Hong Kong Countryside Watch: Development at Pak Lap threatens Hong Kong living fossil Amphioxus

Hong Kong is blessed with the presence of three classic living fossils:

1.Amphioxus (Branchiostoma belcheri, B. belcheri tsingtauense, B. malayanum, Epigonichthys culltellus and E. lucayanus);

2. Lingula (Lingula lingua); and

3. Two species of horseshoe crab (Tachypleus tridentatus and Carcinoscorpius rotundicauda)

Amphioxus

Amphioxus belongs to the Cephalochordata, a very small branch of the animal kingdom.

Also known as lancelets, cephalochordates are small, eel-like animals that spend much of their time buried in sand. However, because of their remarkable morphology, they have proved crucial in understanding the morphology and evolution of chordates in general -- including vertebrates.

Water is taken in through the mouth, drawn in by the beating of cilia located on the wheel organ, a set of ridges lying inside the mouth. The water is first filtered by the oral cirri, slender projections that surround the opening of the mouth.

Since cephalochordates have no hard parts, their fossil record is extremely sparse. However, fossil cephalochordates have been found in very old rocks, predating the origin of the vertebrates. Yunnanozoon, from the Cambrian of south China is over 500 million years old and the oldest known cephalochordate fossil^1.

Study on “The ecology and biology of amphioxus in Hong Kong”

In 2003 - 2004 surveys were conducted by City University of Hong Kong along the coast of Sai Kung at 34 sites in four bays, Nam She Wan, Tai Long Wan, Long Ke Wan, and Pak Lap Wan, to determine the extent of distribution of amphioxus as well as their abundance, habitat preference and community structure.

Results showed that amphioxus are present in shallow water depth between 5 and 23 m in all of the four bays, with high population abundance being found at some specific locations in Tai Long Wan and Pak Lap Wan, with a maximum of 460 and 290 individuals/m², respectively.

Sediment analysis indicated that amphioxus was only confined to sediments containing a high percentage of sand and granule, wth a median diameter, low organic content and low moisture content.

Further research on the habitat requirements of amphioxus was conducted by monitoring the sediment and water quality at one site in Tai Long Wan and another in Pak Lap Wan from November 2003 to October 2004. The results indicate that seawater at the amphioxus habitats had two essential characteristics:

1. Oceanic;

2. Low content of suspended solids.

Such findings were consistent with results obtained from laboratory observations: Amphioxus (B. belcheri) could not survive at low salinity and high suspended solids in the water could induce physical damage on the animal’s oral cirri.

From this, it was concluded that clear oceanic water combined with sand sediment with low organic content are the most important habitat requirements for amphioxus².

Amphioxus body plan and feeding position (in sediment)³