River mussels are the collective name for a class of animals belonging to the Molluscidae family. In some places, they are called clam shells or clams. They live at the bottom of freshwater lakes, ponds, rivers, etc., half-buried in the sand. The meat is edible and can also be used as bait for fish and poultry and as feed for poultry and livestock.

In some areas, freshwater pearl clams are used to raise pearls. The main species used to raise pearls are the triangular sail clam and the ruffled crown clam. The inlet and outlet pipes at the rear end of the clam body are exposed, and water can flow into and out of the mantle cavity, thereby completing functions such as ingestion, respiration, and excretion of feces and metabolites, filtering microorganisms and organic matter particles in the water, and eating small microorganisms.

The shell shape varies, and the two shells are equal. The carvings on the top of the shell are often concentric circles or polygonal lines, but how many Somewhat degraded. The hinge part varies greatly, sometimes with pseudo-primary teeth. With 1 external ligament. The septal membrane between the gill lobes is intact and arranged parallel to the gill filaments. The posterior part of the outer lobes of the outer gills is healed with the mantle, and there are gill water tubes. The openings of the gills and anus are completely separated by a septum.

There is a ligament on the outside, relying on its elasticity to open the two shells. There are muscle marks on the inner surface of the shell where muscles are attached. There are mantle marks parallel to the ventral edge of the shell; there are three muscle marks on the front and upper part of the shell, the largest one is oval, which is the anterior adductor muscle mark; the upper edge of the latter is a small, slightly triangular precontractor muscle mark; The lower edge is the extensor muscle scar. There are two muscle marks at the back end of the shell near the dorsal edge, the large one is the adductor muscle mark, oval shape, and the smaller one on the upper front edge is the retropodial muscle mark.

There are some differences in body shape between male and female. There is a bulge on the abdominal edge. This bulge is related to reproduction and is the temporary residence of the larvae.

habitat

In the natural environment, mussels generally live in rivers, lakes, ponds, small In mud, sand or gravel such as streams. When winter and spring are cold, the axe-foot is used to dig into the mud, so that the mussel body is partially buried in the sand, with the front abdominal edge downward and the back dorsal edge upward; only the rear edge of the shell is exposed for breathing and feeding. When the weather is hot, most of them are exposed to the mud. Toothless clams generally live in still water with a muddy bottom and a pH value of 5 to 9 or in slowly flowing, fatter water.

Food

The food of clams is mainly monocystic algae, protozoa and organic detritus, such as rotifers, flagellates, green eyeworms, green algae, Scenedesmus, Navicula, dinoflagellates, Tetragonal algae, fusiform diatoms, rod star algae, crustacean debris and plant leaves, etc. Easily digested are mainly diatoms. Mussels cannot actively chase food. They rely on the opening and closing of the clam shell and the swinging of cilia on the inner side of the mantle and gill cilia to cause water flow, and food enters the mussel body with the water. Food enters the mantle cavity with water, and the particles move upward along the gill filaments to the base of the gills, and then move forward to the labial valves. After being selected by the labial valves, small particles enter the mouth, while large particles move backward from the edge of the inner gills and settle on the two cheeks. At the intersection, enter the mantle mark to the rear end, and the two shells are suddenly closed and discharged from the body. According to some statistics, up to 40 liters of water flow into the clam body every day.

Action

The clam's ability to move is very weak. When the environment is calm, it is pulled by ligaments, opens its double shell slightly, and slowly stretches out its axe. Generally, the axepods extend toward the front of the shell and are fixed on the mud, and then the clam body shrinks and moves forward. This crawling is very slow, often progressing only a few centimeters per minute. Wherever the clam passes, a shallow ditch is left. When the clam encounters an enemy, it quickly retracts its axe, its adductor muscles contract sharply at the same time, and its double shells are tightly closed to defend against external enemies.

Growth and reproduction

Mussels are dioecious, with gonads located around the dorsal intestine of the feet. The glands are grape-shaped, the testes are milky white, and the ovaries are light yellow. The genital ducts are open, and the genital pore opens very small behind and below the renal pore.

The reproductive season of mussels is generally in summer, and the sperm eggs are fertilized in the gill cavity of the outer gills until the larvae are formed. Due to the adhesion of the mother's body, the fertilized eggs will not be washed out by the water flow and remain in the gill cavity to develop. Therefore, the gill cavity of the external valve gill is also called the marsupium.

The fertilized egg undergoes completely unequal cleavage (spiral type), develops into a blastocyst, forms gastrula by epiboly and invagination, develops into a larvae, and overwinters in the gill cavity. In the spring of the following year, the larvae hatch out and develop into gamma larvae unique to river mussels (equivalent to the facial disc larvae of other lamellar gills).

The larvae have well-developed adductor muscles, and each ventral edge of the shell has a strong hook with teeth. There is a sticky filament called byssus in the center of the abdomen. There are setae on the side edge of the shell, which have sensory function. The larvae have a mouth but no anus, and can swim by opening and closing the double shell.

The freshwater giant catfish (Rhodaus sinensis) inserts its long ovipositing tube into the inlet pipe of the clam and lays eggs in the clam's mantle cavity. Gamma larvae also take advantage of this opportunity to come into contact with the giant catfish and parasitize on the fish's gills, fins, etc. The skin of the fish is stimulated to proliferate abnormally, and the larvae are wrapped in it, forming a cyst. The larvae absorb nutrients from the fish through the mantle epithelium. After 2 to 5 weeks, it metamorphoses into a young clam, breaks out of the fish body, and sinks to the bottom of the water to live. It takes 5 years to reach sexual maturity. It will continue to grow in the future.