This study characterizes the abundance and spatial distribution of siphonophores between the Trinidad Channel (50°06′S) and the Strait of Magellan (52°45′S) in southern Chile, during October–November 2009. Ten species were identified, of which Agalma elegans, Rosacea plicata and Sphaeronectes fragilis are new records for this region. Dominant species showed similar dominance values e.g. Lensia conoidea (26.3%), Dimophyes arctica (24.6%), Lensia meteori (22.2%) and Muggiaea atlantica (20.7%). Eudoxids of L. conoidea and D. arctica represented 97.3% of all eudoxids collected and they were mainly collected in estuarine waters. The highest densities were found in estuarine waters (high vertical stratification and low temperature, salinity and dissolved oxygen values). On the other hand, the lowest densities were found in coastal areas influenced by permanent influx of Sub-Antarctic waters from the Pacific (greater instability and vertical mixing, higher temperatures, salinity and dissolved oxygen values). Temperature and dissolved oxygen were the most important environmental variables. In general, all the dominant species showed a positive association with temperature and a negative association with dissolved oxygen (with the exception of L. meteori). The vertical distribution showed that M. atlantica was mainly distributed in the first 50 m, in association with estuarine waters, while L. conoidea, L. meteori and D. arctica were mainly found in the deeper layer (50–200 m) and in association with modified Sub-Antarctic waters. The comparison of the results obtained in the springs of 1996 and 2009 showed a significant increase in abundance.

Gelatinous zooplankton including salps are an important but often overlooked component of marine ecosystems. This study documents a bloom of Salpa fusiformis at five locations to the north of mainland Scotland. The bloom was observed at Sula Stack, Sula Sgeir, North Rona and at two other locations to the west coast of the Orkney Islands. Salp blooms have the potential to deplete phytoplankton biomass, impacting on other zooplankton, including copepods and fish larvae. New records of two rare salps Cyclosalpa bakerii and Thetys vagina are also documented from North Rona and Skirza Head, Caithness. These specimens were observed in the aggregate (sexual) form. These observations represent the second record in the UK of C. bakerii and the third and fourth records of T. vagina.

Hyperiid amphipods are holoplanktonic marine crustaceans that are known as temporary symbionts of different groups of gelatinous zooplankton. The nature and dynamics of these associations are still poorly understood, particularly in deep waters. The mesopelagic and deep-living planktonic fauna off Monterey Bay, California (down to 4000 m) was surveyed using a remotely operated submersible (ROV) and blue-water diving (BWD) between September 2005 and January 2008. In this work we report our observations on a total of 51 symbiotic associations observed in situ (not from zooplankton samples), between hyperiid amphipods and various taxa of gelatinous zooplankton. We present the first information on the symbiotic relations of the hyperiid Vibilia caeca, and we provide data of 34 previously unknown symbiotic associations. The host range was expanded for several widely distributed hyperiid species. These findings suggest that the symbiotic associations between hyperiid amphipods and gelatinous zooplankton in deep waters deserve further study worldwide.

The spatial and temporal variations of planktonic cnidarians were studied across the inner continental shelf (≤40 m) off the State of Paraná in southern Brazil. Monthly zooplankton samples were taken between November 1997 and March 1999 at five stations. The holoplanktonic Liriope tetraphylla and Muggiaea kochi dominated during the entire period, comprising >80% of the specimens. The coast–ocean gradient became evident due to a near-coastal (≤20 m) assemblage formed by L. tetraphylla, Solmaris corona and meroplanktonic medusae, and a distinct outer (25–40 m) assemblage formed by the medusae Corymorpha gracilis and Aglaura hemistoma and siphonophores. The estuarine runoff during warm rainy periods lowered the salinity (~30) at the inner stations (1, 2 and 3), providing appropriate conditions for the estuarine/coastal species. Wind-driven onshore advection of outer-shelf species to the inner isobaths occurred from late autumn to winter, when the rainfall decreased and salinity increased (>35). Population peaks occurred in both summer and winter at the inner stations. For most species, the seasonal patterns were inconsistent with other studies conducted in the Southern Brazilian Bight, suggesting irregular and unpredictable seasonal distributions of abundance. Whereas in high-latitude ecosystems the dynamics of cnidarian populations follows the seasonal productivity cycle, here, complex hydrographic processes seem to be more important in determining the structure and seasonal dynamics of this community.

Jellyfish (Cnidaria: Scyphozoa) are increasingly thought to play a number of important ecosystem roles, but often fundamental knowledge of their distribution, seasonality and inter-annual variability is lacking. Bloom forming species, due to their high densities, can have particularly intense trophic and socio-economic impacts. In northern Europe it is known that one particularly large (up to 30 kg wet weight) bloom forming jellyfish is Rhizostoma spp. Given the potential importance, we set out to review all known records from peer-reviewed and broader public literature of the jellyfish R. octopus (Linnaeus) and R. pulmo (Macri) (Scyphozoa: Rhizostomae) across western Europe. These data revealed distinct hotspots where regular Rhizostoma spp. aggregations appeared to form, with other sites characterized by occasional abundances and a widespread distribution of infrequent observations. Surveys of known R. octopus hotspots around the Irish Sea also revealed marked inter-annual variation with particularly high abundances forming during 2003. The location of such consistent aggregations and inter-annual variances are discussed in relation to physical, climatic and dietary variations.