Joseph Yip & Patrick Lai, Hong Kong Herbarium

植物分類學家最近根據染色體、分子分支地理區系、生理適應性及葉解剖結構等多方面証據，發現以南中國海為界，秋茄(Kandelia)可分為兩個種群系，代表秋茄屬內兩個種：(1) 南中國海以南及以西的種群系，分布北婆羅洲、馬來半島、泰國、緬甸及印度(模式標本產地，因此本種保留原學名Kandelia candel)；(2) 南中國海以北的種群系，分布於越南北部、海南島、香港、廣東、福建、台灣及日本南部，被定為新種。它具倒卵形的葉片，新學名是Kandelia obovata。經過審閱香港植物標本室所藏採於香港的秋茄樹標本及最近野外觀察所得的初步結論，香港秋茄樹種群的特徵均與新發表文獻所描述的K. obovata相符。

Kandelia obovata in Hong Kong

Kandelia is the most common mangrove plant in Hong Kong. It is widely distributed from western and eastern India, Burma, through the South China Sea region to southern China and southern Japan. The species – first discovered in Malabar, India – was named “Tsjerou-kandel” by van Rheede in 1686. Carl Linnaeus, the father of taxonomy, named it Rhizophora candel as its hypocotyl resembles a candle (as the Latin word “candel” suggests). Until recently, it had the name Kandelia candel and was recognized as the monotypic species of the genus.

Based on the recent studies in molecular phylogeography (Huang & Chen, 2000; Chiang et al., 2001), chromosome number, physiological adaptation and leaf anatomy, botanists found that there in fact existed two quite distinct geographical population groups of Kandelia separated by the South China Sea. The population group to the south and west of the South China Sea (i.e. from India, Burma, Thailand, Malay Peninsula to northern Borneo), to which the type specimen belongs, retain the scientific name Kandelia candel. The northern group (i.e. from northern Vietnam, Hainan, Hong Kong, Gunagdong, Fukien, Taiwan to southern Japan) was recognized as a distinct species by Sheue et al. (2003), who described and named the new species as Kandelia obovata, apparently for the obovate leaves.

Previous studies demonstrated that chromosome numbers of Kandelia differ between populations in Japan (i.e. currently considered as K. obovata) and India (K. candel), which were reported as 2n=36 (Yoshioka et al., 1984) and 2n=38 (Das et al., 1995) respectively. In addition, differences in their tolerance to cold have also been reported in Kandelia populations from different regions. Maxwell (1995) considered that local natural populations of Kandelia species (i.e. currently considered as K. obovata) were able to survive cold winter and reproduced well in Hong Kong, while transplants from Brunei and Thailand (K. candel) to the same site in Hong Kong had only 33% of survival and no fruit was produced during the same period. Similar tolerance of cold in Kandelia populations has also been reported in Japan and China.

Besides the above differences in physiological feature and chromosome number, Kandelia obovata can also be distinguished from Kandelia candel by several external morphological features. Table 1 summarizes some of the major differences between the two species and the characteristic features of Kandelia obovata are illustrated in Figures 1 to 3.

Our preliminary examinations on the specimens kept in the Hong Kong Herbarium and recent observations in the field revealed that the populations of Kandelia occurring in Hong Kong matched the key characters of Kandelia obovata as described by Sheue et al. (2003). In this connection, the proper taxonomic citation for the Kandelia in Hong Kong should be “Kandelia obovata Sheue, Liu and Yong (Kandelia candel auct. non (L.) Druce)”. Perhaps next time when you are out in the field, you may look for such key characters of Kandelia and see if you come to the same conclusion. Another issue to be followed up is providing the corresponding Chinese names for the two species. At present there is only one Kandelia species in Chinese speaking regions. Perhaps, we could still apply the same Chinese name (秋茄樹，亦稱水筆仔) for Kandelia obovata for the time being.
 

Table. 1. Comparison of key characters between Kandelia candel and K. obovata (extracted from Sheue et. al. [2003]) 

表一、兩種秋茄屬植物部份特徵的比較(取材自Sheue et. al. [2003])

Figure 1. Leaves of Kandelia obovata in Hong Kong. Figure 2. Hypocotyls of Kandelia obovata in Hong Kong.

Figure 3. Lateral veins of Kandelia obovata in Hong Kong.

References

Chiang, T. Y., Chiang, Y.C., Chen, Y.J., Chou, C.H., Havanond, S., Hong, T.N. and Huang, S. (2001). Phylogeography of Kandelia candel in East Asiatic mangroves based on nucleotide variation of chloroplast and mitochondrial DNAs. Molecular Ecology 10: 2697-2710.

Das, A. B., Basak, U. C. and Das, P. (1995). Karyotype diversity and genomic variability in some Indian tree mangroves. Caryologia 48: 319-328.

Huang, S. and Chen, Y.-C. (2000). Patterns of genetic variation of Kandelia candel among populations around South China Sea. In: Chiang, T. Y. & Hsu, T. W. (eds.), Wetland Biodiversity, Proceedings of Symposium of Biodiversity in Wetlands, pp. 59-64. Taiwan Endemic Species Research Institute, Nantou.

Maxwell, G. S. (1995). Ecogeographic variation in Kandelia candel from Brunei, Hong Kong and Thailand. Hydrobiologia 295: 59-65.

Sheue, C.-R., Liu, H.-Y. and Young, J.W.H. (2003). Kandelia obovata (Rhizophoraceae), a new mangrove species from Eastern Asia. Taxon 52: 287-294.

Yoshioka, H., Kondo, K., Segawa, M., Nehira, K., & Maeda, S. (1984). Karyomorphological studies in five species of mangrove genera in the Rhizophoraceae. Kromosomo 35-36: 1111-1116.