cystacanth

Cystacanth is the larval stage of acanthocephalans (thorny-headed worms) that resides in the tissues of the intermediate host and is the infective form for the definitive vertebrate host. In the typical life cycle, adults living in the intestine of a definitive host produce eggs that are shed in feces. The eggs hatch into free-living oncospheres that invade an invertebrate intermediate host, most often a crustacean such as an amphipod or isopod, and develop through successive larval stages to the cystacanth. The cystacanth is encased in a cyst and contains a fully formed, retractable proboscis with hooks, enabling attachment to the intestinal wall of the definitive host once transmission occurs. The cystacanth is the infective stage to the definitive host, which is typically a vertebrate such as a bird or mammal that preys on the intermediate host.

The cystacanth resides in the body cavity or coelom of the intermediate host, sometimes in muscle or connective tissue. When a suitable predator consumes the infected intermediate host, the cystacanth excysts in the digestive tract, the proboscis everts and attaches to the intestinal wall, and the worm matures into an adult, completing the cycle.

Ecological and medical relevance: Cystacanths can influence the behavior or physiology of their intermediate hosts in some species to enhance predation by definitive hosts. While infections are usually host-specific, the cystacanth represents a key stage illustrating parasite life-cycle complexity in freshwater and marine ecosystems.