Mai Po Biodiversity: Mangroves
by Thomas Gomersall
The largest of its kind in Hong Kong (at about 450 ha) and the sixth-largest in China, the mangrove forest of Mai Po and Deep Bay is, like all mangrove forests, a tough place for animals and plants to live in. The ground is unstable, the salinity varies wildly and the constantly changing tide makes dry land an extremely fleeting commodity.
In conditions like these, most plants would not survive. But with their unique adaptations such as an extensive root system for better anchorage and a waxy leaf cuticle to help retain water, mangrove trees take it all in their stride. Seven species are found in Mai Po mangroves, providing a vital nursery ground for marine fish and shrimp, as well as food, roosting platforms and even display grounds for a wide range of other animals.
Collared Crow (Corvus torquatus): By day, the collared crow can frequently be seen flying over the fish ponds and gei wai of Deep Bay or foraging at the edge of the water (Viney et al, 2005, p. 218). By night, however, it prefers to roost in the mangroves. A resident species found across Hong Kong, the collared crow’s largest population, at over 100 individuals, live in Deep Bay (Tipper, 2016, p. 100), with Hong Kong as a whole home to almost 20 per cent of the global population. This makes Hong Kong and especially Deep Bay a place of real conservation importance to the collared crow, whose numbers have rapidly declined from 30,000 to fewer than 2,000 individuals in just over a decade, with much of the remaining population being in China. The drivers of this decline are pesticides, hunting and the illegal wildlife trade (Su, 2017).
False Tiger Moth (Dysphania militaris): One of the most commonly quoted differences between butterflies and moths is that butterflies are colourful and active during the day while moths are drab and nocturnal. There are exceptions to every rule, however, and the false tiger moth is one of them. This brightly coloured, diurnal moth is a very common sight in the mangrove forests of Mai Po, as its caterpillars feed on the leaves of the mangrove, Kandelia obovata (Tong et al, 2006).
Bent-winged Firefly (Pteroptyx maipo): Another, much more unique insect found in the mangroves of Mai Po is the bent-winged firefly. Part of a large group of related fireflies (Genus: Pteroptyx), this species was first discovered here in 2010, but was not described as a new species until the following year (WWF-Hong Kong, 2012; Ballantyne et al, 2011). Not much is known about this species, although firefly surveys by WWF have found that it does not congregate in large numbers on the tops of mangrove trees to display its light (unlike other members of the Pteroptyx genus) and that its population peaks in May and September. Confined to Deep Bay, the largest population of this species lives in Mai Po, making the mangroves here vital for its conservation (WWF-Hong Kong, 2012).
Mangrove Mud Crab (Scylla paramamosain): In Mai Po, the most many people tend to see of this large crab is its molted shell lying in the mud. In fact, a more likely place to see the whole crab is on a dinner plate, as it is a popular food animal in China and the aquaculture industry for it there is booming (Ma et al, 2006). However, it is best not to buy it very often, as farmed crabs are usually sourced from wild-caught juveniles, leading to the overfishing of several populations (Shelley, 2008, p. 255). It prefers to hide in burrows away from humans (Fong et al, 2005, p. 64) and feeds on smaller aquatic animals like brine shrimp (Chen et al, 2013), but has also been known to cannibalise members of its own species (Shelley, 2008, p. 259).
Milky Mangrove (Exoecaria agallocha): At first glance, the milky mangrove may seem just as unassuming as the six other mangrove species growing around it. But looks can be deceiving and there is a reason why its other common name is the ‘blind-your-eye mangrove’. Inside its branches is a thick, milky, fluid latex that leaks out when the branch is broken. This latex is highly toxic and will cause severe skin irritation and temporary blindness should it come into contact with a person’s skin or eyes (Fong et al, 2005, p. 23; Mondai et al, 2016.)
Walk for Nature takes places on 7–8 November 2020 at Mai Po Nature Reserve, with the theme “Our Habitat, Our Home”, spotlighting the rich biodiversity of this Ramsar site.
References
· Ballantyne, L., Xin, H.F., Shih, C.H., Cheng, C.Y. and V. Yiu. 2011. Pteroptyx maipo Ballantyne, a new species of bent-winged firefly (Coleoptera Lampyridae) from Hong Kong, and its relevance to firefly biology and conservation. Zootaxa, vol. 2931: 8pp.–34pp.
· Chen, X.L, Lin, Q.W., Wang, G.Z., Li, S.J. and H.H. Ye. 2013. Feeding in the megalopae of the mud crab (Scylla paramamosain): mechanisms, plasticity, role of chelipeds and effect of prey density. Marine and Freshwater Behaviour and Physiology, vol. 46 (5): 321pp.–336pp.
· Fong, T.C.W, Lai V.C.S and H.T.H. Lui. 2005. Photographic Guide Series of Hong Kong Nature (2): Estuarine Organisms — Mangrove, Mudflat and Seagrass Bed. Jan KC Chan, HK Discovery Limited, Hong Kong. 23pp., 64pp.
· Ma, L.B., Zhang, F.Y., Ma, C.Y. and Z.G. Qiao. 2006. Scylla paramamosain (Estampador) the most common mud crab (Genus Scylla) in China: evidence from mtDNA. Aquaculture Research, vol. 37: 1694pp.–1698pp.
· Mondai, S., Ghosh, D. and K. Ramakrishna. 2016. A complete profile on blind-your-eye mangrove Exoecaria agallocha L. (Euphorbiaceae): Ethnobotany, phytochemistry, and pharmacological aspects. Pharmacognosy Review, vol. 10 (20): 123pp.–138pp.
· Shelley, C. 2008. Capture-based aquaculture of mud crabs (Scylla spp.). In A. Lovatelli and P.F. Holthus (eds). Capture-based aquaculture. Global overview. FAO Fisheries Technical Paper. №508. Rome, FAO: 255pp.–269pp.
· Su, X.Q., ‘Look out for collared crows in Hong Kong: new study says numbers are dwindling’. South China Morning Post, 2 August 2017, https://www.scmp.com/news/hong-kong/health-environment/article/2105155/look-out-collared-crows-hong-kong-new-study-says (Accessed: 11 July 2019).
· Tipper, R. 2016. A Naturalist’s Guide to the Birds of Hong Kong, John Beaufoy Publishing, United Kingdom. 100pp.
· Tong, Y.F., Lee, S.Y. and B. Morton. 2006. The herbivore assemblage, herbivory and leaf chemistry of the mangrove Kandelia obovata in two contrasting forests in Hong Kong. Wetlands Ecology and Management, vol. 14: 39pp.–52pp.
· Viney, C., Phillipps, K. and C.Y. Lam. 2005. The Birds of Hong Kong and South China. Information Services Department, Hong Kong SAR Government, Hong Kong. 218pp.
· WWF-Hong Kong, Firefly survey at the Mai Po Nature Reserve, [website], 2012, https://www.wwf.org.hk/en/news/press_release/?uNewsID=8440 (Accessed: 27 August 2019).
