Hong Kong Corals & the Associated Marine Life

Phylum Cnidaria

Cnidarians include a wide range of corals, sea anemones, hydroids and jellyfish. The unique feature of this group of animals is the presence of stinging cells (nematocysts). With a coiled, venom-filled thread of tiny barbs, the stinging cells are used for capturing prey and protecting against predators. When catching a prey or being attacked, the animal will fire the stinging cell powerfully and inject the venom into the target.

Millepora  sp.

There are about 2,700 species of hydrozoans. Most of the animals are small in size and assume a plant-like appearance. They are always ignored or mistaken for seaweed. Hydrozoans occur as either polyps or medusa forms or both. Some of them are capable of producing painful stings, such as the Portuguese Man-of-war (Physalia sp.). Some others, such as Stylaster sp. and Millepora sp. have calcareous skeleton and resemble stony corals. They are generally called Hydrocorals.

Hydroids are the most common form of hydrozoans occurring on coral reefs. They are colonial animals. Each colony has a central stalk and many side branches, displaying a feather-like structure.

Some 200 species of jellyfish have been described from the Arctic to tropical oceans. They are almost entirely marine and mostly inhabit coastal waters. Although most of the jellyfish are harmless but a few species can produce virulent stings that may cause death, such as the Box Jelly (Chironex fleckeri).

With a mucus coating, small fishes can swim among the tentacles of the jellyfish without being stung. They obtain protection from the jellyfish as the clownfishes from their anemone hosts. When the fishes grow large enough to fend for themselves, they will leave and never come back.

Sea anemones are similar to coral polyps but they are always solitary and have no hard skeleton. The body is columnar in shape, with a flattened "foot" at the base for attaching to the substratum. The mouth faces upward and is located at the centre of the body. The surrounding tentacles are used for feeding and cleaning the sediments on the body. As in many species of corals, the tentacles of some anemones contain symbiotic algae. The algae can photosynthesize organic food to the host anemones.

Large anemones in shallow waters always act as hosts for fishes and other invertebrates. So as to prevent the anemone from detecting them as potential prey and avoid being stung, the anemone fishes secret a mucous coating on the surface.