From giant clams to zebra shark, marine biologists want to replace lost and vanishing species at sea but face unique obstacles – not least rampant overfishing

Kneeling on the seabed a few metres underwater, I pick up a clam and begin gently cleaning its furrowed, porcelain smile with a toothbrush. It’s a giant clam but a young one and still just a handful. Here in Fiji, giant clams or vasua as they are known, were so heavily overfished for their meat and shells that by the 1980s they were thought to be extinct locally. Australian clams were imported to start a captive breeding programme, and subsequent generations of their offspring have been released on coral reefs across Fiji. They’re still vulnerable to fishing and poaching, but if carefully guarded the giant clams do well and have become symbols of healthy corals reefs inside well-managed marine protected areas.

A key to their early survival is rearing them in cages to keep them safe from predators until they’re large enough to survive by themselves. However, the cages also exclude herbivorous fish, so the clams can easily get overgrown by seaweed, which is where the regular toothbrushing comes in.