This crab species, endemic to China, lives next to the world's most densely populated city.

Looking through the species calendar, we have talked about crabs so many times, but freshwater crabs, which account for nearly 20% of the more than 7,500 known crab species in the world, have never appeared on the stage. Now, it is here!

A new species of crab near the downtown

The land area of ​​Macao, China is only about 30.8 square kilometers, but it is the most densely populated city in the world, with an average of more than 20,000 people per square kilometer! In such a densely populated city, it is difficult to compare biodiversity with it. However, it was in Macao that an unexpected discovery was made.

Coloane is an island in the south of Macau. The reason for the name is that it is no longer worthy of the name. Due to the ongoing reclamation project, the northern part of Coloane has been connected to Taipa. Macau is too small to form a river, but the numerous mountainous terrains on Coloane have crisscrossed streams, providing a habitat for freshwater crabs.

Here, it is necessary to introduce to you: What exactly is a freshwater crab?

Sinopotamon planum | Zhang Xiaofeng

Not all crabs that can live in fresh water can be called freshwater crabs. For example, although many places call hairy crabs river crabs, and they do live in fresh water most of their lives, adult hairy crabs must descend to the sea and reach the estuary to reproduce the next generation. The entire life cycle of true freshwater crabs is completed in fresh water. Female crabs lay large eggs, which develop directly into juvenile crabs without going through the planktonic larval stage. Most female crabs will continue to wrap the newly hatched juvenile crabs in their abdomens for protection for a period of time before releasing them.

Of the nearly 1,500 species of freshwater crabs in the world, all are distributed in tropical to subtropical regions, especially in Asia, which is the region with the richest freshwater crab species diversity. my country's crisscrossing mountain and river systems provide a rich habitat for freshwater crabs. At present, except for the three northeastern provinces and the northwest region, most provinces and cities have records of freshwater crab distribution. Among them, there are some widespread species, such as the plain Longxi crab Longpotamon planum (Dai, 1992), and a considerable number of species have differentiated into many different genera and species due to natural geographical barriers during millions of years of evolution.

Potamon ibericum | Philipp Weigell / Wikimedia Commons

In 2018, among the freshwater crab specimens preserved in the Municipal Bureau of Macau, researchers found that in addition to the three known freshwater crabs, Cantopotamon hengqinense (Huang, Ahyong & Shih, 2017), Nanhaipotamon guangdongense (Dai, 1997) and Somanniathelphusa zanklon (Ng & Dudgeon, 1992), there is also a new crab that is different from any known freshwater crab. The researchers named this new species after its origin, Macau, and this is the protagonist of today's species calendar - Macau Nanhaipotamon macau.

Relatively introvert, living in a corner

"Nanhai River Crab" is one of the many genera of the family Crabidae. Many species of the family Crabidae are very similar in appearance. To correctly identify the species, you need to examine the male's first ventral limb. The main characteristics of the genus Nanhai River Crab include an oval cephalothorax with a smooth surface, a sharp postorbital angle connected to the upper branchial teeth, and a short and thick male first ventral limb with a swollen end. This genus currently has a total of 22 species, mainly distributed in the Wuyi Mountains and Nanling Mountains in mainland my country and the Penghu Islands and Central Mountain Range in Taiwan.

The Macau South Sea Stream Crab has a dark brown carapace and light orange claws, and its body color varies a lot. Like other species of the genus South Sea Stream Crab, they mostly live in mud holes beside streams in low mountains and hills. Observing the Macau South Sea Stream Crab from the side, you will find that its cephalothorax is higher than those of the swimming crabs and hairy crabs that we usually see on the table. In fact, the cephalothorax of most crabs that are adapted to life on land has such morphological characteristics, the purpose is to make the gill cavity larger, and the gill lamellae are harder than other crabs that rely entirely on the water environment. This can avoid the collapse of the gill lamellae after leaving the water due to the lack of water support, which in turn leads to a reduction in surface area and affects breathing efficiency.

Macao Nanhai River Crab | Huang C et al. / ZooKeys (2018)

This high degree of terrestrial adaptation allows the Macau Nanhai River Crab to live on land without having to immerse its body in water for a long time, as long as the surrounding environment is sufficiently moist. The eight slender walking legs allow them to easily cope with the terrestrial environment and feed on other small invertebrates on land, even small mammals can become their hunting targets.

The ocean is full of abundant plankton, which allows marine crab larvae to obtain enough food during the planktonic stage, but most of them are destined to become food for other predators. Marine crabs usually use the "crab sea strategy" of producing a large number of larvae to ensure the continuation of their population, and most importantly, the planktonic stage of larvae is an important way for their population to spread.

A, male specimen of the Macao Nanhai river crab, number SYSBM 001649, B~D, female specimen of the Macao Nanhai river crab, number SYSBM 001650 | Huang C et al. / ZooKeys (2018)

Freshwater environments, especially mountain streams, lack sufficient support for the floating period. Most importantly, even if there is enough food to ensure the larvae's feeding, the larvae, who have almost no swimming ability, will have difficulty returning to their original habitat after floating downstream. Therefore, freshwater crabs choose to let their eggs develop directly into juvenile crabs. The number of eggs they lay each time is simply incomparable to that of marine crabs that lay tens of thousands or even millions of eggs. In addition, they are extremely dependent on their habitats, and their population diffusion capacity is extremely limited. As a result, most stream crabs only exist in a certain mountain stream in a certain area.

Nanhaipotamon hongkongense, a Hong Kong crab, can be found in Hong Kong and mainland China | nature.catcher / Wikimedia Commons

The Macau Nanhai River Crab is currently found in 12 streams within an area of ​​about 5 square kilometers in Coloane. The population is quite abundant, but due to the small distribution area, habitat protection is crucial for the survival of the Macau Nanhai River Crab. Although researchers have not found that anyone has caught this river crab for consumption or traded in the aquarium market, the relatives of the Macau Nanhai River Crab are "shining" in the aquarium market: the river crabs with cool appearance and varied body colors, especially the Nanhai River Crab, are one of the most sought-after crabs by aquarium enthusiasts. Although freshwater crabs are much simpler to reproduce than marine crabs, all individuals on the market are caught in the wild without exception.

Although habitat protection is more important, over-catching will also put pressure on the survival of stream crabs. This is not only one of the problems that stream crabs themselves have to face, but also a reality that many freshwater crab breeding enthusiasts must face.

Thanks to Huang Chao, the author of the article "Macau South China Sea River Crab", for reviewing this article.

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