Small Majesty - The Long-Armed Shrimp
The Long-armed Shrimp (Macrobrachium spp.), which is a common inhabitant of Hong Kong’s rivers and streams, typically grows to no more than 9 centimetres in length. Its size seems small and insignificant, but it is actually the largest shrimp species found there, as the other creatures tend to be smaller due to the smaller size of such water bodies. Despite its small size, the Long-armed Shrimp is equipped with a pair of impressively long claws. With these powerful weapons at its disposal, it reigns over the local rivers and streams!
The Genus Macrobrachium comprises over 260 species worldwide. It belongs to the Palaemonidae family in the Order Decapoda. Except in Antarctica, the Long-armed Shrimp can be found all over the world. In Hong Kong, Long-armed Shrimps are mainly found in mountain streams, but you can also spot them in intertidal zones or mangroves.
Depending on the species and environment, Long-armed Shrimps may come in varying colours of green, brown, grey, or black. Some shrimps may even have bodies that are transparent or exhibit a rainbow-like lustre. Long-armed shrimps are mostly nocturnal. They hide during the day in crevices between rocks or plants on the stream bed and only come out to forage at night. They have a varied diet that includes both animals and plants, including insects, crustaceans, molluscs, fish, and aquatic plants.
Uniquely long arms
One of the most outstanding features of the freshwater shrimp is undoubtedly its second pair of legs. Arthropods of the Order Decapoda typically have five pairs of legs. One pair of these legs has pincers, while the remaining four pairs are for moving around. This is just like crabs, where the first pair of legs is usually the powerful pincers. However, the first, third, fourth, and fifth pairs of legs of the Long-armed Shrimp are walking legs, while the second pair is especially elongated. For the males, this pair of legs can grow to be as long as the body itself. There are small pincers at the end of the elongated legs, and the shrimps use them to catch their prey, defend against predators, and even compete for mates. The first pair of legs supports the body and provide balance when the shrimps extend their elongated legs.
It has always been difficult to distinguish between the different species of Long-armed Shrimps because even within the same species, there are significant variations in appearance and colour between individual shrimps. Fortunately, we can now do so with the help of genetic analysis. In the past, it was thought that there were only four species of Long-armed Shrimps in Hong Kong. A detailed survey and analysis of local Long-armed Shrimps conducted by three local university scholars last year revealed that there are actually eight species of Long-armed Shrimps in Hong Kong, including the Macrobrachium equidens, M. formosense, M. laevis, M. lantau, M. lar, M. meridionale, M. nipponense and M. venustum. Among them, the M. lantau is a newly discovered species found only in Lantau Island, Hong Kong, and hence, carries the name.