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Size-dependent neural integration between genetically different colonies of a marine bryozoan

Although the formation of genetic chimeras is rare in the animal kingdom, it has long been known that colonial marine invertebrates fuse under natural conditions, forming genetic chimeras. I report here an example of selective, partial fusion. When small colonies of the encrusting marine bryozoan Membranipora membranacea grow into contact, they usually become behaviorally coordinated: if one colony is disturbed, both colonies will simultaneously retract their feeding structures (lophophores). As a first step towards understanding the fitness consequences of this type of apparent fusion, I examine its mechanistic basis. Using assays of zooid behavior, electrophysiological recordings and 14C-labeled metabolites, I demonstrate that physiological integration between M. membranacea colonies is both partial and temporary. Specifically, this study demonstrates (1) that behavioral coordination is the result of neural integration between colonies, (2) that coordinated colony pairs do not exchange metabolites, and (3) that neural integration is a temporary phenomenon that is terminated as colonies grow larger. Additionally, I show that only those zooids at the middle of the intercolony border mediate neural integration. Partial physiological integration between M. membranacea colonies could be a highly specific interaction that increases the potential benefits of fusion (i.e. neural integration) while minimizing the potential costs (i.e. resource parasitism).