Members of this family are heteroxenous, with two hosts. Schizogony takes place in one and sporogony in the other. Development takes place in the host cell proper. The oocysts typically contain many sporocysts. With one dubious exception, the Aggregatidae are parasites of marine annelids, molluscs and crustacea.
In this genus the oocysts contain numerous sporocysts, each with 2 sporozoites. A single species, M. kathae, has been named. It occurs in the kidney of the whelk, Buccinum undatum.
In addition, Paichuk (1953) described oocysts in the feces of several pigs in Kazakhstan which he called Merocystis sp. The oocysts are short-oval, almost spherical, 34 to 43 by 30 to 37 u with a mean of 38.7 by 33.0 u. The oocyst wall is smooth, 2 u thick, composed of 3 layers of which the outer is dark brown, the middle bright green and the inner yellow-green or bright brown. The oocyst wall is very fragile. The number of sporocysts is unknown, but more than 13. The sporocysts are spherical, 9 to 13 u in diameter with a mean of 9.1 u. The presence of an oocyst polar granule is unknown. The oocyst and sporocyst residua are composed of dispersed granules. The sporozoites are spherical, 4.3 u in diameter. Altho Paichuk assigned this form to the genus Merocystis, it is much more probably Adelea or Adelina, both of which occur in arthropods, and it may well be a parasite of some arthropod which the pigs had eaten.