The phylogeny of the Rhopaliophora (Scyphozoa, Cubozoa and Staurozoa) has been discussed and modified extensively up to the present day. Attempting to finally solve this question, the morphology and development of the different early life history stages stages of 17 scyphozoan, 2 staurozoan and 8 cubozoan species were investigated. Due to the magnitude of the study, the findings will be published in two parts. This first part will focus on the comparison of early life cycle stages, the second part (to be published in the following issue of Plankton & Benthos Research) will focus on the evolutionary connection and systematics. The present part of the study focused on the morphology of different types of sessile stages (polyps), the processes of strobilation and metamorphosis, and the resulting products (residues and ephyrae/young medusa) and their further development. The different morphological characters in the polyps helped to categorize them into types. By using the strobilation rates and information on the way morphological characters changed during the process of strobilation/metamorphosis, the strobilae were able to be separately categorized.

The first part of this study focused on the morphology of different early life stages and their further development. This second part of the study demonstrates that the different types of polyps, strobilations/metamorphoses, their products (residues/ephyrae/medusae) and their further development (e.g. gastric system development of ephyrae) explain well the evolutionary development of the Rhopaliophora. By using the strobilation rates and information on the way morphological characters changed during the process of strobilation/metamorphosis, the strobilation and metamorphosis types were newly defined. Species were allocated to types by the combination of this information. The incomplete metamorphosis in Carybdea xaymacana is re-affirmed as a modified monodisc strobilation according to the newly investigated monodisc strobilation type in carukiid species. Complete metamorphosis is a form derived from the modified monodisc strobilation, as well as the metamorphosis type observed in Staurozoa. A clear separation of "Rhizostomeae" into two paraphyletic groups (Kolpophorae and Rhizostomatidea) and the polyphyly of Semaeostomeae is demonstrated. The results lead to a new and reorganised taxonomic system for the class Scyphozoa, which once more includes Cubomedusae and Stauromedusae. Scyphozoa now comprises two subclasses: Coronamedusae and Discomedusae. The latter subclass includes the newly-created infraclasses Pelagimedusae and Centrifugalia. Two new superorders, each containing two new orders (Stipeslongida: Sanderiida, Metamorphida; Opposita: Circulumcanalida, Cyaneida), are contained within the Centrifugalia. The new order Circulumcanalida now contains two newly erected suborders: Antherhizostomatida and Phacellophorida. The former new suborder contains the newly erected infraorders Kolpophorae and Centripetalida. The family Sanderiidae was further erected within the new order Sanderiida.

Research Infrastructures (RIs) are facilities, resources and services used by the scientific community to conduct research and foster innovation. LifeWatch ERIC has developed various virtual research environments, which include many virtual laboratories (vLabs) offering high computational capacity and comprehensive collaborative platforms that supporting the needs of digital biodiversity science. Over its 250 years of history, the taxonomic research community has developed a system for describing, classifying and naming taxa across multiple levels. For the marine biota, taxonomic information is organized and made publicly available through the World Register of Marine Species (WoRMS) that records more than 250,000 described valid species. Although scientists tend to assign an equal status (in terms of contribution to overall diversity) to each taxon used in taxonomy, biogeography, ecology and biodiversity, the question “are all taxa equal?” has never been tested at a global scale. We present evidence that this question can be addressed by applying relatedness indices (Taxonomic Distinctness) over the entire WoRMS metazoan tree. The RvLab, developed by the LifeWatchGreece RI, operating on a high-performance computer cluster, has been used to meet the high computational demands required for such an analysis at a global scale.

Debate exists regarding the number of species of the moon jellyfish (genus Aurelia), a common member of the planktonic community of the coastal shelf seas around the world. Three Aurelia congeners (A. aurita, A. labiata and A. limbata) are currently considered to exist but recent genetic analyses suggested that this is an oversimplification. We analyzed the morphological characteristics of scyphistomae, morphological characteristics of ephyrae and differences in the time span of the strobilation process of Aurelia congeners from 17, 7 and 6 different source populations, respectively, of known species. Morphological characteristics of scyphistomae were similar among the 17 populations but those of ephyrae, such as the shape and form of lappets, were effective discriminators in the 6 cases examined. We recommend identifying species based on differences in 1) the morphological characteristics of scyphistomae and ephyrac (and not only medusae), 2) the genetics of individuals, and 3) the geographical occurrence of the population. This study adds to the growing body of knowledge on scyphozoan scyphistomae and ephyrae, stages of the metagenic life cycle of scyphozoans that have received relatively little study compared to medusae.

Scyphistomae show different modes of propagation, occasionally allowing the sudden release of great numbers of medusae through strobilation leading to so-called jellyfish blooms. Accordingly, factors regulating asexual reproduction strategies will control scyphistoma density, which, in turn, may influence blooming potential. We studied 11 scyphistoma species in 6 combinations of temperature and food supply to test the effects of these factors on asexual reproduction strategies and reproduction rates. Temperature and food availability increased reproduction rates for all species and observed reproduction modes. In all cases, starvation was the most important factor constraining the asexual reproduction of scyphistomae. Differences in scyphistoma density were found according to the reproductive strategy adopted by each species. Different Aurelia lineages and Sanderia malayensis presented a multi-mode strategy, developing up to 5 propagation modes. These species reached the highest densities, mostly through lateral budding and stolons. Cassiopea sp., Cephea cephea, Mastigias papua and Phyllorhiza punctata adopted a mono-mode reproductive strategy, developing only free-swimming buds. Lychnorhiza lucerna, Rhizostoma pulmo and Rhopilema esculentum also presented a mono-mode strategy, but they only developed podocysts. These 3 species had the lowest reproduction rates and polyp densities; not only their reproduction rates but also the need for a 2-fold set of environmental stimuli to produce new polyps (one for encystment, another for excystment) made this reproduction mode the slowest of those observed to be utilized for propagation. We conclude that blooms may be defined phylogenetically by the specific asexual modes each species develops, which, in turn, is regulated by environmental conditions.

With discovery and examination of type specimens in the Natural History Museum, London, UK, we reassign Stephanoscyphistoma simplex (Kirkpatrick, 1890) to the genus Nausithoe Kolliker, 1853, as Nausithoe simplex, comb. nov., and designate a lectotype for the species. Use of morphometric measurements is considered important in coronate systematics, but key features also include the unique whorl of internal cusps and the shape of these cusps. All previous records of N. simplex must be re-evaluated, taking into consideration the morphology of these internal cusps.

A new species of Cubozoa found in an aquarium at Hagenbecks Tierpark (Zoo), Hamburg, Germany, is described herein as Carybdea morandinii. The species is distinguished from all known cnidarians in the anatomy of its polyp stage. The gastric cavity of the cubopolyp is divided horizontally into two chambers by a shutter-lens-like diaphragm. Metamorphosis was total and resulted in a single, four-tentacled medusa as in other known cubozoans. Both polyp and medusa stages bear zooxanthellae, which seemed important in the metamorphic process.

An account is given about the development of the gametes of the holopelagic coronate scyphomedusa Periphylla periphylla (Péron and Lesueur 1809). The gonads of the species are complex and differ from those thus far described in Scyphozoa in having this characteristic composition of trophocytes, follicle cells, gamete-releasing pores, mucus cells, and resorption cells. Our results differ from those of previous coronate studies with respect to the contact of the oocytes with the gonad tissue throughout the duration of development. Among the Medusozoa, follicle cells have thus far been considered as an apomorphy for the Staurozoa, but their presence in the Scyphozoa casts some doubts on this assumption. From morphological structure and examination of the gonads and gametes, it can be deduced that P. periphylla has true organs and that gamete release is continuous. These results supplement our knowledge of the reproductive biology of this mesopelagic species.

Although jellyfish blooms are a focus of recent research, the roles that the developmental stages of species play are underestimated. Planulae, polyps and ephyrae are inconspicuous and often overlooked. The importance of production of ephyrae from the sessile polyps has become more apparent. Our objective was to establish an identification system for early ephyrae of scyphozoan species in plankton samples. We studied ephyrae of 18 species. Standard measurements were introduced and the variability of marginal lappets analysed. Characters differentiating the 18 species are described. Photographs and drawings of each species are presented as a catalogue of ephyrae of these species. We developed a key for identification of the 18 species.

Medusae play an important role in marine ecosystems, as competitors of many invertebrate and fish species. Additionally, jellyfish stings can cause severe pain, inflammation of the affected skin, and allergic reactions in human. Climate and environmental changes are likely to affect the medusae, but it is not yet clear whether these will affect their distribution, physiology, and their toxicity. Very little is known about the effect of biotic and abiotic factors on the proliferation and the distribution of medusan nematocysts. In this study, we compared three types of nematocysts (euryteles and A- and O-isorhizas) and venoms of Cyanea capillata medusae (Scyphozoa) obtained from the North Sea and the Baltic Sea, which have different salinity and temperature ranges. Different types of nematocysts were separated by laser microdissection and pressure catapulting (LMPC), and the proteinaceous contents of the nematocysts were analyzed by matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF MS). Medusae from the brackish Baltic Sea possessed more euryteles than those from the North Sea. The O-isorhizas and A-isorhizas were smaller in the Baltic Sea sample compared to the North Sea samples and the length-to-width ratios were larger in the Baltic Sea sample. Moreover, the pattern of proteins (potential toxins) obtained from the separated nematocysts showed differences among samples and nematocyst types, but no clear pattern was observable. This study displays the novel LMPC/MALDI-TOF MS approach as a useful tool to investigate the function and venom of cnidarian nematocysts types.

Several species of scyphozoan medusae occur in river estuaries and other brackish waters but it is often unknown if the planulae settle and the scyphopolyps reproduce in those low-salinity waters. In the present study, scyphozoan species from the German Bight (North Sea) were tested in laboratory experiments to investigate their tolerance of low salinity. Planula larvae released from medusae in salinity 32 were still active after the salinity was reduced to 10 ( Cyanea capillata , Cyanea lamarckii ) and to 7 ( Chrysaora hysoscella ) in laboratory treatments. Planulae did not settle on the undersides of floating substrates when salinity was reduced to <20. By contrast, planulae released from C. capillata medusae in Kiel Bight (western Baltic Sea) in salinity 15 developed into polyps in laboratory cultures. Polyps reared from planulae in salinity 36 survived a reduction to 12 ( C. capillata , C. lamarckii ) and to 8 ( Aurelia aurita ). Polyps of all tested species strobilated and released young medusae (ephyrae) in salinity 12. These results show a high tolerance of planulae and polyps to low salinity, indicating their possible occurrence in estuaries and brackish waters. In addition to laboratory observations, young C. capillata ephyrae were collected in the western Baltic Sea (Kiel Bight) in salinity 15, which indicates that they were probably released by a local polyp population. We suggest that the polyps of the painfully stinging lion’s mane, C. capillata , may be more widespread in the Baltic Sea than previously assumed and that the occurrence of the medusae may not only depend on inflow of water masses from the North Sea.

The species Thecoscyphus zibrowii Werner, 1984 has an exceptional life cycle, which lacks a medusa stage but develops an extraordinary structure (egg sac) for reproduction. Investigation of the life cycle, as well as anatomical and histological studies of the different developmental stages of T. zibrowii were performed to provide evidence for a possible homology of the egg sac with the medusa stage and to determine whether the reduced metagenesis of T. zibrowii is derived from strobilation. The egg sac showed several characteristics, which were compared to those of coronate medusae. The ectodermis of the egg sac had a plate-like appearance and was completely ciliated as is typical for coronate medusae. The number and the location of the gonads were similar to those of coronate medusae. The cnidocysts were significantly larger in the egg sac than in the polyp. A size difference of cnidocysts in the medusa and the polyp stage is known for several Coronatae. Characteristics of egg sac formation were compared to characteristics of strobilation. The formation of the early operculum was similar in T. zibrowii and N. eumedusoides. The constriction of egg sac and strobila occurred in the same mode and the gastric cavities of two egg sacs stayed in contact in a similar fashion to the gastric cavities of the strobila discs. The developmental zones of cnidoblasts of the egg sac and polyp were separated during the formation of the egg sac which showed a similar developmental gradient to a strobila. The existence of all of these consistent characteristics makes it very likely that the egg sac structure was homologous to a medusa. The species T. zibrowii would therefore be derived from a metagenetic ancestor. This species has reduced the medusa generation to the greatest extent within the Nausithoidae and has demonstrated thus far the endpoint of a regressive evolution of the medusa generation.

Asexual propagation is an underappreciated way of increasing the sessile population of the metagenetic Scyphozoa. Our studies on Sanderia malayensis have shown a great variety of asexual reproduction modes. A new mode of polyp asexual propagation is described. By integrating earlier reported modes occurring in other species we established a classification which may serve as a standard tool to describe asexual propagation in Scyphozoa.

Some of the most interesting and enigmatic cnidarians are classified within the hydrozoan subclass Trachylina. Despite being relatively depauperate in species richness, the clade contains four taxa typically accorded ordinal status: Actinulida, Limnomedusae, Narcomedusae and Trachymedusae. We bring molecular data (mitochondrial 16S and nuclear small and large subunit ribosomal genes) to bear on the question of phylogenetic relationships within Trachylina. Surprisingly, we find that a diminutive polyp form,Microhydrula limopsicola(classified within Limnomedusae) is actually a previously unknown life stage of a species of Stauromedusae. Our data confirm that the interstitial formHalammohydrasp. (Actinulida) is derived from holopelagic direct developing ancestors, likely within the trachymedusan family Rhopalonematidae. Trachymedusae is shown to be diphyletic, suggesting that the polyp stage has been lost independently at least two times within trachyline evolution. Narcomedusae is supported as a monophyletic group likely also arising from trachymedusan ancestors. Finally, some data, albeit limited, suggest that some trachyline species names refer to cryptic species that have yet to be sorted taxonomically.

Coronates occur in all oceans from coastal to abyssal waters. In the Mediterranean, three of the six local nausithoid species inhabit marine caves. They have a reduced pelagic life stage in common, which presumably offers them advantages in cave or shallow water habitats, though the mechanisms differ. Within that small number of species, apogamy, parthenogenesis, hermaphrodity and brood care are encountered. In this study, knowledge of habitat and ecology of the Mediterranean Nausithoidae is compiled and the relationships of the species are investigated through a combined approach of morphological and molecular character analysis. Three clades are evidenced. These results are corroborated by studies of the life cycles and observations on the behaviour.

The settlement behaviour of planula larvae and their development to young polyps was investigated in laboratory experiments in five scyphozoan species [Aurelia aurita (L.), Cyanea capillata (L.), Cyanea lamarckii Péron and Leseur, Chrysaora hysoscella (L.), and Rhizostoma octopus (L.)]. The undersides of settling plates were strongly preferred for settlement. Shells, the only natural substrate type offered, were less attractive than artificial substrates (concrete, machined wood, polyethylene, and glass). The advantages of colonization of substrate undersides for survival and reproduction of polyps are discussed. It is supposed that the increase of artificial substrates in our seas, due to marine litter pollution and submarine building activities, enlarge the areas of distribution of scyphozoan polyps, in coastal as well as in off-shore regions. Subsequent increases in ephyra production by polyps are probably one reason for the increase in mass occurrences of jellyfish recognized worldwide during the last few decades. It is suggested that the early developmental stages in the cnidarian life cycle, the planula larvae, and the polyps, play the key role in the development of jellyfish outbursts.