Predation by a tiny phoxichilidiid sea spider on a juvenile terebellid polychaete <i>Thelepus japonicus</i>

Most species of sea spiders (pycnogonids) have been identified as predators, scavengers, or parasites, with some consuming polychaetes, although their prey is unknown in most species. A tiny (∼3 mm) pycnogonid preying on a juvenile polychaete was found in Aomori, Japan. The pycnogonid and polychaete were identified as Phoxichilidiidae sp. (a member of <i>Anoplodactylus</i> or <i>Phoxiphilyra</i>) and <i>Thelepus japonicus</i>, respectively, based on their morphological characteristics and mitochondrial 16S rRNA and/or COI gene sequence analysis. Our video demonstrates the pycnogonid feeding on the polychaete at around chaetiger 1, which is consistent with a previous report of a pycnogonid feeding on a nereidid polychaete. Our findings provide additional evidence of pycnogonids preying on polychaetes and signify the importance of polychaetes as a food source for pycnogonids.