New and poorly known benthic foraminifera (Protista, Rhizaria) inhabiting the shells of planktonic foraminifera on the bathyal Mid-Atlantic Ridge

Based on sediment samples from the Mid-Atlantic Ridge (MAR; 49–54°N, 2508–2772 m water depth) we redescribe Hospitella fulva Rhumbler, 1911, an organic-walled foraminiferal species that inhabits the empty shells of planktonic foraminifera and has not been reported since its initial description more than 100 years ago. This poorly known species is characterized by a series of more or less globular ‘pseudo-chambers’ joined by tubular stolons and with an organic, brownish-orange, somewhat brittle test wall. It is probably related to Placopsilinella aurantiaca Earland, 1934, a species that is commonly attached to the external surface of planktonic foraminiferal shells at our MAR sites. A second inhabiting foramininferan, which we describe as Incola inculta gen. & sp. nov., is distinguished from H. fulva by the very thin and flexible organic test wall and the distinct tubular to somewhat conical agglutinated structure, which extends above the surface of the host shell and terminates in an aperture. A large number of xanthosomes (waste products) are distributed throughout the cell body of the new species. These and other inhabitants of planktonic foraminiferal shells make an important contribution to benthic foraminiferal assemblages on the bathyal MAR.