Biology of Hydractiniid Hydroids. 2. Histocompatibility Effector System/Competitive Mechanism Mediated by Nematocyst Discharge

Intraspecific encounters between colonies of the athecate, colonial hydroid Hydractinia echinata result in contact between mat or stolonal tissues. We have monitored colony ontogeny in five clones of H. echinata and initiated experimental encounters between the two tissue types in both isogeneic and allogeneic combinations. All isogeneic interactions result in fusion, all allogeneic interactions in rejection. Transmission electron microscopy shows that fusion results in the establishment of a common gastrovascular system, whereas rejection is characterized by an electron-dense, fibrous layer separating the two colonies. Rejection involves either the passive cessation of growth along the contact zone or the development of hypertrophied stolons. These hyperplastic stolons destroy foreign tissues and can develop only from existing stolons. Scanning and transmission electron microscopy demonstrates that stolons become hyperplastic through the differentiation of interstitial cells into nematocytes and that the destruction of foreign tissue is effected by nematocyst discharge. Experimental elimination of interstitial cells removes the capacity of a colony to produce hyperplastic stolons, but does not affect historecognition. A comparison between these results and similar studies in anthozoans suggests the need to distinguish between the evolution of historecognition and the evolution of mechanisms of interference competition