Studies on species-and self-recognition in scleractinian corals

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Developmental changes in histocompatibility reactions and the cellular mechanism of transformation from incompatible fusion to nonfusion in the coral Pocillopora damicornis were studied.When primary polyps or young colonies derived from different colonies of a coral, P.damicornis were brought into contact within a few days after settlement, they showed either fusion or incompatible fusion. When branches isolated from adult colonies were brought into contact they showed either fusion or nonfusion. The present results suggest that the coral do not have functional histocompatibility system to avoid tissue fusion at an early stage of development.Pairs that show incompatible fusion transformed into nonfusion or disconnection after several months. Large extracellular spaces and cells with condensed nuclei and small amount of cytoplasm were observed at the interface of incompatibly fused pairs. This suggests that cells at the interface died through apoptosis and were phagocitized during the transformation from incompatible fusion to nonfusion.Sensitivity of nematocyst discharge response and that of extracoelenteric digestion response were compared using various coral species. In some combination pf coral species, corals did not discharge nematocysts against target corals, while they display aggressive behavior using mesenterial filaments. This suggests the possibility that coral use different markers to distinguish allogeneic from xenogeneic corals.