Choanoflagellates are either free-swimming in the water column or sessile, adhering to the substrate directly or through either the periplast or a thin pedicel. Although choanoflagellates are thought to be strictly free-living and heterotrophic, a number of choanoflagellate relatives, such as members of Ichthyosporea or Mesomycetozoa, follow a parasitic or pathogenic lifestyle. The life histories of choanoflagellates are poorly understood. Many species are thought to be solitary; however coloniality seems to have arisen independently several times within the group and colonial species retain a solitary stage.