This column aims to introduce interesting species of Hong Kong flora and fauna that might be encountered during fieldwork. Distinctive physical characteristics and some interesting ecological facts are included for each example.

	Macrobrachium hainanense
	Nerita

Nerita
by Ariel Yeung

When you walk along the coastal region of Hong Kong, you can find one group of animals easily, species of Nerita. They belong to the family of prosobranch gastropods Neritidae that is widely distributed in tropical regions. The shell of Nerita are globular or depressed. They have a flattened base. The colour of Nerita differs from species to species. Typical Nerita albicilla shells are black in colour (but some are eroded so white colour can be seen) while Nerita yoldii shells are creamy yellow with black bands.

These animals are grazers, feeding on algae attached to the shore with their radulae. Because of their high abundance, they are important to the structure of the algal community in the coastal region. For example, halos from absent biofilm are common on western rocky shores, due to the grazing effect of Nerita.

Nerita has separate sexes. Unlike some other gastropods, they pocesses internal fertilization. You can identify the sex of a Nerita by seeing the presence or not of a triangle-shaped penis beside its right tentacle. During autumn, white egg capsules ('white disks' with diameter about 5 mm) can be seen easily on Hong Kong shores. Within each egg capsule, more than 100 babies can be found!

Fig.2. Nerita costata

There are more than 100 Nerita species in the world, while 8 of them can be found in Hong Kong. Most live on rocky shores. The commonest is Nerita albicilla in eastern oceanic waters, while in the western estuary region, a high density of Nerita yoldii can be seen. Some big guys such as Nerita costata (black in colour, Fig. 2) and Nerita plicata (white in colour) can also be found on the rocky shores of Cape d'Aguilar. Apart from rocky shores, Nerita spp. can inhabit mangrove regions, such as Nerita lineate, and sandy shore regions, such as Nerita chameleon. Both are abundant in Three Fathoms Cove.

In order to live in the intertidal region, a highly variable environment, they have a particular strategy. They hide in crevices most of the time to escape from predators and physical stress. They also have an endogenous rhythm to ensure they forage at the right time. Next time when you walk along the shores, try to search out these animals. Try to observe their behaviour and see how they have adapted to the environment.