The waters of the Southern Ocean, which surrounds Antarctica, are considered to be a hotspot for marine biodiversity, yet only their macrofauna has been extensively studied. The meiobenthos (bottom dwelling organisms larger than single-celled microbes, but still difficult or impossible to see with the naked eye) of this region has been particularly poorly studied, and almost all of the studies which have occurred have been of Nematodes or Crustaceans. To date, twenty species of Tardigrades have been described from the waters around Antarctica, two Gnathostomulids, one of which was only identified to family level, and five Kinorynchs (although a study which should be published later this year is expected to add to this) have been described from this region, but not a single Scalidophoran, Loriciferan, or meiobenthic Priapulid.
Gastrotrichs are a phylum of minute animals, generally less than a millimetre in length, found in interstitial spaces in sediments. Their small size meant that they went unnoticed until the event of microscopy, with the group not being discovered until the 1860s. Despite this unfamiliarity they seem to be ubiquitous in marine sediments, and are also often found in non-marine settings. To date, only a single Gastrotrich, Thaumastoderma antarctica, has been identified from Antarctic waters, although there have been several reports of unidentified Gastrotrichs.
In a paper published in the journal Zootaxa on 17 June 2025, Martin Sørensen of the Natural History Museum of Denmark at the University of Copenhagen, Thiago Araújo of the University of Massachusetts Lowell, Lara Macheriotou and Ulrike Braeckman of the Marine Biology Research Group at Ghent University, Craig Smith of the Department of Oceanography at the University of Hawai’i at Mānoa, and Jeroen Ingels of the Coastal and Marine Laboratory at Florida State University, and the National Institute for Water and Atmospheric Research in Wellington, New Zealand, describe a second species of Gastrotrich from Antarctic waters.
The new species is described from specimens collected in December 2015 and April 2016 from sediment cores collected from depths of between 532 and 701 m in Andvord Bay the west coast of Graham Land, and the Gerlache Strait, which separates the Palmer Archipelago from the Antarctic Peninsula. It is placed within the genus Musellifer, and given the specific name leasiae, in honour of marine biologist Francesca Leasi in recognition of her numerous contributions to Gastrotrich taxonomy and morphology.
Specimens of Musellifer leasiae are between 322 and 415 μm in length, and have a body with a pointed head, a weakly defined neck, a parallel-sided body, and a pair of tapering furcal branches ('tails'). This body is covered by approximately 26 columns of scales, with an average of 45 scales per column. The columns can be divided into eight dorsal columns, two sets of five ventral columns, and eight ventral columns. The ventral surface also has two rows of locomotory cilia.
Five species of Musellifer have been described previously; Musellifer delamarei and Musellifer profundus from the Mediterranean, Musellifer tridentatus from the Caribbean, Musellifer reichardti from the Atlantic coast of Florida, and Musellifer sanlitoralis from the San Juan Archipelago in Washington State. Only a single specimen has previously been described from the Southern Hemisphere, a possible specimen of Musellifer profundus, making Musellifer leasiae the first known species in the genus with a Southern Hemisphere, as well as the first species from the Antarctic.
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