Kinorhyncha, Reinhard, 1885

4.5. Community structure of Kinorhyncha View in CoL along the vertical profile in the South Orkney Trench

Regarding the distribution of the whole kinorhynch community along the sediment, the specimens are homogeneously distributed in the axis but unevenly in the terrace (concentrated up to 2 cm depth). This could be due to a higher oxygen availability in the deeper layers of the axis caused by bottom sea currents and bioturbation phenomenon of mega and macrofauna ( Callaway, 2006). Our hypothesis is supported by the unusual abundance of macrofauna organisms found by Sokolova et al. (1996) in the bottom of the hadal zone of the South Orkney trench, at depths over 6.000 m.

The high proportion of juveniles compared to adults as well as the uneven distribution of juveniles along the vertical profile of the sediment has already been observed in previous studies (Alvarez-Castillo´et al., 2015; S´anchez et al., 2022). The fact that only juveniles appear in the first sediment horizons while adults are found at greater depths, independent of the site, could be related to physical-chemical factors and the anatomy of the animals. It is well known that most meiofaunal organisms must inhabit at the first few centimetres of the sediment since the oxygen concentration decreases rapidly along the vertical profile of the seafloor ( Higgins and Thiel, 1988; Sørensen and Pardos, 2020; Neuhaus, 2013). Considering that oxygen availability acts as a limiting factor for these organisms, we hypothesize that juveniles may have a greater oxygen demand due to a higher metabolic activity than adults, leading them to inhabit only the uppermost layers of the sediment. Another explanation would be related to a lesser development of juvenile anatomical structures involved in locomotion (Neuhaus, 2013). As the number of introvert structures increases in the ontogeny, juvenile stages would find it more difficult to move through the sediment particles of deeper, more compact layers, relegating these specimens to remain in the upper horizons.

CRediT authorship contribution statement

Nuria Sanchez ´: Writing – review & editing, Writing – original draft, Visualization, Investigation, Data curation, Conceptualization. Marta García-Cobo: Writing – review & editing, Visualization, Resources, Data curation. Mauricio Shimabukuro: Writing – review & editing, Visualization, Resources. Daniela Zeppilli: Writing – review & editing, Project administration, Funding acquisition. Hidetaka Nomaki: Writing – review & editing, Funding acquisition. Alberto Gonz´alez-Casarrubios: Writing – review & editing, Writing – original draft, Visualization, Investigation, Data curation.