Recently, interest in Southeast Asian crustaceans, especially freshwater crabs, has increased. I originally loved crustaceans and specialized in freshwater shrimp, but the more I learned about crabs, the more interesting I became. When doing fieldwork in Southeast Asia, crabs almost always end up in the net along with fish and shrimp. In the past, they didn't want their precious fish or other prey to be harmed by crabs, so they would release them into the river as soon as they got into their nets. However, when we collect crabs from various places, we find that even similar species have slightly different body colors, patterns, and the shape of their claws. Once I became interested in them, I began to see more differences, and I was made to realize the depth of the two. Please understand that this column will often feature crabs.

Thailand, the base of my fieldwork, is home to many freshwater crabs. There are overwhelmingly many species of Japanese freshwater crabs, but among them, the species that specializes in living on land is called the mountain crab. It is so called because it lives not in the water, but in the mountains, far from the water's edge.
However, when it comes to mountain crabs, there are some types that are quite adapted to life on land, and others that are still highly dependent on water. Sometimes when I drive through the mountains, I see crabs in places where there are no rivers or water bodies nearby. These species are probably highly adaptable to land.

This time, I would like to introduce the mountain crab that lives in Chanthaburi, eastern Thailand. This species is still relatively dependent on water, and even the parent crabs live not far from rivers. I've been observing fish, crabs, and shrimp in Chanthaburi for some time, and every time I've been there, I've captured this mountain crab on camera. This year, I was fortunate enough to be able to film the mating scene and the protection of the young crabs, and was able to introduce their life history, so I thought I'd share it here.

The Chanthaburi wild crab has a pale yellow to brown body color. The wild crab in Talat, not far from Chanthaburi, has a deep purple body color, but the body shape is very similar. In the case of crustaceans, it would be a big mistake to classify them as different species just because of the difference in color. Japanese freshwater crab is a good example of this. Depending on where they live, they range in color from bright orange to brown, or even white or blue. Classifying crabs seems to be more difficult than shrimp, and there is not much information available. Well, maybe it's because of my own laziness, but I haven't yet identified the species of the Chanthaburi mountain crab I'm introducing this time. Please bear with me as I will report again once I know the scientific name and other details.

Here in Chanthaburi, wild crabs live in holes dug on the mountain slopes not far from the river. Even though they are wild crabs, they are probably more comfortable when they have water, and they are often seen walking on riverside roads on rainy nights.Females during the breeding season seem to need water, so they are more comfortable in rivers. It seems that they live by digging holes near the. This is thought to be because when young crabs prefer living in water rather than on land. Large adult crabs can withstand some dryness with their strong shells, but small juvenile crabs quickly lose water from their bodies and die. Females with eggs are often seen in holes, but this time I happened to wake up a stone in the water and found a large female with a belly full of young crabs. The female may protect the eggs in the hole while they hatch, and then move to the river when it is time to release the young crabs into the river.

Mating appears to take place at night near the burrow where the female lives. It is performed in the missionary position with the abdomen touching the abdomen. The males of many crabs are larger than the females, and during mating the males use their pincers to hold the females immobile. Mating takes a relatively long time, lasting several tens of minutes. The two mating birds that I found this time did not run away from the flashlight and continued mating, making it very easy for me to photograph them.

After mating, the male and female live separately in burrows, and the female lays eggs inside the burrow. The eggs are carried in the female's abdomen and are carefully protected. Newly laid eggs are bright orange in color. The eggs are large, about 3mm in size. The Japanese freshwater crab and wild crabs are land-locked crabs that spend their larval stage inside eggs and are born as young crabs. Although the number of eggs laid is small compared to marine crabs, the survival rate of the offspring is high. As the eggs develop, their color changes to black. It seems to take more than a month for them to hatch. After hatching, young crabs grow up in the female's abdomen for a while, but eventually separate and start living independently. In the river near the habitat of this wild crab, there are many small crabs that are probably the young of the wild crab. However, there are no large individuals with carapace widths exceeding 5 cm, so they probably transition to a life on land at around that size.

Many wild crabs live in Southeast Asia. In addition to the plain colored types introduced this time, there are also many types with bright colors. Due to their limited habitat, many species, like the queen crab introduced earlier, are protected species. If I have a chance in the future, I will continue to introduce the interesting ecology of mountain crabs.