After lunch Khamis the head ranger led me through the forest that covers Chumbe, holding aside trailing creepers and pointing out the initials carved by British sailors on a huge baobab tree, and the date - 1942. The baobab was already old by then - it's been growing here for 500 years. Its roots climb down into an intertidal pool, inky and dank-smelling, and mingle with the roots of the black mangrove trees that grow up through the water. Like most of the species on Chumbe, the black mangrove has adapted to an unpromising environment. To maximise oxygen intake, its roots grow up out of the ground and form 'noses' that stick out of the soil for a breath of air. Mangrove seeds are long and pointed, weighted at one end so they'll fall vertically and start to take root immediately if they find themselves in soil. If they fall into water, they float, sometimes up to 200 km without rotting, until they find a more promising place to grow.
Chumbe's forest also provides a sanctuary for the endangered Ader's duiker, a small antelope also found in the Jozani Forest reserve in the centre of Zanzibar Island. Studies were carried out which showed the forest habitat on Chumbe to be ideal for the duiker, and several adults were relocated by boat from Jozani to the island to start a new colony.
Periodically, as we picked our way through damp loam and fallen leaf matter, trying to avoid the drips from the tree canopy above us, we come across a neat square of string on the forest floor, and some plastic labels on which notes had been scrawled in waterproof ink. As well as its unique mixture of plant, fish and coral life, the island also supports a small seasonal colony of American biologists and college students, who live in gaily-coloured tents and come out early in the morning to measure and record the progress of the forest or the reef.
I barely had a chance to doze off in my hammock before the tide was in and the time was ripe for snorkelling over Chumbe's coral reef - described by Professor Charlie Veron, chief scientist at the Australian Institute of Marine Science, as 'one of the most spectacular coral gardens to be found anywhere in the world'. I floated for hours, entranced, above a science-fiction world of green and blue forests, valleys of puce, spiky trees and plains of yellow feathery grass. Outlandishly coloured and patterned fish pottered busily around, nibbling fussily at the coral garden or grazing in herds, like cows. The stillness was so complete I could hear the sounds they made with each mouthful as they cropped at choice tentacles. A huge, impossibly patterned lobster moved slowly across the sea bed, waving feelers a foot long at the tiny purple jellyfish that floated along the surface and bumped against my mask. An underwater field guide, floating from a rubber ring, gave the fish names as outlandish as their appearance - Golden Trumpetfish (long and tubular), Goldsaddle Goatfish (with beard), Bullethead Parrotfish (belligerent and beaked), Picasso Triggerfish (appears to be made out of bits of other fish) and lastly, lugubriously staring - Bigeye.
Later, after a dinner lit by flickering candles in jars and accompanied by the sound of the sea, Khamis took me out looking for coconut crabs. These are the world's biggest land-dwelling crustacea, endangered in much of the world, shy and nocturnal. They start their lives in the sea as humble hermit crabs, then leave their borrowed shells behind, grow until they weigh up to 4kg, and start to feed on coconuts that have fallen from trees, cracking them open with enormous claws. If coconuts get a bit scarce on the ground, they are said to climb the palm trees and snap off the fruit at the top.