Hydrozoans, scyphozoans, larvaceans and ctenophores observed in situ at hadal depths

Abstract

Hydrozoa, Scyphozoa, Larvacea (Appendicularia) and Ctenophora are not typically associated with hadal communities. Here, we report observations of these groups based on 136 benthic camera lander deployments that spanned all five oceans, encompassing 14 deep sites, culminating in >1000 h of video in the near-bottom waters between 5000 and 10 925 m. Of the Hydrozoa, trachymedusae had a maximum depth of 9066 m in the Mariana Trench, narcomedusae were recorded to a maximum depth of 7220 m in the San Cristobal Trench and a single siphonophore was seen at 7888 m in the Mariana Trench. Scyphozoans were seen as deep as 6898 m in the New Hebrides Trench. The deepest ctenophore was seen at 6037 m in the Kermadec Trench. Larvaceans were seen in the Agulhas Fracture Zone and the Puerto Rico, Kermadec, South Shetland and Java trenches, with the deepest being 7176 m in the Java Trench. None of these groups were seen in the deep Arctic or Antarctic deeper than 6000 m. Narcomedusae, siphonophorae, Scyphozoa and Ctenophora appear very rare at hadal depths, while the larvaceans and trachymedusae appear to be relatively conspicuous in the benthopelagic at hadal depths.