St Piran was a young Irish priest, it is said, who preached against King Aengus of Munster for planning to ditch his wife for a younger lady of the court. Piran was tied to a millstone and thrown off the highest cliffs into the sea.

But the stone floated and Piran was blown over to Cornwall, where he made another new home, enjoying feasting and fine wines and bringing Christianity to the druidic masses. His spells as a hermit attracted particular admiration and, when he rediscovered tin, his popularity among the locals became legendary.

Fifteen centuries later, a competition on BBC Springwatch led to (one of) Cornwall’s patron saints lending his name to an equally charismatic opportunist and hermit.

St Piran’s hermit crab only grows to 15mm and, like its saintly namesake, has a similar aptitude to making himself a new home. The crab takes up residence in empty periwinkles, dog whelks and other gastropod shells.

Historically, it has been a creature of warmer Mediterranean and eastern Atlantic waters. The coast of Cornwall became its new northern-most outpost when it colonised them in the 1960s. But it vanished not long after it arrived: populations entered a death spiral in the years after the Torrey Canyon oil spill in 1967. The detergents used to clean up the oil-wrecked rock pools and shoreline of the west coast didn’t help it either.

In 2016, it was discovered having returned to the South Devon coast – probably swept by currents from its nearest colonies in Brittany and the Channel Islands. Now warming seas appear to be assisting its spread around the south-west coast. It was recently found on Newquay beaches.

This crab is a good looker, with striking black-and-white-spotted eyes mounted on red eye stalks, bright red antennae and vivid blobs of red and electric blue on its legs and claws.

Unsurprisingly, given its size, it is shyer than the common hermit crab, and likes to curl up inside its shell and wait out all dangers.

So why not vote for an invertebrate that shows the resilience of the natural world, and cheer on the return of St Piran’s hermit crab – and our own capacity to appreciate such small miracles of life.

• 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.

Dotted along the length of Britain’s rivers are various obstacles – some as large as dams, others as small as weirs (which bisect a river like steps) – stopping creatures, sediment and plants from moving along the watercourse. Only 1% of the UK’s rivers are free of artificial barriers.

Moves are under way in many countries to remove such obstacles and let rivers “re-naturalise” and follow their own paths. But while many scientists agree that river barriers need to go in the UK too, other people are hesitant – concerned about creating unpredictable water flows in already flood-prone regions.

“Our rivers and our lakes are the most damaged of all ecosystems,” says Paul Kemp, a professor of ecological engineering at the University of Southampton. Globally, freshwater ecosystems are home to about a third of the planet’s vertebrate species, and these have been declining at twice the rate of marine and land-based animals. The situation in England is dire, with about 15% of rivers achieving good ecological status, according to the Rivers Trust.

Removing barriers “results in the greatest impact the most rapidly and for the least amount of money,” says Kemp.

In England, there are more than 50,000 barriers disrupting the passage of the country’s beleaguered rivers, according to the Environment Agency database. Scientists within the organisation suspect that there are many more. The overwhelming majority of these are relatively small: about 27,500 culverts, usually round concrete pipes, and around 16,300 weirs, which adjust the water level and effectively create a small dam.

These barriers are not only costly to maintain – they also cause avoidable environmental damage.

“A few decades ago, we thought only migratory species needed to move,” says Dr Perikles Karageorgopoulos, a senior technical specialist at the Environment Agency. “But we increasingly understand that all species need to move, from the tiniest ones that will have local migrations through to the others that migrate for many kilometres to overwinter or reach their spawning grounds.” Having so many barriers effectively creates a network of small dams or lakes, which offer plants and animals very different habitats from those found in free-flowing rivers, and are often not appropriate for river species.

Sediments also need to move. Barriers trap soil and geomorphological materials upstream, depriving downstream areas of sand and gravel, which are crucial for spawning creatures and many plants.

“Removing weirs is the most effective way of restoring a river,” says Karageorgopoulos. This is what happened in 2010, when a disused mill weir on the River Ouse in East Sussex failed and had to be removed. That stretch of the river had become a “ponded river with lilies”, he says. Now “there is a huge physical diversity that supports a large variety of species” – which also makes the river and its creatures more resilient to climate change.

“A diverse habitat is much more resilient to high or low [water] flows and extreme temperatures, and can provide a refuge for many species,” he explains. “In the past, the same river reach would have warmed up and become deoxygenated during low flows and warm weather. The warm water flowing downstream would have affected the ecology downstream too.”

In addition to that particular section, the weir removal had a positive impact on the river more than a kilometre upstream, Karageorgopoulos says. Within two years, the stretch of river was reclassified, moving from poor to good.

“It’s a good technique for focusing your efforts around restoration,” says Jesse O’Hanley, an environmental systems specialist and current associate dean of research and innovation at the University of Kent. “A lot of restoration efforts happen at a very small scale like planting some trees, bending the river in a little way or putting in some rocks. It’s a very hyper-localised solution that doesn’t really scale up and it’s expensive. It’s usually easier to let the river take care of it on its own.” This is what removing river barriers does: it lets rivers re-naturalise.

But letting a river choose its own path is a risky business, especially in flood-ravaged parts of England.

The Environment Agency estimates that about 3.4m properties in England are in areas at risk from surface flooding. Would removing river barriers make the situation worse, or improve it?

“Flooding is a natural process,” says Karageorgopoulos. “When you get really big floods, there just physically isn’t space in the river to contain the water.”

Rivers run in three dimensions: there is the flow we usually see in which water follows the channel of the river; but rivers also run laterally, breaching their banks and dispersing sideways on to flood plains, as well as vertically, linking the riverbed to the water table below. Obstacles and barriers have several consequences for a river’s passage in all three of those directions, both up- and downstream from the impediment.

The risk of flooding is always context specific, says Carlos Garcia de Leaniz, a professor of aquatic biosciences at Swansea University. Weirs and culverts can make flooding worse upstream, because they slow the water in the river, collecting it in mini-ponds and stopping it from flowing downstream. It is also quite easy for culverts to be blocked by trees and debris.

Additionally, the barriers stop sediment from moving down the river, causing it to collect in specific places. This substrate acts as a blanket on the riverbed, cutting off the link between the river and the water table, as the water cannot filter through. “That means less water is able to reach the water table and more water needs to be carried by the river channel,” says Garcia de Leaniz. “That means the risk of flooding may actually increase.”

The reality is that flooding is necessary and will happen – it’s about deciding where that water will go.

All the scientists the Observer contacted agreed that it is possible to predict the outcomes of taking away a culvert or weir. “There will be localised changes in flooding, and that is one of the consequences [of barrier removal],” says Hannah Cloke, a professor of hydrology and co-director of Water@Reading at the University of Reading. “But you can predict where those will be, and those are the kinds of things we should be doing anyway – making space for water on the flood plain.”

In a 2019 report, the Environment Agency warned that if current development continues on flood plains, the number of properties at risk of flooding could double in the next 50 years.

“We can only engineer our way out of some level of flooding… We can’t keep building giant concrete walls and structures in our rivers to control the water – that’s impossible. Working with the landscape is a much more sensible way of managing our rivers, and also has benefits for ecosystems and water quality,” says Cloke.

Europe and the US are leading the global charge on barrier removal, particularly when it comes to dam removal. (Britain has about 2,800 dams.)

The European Union has decided that by 2030 it wants 25,000 kilometres of rivers in Europe to be free-flowing, says Garcia de Leaniz. He headed the EU-funded Amber (Adaptive Management of Barriers in European Rivers) project, which found that there are more than 1m barriers fragmenting rivers in Europe and the UK. “Europe is making great headway; the UK, not so much,” he says.

A significant reason for this is that the UK, particularly England, is much more densely populated. “It is much easier to restore rivers when few people live around,” says Garcia de Leaniz. He also has a pragmatic approach to barrier removal. “We have so many obstacles to choose from; let’s start with those that are obsolete and those that pose a flood risk or hazard,” he says. “And in the bigger scheme of things, some people would argue that for the cost of removing one big dam, you can remove 100 or more barriers, which is going to be much more beneficial.”

In England, the Environment Agency tends to work on a more ad hoc basis, says Karageorgopoulos. While it has a list of priority barriers to remove – such as those that are costing money to maintain and which serve no function – it has to move when opportunities present themselves. Although the Environment Agency owns or maintains the vast majority of these barriers, many of them are on private land or affect private land.

A major challenge, says Kemp, is land and river ownership. “You can’t just go and do a strategy on a river because you’ve got multiple owners of that land and you have to work with them collegiately, and try to find a solution,” he says.

Often, the Environment Agency “will be able to do things when they find a landowner who is receptive to the idea, so it is very opportunistic,” says Kemp.

Even if the owner is on board, the process still takes time. Many years of planning and stakeholder consultation are required, says Karageorgopoulos. “Money is the obvious [obstacle],” to barrier removals, but communities and anglers are often vocal opponents. “We are human – we like routine. We’ve even had opposition from people who like to feed the swans and ducks in a specific location,” says Karageorgopoulos. But when there are no land restrictions and “everyone is onboard, it is the easiest way” to improve river health.

It will not “fix” England’s rivers, though, warns Cloke. “Naturalising rivers is always a good idea because natural flows are what rivers are designed to do,” she says. “In the long term, it’s probably helpful and is one of the lowest hanging fruits.”

However, none of this will address the major threats to British rivers – namely huge quantities of pollution in the form of sewage and agricultural and industrial waste, and development. “Should we be building on floodplains? No. Should we be polluting our rivers? No. Those are the giant questions that need careful thinking from government,” she says.

Do you pay with crypto, dig the blockchain and drive a Tesla? You’re an early adopter, a future-chaser, right? So, naturally, you’ll vote jelly.

These animals are taking over the ocean. They are the great survivors of evolution – and of the Anthropocene. They are thriving in an era of warming oceans and algal blooms, and when humans have wiped out larger marine predators. Jellyfish can survive when human pollution reduces oxygen levels in the ocean. And so we will embrace their future, our grandchildren tucking into jellyfish and chips.

British seas are mostly too cold to be swarming with jellyfish species but the barrel jellyfish is Britain’s largest jelly, and most frequently found in south-western seas.

Its hefty, translucent, mushroom-shaped bell can grow to the size of a dustbin lid. A pretty dustbin lid, sometimes shaded in yellow, pink or blue. The bottom edge of the bell is fringed with violet, and this contains the jelly’s sense organs. Below are eight frilly, stocky arms, which look a little like elongated cauliflowers. These contain small, stinging tentacles, which deliver food to surrounding tiny mouths, hundreds of them. The stings are designed to disable zooplankton – tiny sea creatures – and so don’t harm us.

Watch this gentle giant swim, pulsing in the blue depths, mesmerising us with its sensuous curves and extraterrestrial strangeness.

If you’re a traditionalist and lover of history, the barrel jellyfish deserves your vote too. Jellyfish have floated through Earth’s oceans for more than 500m years, miraculous beings with bodies made up of 90% water and living without brain, heart or blood.

Sadly, we tend to encounter them washed up, dead, on our beaches in early summer months. These jellies have underestimated their own size, following their tiny prey into shallow water and become stranded, washed in, washed up by the waves and tides.

In some parts of the world, people have eaten similar species from the Rhizostomae order for thousands of years. Its bland flesh is livened up with strong sesame oil and soy, and served in sushi, noodles and even ice-cream.

But this 35kg behemoth is the favourite food of the world’s largest sea turtle, the leatherback. It, of course, is struggling to survive in an ocean filled with plastic bags that can, fatally, resemble its favourite jellyfish prey.

So vote jelly. Vote for the great the survivor, food for the gods of the sea, and the real deal – definitely not a plastic bag.

• 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.