together; many people do not realise that sponges are animals, let alone that there are thousands of different species,” she says “This leaves a huge risk that species are going extinct before we even know that they are there, with surprising and potentially disastrous consequences.”

Upturning predator food chains

The spread of invasive invertebrates can have major consequences for the whole ecosystem. In January, a study found that the arrival of invasive big-headed ants in Kenya had set of an ecological chain reaction that led to lions making fewer zebra kills. Tree cover had fallen in areas where the big-headed ants had spread, providing less cover for lions to ambush zebras. Researchers said their findings had a global lesson about the importance of invertebrates.

“Although ants might seem small and unimportant, for holding together entire ecosystems this couldn’t be further from the truth. This recent study shows that native mutualistic ants are the fabric that holds together the African savanna,” says evolutionary biologist Dr Corrie Moreau, an expert on ants at Cornell University’s Moreau lab.

“When the native ants are displaced from their plant partners by invasive species this causes ripple effects across the entire landscape,” she says. “It is amazing to think that the small but mighty ant can influence the diets of top predators.”

Find more age of extinction coverage here, and follow biodiversity reporters Phoebe Weston and Patrick Greenfield on X for all the latest news and features

More than 200 of the nation’s hazardous chemical plants will be mandated to reduce toxic emissions linked to cancer, the Biden administration announced on Tuesday.

The long-awaited rule finalized by the Environmental Protection Agency (EPA) will strengthen protections for communities living near industrial sites, especially along the Gulf coast.

The new update focuses on ethylene oxide, used to produce antifreeze, pesticides and sterilizing agents, as well as chloroprene, which is used to make synthetic rubber for shoes and wetsuits.

“President Biden believes every community in this country deserves to breathe clean air,” said the EPA administrator, Michael Regan, highlighting communities such as St John the Baptist parish in Louisiana. “We promised to listen to folks that are suffering from pollution and act to protect them. Today we deliver on that promise with strong final standards to slash pollution, reduce cancer risk and ensure cleaner air for nearby communities.”

Last time the government updated the pollution limits from chemical plants was 2006. The strengthened rule would lower toxic pollutants by 6,200 tons a year, and slash ethylene oxide and chloroprene emissions by 80%. The new update under the Clean Air Act would also require fence-line monitoring of six toxic air pollutants: ethylene oxide, chloroprene, vinyl chloride, benzene, 1,3-butadiene and ethylene dichloride.

“This shows the administration’s commitment to the issues of environmental justice,” said Adam Kron, a senior attorney with Earthjustice. “This rule will reduce a lot of hazardous air pollutants. There will be less cancer based on these emissions, there will be lives saved.”

There is only one facility in the US that produces the pollutant chloroprene, which is operated by the Japanese chemicals giant Denka and is situated in St John the Baptist parish in the heart of the heavily industrialised region in Louisiana known as “Cancer Alley”.

The EPA lists chloroprene as a likely human carcinogen and has long suggested a safe lifetime exposure limit of 0.2 micrograms per cubic metre. The agency has been monitoring the air around the Denka facility since 2016 and readings have regularly exceeded this limit by dozens of times.

Denka, a Japanese company that bought the former DuPont rubber-making plant in 2015, said it “vehemently opposes” the EPA’s latest action.

If elegance is your thing then consider the ash-black slug. Its nightly forays through the forest in search of fungi are the epitome of grace, a stately glide not only along the woodland floor but up and along the mossy branches of trees.

If size is your thing, then the ash-black slug must get your vote. Britain’s biggest slug is one of the largest land slugs in the world. The ash-black slug can grow up to 30cm (11.8in) in length, although most measure up at a still impressive 10-20cm.

This magnificent gastropod (meaning “belly-foot”) is a keeled slug with a pale-coloured ridge running along its back contrasting with its dark-grey colouration. It has a pale stripe along its foot as well.

Slugs are disliked for devouring gardeners’ lettuces but the ash-black slug is not a garden pest but an ancient woodland dweller. Here it feeds on fungi, algae and lichen and lives for an unusually long time, up to five years.

The ash-black slug’s elegance in motion turns into something stranger when it is mating time.

Ash-black slugs are hermaphrodites, so each individual is male and female at the same time. They can reproduce alone but prefer to find a partner, attracting them with a trail of scented slime as they glide up into the trees.

When they find a partner, the pair perform a twirling dance, eventually suspending themselves with mucus from a branch. A striking pale blue corkscrew-like appendage unfurls from the side of each slug’s head. Each corkscrew is actually its penis, and it is as long as the slug’s body.

The slugs pass a package of sperm to each other, and fertilise it themselves. Eggs are laid in the damp woodland floor and young emerge anytime from autumn to spring.

African savannahs may have their “big five” charismatic beasts but on Dartmoor national park there is a “little five” including the ash-black slug. The wet woodlands on the fringes of Dartmoor are a stronghold for the creature.

Threats include natural ones such as the blue ground beetle, which is Britain’s largest ground beetle. Its 3cm size is tiny compared with the slug but it ferociously attacks the bigger animal.

The ash-black slug is not endangered but unfortunately appears to be in decline, possibly because increased air pollution is reducing the ubiquity of sensitive lichens that are a key part of its diet.

So do different, vote slug.

• Welcome to the Guardian’s UK invertebrate of the year competition. Every day between 2 April and 12 April we’ll be profiling one of the incredible invertebrates that live in and around the UK. Let us know which invertebrates you think we should be including here. And at midnight on Friday 12 April, voting will open to decide which is our favourite invertebrate – for now – with the winner to be announced on Monday 15 April.