The marine sponge, Hymeniacidon sinapium, displays allorecognition of siblings during post-larval settling and metamorphosis to juveniles

Marine sponges, including the crumb of bread sponge, Hymeniacidon sinapium, display allorejection responses to contact with conspecifics in both experimental and natural settings. These responses have been used to infer immunocompetence in a variety of marine invertebrates. However, larvae and juveniles from several marine sponge species fuse and form chimeras. Some of these chimeras persist, whereas others eventually break down, revealing a period of allogeneic non-responsiveness that varies depending on the species. Alternatively, for H. sinapium, most pairs of sibling post-larvae and juveniles that settle in contact initiate immediate allorecognition and show the same morphological response progression as the adults. This indicates that allorecognition and response occurs during early metamorphosis. Results from H. sinapium and other sponge species, in addition to annotations of sponge genomes, suggest that allorecognition and immunocompetence in sponges are mediated by distinct systems and may become functional at different times during or after metamorphosis for different species. Consequently, allorecognition may not be a good proxy for the onset of immunocompetence.