Hydractinia symbiolongicarpus is a colonial hydrozoan (family Hydractiniidae) found on hermit crab shells along the Atlantic coast of North America, serving as a major model organism for studying cnidarian allorecognition, where two colonies either fuse (compatible) or reject (incompatible) each other based on polymorphic allorecognition loci. The species also provides a model for sexual reproduction, coloniality, and stem cell biology in Hydrozoa. Its genetics has contributed significantly to understanding the origins of immune self-non-self recognition systems in early-diverging animals. Genomic data have been generated for this species and are available in public databases, enabling comparative studies of cnidarian evolution and ecology.