By Peter Yeung HONG KONG — Thousands of grayish, oval-shaped oysters pepper the mudflats of far northern Hong Kong, clustering together on fragments of rock lining the shoreline or clamping onto abandoned concrete posts. Despite being stuck in between a proverbial rock and a hard place — the bustling megalopolis of Hong Kong sprawling behind and the spiky skyline of Shenzhen and its 17 million inhabitants just across the water — the area is rippling with activity. Crabs scamper between shells and burrow holes underground; mud hoppers splash between the brackish puddles left by the low tide; an…