The Chinese mystery snail, black snail, or trapdoor snail, scientific name Bellamya chinensis, synonym Cipangopaludina chinensis, is a large freshwater snail with gills and an operculum, an aquatic gastropod mollusk in the family Viviparidae. The Japanese variety of this species is black and usually a dark green, moss-like alga covers the shell. The name, “trapdoor snail” refers the operculum, an oval corneous plate that most snails in this clade possess. When the soft parts of the snail are fully retracted, the operculum seals the aperture of the shell, providing some protection against drying out and predation.
Though native to East Asia from the tropics of Indochina to northern China, this species has established itself in North America. The native range is from Southeast Asia to Japan and eastern Russia. This species is widely distributed in China including the Chinese Loess Plateau.
Species of the genus Bellamya can be identified by their relatively large globose shells and concentrically marked opercula. The shell is conical and thin but solid, with a sharp apex and relatively higher spire and distant body whorl. This species has a small and round umbilicus and the spire is produced at an angle of 65–80°. Bellamya chinensis exhibits light coloration as a juvenile and olive green, greenish brown, brown or reddish brown pigmentation as an adult. The inner coloration is white to pale blue. The surface of the shell is smooth with clear growth lines. The shell has 6.0–7.0 whorls. Bellamya chinensis is a large gastropod species generally 40 mm in shell height and 30 mm in shell width, the largest being 60 mm in height and 40 mm wide. The shell height can reach up to 65 mm. Bellamya chinensis has a width to height ratio of 0.74–0.82. The aperture is ovoid with a simple outer lip and inner lip.
In juveniles, the last shell whorl displays a distinct carina, and the shell contains grooves with 20 striae/mm between each groove. Juveniles also have a detailed pattern on their periostracum consisting of 2 apical and 3 body whorl rows of hairs with long hooks on the ends, distinct ridges and many other hairs with short hooks. The shell of Bellamya chinensis grows allometrically (the height increasing faster than the width) and does so at a decreased rate in comparison with Bellamya japonica, such that the adult shell is less elongate than that of its congener. The radula also may differ between Bellamya japonica and Bellamya chinensis, but there is so much variation even within one species that it is not a good diagnostic characteristic. However, as a general guide, in one North American population, the radula of Bellamya chinensis had seven small cusps on the marginal tooth and a large central cusp with four small cusps on either side. The Chinese mystery snails (unlike apple snails) do not possess a siphon. They give live birth, and like all aquatic snails they only have one set of tentacles.
This species prefers freshwater lakes with soft, muddy or silty bottoms, reservoirs, slow-moving freshwater rivers, streams, paddy fields, and ponds with aquatic grass, creeping at the bottom of the water or on aquatic grasses. It prefers lentic water bodies with silt, sand, and mud substrate in eastern North America, although it can survive in slower regions of streams as well. It can tolerate conditions in stagnant waters near septic tanks. This species has been found in waters in eastern North America with pH 6.5–8.4, calcium concentration of 5–97 ppm, magnesium concentration of 13–31 ppm, oxygen concentration of 7–11 ppm, depths of 0.2–7m, conductivity of 63–400 μmhos/cm, and sodium concentration of 2–49 ppm. The optimal water temperature for it to grow and develop is between 20 and 28 °C. It will hibernate while water temperature is lower than 10 – 15 °C or higher than 30 °C.
Bellamya chinensis feeds non-selectively on organic and inorganic bottom material as well as benthic and epiphytic algae, mostly by scraping, but diatoms are probably the most nutritious food it ingests at sites in eastern North America. This species is primarily an algae eater in an aquarium context. These snails are popular in freshwater aquariums because they do not eat fish eggs or plants, they do not overpopulate the aquarium, and they close up if there is a water problem, giving people an indication that something is wrong a few weeks before the fish die.
Reproduction is initiated sexually. This species is ovoviviparous. Females live up to 5 years, while males live up to 3, occasionally 4 years. Female fecundity is usually greater than 169 young in a lifetime, and may reach up to 102 for any given brood. All females generally contain embryos from May to August and young are born from June through October in eastern North America in shallow water, then females begin migrating to deeper water for the winter in the fall. Females bear more young in their 4th and 5th years than in other years.