Kurusadai Island

Kurusadai Island — also spelt Krusadai — is a low, uninhabited islet of sand, scrub and coral rubble lying off the Mandapam coast on the Gulf of Mannar side of the Ramanathapuram mainland, a short way south of the Pamban channel that separates the mainland from Rameswaram island.

• To a passing traveller it is an unremarkable green smudge on turquoise shallows; to Indian science it is one of the most famous few hectares of shoreline in the country.
• In the 1920s the Madras Fisheries Department established a marine biological station on the island, giving researchers a permanent base beside reef, sandflat and seagrass habitats that could be walked at low tide.
• From the 1930s the Madras Government Museum began publishing the survey monographs known collectively as the "Fauna of Krusadai Islands", which catalogued the sponges, corals, molluscs, echinoderms and worms of these flats in a level of detail then unusual for Indian waters.
• Those volumes turned one small islet into a fixture of zoology teaching across Indian universities, and they are the reason its name is familiar to many people who will never see it.
• The island's signature animals are its acorn worms — the hemichordates Ptychodera and Balanoglossus, soft, burrowing creatures that older textbooks celebrated as a "living fossil" and as a connecting link between invertebrates and vertebrates.
• Modern classification is more sober: hemichordates are deuterostomes, more closely related to starfish and sea urchins than to any vertebrate ancestor.
• What has not changed is their abundance here.
• It was the sheer density of these worms and their neighbours in Kurusadai's intertidal flats, rather than any rarity, that made the island a teaching and collecting ground in the first place, and later a place to be protected rather than harvested.
• Kurusadai sits inside the Gulf of Mannar Marine National Park, and its fringing reef, sandflats and seagrass are managed as part of that protected island chain.
• What matters for a visit is the access regime.
• The island is uninhabited and strictly protected, and trips are permitted only with prior clearance from the Tamil Nadu Forest Department, usually as a short guided run from the Mandapam coast in calm, clear-sea months.
• Nothing may be collected, and the reefs are best appreciated from a Forest Department-approved glass-bottom or country boat rather than by landing; swimming, snorkelling and any contact with the coral are not part of a permitted visit.
• Sea conditions, not the calendar, decide whether a boat goes out at all, so treat the outing as a possibility to confirm on the day rather than a fixed booking, and expect a naturalist's half-day rather than a beach outing.
• Eat before or after, never on the island, which has no shops, stalls or drinking water.
• Mandapam is a working fishing town and Rameswaram a working fishing island, so on both sides of the bridge look for an eatery that clearly displays a "pure vegetarian" or "sudha saivam" board and confirm at the counter before ordering, rather than assuming any street or class of eatery is meat-free.
• The widest choice is in the temple town, where unbranded South Indian "meals" hotels near the Ramanathaswamy gopurams serve banana-leaf plate meals and tiffin through the day — check the board there too.
• Carry water and a packed vegetarian snack for the boat.