Marine ecologists discover and name the first endemic tree-climbing crab

The Mangrove Ecology and Evolution Lab, led by Dr Stefano Cannicci at the Swire Institute of Marine Sciences (SWIMS) and School of Biological Sciences, the University of Hong Kong (HKU), has recently discovered, described and named a new species of mangrove-climbing micro-crab from Hong Kong, Haberma tingkok, and published the description in ZooKeys, a peer-reviewed and open access international journal dedicated to animal taxonomy.

The new species has been given the scientific name of Haberma tingkok, since all the specimens found at present were spotted at a height of approximately 1.5 to 1.8 metres above chart datum, walking along the branches of the mangroves of the Ting Kok area. The crabs are small, less than a centimetre long, predominantly dark brown, with a squarish carapace, very long legs and orange claws. It represents the second endemic mangrove crab species described in Hong Kong. The previous one, Pseudosesarma patshuni, was described in 1975.

Discovery of a new endemic mangrove crab in over 40 years

This endemic tree-crab was previously unknown to science and it is only known, at present, to have come from Tolo Harbour, Hong Kong. It is Hong Kong's first truly arboreal crab, i.e. living on the branches and canopies of mangroves and breathing air. Its closest relatives, i.e. crabs of the same genus, are only known to be found in the mangroves of Singapore and Indonesian New Guinea, but they are all normal crabs that live in the mud - they do not climb trees.

The first known individual of Haberma tingkok was found in the mangroves of Ting Kok at the end of last summer, during one of the routine biodiversity samplings carried out by a group of HKU undergraduate students majoring in Ecology & Biodiversity and research assistants. At the time, they were conducting a multi-institutional project funded by the Hong Kong Environment and Conservation Fund (ECF) "Assessing the Marine Biodiversity and Ecology of Tolo Harbour and Channel, with particular Reference to Coastal Marine Environments of Ting Kok and Shuen Wan Hoi - Phase I." The Project is coordinated by the Director of SWIMS, Professor Gray A Williams, and managed by Dr Kevin Ho King-yan.

During the sampling, Mr Steven Wong Ho-tin, a year four undergraduate student majoring in Ecology and Biodiversity at HKU, found a mature, female crab of unusually small dimensions on the mangrove branches and showed it to Dr Cannicci, Associate Professor at HKU School of Biological Sciences, who immediately realised that the crab was not a common one in Hong Kong as it had some much-specialised features. Although the vast majority of crabs are marine and freshwater species, there are some groups that, along their evolutionary history, developed the uncommon ability to climb trees, especially in a mangrove habitat. This crab had the characteristics of a tree climber, extremely elongated legs, with respect to the body, which is very flattened, and she was carrying eggs, a clear indication that it had extreme confidence in living up in the branches of tress.

Since the identification of this group of crabs, i.e. the common mangrove crabs known as the Sesarmidae, can only be finalised by studying males, a large scale crab-hunt started, involving the whole group of students and researchers from HKU. It was Miss Cherry Cheung Cheuk-yiu, a research assistant at HKU, who finally spotted the first known male of this species. Under the microscope, in Dr Cannicci's laboratory, it was clear that the crab was the first record of the crab genus Haberma in Hong Kong, since in this group the males have a peculiar locking-system on the second and third walking, probably for grasping females while mating. This new identification was confirmed by Dr Cannicci during a visit to the Lee Kong Chian Natural History Museum of the National University of Singapore and published by Dr Cannicci and Professor Peter Ng Kee-lin, Director of the Museum.

The discovery of this new crab species shows how little is known about the diversity of crabs in Hong Kong. On the basis of the number of known species from Japan and Taiwan, marine biologists from all over the world estimate that we only know up to 50-60% of the real diversity of coastal and littoral crabs in Hong Kong, which is far less than the ratio known for many other marine species. That is why the above-mentioned project on marine biodiversity in the Tolo Harbour area led by HKU and involving five other universities in Hong Kong is of paramount importance, and echoed the development of the Biodiversity Strategy Action Plan (BSAP) initiated by the Government of the Hong Kong Special Adminsitrative Region under the United Nations' Convention of Biological Diversity.