Arthropoda is the largest phylum of living creatures. It is composed of roughly one million species, accounting for over three-fourth of all animals described. Class Crustacea of this phylum is a dominant group found in coral areas. It includes not only the well-known lobsters, shrimps and crabs, but also the land-dominating insects. This group of organisms is incredibly diverse with regard to sizes, shapes, colours and life styles.

Crustaceans are characterized by the presence of a rigid external skeleton, a segmented body, jointed limbs with internal muscular attachment, two pairs of antennae and compound eyes. Since they grow continuously throughout the life cycle, their shells are molted and replaced at regular intervals.

Some shrimps and crabs form symbiotic associations with a large variety of organisms, including sea urchin, starfish, sea anemones, coral and so on. The basis of the association is variable depending on the partners involved. In most of the cases, shelter, protection and/or food can be obtained from each other.

An interesting symbiosis exists between alpheus shrimp and goby. Since the shrimp has extremely poor vision, it needs a partner as "watchdog". The goby stands guard at the entrance of their living burrow and signals the shrimp for danger by flicking the tail. As a return, the shrimp excavates continuously to maintain their living place.

More than 240 species of pontoniine commensal shrimp are known from the Indo-Pacific and 20 species are known from local waters. There are over 800 species of marine crab in waters around China but the number of species found in Hong Kong is still unknown.