The colonial hydroid Hydructiniu echinata is able to discriminate between self and non-self. Encounters between isogeneic, or allogeneic but histocompatible, colonies of this hydroid result in fusion, whereas incompatible colonies do not fuse but display a peculiar mode of rejection. A special type of nematocyte, the microbasic mastigophore, which occurs exclusively in the stolon compartment of a colony, migrates to and accumulates at contact sites. Here these nematocytes adjust their cnidocils toward the alien tissue and discharge their threads nearly synchronously. Later incoming nematocytes discharge without delay upon arrival. Nematocytes move to and accumulate at contact sites in both compatible and incompatible encounters. However, only contact between incompatible animals causes the discharge of the nematocytes.
Permanent contact between incompatible competitors leads to an increase in the density of microbasic mastigophores from 12 to a maximum of 450 per mm stolon length. As a consequence the stolons acquire a hyperplastic appearance.
Labeling experiments indicate that stem cells or precursor cells are sensitive to the contact stimulus shortly before or during the S-phase of the cell cycle. Differentiation to mature microbasic mastigophores takes 4 days.
Upon interruption of contact, the density of microbasic mastigophores is regulated down to the normal level. Supernumerary cells are removed by phagocytosis.