Trophic ecology of Ophiuroidea and Asteroidea in the Clarion-Clipperton-Fracture Zone (Central Pacific)

Abyssal seascapes represent over 50 % of the Planet's surface, but the life history traits of fauna present in these ecosystems remain poorly understood. Ophiuroidea constitute about one third of the invertebrate megabenthos assemblage between 3800 m and 4800 m water depth in the Clarion-Clipperton Fracture Zone (CCZ); Asteroidea are present in lower densities. We hypothesize that (1) Ophiuroidea, Xenophyophoroidea, and Hexactinellida have a predator-prey relationship, where Ophiuroidea feed on foraminifera- and sponge-derived organic matter (OM). (2) Ophiuroidea have a reduced dependency on fresh phytodetritus. (3) Brisingida (order of Asteroidea), often cling to stalks to have easier access to particulate OM sinking to the seafloor.

To test these three hypotheses, we combined bulk and compound-specific stable isotope analyses of fauna (Ophiuroidea, Asteroidea) and sediments with analyses of seafloor images from the eastern CCZ.

All investigated Echinodermata species had a high trophic level. Phospholipid-derived fatty acids (PLFAs) used as biomarkers suggest that Silax daleus consumes sedimentary detritus that is processed by its gut microbiome. Ophiacantha cosmica is likely a top consumer or scavenger, Ophiosphalma glabrum is an opportunistic omnivore ingesting phytodetritus, bacteria, Crustacea, and Foraminifera, while Ophiuroglypha cf. polyacantha (sp. 6) is a more selective omnivore. Freyella benthophila sits mostly on stalks of Hexactinellida and uses this elevated position to catch phytodetritus and zooplankton. Freyastera cf. tuberculata, in comparison, sits mostly on polymetallic nodules from where it preys upon Crustacea moving on the sediment surface. We confirmed that Ophiuroidea are less dependent on phytodetritus, and they consume foraminifera- and sponge derived OM.