Palk Bay. Researchers observed a pattern of bleaching in corals when the temperatures rose to between 32 °C and 36 °C. Marine activities The Gulf of Mannar is known for its pearl banks of Pinctada radiata and Pinctada fucata for at least two thousand years. Pliny the Elder (23–79 CE) praised the pearl fishery of the gulf as one of the most productive in the world. Although extraction of natural pearls is considered too expensive in most parts of the world, it is still carried out in the gulf. The chief seaports on the Gulf of Mannar are Thoothukudi (Tuticorin) in Tamil Nadu, and Colombo in Sri Lanka. While these ports can accommodate deep-draft vessels, the shallow Palk Strait can only accommodate small shallow-draft vessels. In July 2005, the Indian Government took preliminary steps to go ahead with the Sethusamudram Shipping Canal Project, which would create a deep channel linking the Gulf of Mannar to the Bay of Bengal. Project boosters emphasize the benefits of a direct shipping route that connects India's east and west coasts without the long trip around Sri Lanka; environmentalists have warned against the grave damage such a project could cause to the sea life and fisheries of the Palk Strait and the Gulf. See also Coral reefs in India References Bodies of water of Mannar District Coromandel Coast Mannar Mannar India–Sri Lanka border Landforms of Tamil Nadu