Many animals permanently inhabit the marine interstitium, the space between sand grains [ 1, 2 ]. Different evolutionary scenarios may explain the existence of interstitial animals [ 3, 4 ]. These scenarios include (1) that the interstitial realm is the ancestral habitat of bilaterians [ 5, 6 ], (2) that interstitial taxa evolved from larger ancestors by miniaturization, or (3) progenesis [ 3 ]. The first view mirrors the former hypothesis that interstitial annelids, called...