Mountain Crab (Nanhaipotamon hongkongense)

Lung Fu Shan Environmental Education Centre x Little Woods Nature Education

A small city Hong Kong, nurtured millions of people and biodiversity. Some of them are named after Hong Kong, meaning that they were first identified here. Let's find out who are they? ┉┉┉

▷Potamidae ▷Nanhaipotamon ▷IUCN Least Concerned

Huge claws and vibrant red colour, is Mountain Crab's (Nanhaipotamon hongkongense) signature. Hong Kong is located at the sub-tropical area, estimated to have around 300 species of crabs, 1/3 of China's total crab species. In which, 4 of the freshwater crab species are endemic to Hong Kong, including Somanniathelphusa zanklon, Cryptopotamon anacoluthon, Nanhaipotamon aculatum and Nanhaipotamon hongkongense (Mountain Crab).

Although Mountain Crab is one of the "freshwater" crabs, as you may tell from its name, it also enjoy living in damp woods, literally a "mountain" crab. Damp environment allows crabs to keep their gills moisturised, so they can still undergo gas exchange (breathing with gills) on land. During the wet seasons, they like to hang out near clean rapids; and dig sloped caves to hide, only come out to forage at night or when there's rain. These caves also act as nurseries, females brood (similar to hatching in birds) in these caves until babies hatch.

Fun fact, sea crab babies are larvae floating in water, through multiple molting process (similar to shedding in reptiles), they slowly turn into crabs; freshwater crab babies are different, they are already in "crab form" when hatched from eggs, therefore not a must for them to lay eggs in water.

Adult freshwater crabs are quite territorial and aggressive, sometimes cannibalistic. Therefore baby crabs are carefully raised by their mother until their shells are calcified (hardened), where they finally live on their own. Being omnivorous, they do not only prey on crabs, but also fallen leaves, centipedes and fishes... basically anything! They are cleaner of the river ecosystem.

In 1930, HKU reader (professor) Geoffrey Herklots collected specimens of a female and male freshwater crab and sent them to a laboratory at the Peking University. In 1940, a arthropod professor CJ Shen published an article confirming and describing this new species, and named it after Hong Kong, Nanhaipotamon hongkongense. It was said that there's one observation record at Guang Dong, outside Hong Kong, but since it wasn't proven, Nanhaipotamon hongkongense is still said to be endemic to Hong Kong.

Mountain Crab is widely distributed across Hong Kong Island, New Territories, Lamma Island and Lantau, with 34 reported locations, including Lung Fu Shan. However, channelisation has caused streams to be separated, limiting the living areas of Mountain Crab. Same as other freshwater crabs, Mountain Crab juveniles lack plankton stage, which is harder for them to spread to other locations. We should develop natural stream management policies in the long run, in order to protect this endemic species.

Crab molting

插畫、撰文：小樹林自然教育 Little Woods Nature Education