A Complete Analysis of Freshwater Mussel Breeding Techniques and Methods

Freshwater mussels are dioecious, with the gonads located around the intestine on the dorsal side of the foot, appearing as grape-like glands. The reproductive season is generally in summer. Sperm and eggs are fertilized in the gill cavity of the outer gill, until the glochidia form. Because of the adhesive action of the mother, the fertilized eggs will not be washed out by the water flow but remain in the gill cavity to develop, which is why the gill cavity of the outer gill is also called the brood pouch. Let us take a look at the breeding techniques of freshwater mussels together below!

Selection and Cultivation of Parent Mussels

1. Selection of Parent Mussels

① Parent mussels are required to have bright shell color, appearing blue-green, with intact shells free of defects, a bulging and round "abdomen", a robust and plump body, and strong shell-closing ability. A mussel age of 6 to 8 winters is preferable, and female mussels are best if they are mussels that have already reproduced.

② The sex of pearl-culturing mussels is very difficult to distinguish by appearance; it can only be accurately determined based on the density and number of internal gill filaments. Generally, female mussels are slightly larger, with wider growth rings, and the shell is also slightly wider and thicker. When a freshwater mussel is opened, the gill filaments of the female are tightly arranged, and after sexual maturity, the number of gill filaments on the outer gill lamella reaches 100 to 120. The gill filaments of the male are loosely arranged, and after sexual maturity, the number of gill filaments on the outer gill lamella is only 60 to 80.

2. Parent Mussel Cultivation: The parent mussel cultivation pond should preferably have an area of over 2000 sq m, a water depth of 1.5 m, and a moderate thickness of mud at the bottom. The oxygen content of the culture water layer should be 4.0 to 8.0 mg/L, the pH should be 6.5 to 8.0, and the bait biomass should be 10 to 20 mg/L. The water quality should not be too fertile, so as to avoid poor development of female reproductive cells due to lack of oxygen or the occurrence of sex reversal. If a small area of water is used to cultivate parent mussels, there must be slow-flow conditions.

3. Stocking Ratio: Generally, males and females are stocked at a ratio of 2:1 (that is, 2 males flanking 1 female as 1 combination), hung in the water layer with their tails leaning against each other for sex-ratio combination culture. Parent mussel cultivation work should begin in autumn, with regular water injection and drainage, and timely cultivation of bait organisms, to promote the development and maturation of the parent mussels' gonads.

Fertilization and Embryonic Development

1. Fertilization Season: Taking the triangle sail mussel as an example. Under normal circumstances, the triangle sail mussel already has reproductive capacity at 3 years of age. Sexual maturity begins every year in May. At sexual maturity, the gonads of the female triangle sail mussel appear orange-yellow in appearance, while the gonads of the male triangle sail mussel appear white. The spawning season is from May to July. Mature eggs are spherical, and mature sperm can be seen under a microscope swimming with tail movement. When the vast majority of egg cells in the reproductive nest mature, the female mussel ovulates and fertilizes. The triangle sail mussel spawns 5 to 8 times per year, with a spawning quantity of 400,000 to 500,000 eggs.

2. Fertilization Methods: There are two methods of freshwater mussel fertilization: natural fertilization and induced insemination.

① Natural Fertilization: During the mussel's reproductive season, the mature eggs of the female mussel are discharged through the genital pore into the body's mantle cavity, and the mature sperm of the male mussel are discharged from the vas deferens through the genital pore into the suprabranchial cavity, then through the excurrent siphon into the external water body. The sperm enter the female mussel's mantle cavity with the water and combine with the waiting eggs, completing fertilization. The fertilized eggs then undergo embryonic development on the left and right outer gill lamellae of the mother mussel.

② Induced Insemination: The reproductive cells of male mussels usually mature before those of female mussels. When some eggs of the female mussel have matured, the male mussel's semen can be manually extracted, diluted with reproductive saline, and injected into the female mussel's mantle, and the female mussel is placed in advance in a fertilization basin with a water temperature 3 to 5°C higher than the original pond water temperature to carry out induced insemination. The first injection of semen only serves to induce the female mussel to ovulate; the second injection of diluted semen must be done in the same way, and the sperm in this semen will exactly fertilize the mature eggs that have been induced out. Through induced insemination, freshwater mussels can be brought into the reproductive season ahead of time.

3. Embryonic Development: From cleavage onward, the fertilized egg goes through the embryonic development process of fertilized egg → morula stage → blastula stage → gastrula stage → glochidium, forming the glochidium larva. The entire embryonic development process generally takes 35 to 50 days.

Collecting Glochidia Seedlings

After the glochidia mature on the outer gill lamellae of the mother mussel, they have a byssus and hooks, and can parasitize fish bodies, and indeed must parasitize fish bodies, in order to complete the metamorphosis process and become young mussels. Therefore, when the glochidia are about to shed their membranes and emerge, fish must be used as "seedling collectors" to collect the glochidia on the fish bodies.

1. Selection of Seedling-Collecting Fish: Silver carp, bighead carp, grass carp, and common carp can all collect glochidia, but the gentle-tempered bighead carp and grass carp fingerlings are preferable. Each mussel requires 300 to 500 fish fingerlings of 9.9 cm size per seedling collection. Healthy and superior fingerlings must be selected so that they can tolerate the parasitism of a relatively large number of glochidia.

2. Maturity Assessment: Immature glochidia do not have parasitic ability, so the maturity of the glochidia must be assessed. The assessment method is to take out a few cultivated mother mussels, first use a shell opener to pry open the freshwater mussel, add a wedge to fix it to a certain extent, then use a probe to pick out a small amount of glochidia larvae from the middle of the mussel's gill lamellae. If the picked glochidia larvae can stick to each other and form a chain, they are mature glochidia.

3. Seedling Collection Methods

① Indoor Seedling Collection: Pour 10 to 15 cm of clean fresh water into a basin, place the sampled mother mussel flat on the bottom of the basin, then put in the seedling-collecting fish, and run a water pipe so that the water becomes flowing water; after several hours this will stimulate the mother mussel to discharge glochidia.

② Outdoor Seedling Collection: In the clear water quality of rivers and lakes, fish boxes can be used for seedling collection. The quantity of collected seedlings is large, suitable for mass production. During seedling collection, the collection point must be changed in a timely manner, and it is appropriate for each fish to be parasitized by about 200 glochidia; too large a number will cause the death of the seedling-collecting fish. One day before seedling collection, the seedling-collecting fish must be given seine-net exercise to strengthen their physique and to discharge feces, so as to avoid the seedling-collecting fish contaminating the glochidia.