An assessment of polychaete and pygnogonid diversity across different habitats of Helgoland

In a one-week survey (24–29 Sept 2025) around Helgoland we quantified polychaete and pycnogonid diversity across three different habitats: intertidal rocky shore, the subtidal shell mosaic at Tiefe Rinne, and coastal plankton hauls. Our aim was to document contemporary pycnogonid diversity and to place the 2025 polychaete assemblage in context by comparing our findings with earlier surveys from 1939 to 2023. We recorded 17 polychaete species from nine families (intertidal n = 11; Tiefe Rinne n = 4; plankton hauls n = 2). Overall, 14 of the 17 polychaete species recorded in 2025 were shared with at least one of the four datasets, and two species (Spirobranchus triqueter and Lepidonotus squamatus) occurred in all five surveys, indicating a persistent core polychaete fauna around Helgoland over nearly 90 years. Five pycnogonid species (three families) were recorded; all but a single Pycnogonum litorale specimen came from dredged material. Further investigations and long-time studies from different spots and with alternative methods would be needed to enhance the insight in the diversity of polychaetes and pycnogonids.