During this sex change, the testis change in colour and decrease in size. Sex differences in adult Pomacea flagellata flagellata snails. The sex ratio within apple snail populations is around 0. 5, which means that there is an equal amount of males and females. However, the sex ratio between the broods of a population varies a lot. Some egg masses give almost exclusively male hatchlings, while other give exclusively female offspring. It was first thought that environmental factors influence the sex ratio in apple snails.
It turns out that different spawns of a single mating pair shows little variation in sex ratio between the different broods, despite variations in environmental circumstances. This supports a genetic basis for sex determination. However, there is no clear evidence for the presence of X and Y chromosomes in apple snails. The combination of at least three and most likely more genes determine the sex ratio. The male reproductive tract consist of the testis and vas deferens, the seminal vesicle and the prostate gland, the penial sac, the penis and the penis sheath. The vas deferens passes down to the seminal vesicle, beneath the kidney chamber. In this seminal vesicle, the sperm cells are stored.
The prostate gland passes down the right margin of the mantle skirt, along the rectum and ends in the penial papilla, next to the anal papilla at the roof of the right mantle cavity. During mating activities, this penial papilla bends towards the sperm groove in which the sperm is conducted. From this sperm groove the sperm is conveyed in the penial duct at the base of the penis. On erection the penis comes out of the penial sac and is grasped around on the lower thirth by a muscular penial sheath from the mantle. It’s the latter, which can be seen when the snails are mating. The real penis is rather thin, whip-like and often stays out of sight. Close up of the picture at the left, with the penis sheat inserted in the female snail.
In the sperm cell production in Pomacea canaliculata and perhaps also all other members of the Ampullariidae family, two types of sperms are produced: eupyrene sperm, or normal, viable sperms cells, and apyrene parasperm. The function of these sterile sperms cells is not fully understood, but several possible functions are proposed. According to this viewpoint, the apyrene sperm cells support the normal sperm cell in their travel trough the female reproductive tract by releasing their glycoproteins. These glycoproteins are an important energy source for the moving sperm cells.
The egg cells of the female snail are stored in the ovaria, which are located in the top of the spine, closely to the digestive gland. To produce an egg, the egg cell is brought to the receptaculum seminis. In this place, where the sperm from the male is stored, the fertilisation of the egg cells takes place. These glands increase considerably when the female is about to produce eggs. In this bursa the sperms of the male are received and the defect or incomplete sperm cell that aren’t able to swim towards the receptaculum seminis, are absorbed here.
At the end of the egg tube, the egg is finished and leaves the genital tract at the gonopore. The apple snail species from the genera Pila and Pomacea deposit their eggs above the waterline. They are guided to the egg clutch in an ovipositor groove over the snail’s body. This groove lays on top of a swollen part of the foot, which is formed by inflating the tissue below with fluid. The ovipositor structure is, as result of this, visible as a semi-transparant lump on the snails body. The eggs of the species that deposit their eggs above the waterline are soft when they appear from the snails body, but the eggshell hardens within a few hours after they are laid. The rate of which the eggs are laid is about 1 every 30 seconds in Pomacea canaliculata and Pomacea diffusa.