This study provides the first records of the upside-down jellyfish Cassiopea andromeda (Forskål, 1775) in the eastern Atlantic supported by molecular analysis. Specimens were observed, recorded, and sampled in an inland aquaculture facility in September 2023 in Tenerife Island (Canary Islands). This new record officially demonstrates the geographical expansion of C. andromeda, and the introduction of a new potential invasive species in the Macaronesia oceanic island system.

A single Stygiomedusa gigantea was caught by demersal crab trawl off Namibia at a fishing depth of 750 m. Although widely considered to be globally distributed in meso- and bathy-pelagic waters, this represents the first record of the species from the SE Atlantic and only the second from around Africa. The photographed specimen differs slightly from previous observations and attention is drawn to accurately recording future material.

We investigated how environmental conditions translate into reproductive success or failure in Aurelia aurita from the medusa to the polyp life stage. This study examined how: (i) settlement success and development of planula larvae and polyps vary across the year, (ii) the role of temperature in determining the successful settlement of larvae and growth of polyps, and (iii) the influence of maternal provisioning in the successful settlement of larvae and growth of polyps. Medusae were collected monthly from February to December 2019 from Horsea Lake, UK. Planula larvae were settled in conditions mimicking the in situ temperature and salinity of collection. For the individual treatments, planula collected in August settled most rapidly. Early development rates (<8 tentacles) were significantly higher than later growth rates (>8 tentacles) and were positively correlated with temperature, unlike later growth rates. Planula length, used as an indicator of maternal provisioning, varied significantly across the year. In July 2019, a high temperature anomaly coincided with an increased time spent by planula larvae in the water column. Increasing temperatures past thermal limits through the increasing occurrence of temperature anomalies is likely to be detrimental to larval settlement and indirectly to the replenishment of temperate polyp populations.

This study focused on a jellyfish monitoring and sampling programme along the eastern Qatar seawaters and reports the first jellyfish records for the Qatar pelagic ecosystem and the occurrence of three scyphozoans: the semaeostome Chrysaora cf. caliparea and the rhizostomes Marivagia stellata and Catostylus perezi, considered as first records of Scyphomedusae for Qatar. Jellyfish monitoring, underwater photography and sample collection were carried out along an inshore-offshore transect composed of five stations along the eastern coast of Qatar. Jellyfish material examined, systematic description, general distribution and sting risk of the three species are provided. The discussion section highlights the blooming capabilities of Chrysaora cf. caliparea and Catostylus perezi, while Marivagia stellata seems scarcer, being spotted for the first time since we started our jellyfish investigation programme in August 2018.

Since the mid-1980s, swarms of the rhizostome Rhopilema nomadica have been an annual phenomenon in Israeli Mediterranean coastal waters during the summer months. Despite its annual prominence and the potential impact on food webs and ecosystem services, studies concerning its feeding ecology and its interactions with other biota in the marine food web have not been conducted. During summer 2015 gut contents of 41 R. nomadica were analysed as well as ambient plankton assemblages. More than 60% of the medusae diet was found to consist of microzooplankton <150 μm. Size correlations revealed that larger R. nomadica consumed faster swimming prey while smaller medusae relied more on the slower swimming taxa. The medusan diet reflected most of the ambient plankton taxa, but no statistically significant correlations between the relative abundance in diet and ambient plankton were found. As summer progressed, there was a gradual decrease in both mean medusa bell diameter (from 42.2–16 cm) and integrity of feeding structures. These findings suggest that R. nomadica, at least at the time of its appearance in Israeli coastal waters, may exert less predatory pressure on the plankton than we might otherwise expect.

Sampling at four salmon aquaculture sites along the west coast of Scotland has identified short-lived aggregations of planktonic hydrozoans (>280 individuals m−3), here termed blooms. Several such blooms were linked with increases in gill pathology and mortality in caged fish. Two types, Obelia sp. and Lizzia blondina, were found to cause blooms regularly and often concurrently. Species composition of hydrozoan populations and fluctuations in population sizes were spatially and temporally heterogeneous, with adjacent sites (within 30 km of one another and with similar oceanic exposure) experiencing no correlation between species composition and population density. Blooms appeared temperature-mediated, with all identified blooms by Obelia sp. and L. blondina taking place in water above 12 °C; however, temperature alone was not found to be predictive. Blooms were not significantly associated with change in salinity, water clarity, or photoperiod. Due to the apparent lack of broadly applicable predictors, we suggest that localized, targeted sampling and examination of planktonic hydrozoan populations is required to discern the presence or absence of a bloom. It is likely that many blooms have historically caused harm in salmon aquaculture while remaining unrecognized as the root cause.

Animals often select for habitats that increase their chance of survival by balancing the need to acquire food, reproduce and avoid predation. Perennial blooms of golden jellyfish (Mastigias papua etpisoni) are present in Jellyfish Lake, Palau, a popular tourist destination. Based on the species’ economic importance and unusual behavioural complexity, increased understanding of jellyfish habitat selection is necessary. We used a novel approach, a REMUS autonomous underwater vehicle, to quantify jellyfish distribution, abundance and habitat, and compared these findings to traditional methods. Midday acoustic surveys showed jellyfish distribution was patchy and the population resided mainly on the eastern side of the lake, as it is known that jellyfish migrate eastward towards the sun. Highest vertical densities of jellyfish were at 6–7 m, potentially to mitigate UV damage or photoinhibition of their photosymbionts, suggesting a coupling exists between their vertical distribution and water properties. Abundance estimates of jellyfish were ~2.75 and ~7.1 million (~2 million excluding bell diameters <1 cm) from acoustic and net samples, suggesting the methodology employed underestimated the population's smaller size fraction and non-synoptic surveys could impact estimates due to unresolved patchiness. Our approach could investigate population dynamics, behaviour or habitat associations on fine scales.

Discoidal macrofossils reported herein from the lower Cambrian Zabriskie Quartzite (Great Basin, western United States) record the oldest Phanerozoic medusozoan body fossils, as well as the oldest medusozoan stranding event on record. Moreover, these fossils provide evidence of a significant shift in the taphonomic mode characteristic of preservation of nonmineralized taxa in coarse-grained siliciclastic successions near the onset of the Phanerozoic. Taphonomic and sedimentological evidence recorded by these and younger examples of stranded Cambrian medusae is consistent in suggesting that several of the requirements for preservation of these fossils were holdovers from the Ediacaran Period, including the presence of microbial mats and a lack of carcass disturbance by scavenging and/or bioturbating taxa. To shed further light upon the taphonomic factors necessary for the preservation of Cambrian medusae, we compared the biostratinomy and sedimentology of Cambrian medusa strandings to those of Ediacara Biota assemblages from lithologically similar successions. We find key secular disparities in the taphonomic histories of these two types of fossil assemblage. Inconsistencies between the preservational styles characteristic of fossil assemblages preserved in sandstone lithofacies on each side of the Precambrian–Cambrian boundary are explained by a considerable change in the preferred depositional setting in which these macrofossil assemblages are preserved. Thus, rather than documenting a single taphonomic continuum through the Precambrian–Cambrian transition, the Zabriskie and younger medusozoan body fossil assemblages record the advent of an entirely new, yet still very rarely exploited, taphonomic window exclusive to the Cambrian Period.

Upside-down jellyfish (genus Cassiopea) can be found in tropical coastal waters worldwide. Until now reports of the genus from Brazilian waters have been scant. We report here medusae and scyphistomae collected from Cabo Frio, Rio de Janeiro state. Although we could not unambiguously identify the material using morphological criteria, genetic sequence data (COI) indicate that the Brazilian jellyfishes are genetically similar to those from Bermuda, Hawaii and Florida, which are related to specimens from the Red Sea (Cassiopea andromeda). We hypothesize that the presence of C. andromeda in Brazil is due to an invasion event, as the scyphistomae were found growing over the known invasive ascidian Styela plicata. Estimation of divergence time between Brazil (Cabo Frio) and Florida/Bermuda populations is that it occurred at the beginning of ship movement to South America.

We present data on the occurrence of the invasive ctenophore Mnemiopsis leidyi in Norway after the initial observations made in 2005. Our data comes from several net sampling investigations conducted along the Norwegian coast in 2008–2014, as well as beach seine bycatch from the south coast (September–October 2005–2014). In 2008–2010, M. leidyi occurred in moderate abundances (≤0.56 lobate ind m−3) during autumn, with northernmost observations from Trondheimsfjord. Mnemiopsis leidyi was not observed in 2011–2012 and was scarce in 2013, but in 2014 it was again abundant along the south and west coasts. While temperature and salinity conditions along the Norwegian south coast and its fjords are sufficient for survival and reproduction by M. leidyi, temperature may limit egg production rates. Biological factors including food limitation as well as competition and predation by native gelatinous predators can also constrain populations. Mnemiopsis leidyi populations in Norway are likely to exhibit source–sink dynamics, with advective losses and suboptimal conditions preventing overwintering in large areas along the coast. The presence of M. leidyi in the southern North Sea, coupled with the cyclonic circulation pattern, suggests that outbreaks may nevertheless be expected in years with favourable conditions and/or significant inflow from the southern North Sea. Climate change could enhance reproduction of M. leidyi in Norway and protected inner fjords may offer a suitable habitat for establishment of local populations in the future.

We report the first record of the scyphomedusa Rhopilema hispidum from the coast of Pakistan. The jellyfish is briefly described and the diagnostic characters are discussed. The morphological features used for the genera identification within the family Rhizostomatidae and various species of the genus Rhopilema are highlighted. We also question the previous record of Rhizostoma pulmo in Pakistan, assuming it to be a Rhopilema jellyfish and possibly the given species.

We describe a new species of Crambionella, C. helmbiru, from central Java, Indonesia. The combination of the mean number of lappets per octant (14), presence of foliaceous appendages amongst frills on oral-arms, absence of tubercles on the velar lappets, proportion of terminal club length to oral-arm length (0.28), and the body colour distinguish this species from three previously described congeners. In addition, the analysis of partial sequences of the cytochrome c oxidase subunit I gene indicate substantial genetic differences from both Crambionella orsini and Crambionella stuhlmanni, supporting the validity of this new species. A combination of morphological and genetic approaches determined that the remarkable differences in exumbrellar colours observed in specimens are simply intra-specific variation. Surprisingly, this species has been commercially harvested for more than 20 years and is well-known to the local people in the region, yet it had remained unknown to science until this point. The commercial fisheries targeting this formerly unknown species are also described in detail.

Olindias spp. medusae are found worldwide in sublittoral tropical and sub-tropical coastal regions; their occurrence near shore can result in human envenomation events. While behaviour of medusae and human contact with medusae has been documented for the genus, the hydroid (polyp) phase of the Olindias life cycle has eluded investigators for over a century. Given the recent debate among public media and scientific communities that jellyfish blooms are increasing worldwide, there is a growing urgency to understand how and why jellyfish populations bloom. In order to understand jellyfish population dynamics, the asexual benthic phase must be studied to determine when, where, and how juvenile medusae are produced. In this study, husbandry management strategies, including the creation of artificial habitat for Olindias formosus medusae in aquaria were developed to encourage spawning and larval settlement. The resultant hydroid colony of Olindias formosus was discovered in November of 2012, utilizing the natural fluorescence of the medusa as a detection method. A description of the hydroid and early medusa stage is presented. These techniques provide a basis for locating in situ the benthic hydroid phases within this genus and other fluorescent medusae, the discovery of which may lead to a better understanding of the causative factors for jellyfish blooms.

Concern has been expressed over future biogeographical expansion and habitat capitalization by species of the phylum Cnidaria, as this may have negative implications on human activities and ecosystems. There is, however, a paucity of knowledge and understanding of jellyfish ecology, in particular species distribution and seasonality. Recent studies in the UK have principally focused on the Celtic, Irish and North Seas, but all in isolation. In this study we analyse data from a publicly-driven sightings scheme across UK coastal waters (2003–2011; 9 years), with the aim of increasing knowledge on spatial and temporal patterns and trends. We describe inter-annual variability, seasonality and patterns of spatial distribution, and compare these with existing historic literature. Although incidentally-collected data lack quantification of effort, we suggest that with appropriate data management and interpretation, publicly-driven, citizen-science-based, recording schemes can provide for large-scale (spatial and temporal) coverage that would otherwise be logistically and financially unattainable. These schemes may also contribute to baseline data from which future changes in patterns or trends might be identified. We further suggest that findings from such schemes may be strengthened by the inclusion of some element of effort-corrected data collection.

Marivagia stellata, a cepheid jellyfish was previously recorded off Kerala, India in the south-east of the Arabian Sea. Now, the species has been found off the mouth of the River Indus, Pakistan for the first time. This new record greatly extends the range of its distribution to the north-east of the Arabian Sea along the coast of Pakistan.

Six new records of hydromedusae are reported as secondary hosts of Opechona sp. (Lepocreadiidae) for the Brazilian coast. Among the 392 hydromedusae sampled, 43 (~11%) were harbouring metacercaria. Prevalence (P) and intensity of infection (I) of Opechona sp. varied considerably among the different host species. Gossea brachymera (Limnomedusae) was highly parasitized (P = 30%; I = 1–7), while the other five hydromedusae species were not (P = 1.5–10; I = 1–2). The high parasitism in G. brachymera suggests the importance of this species in the transmission of Opechona sp. to fish, the definitive hosts, and highlights the hydromedusae as a probable noteworthy food item for zooplanktivorous fish in the area.

The present paper reports two scyphozoan species for the coast of Pakistan: the semaeostome Pelagia cf. noctiluca and the rhizostome Catostylus perezi. Both species are described and systematic characters used for identification are discussed for both. We also questioned the previous record of C. mosaicus in Pakistan assuming it to be of C. perezi.

Stable carbon and nitrogen isotopes and fatty acid compositions of Aurelia aurita, Stomolophus meleagris and Cyanea nozakii collected from the Yellow Sea in August 2009, were analysed. δ13C and δ15N values of the jellyfishes were in the range of −22.0‰ to −17.4‰ and 4.8‰ to 10.6‰, respectively, which indicated that jellyfishes in this study area were mainly dependent on marine-based carbon. The trophic levels of jellyfishes were estimated as between zooplanktons and high level carnivorous fishes. Low C18:1n-7/n-9 ratios and high C20:1 (n-11 and n-9) and C22:1 (n-11 and n-9) concentrations suggested jellyfishes in the Yellow Sea were carnivorous and zooplanktons were an important diet for them. Fatty acid compositions of jellyfishes were characterized by high C20:4n-6 concentration (>10%), with ratios of C20:5n-3/C22:6n-3 >1. Specific fatty acids (C20:1 + C22:1 and C20:4n-6) indicated that both pelagic-derived material and benthic detritus-derived food were diet of jellyfishes. Variations of certain biomarkers (C15:0 + C17:0, C20:1 + C22:1 (n-11 and n-9) and C20:4n-6) in jellyfishes of different body sizes suggested that ontogenetic diet shifts may take place during the growth of jellyfishes. Larger individuals with higher C20:1 + C22:1 (n-11 and n-9) concentration, tended to consume more zooplanktons; while the detritus-derived material made a more significant contribution to the small individuals, as suggested by higher C15:0 + C17:0 and C20:4n-6 concentrations. According to our study, besides the diet, species seemed to influence variations in stable isotopes and fatty acid compositions in jellyfishes.

The present study reports the first record of predation of the banded butterflyfish Chaetodon striatus on the jellyfish Chrysaora lactea on Brazilian rocky reefs. The activity involved a pair of C. striatus and was photographed and video-recorded. We believe that temporarily available food resources benefit species with a more generalist diet.

Observations on gonad morphology and the structure of ovaries and testes of the coronate scyphozoans Atolla wyvillei and Periphylla periphylla are described based on samples collected from the Gulf of Mexico and Cape Hatteras (north-western Atlantic). In A. wyvillei, gonads of distinguishable sex were observed in medusae as small as 17 mm bell diameter (BD). Spermatogenesis occurred within follicles (average 366 × 254 μm) that were evenly distributed throughout the gonad. Oocytes in different stages of development were observed in all the females with gonads. Oocytes arise from the gastrodermis and migrate into the mesoglea to develop from early-mid to late vitellogenic oocytes characterized by a large nucleus and granular (organic-rich) cytoplasm. The largest oocytes measured were 543 μm and 263 μm from the Gulf of Mexico and Cape Hatteras respectively. Possible reasons for this difference are discussed. In P. periphylla gonads were also initially observed in medusae 17 mm BD, although not all larger medusae had obvious gonads. Unlike A. wyvillei sperm follicles were arranged in long convoluted rows normally only one follicle thick. The organization of ooytes in female P. periphylla was very similar to A. wyvillei, although the gonads were small and the number of oocytes present in each gonad very low (<22). The largest oocyte measured was 777 μm in a 53 mm BD medusa. Although medusae were collected from one time period only (September) in this study, our findings appear to be in agreement with literature evidence indicating that coronate jellyfish produce few eggs continuously over a long time period. Aspects of gonad development and gametogenesis are discussed with respect to potential differences in site productivity and species identification.