Oysters may be among the most favoured gourmet delicacies for many people. However, our focus today is not the juicy species on buffet tables, but a smaller type of oyster - Saccostrea cucullata, commonly known as Rock Oyster or Hooded Oyster. As dull-looking as it may seem, the species plays an important role in marine ecology. Every day, these oysters stand against the waves in the intertidal zone, and dutifully filter the water and stabilise the coastline. They are indispensable to nature and to us humans!
Rock oyster is a small (about 4cm) member of the bivalve family, which include relatives such as clams and scallops. It has a rough and irregular outer shell, sometimes rounded and sometimes in a wavy shape. The colour does not vary much though; typically, the centre of the shell is opaque white while the periphery is purple-black.
Rock oysters usually grow in groups. The species has a wide distribution and can be found even in Australia, Africa and South America. They are highly adaptive and can tolerate a wide range of temperature and salinity. By adopting external fertilisation, the female can produce a large number of eggs during high tide, which helps the spread of their species. They can be found along almost all coasts around the world. In Hong Kong, you can find rock oysters in most intertidal zones, such as Tai Tam Bay, Tolo Harbour, Starfish Bay and Ha Pak Nai.
Like most other oysters, rock oysters are filter-feeders that suck in organic matter in the water through their gills. In the process, the water is filtered and cleaned. Research has shown that commonly an oyster can filter 240-400 litres (the volume of a standard bathtub) of seawater a day. An oyster functions as a filter as well as a sponge, and will retain toxic substances (such as heavy metals) in its body. Scientists have monitored pollution levels in the sea by using oysters as biological indicators. In some places, oysters are even used for biological control. For example, Tokyo Bay – where the water was severely polluted since the 1950s, and swimming was banned – was reopened after six years of a purifying scheme, by culturing oysters since 2009.
Protecting the Coastline
Have you noticed that rock oysters always grow along the coastline? As a sessile animal, rock oyster attaches to a fixed location throughout its life cycle. The typical substrate is a rock or the base of a mangrove tree. In recent years, experts have proposed that the wavy surface of oyster can disperse wave energy and hence function better than concrete breakwaters. Natural oyster breakwaters may be used to protect coastlines from floods, typhoons and rainstorms.
Rock oysters filter harmful substances and clean up water for other marine organisms. After death, their shells become home to other coastal animals such as sand crabs and sea snails. The modest species is indeed the indispensable guardian angel of the ocean that we cannot live without!