Ontogenetic changes in fusibility in the colonial hydroid Hydractinia symbiolongicarpus

Invertebrate fusion and rejection responses are typically assumed to be mutually exclusive and temporally stable states. We here describe a segregant class appearing in an F1 cross of the colonial hydroid, Hydractinia symbiolongicarpus, that displays an ontogenetic change in fusibility. Reproductively immature offspring fuse to parental strains and have been observed to remain continuously fused for up to 76 days. By contrast, reproductively mature offspring initially fused with parental strains, only to separate into distinct colonies after a period of 3–21 days postfusion. Experimental manipulations demonstrate that (1) ontogenetic changes in fusibility cannot be removed by resetting a colony to a reproductively immature state, and (2) secondary contact does not result in an accelerated response.