Jellyfish can often be found in abundance in communities living in the benthic boundary layer, the water directly above the seafloor. The cold high-latitude systems surrounding the poles are no exception. A recent study published by Grange et al. in PLOS One reports on unusually high abundances of Ptychogastria polaris Allman in fjords in the glacier-rich West Antarctic Peninsula.
P. polaris is a cold-water species that has been found in a variety of locations in the high latitudes of the Northern and Southern Hemisphere. It was first described in 1878 by A.G. Allman, based on a single specimen collected off East Greenland. Since then, it has been found to have a patchy, circumpolar distribution in Arctic and sub-Arctic areas, while only a few specimens have been documented in Antarctica.
Between January and February 2010, Grange et al. conducted surveys of benthic megafauna in three subpolar fjords along the West Antarctic Peninsula – Andvord, Flandres and Barilari Bays.
“Arctic fjords are heavily impacted by meltwater inputs and sedimentation that yield low seafloor abundance and biodiversity, so we wanted to see if that was also the case in the Antarctic,” Grange explained to GlacierHub.
They analyzed live specimens, conducted photosurveys of the seafloor, and measured background environmental conditions to gain a better understanding of the distribution of P. polaris. Molecular analysis and DNA sequencing were also used to confirm the species identifications of specimens.
P. polaris was found to be a common component of seafloor communities in both Andvord and Flandres Bays, but was absent in Barilari Bay. “We noted the conspicuous occurrence and high abundance of P. polaris,” Grange stated. She noted that the densities in these locations up to 400 times higher than previously recorded in northeast Greenland and the Barents Sea.
These levels could be a result of higher productivity within the benthic boundary layer in the fjords. Reasons for this productivity include higher nutrient inputs that occur when the remains of sustained phytoplankton blooms sink to the ocean floor, or when macroalgae (large-celled algae such as seaweed) cascade down fjord walls, providing food sources that support larger populations of P. polaris. In addition, migrating Antarctic krill and baleen whales can transport nutrients to these regions in the form of feces and krill carcasses.
P. polaris was also observed in smaller densities in the water column in all three bays. Although this species is known to undertake short swimming expeditions of up to fifteen seconds, these observations were relatively frequent, suggesting that P. polaris in Antarctica may behave differently from counterparts in Arctic and boreal environments. This could be driven by feeding opportunities, localized regions of turbulent mixing at the seafloor, or distinct circulation patterns, but further research is needed, according to Grange et al.
Both findings also suggest that P. polaris may form a link between pelagic (open water) and benthic food-webs within the region. For example, they may play an important role as ecological predators of benthic organisms like zooplankton, while providing food inputs to the seafloor when they die. This contributes to nutrient and energy transfers between the ecosystems, helping to integrate the dynamics of food-webs in different layers of the marine environment.
This study was also the first to provide a phylogenetic (evolutionary history and relationship) analysis of the Ptychogastriidae family, to which P. polaris belongs. “We found relatively large genetic differentiation among P. polaris compared to that for other hydrozoan (the larger taxonomic class of organisms) species,” Grange explained. “This discovery may suggest the species contain multiple cryptic species (different species with identical physical forms) or an unusually high degree of sequence variation between the extreme ends of its distributional range.”
Further research will help to elucidate the findings of this study. The complex interplay between wind, tidal and glacial processes in subpolar fjords also creates a variety of conditions in different fjords, suggesting that glacier-related environments such as these may yield more surprising discoveries.