Jellyfish (Scypho- and Hydromedusae) are widespread and diverse representatives of the gelatinous zooplankton and thus attract attention in many ecological and taxonomic studies.
However, the correct identification of different developmental stages and the adequate fixation of their gelatinous bodies are challenging tasks in jellyfish taxonomy. Most diagnostic features are still recognizable in formalin preserved samples. In contrast, fixation in ethanol, the most common preservative for later DNA analysis, causes distortion of morphological features of the gelatinous and fragile medusae.
Molecular taxonomy provides an additional tool for species identification, particularly useful for the identification of life stages and species with few diagnostic morphological features. In addition, genetic analyses are helpful for the discrimination of closely related species with a similar or identical morphology. Our studies provide the link between morphological and molecular genetic discrimination and identification methods, demonstrating the power of this integrated approach in cnidarian taxonomy. This approach was realized successfully for scyphozoan, hydrozoan and staurozoan species occurring in the German Bight, North Sea (Laakmann & Holst 2014, Holst & Laakmann 2014, Holst et al. 2019). The combination of morphological and molecular genetic investigations confirmed the occurrence of the stalked jellyfish Haliclystus tenuis (Staurozoa) in the North Sea (Holst & Laakmann 2019). It was previously assumed that the geographical range of this staurozoan species would be limited to the northwest Pacific.