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Mangrove is one of the most ecologically and socioeconomically important types of natural habitats found in the intertidal zone of tropical and subtropical regions. It occurs on soft shores that receive inputs from regular tidal flushing and from freshwater streams and rivers. Mangroves have high productivity, particularly the detritus from mangrove plants (e.g. fallen leaves and branches) provides a massive amount of food for marine life. As such, mangroves are important breeding sites and feeding grounds for a large variety of invertebrates, fish and other coastal species. In some parts of the world, coastal and indigenous communities rely on the wood of mangrove plants for construction and fuel. Mangroves also protect shorelines from erosion due to currents, waves, wind and rain. Besides, the root systems of mangroves filter and trap sediments and pollutants, and hence are essential to maintaining water quality. Although mangrove environments are highly dynamic (e.g. high temperature, fluctuating salinity, alternating aerobic and anaerobic conditions, periodically wet and dry, unstable and shifting substratum), diverse groups of flora and fauna have adapted to thrive in this habitat.

The massive freshwater influence of the Pearl River, especially during the wet season, promotes the development of mangroves in Hong Kong in the western region (e.g. Deep Bay). Nevertheless, due to the cold winter and a lack of suitable substrate, mangroves in Hong Kong are relatively scattered and their extent is also limited compared to most tropical areas. Currently, Hong Kong has more than 60 mangrove stands which cover a total area of about 624 ha. They are mainly distributed in six districts: Sai Kung, Northeast New Territories, Tolo Harbour, Deep Bay, Lantau Island and Hong Kong Island (Distribution Map). Mai Po Nature Reserve hosts the largest local mangrove stand, and particularly some of the mangrove areas at this site were traditionally converted to gei wais (traditional shrimp farming ponds) and fish ponds.

Local mangrove stands are formed by 8 true mangrove species including Kandelia obovata, Black Mangrove (Avicennia marina), Aegiceras corniculatum, Leather Fern (Acrostichum aureum), Milky Mangrove (Excoecaria agallocha), Coastal Heritiera (Heritiera littoralis), Many-petaled Mangrove (Bruguiera gymnorhiza) and Lumnitzera racemosa. Among these species, Kandelia obovata is the most common species dominating most of the mangrove stands.

Pak Nai Mangrove, one of the largest local mangrovesSome mangrove plants (e.g. <em>Kandelia obovata</em>) reproduce by vivipary ('seeds' start to germinate before they detache from parent plants), a special reproductive mode to increase survival in the unstable substratum.Black Mangrove (<em>Avivennia marina</em>) develops extensive 'pencil roots' (pneumatophores) to facilitate gaseous exchange under water-logged soil conditions. <em>Aegiceras corniculatum</em> develops salt glands to concentrate and actively excrete the absorbed salts so as to regulate internal salt concentration. Sesarmid crabs are the most diverse crabs (more than 20 species) in local mangroves. They feed mainly on mangrove leaves but also on other organic matters such as sediment detritus and faunal tissue. The video shows the feeding behaviour of <em>Sesarmops intermedius</em>.Fiddler crabs (e.g. <em>Austruca lactea</em>) are common in most of the local mangroves. Male fiddler crabs are characterised by a striking large cheliped which is used mainly for courtship and combat against other males.Ting Kok Tree Climbing Crab (<em>Haberma tingkok</em>) is the only exclusively arboreal crab found in local mangroves, and they have been recorded in Hong Kong only so far.<em>Perenella</em> spp. are among the most common mud snails found in local mangroves.Mangrove plants provide a 3-dimensional environment for species to live. <em>Cerithidea moerchii</em> is a common arboreal snail species inhabiting local mangroves. Mudskippers (e.g. <em>Periophthalmus modestus</em>) are a group of unique fish that can survive both in and out of water. Their pectoral fins function similarly to legs that allow them to walk on the mud of mangroves.

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