by Liu Min and Valerie Ho

The pearlscale butterflyfish (Chaetodon xanthurus) has been recorded in the western Pacific, from Indonesia, Palau and the Philippines to the Ryukyu Islands (Map 1). Along coastal waters of China, the species has been noted in Hainan and Taiwan, and also in surrounding waters of Hong Kong, such as the Lema Islands. A pearlscale butterflyfish in eastern Hong Kong waters was first spotted at Bluff Island (Ung Kong Wan) on the 2 October 2005 during the Big Fish Count (see p.11), organized by the World Wide Fund for Nature Hong Kong; it was a single individual of 7 - 9 cm total length, swimming around stag-horn corals (Acropora sp.) at the depth of 4 m.

Map 1. Global distribution of the pearlscale butterflyfish (Chaetodon xanthurus).

The pearlscale butterflyfish can be readily distinguished from other members of the genus Chaetodon by the cross-hatched pattern on body sides due to black scale margins (Fig. 1). The recorded maximum size of the species is approximately 14 cm standard length. Juveniles are particularly restricted to live coral areas and remain close to shelter; adults can be found in outer reef slopes and drop-offs to a depth of 50 m (Allen et al 1998). Fish usually occur singly or in pairs and are active during daylight hours feeding on small benthic invertebrates and algae. Little is known about the biology of this species.

Fig. 1. The pearlscale butterflyfish (Chaetodon xanthurus) (Photo: Robert Yin).

There are another two species in the genus Chaetodon which share a similar appearance with the pearlscale butterflyfish; the atoll butterflyfish (C. mertensii) (Fig. 2) from the Indo-west Pacific, and the Eritrean butterflyfish (C. paucifasciatus) (Fig. 3) from the Red Sea and neighbouring Gulf of Aden. However, neither has the strong crosshatched pattern seen in the pearlscale butterflyfish.

Fig. 2. The atoll butterflyfish (Chaetodon mertensii) (Photo: John E. Randall).

Temperature and salinity of nearshore waters of Hong Kong, especially in the east, are determined by three major water currents; the Kuroshio Current from the Pacific and the Taiwan Current from the East China Sea in the winter, and the Hainan Current from the South China Sea in the summer, which support subtropical and tropical marine fish species in the area. It is likely that these currents bring some unusual marine fish larvae or juveniles to settle in Hong Kong waters; however, it is not clear whether these species are able to maintain a population locally (Sadovy & Cornish 2000).

Fig. 3. The Eritrean butterflyfish (Chaetodon paucifasciatus) (Photo: John E. Randall).

Bibliography

Allen, G.R., Steene, R. & Allen, M. (1998). A guide to angelfishes and butterflyfishes. Odyssey Publishing/Tropical Reef Research.

Sadovy, Y. & Cornish, A.S. (2000). Reef fishes of Hong Kong. Hong Kong University Press, Hong Kong.

More information about the Big Fish Count and on butterflyfishes is available from http://www.wwf.org.hk and http://www.fishbase.org, respectively.