Improving biodiversity records of Hydrozoa (Cnidaria) in Helgoland, North Sea

Hydrozoa is a highly diverse class within Cnidaria and can be found in every ocean at most latitudes. Their diversity, abundance and ecological significance have been largely overlooked. In this study, the species richness of colonial hydrozoans was investigated at two localities around Helgoland, North Sea; the rocky shore of the northern most point of Helgoland, as well as the up to 60m deep Tiefe Rinne, south of Helgoland. Out of 22 colonies found, 21 were identified to species level and one to family level. Species richness was compared to previously published data from the region (Harms, 1993; Reichert & Buchholz, 2006). In the rocky shore, four species not mentioned in either paper for the intertidal were identified, while eight of the 11 reported species were not found. Although the species richness might have decreased in recent years, factors such as examination time, sampling accuracy and seasonal variation in density are also possible reasons as to why the eight species were not found. Species richness of hydrozoans in the Tiefe Rinne is mostly unexplored, with only two genus-level identifications being mentioned in the Harms review. In the current study, 15 previously unreported species were identified from the Tiefe Rinne; of those, eight were new for the entire Helgoland area, as compared to Harms and Reichert & Buchholz. The newly recorded species likely represent either previously overlooked members of the regional fauna or natural range expansions within the North Sea, supported by gradual environmental changes rather than recent introductions.