Soft-shelled benthic foraminifera from a hadal site (7800 m water depth) in the Atacama Trench (SE Pacific): preliminary observations

Abstract. Soft-shelled foraminifera (organic-walled allogromiids and agglutinated saccamminids) are an important component of the deep-sea meiofauna. Although these largely monothalamous taxa are common at bathyal and abyssal sites in almost all oceans, there are only two records from hadal depths. Here we report the occurrence of numerous allogromiids and saccamminids in a sample collected at 7800 m water depth in the Atacama Trench. The >20 μm fraction of the core sample (0–6 cm layer) yielded a total of 546 soft-walled specimens, the vast majority of them Rose Bengal stained, belonging to 20 morphospecies. Most specimens were allogromiids (82.0%), followed by saccamminids (11.0%) and psammosphaerids (6.0%). Allogromiids, particularly Nodellum- and Resigella-like forms, were responsible for a distinct peak around 120–160 μm in the size distribution, while the spherical Allogromiid sp. 1 dominated the larger-size classes. This sample provides further evidence for the widespread occurrence of soft-walled monothalamous foraminifera in marine habitats. A form resembling Resigella is common in the Atacama Trench sample but has not been observed at abyssal sites in the Pacific Ocean or Atlantic Ocean.