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Introduction

Mangroves are habitats of enclosed intertidal mud flats where wave action is greatly reduced and which receive an influx of freshwater. Major mangrove habitats in Hong Kong are located along Deep Bay (e.g. Pak Nai and Tsim Bei Tsui, where salinity is very low under the influence of freshwater from the Pearl River) and along some mud flats where salinity is lowered by surrounding streams (e.g. Three Fathoms Cove and Ting Kok). Trees living in this habitat are referred to as mangrove trees. During high tides, parts of the trunks of these trees are immersed whilst they are completely exposed during low tides. Mangroves, therefore, pose a very challenging environment. Plants living in mangroves will face difficulties in gaining strong anchorage in the soft substratum, obtaining enough oxygen from the anoxic mud and also need to tolerate fluctuating salinity and desiccation stress during low tides. Organisms living in this habitat have, therefore, adaptive features enabling them to survive in such a stressful environment.

Common mangrove tree species in Hong Kong (Table 1, and also refer to the laminated common species identification sheet) include Kandelia candel, Avicennia marina, Acanthus ilicifolius, Bruguiera gymnorrhiza and Aegiceras corniculatum, which are located near the seaward areas of the mangrove, and Lumnitzera racemosa and Excoecaria agallocha, which are commonly found near the landward side where there is more freshwater input. In the landward areas there are also a variety of shrubs and trees which are never immersed by the high tides which can be described as mangrove associates. Common mangrove associates in Hong Kong include Hisbiscus tiliaceus, Clerodendron inerme and Pandanus sp.

 

Species	Family	Distinguishing features
Acanthus ilicifolius	Acanthaceae	Prickly leaf margin (spiny mangrove)
Aegiceras corniculatum	Myrsinaceae	Red petiole, salt glands on leaf
Avicennia marina	Avicenniaceae	Pneumatophores present
Kandelia candel	Rhizophoraceae	Opposite, elliptical leaves
Bruguiera gymnorrhiza	Rhizophoriceae	Large, lanceolate leaves
Excoecaria agallocha	Eurphorbiaceae	White latex exuded when damaged
Lumnitzera racemosa	Combretaceae	Small, thick oval leaves

Table 1. Common mangrove tree species in Hong Kong and their distinguishing features.

 

Mangroves provide a complex, 3-dimensional habitat for organisms to live in. Organisms can inhabit the mangrove tree trunk, leaves, root surface, the mud surface and also burrow into the mud. The mud snail Terebralia sulcata, for example, inhabits both the mud surface and the mangrove tree trunk. Sessile organisms, including the oyster Saccostrea cucullata and barnacles Euraphia withersi, attach onto the mangrove tree trunks and root surfaces. Bivalves such as Anomalocardia squamosa and Gafrarium pectinatum burrow inside the mud. Crabs such as Scylla spp. and Perisesarma bidens burrow inside the mud but to feed they emerge from their burrows, and Perisesarma actually climb into the trees!

Hong Kong mangroves are, however, under severe stress due to reclamation and urbanization and as a result the conservation of these habitats has begun to receive increased attention. The Agriculture, Fisheries and Conservation Department (AFCD) of the Hong Kong SAR Government has assigned marine parks and marine reserves to protect marine habitats in Hong Kong. At present, the mangrove at Hoi Ha Wan Marine Park is protected so that collecting organisms is prohibited (see Learning Support Centre for further information). Apart from assigning marine parks, the AFCD and some local conservation groups have also planted mangrove seedlings at Tin Shui Wai, Kei Ling Ha Lo Wai and Pak Sha Wan (Hebe Heaven). In the year 2003 - 2004, there will be another extensive artificial mangrove stand created by the AFCD in Tai O.