Maximum depth extensions for Hydrozoa, Tunicata and Ctenophora

The observation of singleton or rare species in the deep sea is extremely valuable for gaining a census of biodiversity. At hadal depths (> 6000 m), these records provide a more complete picture of the vertical distribution of fauna. In this study, we present new in situ video records for Trachymedusae (Hydrozoa), Ascidiacea (Tunicata), and Tentaculata (Ctenophora) taken by submersible and supporting landers in the western Pacific Ocean. Together, these three taxonomic groups are present at depths far deeper than previously known. Observations of the rhopalonematid trachymedusa (Pectis cf. profundicola) from both lander and submersible dives at 10,063 and 10,040 m in the Philippine Trench (NW Pacific Ocean) extend the maximum depth of the Hydrozoa by a further 997 m and is the first record of Hydrozoa > 10,000 m. The predatory tunicate Octacnemidae sp. was observed nine times at 7799 m in the Mariana Trench and once at 8077 m in the Izu-Ogasawara Trench (NW Pacific Ocean), therefore extending its maximum depth by 1726 m, and 1002 m to historical accounts of what might also be in the Phlebobranchia order of Ascidiacea. Several observations of large, globular ctenophores with long filamentous tentacles and lacking oral lobes from 10,040 m in the Kermadec Trench (SW Pacific Ocean) increase the maximum depth of the Ctenophora by 2823 m and is the first record of Ctenophora > 10,000 m. Benthic ctenophores were also recorded from the Japan Trench at 8001 m, extending their known depth range by a further 2750 m.

Automated summary

The observation of singleton or rare species in the deep sea is of great value for biodiversity census. This study presents new in situ video records for Trachymedusae, Ascidiacea, and Tentaculata taken by submersible and supporting landers in the western Pacific Ocean, extending their known depth ranges. The findings include the first record of Hydrozoa > 10,000 m, an extended depth range of the predatory tunicate, and the deeper observation of Ctenophora.