The biggest feature of concave finger fiddler crab (scientific name: UCA vocans) is that the male crab has a pair of claws of different sizes, and the large claws placed on the chest are like the shield of a warrior. It will make the action of dancing the big pincers, which is called “attracting tide”, in order to intimidate the enemy or courtship. In addition, the crab has a pair of matchstick like eyes, which is very special. They feed on algae, can swallow sediment to absorb organic matter, and spit out the inedible part.
Concave fingered fiddler crab has a pair of protruding rod-shaped eyes with blue and transparent top, which is very crystal clear. The big claw of male crab is orange, which is very similar to the northern calling fiddler crab (scientific name: UCA borealis), but they are two different species. Its head and chest is a trapezoid. The front is wide and the back is narrow, the forehead is narrow, the orbit is wide, and the eye handle is slender. One male claw is always much larger than the other (called mating claw). The large claw is very large, even larger than the body, weighs almost half of the whole, and the small claw is very small, which is used for feeding (called feeding claw). The two claws of the female body are quite small and symmetrical, and the knuckles are spoon shaped. They are feeding claws. If the male loses the big claw, a small claw will grow out, and the original small claw will grow into a big claw to replace the lost big claw. The color of male is brighter than that of female.
It is widely distributed from the Western Pacific to China, Myanmar, Thailand, Indonesia, the Philippines and Malaysia. It is a warm water clustered crab.
Fiddler crabs rely on vision and hearing to receive communication, contact and warning signals to realize social aggregation behavior. They live in caves and often have specific caves, but they are often replaced every few days. The depth of the hole excavation is related to the groundwater level. The hole depth can reach 30 cm. Generally, the bottom of the hole needs to reach the wet soil. Many male crabs also build a semi-circular umbrella cover to cover the hole. The activity of the crab has a certain law with the rise and fall of the tide. At the high tide, it stops at the bottom of the cave. After the ebb tide, it moves on the beach, feeds and repairs the cave. Finally, it occupies the cave and is ready to mate. The cave is the center of this kind of crab’s life. It can not only avoid the invasion of various predators on land and water, but also avoid tide flooding or direct sunlight.
The crab scrapes the small particles on the silt surface with a small claw and sends them to the mouth. These small particles contain a lot of debris, algae, bacteria and other microorganisms. After they are sent to the inlet, they are absorbed by the body. There is a special organ in the mouth, which can classify and filter the food. The unusable residue is taken out by the small claw and placed on the ground to form the small earth ball seen by people. It is called “pseudo feces”, which is different from the feces really discharged from the anus through the digestive tract.
Growth and reproduction
During the mating season, mature male crabs will wave their big claws near the hole for various show off performances. When the female crab approaches, the big claws of the male crab will wave faster and faster. If the courtship is successful, the female will follow the male into the cave for mating. At night, males often use big pincers to tap the ground rhythmically to attract females.