Weston Johanna N. J.

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Johanna N. J.

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  • Preprint
    Eurythenes sigmiferus and Eurythenes andhakarae (Crustacea: Amphipoda) are sympatric at the abyssal Agulhas Fracture Zone, South Atlantic Ocean, and notes on their distributions
    (Elsevier, 2023-04-25) Weston, Johanna N. J. ; Stewart, Eva C. D. ; Maroni, Paige J. ; Stewart, Heather A. ; Jamieson, Alan J.
    Cryptic species in the deep ocean are rapidly being identified with molecular evidence and as a result, new species are being described. Consequently, our understanding of distributions among the revised landscape of species needs to be reassessed. A model example is the large scavenging amphipod, Eurythenes gryllus (Lichtenstein in Mandt, 1882), which historically was thought to have a eurybathic and cosmopolitan distribution. Molecular evidence has since led to the separation of E. gryllus into ten named species and truncating its range to bi-polar bathyal depths. This study focuses on two species; Eurythenes sigmiferus and Eurythenes andhakarae d'Udekem d'Acoz and Havermans, 2015, and presents new records of both species from 5,493 m in the previously unsampled Agulhas Fracture Zone, South Atlantic Ocean (42.77°S, 10.05°E). We paired morphology with DNA barcoding at two mitochondrial regions to achieve robust identification and assessed their wider geographic range by reassessing historical records. Their overlapping presence at the Agulhas Fracture Zone expands their known ranges to the non-polar South Atlantic Ocean. Specifically, for E. sigmiferus, the data suggests this species has a multi-ocean tropical to temperate distribution from abyssal to shallow hadal depths (3,410–6,097 m). Eurythenes andhakarae is not restricted to the Southern Ocean but is distributed across the Antarctic Polar Front to the temperate South Atlantic Ocean between abyssal and hadal depths (3,069–7,099 m), with a presence at bathyal depths requiring molecular confirmation. This study highlights that pairing new expeditions with a re-inspection of rich historical collections exploration can fill in data gaps across species ranges and, ultimately, biogeography.
  • Article
    Barriers to gene flow in the deepest ocean ecosystems: Evidence from global population genomics of a cosmopolitan amphipod
    (American Association for the Advancement of Science, 2022-10-26) Weston, Johanna N. J. ; Jensen, Evelyn L. ; Hasoon, Megan S. R. ; Kitson, James J. N. ; Stewart, Heather A. ; Jamieson, Alan J.
    The deepest marine ecosystem, the hadal zone, hosts endemic biodiversity resulting from geographic isolation and environmental selection pressures. However, the pan-ocean distribution of some fauna challenges the concept that the hadal zone is a series of isolated island-like habitats. Whether this remains true at the population genomic level is untested. We investigated phylogeographic patterns of the amphipod, Bathycallisoma schellenbergi, from 12 hadal features across the Pacific, Atlantic, Indian, and Southern oceans and analyzed genome-wide single-nucleotide polymorphism markers and two mitochondrial regions. Despite a cosmopolitan distribution, populations were highly restricted to individual features with only limited gene flow between topographically connected features. This lack of connectivity suggests that populations are on separate evolutionary trajectories, with evidence of potential cryptic speciation at the Atacama Trench. Together, this global study demonstrates that the shallower ocean floor separating hadal features poses strong barriers to dispersal, driving genetic isolation and creating pockets of diversity to conserve.
  • Article
    Amphipoda from depths exceeding 6,000 meters revisited 60 years on
    (Oxford University Press, 2023-05-15) Jamieson, Alan J. ; Weston, Johanna N. J.
    In the 1950s, the Danish Galathea Expedition undertook one of the first and most comprehensive explorations of our ocean’s hadal zone, depths extending from 6,000 to nearly 11,000 m, and presented a rich collection of the diversity of Amphipoda. The subsequent papers, however, concluded that these established ‘nothing essentially new’ to the existing knowledge of amphipod biology. Since Dahl’s foundational paper in 1959, amphipods, primarily from the superfamilies Lysianassoidea and Alicelloidea, emerged as one of the best-sampled hadal fauna, as these mobile invertebrates are readily recovered by different sampling techniques. Importantly, amphipods have become the model taxon, helping us to unlock knowledge about life in the hadal zone. In this review, we collate the knowledge gained since the Galathea Expedition and summarise the current understanding of how amphipods that appear during hadal exploration survive the trench environments. We discuss population structures across depth, inter-trench distribution and connectivity, applications in hadal microbiology, and, critically, how the hadal zone is being impacted by anthropogenic activity.