Gelatinous and soft-bodied zooplankton (GZ) have long been considered to have low energetic value (‘trophic dead end hypothesis’) and be insufficient to sustain higher trophic levels. However, the nutritional composition and energy content of GZ are often poorly known for entire groups, ignoring species-, size-, and stage-specific differences. In this study, organic matter and elemental composition (carbon and nitrogen) were measured for >1000 specimens from 34 GZ species collected from neritic and oceanic waters of the Northeast Pacific between 2014 and 2020. Species included 3 gastropods, 16 hydrozoans, 2 nude ctenophores, 6 scyphozoans, 3 tentaculate ctenophores, and 4 thaliaceans. Organic content and elemental composition were used to estimate energy content using published conversion factors and differed between and within taxonomic classes. Size-dependent variability was shown for several species. Differences in organic content and elemental composition by development stage were observed in a salp and scyphomedusa species, highlighting the need to consider life cycle stages separately. The relative energy values of GZ were generally low and highly variable, although some taxa were comparable to crustaceans. The findings of the present study emphasise the need for a more detailed consideration of GZ in marine food web models and time series analyses, to take into account their inter- and intraspecific variability.

Freshwater jellyfish species of the genusCraspedacustapurportedly originated from the Yangtze River catchment area, China, and have now been observed on all continents except Antarctica. Sightings ofC. sowerbiiin the Pacific regions of Canada were compiled to document some of the northernmost records of this species in the Americas. Species identification has been difficult in the past. Therefore, field collection of specimens was carried out on southern Vancouver Island. The morphology was described macro- and microscopically as well as molecularly using mitochondrial and nuclear markers. Collected hydromedusae from British Columbia (BC) did not deviate morphologically fromC. sowerbiispecimens from other continents, but molecular analyses support the idea of 2 main widely distributed lineages hidden under similar morphological features (i.e. a species complex). Through a community science approach, an understanding of the extent ofC. sowerbiidistribution in western and southern BC (present in 24 lakes from as early as 1990) has been established. Results showed that the number of sightings increased considerably in the period after 2010. Recent increases in sightings ofC. sowerbiiin BC and worldwide could be indicative of a climate warming-related range extension or growing public awareness and/or increased observational efforts. Even after more than 120 yr ofCraspedacustaresearch, much about their biology and ecology remains unknown, which motivated us to compile a list of knowledge gaps based on an extensive literature survey.

The freshwater jellyfish Craspedacusta sowerbii is among the most widespread invasive species, observed across a wide temperature range. The aim of this study is to analyze the polyp and medusa stages response to different temperatures by using (i) an experimental study on the polyp colony growth at 19 and 29 °C, and (ii) prediction of the Thermal Habitat Suitability (THS) based on the thermal tolerance of the medusa stage. The total number of polyps and colonies was greater at high temperature. At 19 °C, colonies with 1 to 5 polyps were present, with colonies of 1 to 3 polyps numerically dominating. At 29 °C, colonies were 80% composed of 1- to 2-polyps. Based on the published medusa pulsation rhythm data, a Thermal Performance Curve (TPC) regression was performed and used to monthly predict the THS for current and future (2050 and 2100) scenarios. The southern hemisphere offered optimal conditions (THS > 0.6) year-round. In the northern hemisphere, the optimum period was predicted to be between June and September. The future THS were considerably larger than at present with an increase in optimal THS at higher latitudes (up to 60° N). The combination of experimental and modeling approaches allows to identify the optimal thermal conditions of the polyp and medusa stages and to predict their invasive capacities.

The freshwater hydrozoan Craspedacusta sowerbii was introduced to habitats around the world and they spread into the surrounding water bodies thereafter. Despite an increase in reported observations, little is known about its genetics, physiology and ecology. The present study examined the organic content, elemental composition and energy content of jellyfish collected in Canada and the USA and compared these metrics with their marine counterparts and other co-occurring zooplankton. Specimens from the sampled lakes were sexed and molecularly characterized. Molecular, stoichiometric and energetic information is critical in determining distribution pathways and trophic connections. Molecular analyses showed two closely related sequences in North America, one shared by the Canadian and Japanese specimens, while another one is widely present across the Americas. Organic and all elemental contents were higher compared to marine jellyfish species. Estimated energy content of C. sowerbii was 20–27 kJ g DW⁻¹, which is 2-fold higher than for marine hydrozoans and compared well with cladocerans, copepods and rotifers. Estimated C/P values indicated that cladocerans and copepods are suitable prey for C. sowerbii and that jellyfish may compete with juvenile fish to fulfill similar nutrient demands. Freshwater jellyfish were stoichiometrically and energetically different from marine hydrozoans, indicating higher importance in freshwater food webs than previously thought.