Abstract
Medusae can be conspicuous and abundant members of seafloor communities in deep-sea benthic boundary layers. The epibenthic trachymedusa, Ptychogastria polaris Allman, 1878 (Hydrozoa: Trachylina: Ptychogastriidae) occurs in the cold, high latitude systems of both the northern and southern hemispheres, with a circumpolar distribution in Arctic and sub-Arctic areas, and disjunct reports of a few individuals from Antarctica. In January-February 2010, during benthic megafaunal photosurveys in three subpolar fjords along the West Antarctic Peninsula (Andvord, Flandres and Barilari Bays), P. polaris was recorded in Antarctic Peninsula waters. The trachymedusa, identified from megacore-collected specimens, was a common component of the epifauna in the sediment floored basins at 436–725 m depths in Andvord and Flandres Bays, reaching densities up to 13 m-2, with mean densities in individual basins ranging from 0.06 to 4.19 m-2. These densities are 2 to 400-fold higher than previously reported for P. polaris in either the Arctic or Antarctic. This trachymedusa had an aggregated distribution, occurring frequently in Andvord Bay, but was often solitary in Flandres Bay, with a distribution not significantly different from random. Epibenthic individuals were similar in size, typically measuring 15–25 mm in bell diameter. A morphologically similar trachymedusa, presumably the same species, was also observed in the water column near the bottom in all three fjords. This benthopelagic form attained abundances of up to 7 m-2 of seafloor; however, most P. polaris (~ 80%), were observed on soft sediments. Our findings indicate that fjords provide a prime habitat for the development of dense populations of P. polaris, potentially resulting from high and varied food inputs to the fjord floors. Because P. polaris resides in the water column and at the seafloor, large P. polaris populations may contribute significantly to pelagic-benthic coupling in the WAP fjord ecosystems.
Highlights
Pelagic organisms, medusae, are common members of benthic boundary layer communities, where some species may shift to benthic life stages, scavenge epibenthic food sources and attain appreciable seafloor abundance (e.g. [1, 2, 3, 4, 5, 6])
Apart from the gonads being more rugose in the specimens collected in 2012 in the Japan Sea, no other differences were apparent, confirming our Antarctic trachymedusae to be P. polaris based on morphological characters and agreeing with Kramp’s assertion that the two species are likely synonymous
cytochrome oxidase I (COI) sequences generated for this study show a large degree of differentiation between the Japan Sea and the Antarctic Peninsula with some 27%
Summary
Medusae, are common members of benthic boundary layer communities, where some species may shift to benthic life stages, scavenge epibenthic food sources and attain appreciable seafloor abundance (e.g. [1, 2, 3, 4, 5, 6]). Medusae, are common members of benthic boundary layer communities, where some species may shift to benthic life stages, scavenge epibenthic food sources and attain appreciable seafloor abundance The fragility of gelatinous zooplankton, and absence of suitable sampling gear and preservatives, have hindered effective collection of intact specimens and contributed to their poor representation in deep-sea faunal inventories (summarised in [10]). Plankton nets tows [15] and sediment traps [16, 17], as well as gear adapted for dedicated sampling of medusae (e.g. nets on epibenthic sleds and bottom trawls as described in [18]), have facilitated collection of gelatinous organisms from deep benthopelagic habitats
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