Cydippid ctenophores in the coastal waters of Svalbard: is it only Mertensia ovum?

Pelagic deep-sea fauna observed on video transects in the southern Norwegian Sea

Observations of the diversity, distribution and abundance of pelagic fauna are absent for many ocean regions in the Atlantic, but baseline data are required to detect changes in communities as a result of climate change. Gelatinous fauna are increasingly recognized as vital players in oceanic food webs, but sampling these delicate organisms in nets is challenging. Underwater (in situ) observations have provided unprecedented insights into mesopelagic communities in particular for abundance and distribution of gelatinous fauna. In September 2018, we performed horizontal video transects (50–1200 m) using the pelagic in situ observation system during a research cruise in the southern Norwegian Sea. Annotation of the video recordings resulted in 12 abundant and 7 rare taxa. Chaetognaths, the trachymedusa Aglantha digitale and appendicularians were the three most abundant taxa. The high numbers of fishes and crustaceans in the upper 100 m was likely the result of vertical migration. Gelatinous zooplankton included ctenophores (lobate ctenophores, Beroe spp., Euplokamis sp., and an undescribed cydippid) as well as calycophoran and physonect siphonophores. We discuss the distributions of these fauna, some of which represent the first record for the Norwegian Sea.

Hiding in plain sight-Euplokamis dunlapae (Ctenophora) in Norwegian waters

Cydippid ctenophores of genus Euplokamis have been rarely reported from the north-east Atlantic in the scientific literature. The conspicuous lack of previous records is likely attributable to methodological constraints detrimental to sampling ctenophores, including the use of plankton nets and preservation of samples as well as poor identification literature and a lack of taxonomic expertise on gelatinous zooplankton. Here, we have compiled published and novel records as well as documented diver observations, of Euplokamis spp. in Norwegian waters. Despite scant earlier reports, our data suggest that the genus Euplokamis is widely distributed and relatively common along the entire Norwegian coast, including Svalbard. Euplokamis was recorded from samples taken from several hundred meters depth to surface, from fjords as well as offshore. Most of the observations reported in this study are from the period between April and July, whereas specimens have been found nearly throughout the year. Specimens from Norwegian waters were morphologically most similar to Euplokamis dunlapae, and conservative 18S rDNA sequences of some specimens had a 100% match with an E. dunlapae specimen from Friday Harbor, USA, the type locality for the species. However, the morphological and molecular variation of Euplokamis demonstrates the need for systematic global sampling of multiple individuals of many ctenophore species.