Hong Kong Corals & the Associated Marine Life

Phylum Chordata includes both vertebrates and invertebrates, comprising three sub-phyla: Urochordata (Sea squirts & Salps), Cephalochordata and Vertebrata (vertebrates)

Sea squirts are the invertebrates related most closely to vertebrates, though they are most often confused with sponges, the simplest living group of multicellular organisms. Some sea squirts are entirely pelagic while those commonly found in coral areas are benthic, in both solitary and colonial forms.

Both solitary and colonial sea squirts have a body wall with two openings, the oral and cloacal siphons. Long bundles of muscle fibres in the body wall can contract the body quickly to produce a fine stream of water coming in and going out the siphons. Within the body wall are the branchial sac for gas exchange, food-gathering structures, stomach, circulatory system, gonads and other internal organs.

There are four major classes of fishes alive today, the lampreys (Cephalaspidomorphi), the hagfish (Pteraspidomorphi), the cartilagenous fish (Chondrichthyes) and the bony fish (Osteichthyes). Only the cartilagenous fish (sharks and rays) and the bony fish (most other fish) inhabit reef. There are about 350 species of shark and 500 rays worldwide. Among the estimated 23,000 species of bony fish, around 13,000 are marine.

 Reef fish are those that spend part or all of their lives on or close to areas of hard substrata. Most "reefs" in Hong Kong are rocky although there are a number of small coral communities, particularly in eastern waters. They provide food and shelter to the fish that inhabit them.

The biodiversity of reef fish in Hong Kong is relatively high, being greater than that of Hawaii for instance. It is similar to other locations in the South China Sea such as Hainan Island. Hong Kong has an interesting mix of tropical species that are distributed widely throughout the Indo-Pacific and temperate species which mostly occur from Hong Kong north to Japan and Korea. No reef fish are known to be endemic to Hong Kong.

Although the diversity of local reef fish is high, some of them such as the surgeon and triggerfish that are normally common on coral reefs are rare now. This is likely because local conditions are not suitable for them, or there are few larvae of these species settling onto local reefs. Reef fish are heavily dependent on the continued survival of hard corals. Having healthy coral communities in local waters is essential for the continual survival of the reef fish.