adapted to freshwater and intolerant of salt water – are native to the island, on account of the islands' oceanic origin and relatively recent geologic origins. Marine ecology Mayotte, the oldest island, is surrounded by a barrier reef, with extensive reefs and lagoons between the reef and the island. The reef and lagoon cover over 1,000 km2. They are home to more than 760 species of marine fish, including 17 threatened species, 581 species of marine arthropods, more than 450 cnidarian species, and 24 marine mammal species. The other Comoro Islands are surrounded by fringing reefs, which form a narrow platform extending a short distance from the coast. Reefs fringe about 60% of the coast of Grande Comore, 80% of Anjouan's coast, and 100% of Mohéli's coast. The Sima Peninsula on Anjouan is surrounded by a vast reef. The islands also have rocky reefs on basalt substrates. Mayotte has 7.6 km2 of seagrass beds. The extensive seagrass beds in Moheli Marine Park account for about 90% of the remaining seagrass beds in the Comoros. Other seagrass beds occur at Mitsamiouli, Malé and Ouroveni around Grande Comore and at Bimbini and Ouani around Anjouan. There are eight seagrass species in the Comoros. The islands' seagrass beds have been degraded by sedimentation and climate change. Between 1993 and 1998 the Thalassodendron ciliatum beds at Moheli Marine Park were destroyed by a large sediment influx into the lagoon from upland deforestation coupled with high rainfall. Mayotte has 8.5 km2 of mangroves. There are approximately 120 ha of mangroves on the other Comoro islands, 75% of which are on the south coast of Mohéli, particularly around Damou and Mapiachingo. Smaller areas of mangrove are also found on Grande Comore and Anjouan. There are seven mangrove species in the Comoros, with Rhizophora mucronata and Avicennia marina the most abundant. Marine mammals in the Comoros' seas include the humpback whale (Megaptera novaeangliae) sperm whale (Physeter macrocephalus), dugong (Dugong dugon) spinner dolphin (Stenella longirostris), pantropical spotted dolphin (Stenella attenuata), Fraser's dolphin (Lagenodelphis hosei), bottlenose dolphin (Tursiops sp.), Risso's dolphin (Grampus griseus), melon-headed whale (Peponocephala electra), short-finned pilot whale (Globicephala macrorhynchus), pygmy killer whale (Feresa attenuata), Blainville's beaked whale (Mesoplodon densirostris), and Longman's beaked whale (Mesoplodon pacificus). Other marine fauna include the green sea turtle (Chelonia mydas), most abundantly on Mohéli and Mayotte where they still come to lay eggs, and the coelacanth, a deepwater fish known from