Diversity of nudibranchs in shallow water habitats in the region of Tromsø, northern Norway

Baseline data on the distribution of marine species is crucial to be able to address biogeographical patterns and to monitor  changes in species occurrences in marine systems. Nudibranch mollusks have proved to be useful bioindicators for monitoring  shifts in distribution and have received much attention by the scientific community in recent years. Being positioned in a zoogeographic transition zone between boreal and Arctic regions, northern Norway is an important area for detecting and tracking early distributional shifts. Despite this, no comprehensive knowledge on current biodiversity and distribution of nudibranchs exists from the region. This work presents, for the first time, an annotated and illustrated inventory of nudibranchs in shallow water habitats of the Tromsø region in northern Norway. In total, 49 different nudibranch species or taxa belonging to 19 different families were recorded during the time period May 2020 – December 2023. Compared to occurrence data from literature records and online data sources, 31 species are here reported from the region for the first time. In addition, northern range extensions are presented for a significant part of the Norwegian nudibranch fauna. By documenting current biodiversity and distribution the present study hopes to serve as a baseline for studies focused on monitoring biodiversity in the Arctic region in the future.