Hidden from sight within the meandering lower reaches of the Torridge, one of Britain’s most endangered species is clinging on to life, holding fast among the rocks in the ever-changing flow.

By a secluded wooded section of the bank, the two of us pulled on waders before edging into the current. My expert guide, Izzy Moser of the Devon Wildlife Trust, went first, using a bathyscope – a glass-bottomed funnel – to view the stony riverbed; I followed, carefully matching her footfalls as we slowly made our way downstream.

Izzy Moser of the Devon Wildlife Trust uses a bathyscope to search the riverbed for pearl mussels. Photograph: Charlie Elder

“There’s one,” she said eventually, pointing to the shallows. It took a while to get my eye in, but under the surface I could make out the distinctive shape of a large mussel, hinges pressing its dark shell halves together in a flat smile. Another two palm-sized mussels hugged a rock a couple of feet behind.

A 12cm freshwater pearl mussel, half-hidden under sandy sediment in the river Torridge. Photograph: Charlie Elder

Larger than seaside mussels, these individuals were freshwater pearl mussels (Margaritifera margaritifera), a protected species. Once numerous across Britain and harvested for the pearls a few contain, they have disappeared from areas where water quality has declined; the estimated 2,000 mussels in the Torridge make up the largest remaining stronghold south of Yorkshire.

“These mussels are real survivors and up to 80 years old,” said Izzy, “but they are not thought to have successfully reproduced here since the 1960s.” She is project officer on the trust’s Biffa award-funded scheme to save these threatened colonies.

The pearl mussels’ lifecycle, and their ability to distribute upstream as well as down, is intimately tied to the presence of juvenile salmon and trout. Millions of mussel larvae are ejected into the water in late summer in the hope that a few latch on to the gills of passing fish, where they spend months in this oxygen-rich environment, before dropping off and developing into adults on the riverbed.

From drifting hitchhiker to sedentary filter-feeder, the mussels lead a precarious existence, their survival dependent on clean water, plentiful host fish – and good fortune. Those that make it can endure to become one of Britain’s longest-lived animals. All being well, the aged mussels I saw could keep gently sieving the river flow where we left them for decades.