The effect of salinity on the filtration rate of blue mussels, Mytilus edulis, from the brackish Great Belt (Denmark) and the low-saline Central Baltic Sea, respectively, was studied. First, we measured the effect of longterm (weeks) constant ambient salinities between 5 and 30 psu on the filtration rate of M. edulis collected in the Great Belt where the mean salinity is 17 psu. At salinities between 10 and 30 psu, the filtration rates did not vary much, but at 5 psu the filtration rates were significantly lower. Next, we studied dwarfed M. edulis (\25 mm shell length) from Central Baltic Sea (Askö, Sweden) where the mean salinity is 6.5 psu. The maximum filtration rate (F, ml min -1 ind.-1 ) as a function of shell length (L, mm) and dry weight of soft parts (W, mg) were found to be: F = 0.003L 2.71 and F = 0.478W 0.92

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Gelatinous zooplankton can be present in high biomass and taxonomic diversity in planktonic oceanic food webs, yet the trophic structuring and importance of this "jelly web" remain incompletely understood. To address this knowledge gap, we provide a holistic trophic characterization of a jelly web in the eastern tropical Atlantic, based on δ 13 C and δ 15 N stable isotope analysis of a unique gelatinous zooplankton sample set. The jelly web covered most of the isotopic niche space of the entire planktonic oceanic food web, spanning > 3 trophic levels, ranging from herbivores (e.g., pyrosomes) to higher predators (e.g., ctenophores), highlighting the diverse functional roles and broad possible food web relevance of gelatinous zooplankton. Among gelatinous zooplankton taxa, comparisons of isotopic niches pointed to the presence of differentiation and resource partitioning, but also highlighted the potential for competition, e.g., between hydromedusae and siphonophores. Significant differences in spatial (seamount vs. open ocean) and depth-resolved patterns (0-400 m vs. 400-1000m) pointed to additional complexity, and raise questions about the extent of connectivity between locations and differential patterns in vertical coupling between gelatinous zooplankton groups. Added complexity also resulted from inconsistent patterns in trophic ontogenetic shifts among groups. We conclude that the broad trophic niche covered by the jelly web, patterns in niche differentiation within this web, and substantial complexity at the spatial, depth, and taxon level call for a more careful consideration of gelatinous zooplankton in oceanic food web models. In light of climate change and fishing pressure, the data presented here also provide a valuable baseline against which to measure future trophic observations of gelatinous zooplankton communities in the eastern tropical Atlantic.

Experimental feeding and growth studies on filter-feeding organisms often rely on constant algal concentrations maintained over extended periods of time. Here we present a fluorometer controlled apparatus (FCA) designed for feeding experiments with suspension-feeding mussels at naturally low chlorophyllaconcentrations above 0.5 µg L−1. The principle used is feedback regulation of the algal concentration based on continuous monitoring of the fluorescence intensity of chlorophyllain water pumped through the apparatus from an aquarium with mussels. The filtration rate is monitored continuously as the rate of change of measured volume of an algal stock added to the aquarium for keeping a constant algal concentration. As an example, the FCA has been used to study the filtration rates of blue mussels (Mytilus edulis) at algal concentrations both near and above the incipient saturation level for reduced filtration activity. As another example to put the FCA into perspective as a reliable method for environmental effect studies, the apparatus has been used to demonstrate the acute effect of changing salinity on the filtration rate ofM. edulis.

Freshwater jellyfish species of the genus Craspedacusta purportedly originated from the Yangtze River catchment area, China, and have now been observed on all continents except Antarctica. Sightings of C. sowerbii in 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 from C. sowerbii specimens 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 of C. sowerbii distribution 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 of C. sowerbii in 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 of Craspedacusta research, much about their biology and ecology remains unknown, which motivated us to compile a list of knowledge gaps based on an extensive literature survey.

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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.

The freshwater jellyfish Craspedacusta sowerbii is one of the most widespread invasive species, but its global distribution remains uncertain due to ephemeral appearances and general lack of information in various aquatic environments. The aim of this study was to map current and future distributions (2050 and 2100) using Species Distribution Models allowing to visualize the habitat suitability and make projections of its changes under potential climate change scenarios. Except in Oceania where the range decreased, an expansion of C. sowerbii was projected during the next century under modeled future scenarios being most intensive during the first half of the century. The present study shows that the expansion of C. sowerbii worldwide would be facilitated mainly by precipitation, vapor pressure, and temperature. The predictions showed that this species over the eighty years will invade high-latitude regions in both hemispheres with ecological consequences in already threatened freshwater ecosystems.