Predation by a tiny phoxichilidiid sea spider on a juvenile terebellid polychaete Thelepus japonicus

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 Anoplodactylus or Phoxiphilyra) and Thelepus japonicus, 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.