Adult males and females live in the intestine of freshwater fish. Females deposit embryonated eggs which pass in the feces and are eaten by an ostracod intermediate host. The acanthor hatches from the egg and uses its hooks to penetrate into the body cavity. It first develops into an acanthella.
What happens next depends on when the ostracod is eaten.
If the ostracod is eaten by a fish while the parasite is still in the acanthella stage, the acanthella will penetrate the gut of the fish, enter the tissues, and complete development into a cystacanth in the tissues of the fish. Then, if that fish is eaten by a second fish, the parasite will mature in the gut of the second fish. The first fish seems to behave as an intermediate host, because the parasite undergoes development in it. But, the first fish is not necessary for development, so in categorizing this life cycle, the first fish would more properly characterized as a paratenic host than an intermediate host. Many acanthocephalans are capable of using paratenic hosts.
On the other hand, if the infected ostracod manages to avoid being eaten for a while, the parasite will simply continue its development to the cystacanth stage. Then, when eaten by a suitable fish, that fish becomes the definitive host and the parasite matures. this is the typical type of 2-host life cycle exhibited by most acanthocephalans.