cercaria

Cercaria is a free-swimming larval stage in the life cycle of most digenetic trematodes (flukes). These larvae are formed inside the molluscan intermediate host, typically a snail, as part of the snail-derived generations (sporocysts or rediae). They are released from the snail into water and then continue development toward the next host.

Morphologically, cercariae have a body adapted for swimming and, in many species, a posterior tail that may be forked or otherwise specialized for locomotion. The tail aids in seeking a suitable host, and the larva often possesses structures such as a mouth, pharynx, and digestive glands appropriate to its parasitic lineage. After locating a target host, cercariae either penetrate directly or encyst as metacercariae on vegetation, in fish flesh, or within the tissues of another intermediate or definitive host.

The life cycle typically proceeds when a cercaria enters the next host, at which point the tail is resorbed and the larva metamorphoses into the next stage (often a schistosomula in blood flukes or a metacercaria in other groups). In some life cycles, the cercariae that encyst become the infective form for the definitive host.

Ecologically and medically, cercariae play a crucial role in transmission of trematode diseases, including schistosomiasis and various liver and intestinal fluke infections. Their release is influenced by environmental factors such as light, temperature, and water quality, and they may be produced in large numbers in suitable aquatic habitats.