Our research in the field of Cnidaria covers various taxa, with a focus on species of Scyphozoa, Hydrozoa and Staurozoa.
Dr. Sabine Holst
Fachgebietsleiterin Cnidaria
Cnidaria
S.Holst
Cnidaria belong to the oldest multicellular animals. Many species are capable to adapt to changing environments because of their high regeneration capacity and variable reproduction strategies.
Research
We use life cultures of selected cnidarian species to investigate their life cycles and the effects of environmental conditions on their survival and reproduction.
Our taxonomic studies are based on material reared in laboratory cultures as well as on field samples. Preservation of the gelatinous soft bodies can cause massive shrinkage leading to difficulties in the identification of morphological features. Therefore, molecular identification methods provide important additional tools for species identification and species discrimination in the Cnidaria research. In our integrative taxonomic studies, we combine descriptions of morphological features of selected species with molecular analyses.
To investigate particular morphological features of Cnidaria, as sting capsules and statoliths, we use various techniques including scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), confocal laser scanning microscopy (cLSM) and 3D-reconstructions from X-ray micro-tomographic data (micro-CT).
Laboratory cultures of living Cnidaria species, realized at the DZMB in collaboration with the University of Hamburg (Dr. Ilka Sötje), is an important method to investigate the life cycles of selected species and to document morphological characters of different life stages.
Many cnidarian species have a complex insufficient investigated life cycle with several asexual and sexual reproduction strategies. In most scyphozoan jellyfish species the large medusae are produced asexually by small and inconspicuous, sessile polyps which are often poorly known.
Environmental factors, like climate change, overfishing and eutrophication affect the development of medusae. These factors are discussed as the main causes for the increase of jellyfish mass occurrences which have been noticed worldwide in the last few decades. In temperate climate zones scyphozoan medusae usually occur from spring to autumn before they degenerate after the sexual reproduction phase. Polyps however, can survive for several years, producing young medusae (ephyrae) every year in a process called strobilation. Therefore, the development of the polyp populations plays a crucial role for the development of the medusa populations. Live cultures are useful to investigate the effects of environmental factors on the development of polyp and medusa populations (Holst 2012, Lesniowsky et al. 2015, Alguéro-Muñiz et al. 2016, Goldstein et al. 2017).
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.
All cnidarian species have sting capsules (cnidocysts). The name of the taxon Cnidaria is derived from this common character of all cnidarian species. In many cnidarian taxa the different cnidocyst types are an important taxonomic character used to identify and differentiate the species.
Cnidocysts have various functions. They are used for prey capture, defense or to attach to the ground. Each cnidocyst is a formation of a single cell which is eliminated and replaced by a new sting cell after use. During the explosion-like discharge of the cnidocyst a long tubule is released from the capsule. Various capsule types can be distinguished by differences in the tubule shape (Sötje & Holst 2020). The contact of jellyfish sting capsules with the human skin can cause painful irritations (Hoffmann et al. 2017).
Live-observation on the inconspicuous polyp generation of the upside-down jellyfish Cassiopea andromeda highlight their fascinating asexual reproduction strategies resulting in the offspring of either young polyps or tiny medusae (ephyrae). The examination of fresh tissues under 1000fold magnification provided new insights in variation range of the cnidocysts of the species. Focusing on the tubule shape of discharged sting capsules revealed a higher diversity of cnidocyst types than described in previous studies. Differences in abundance and size of cnidocyst types between five examined developmental stages indicates an adaptation to changing prey item preferences (Heins et al. 2015).
Statocysts, the balance organs of jellyfish, are located at the bell rim of the medusae. They contain tiny crystals, the statoliths, representing the only solid and inorganic structures in the gelatinous medusa bell. Statoliths are composed of calcium sulfate hemihydrate (basanite), a substance with an extremely hygroscopic nature leading to difficulties in investigations on statoliths.
Our studies on scyphozoan and cubozoan statoliths and statocysts in collaboration with the University of Hamburg (Dr. Ilka Sötje), funded by the German Research foundation (DFG), provide new insights into the scientific potential of these structures (Holst et al. 2016, Sötje et al. 2017, Heins et al. 2017). We used a combination of methods, modified for the particular requirements of statoliths: light microscopic studies with a slide scanner, application of the fluorescent marker calcein on statoliths combined with confocal laser scanning microscopy (cLSM), 3D-reconstructions from X-ray micro-tomographic data (µCT), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), and Single-Crystal Analysis.
The size, growth, and number of scyphozoan statoliths increase with increasing medusa size, indicating the potential of these crystals for medusa age determination. Moreover, investigations on scyphozoan statocysts indicate taxon-specific differences in the morphology of the statolith arrangement and in statolith shapes.
Team
Lisa Kaiser
Bachelorstudentin (Universität Hamburg)
