Biogeographical distribution of Hyrrokkin (Rosalinidae, Foraminifera) and its host-specific morphological and textural trace variability

The parasitic foraminifer Hyrrokkin sarcophaga predominantly infests the cold-water coral Lophelia pertusa and the co-occurring bivalve Acesta excavata, showing a commensal or parasitic behaviour. It occurs also on some other corals (e.g., Caryophyllia sarsiae), bivalves (e.g., Delectopecten vitreus) and sponges (Geodia sp.), typically in aphotic environments. The aim of the study is to describe its traces from various host substrates, to characterise its parasitic behaviour and to map the geographical distribution of the genus Hyrrokkin. Epoxy-resin casts of H. sarcophaga traces in A. excavata C. sarsiae, D. vitreusand L. pertusa, and of H. carnivora traces in A. excavata, were SEM analysed. The boring pattern is in all cases characterised by a shallow groove of up to 7 mm in diameter (max. 2 mm deep), from which ‘whip’-shaped extensions protrude vertically into the substrate. In A. excavata the foraminifer can penetrate the entire valve to the mantle cavity, producing a thick shaft of fused ‘whips’. This parasitic attack is answered by a strong callus formation of the mollusc. One individual foraminifer can repeatedly bypass the organic-rich callus, resulting in a thick aragonite pinnacle. The trace surface texture is xenoglyph and changes with the penetrated host-microstructures. This is especially obvious on deeply penetrating trace portions (e.g., ‘whip’-shaped filaments) and is a strong indication for a chemical penetration mode (etching). The trace of Hyrrokkin is described as Kardopomorphos polydioryxigen. n., isp. n. On the substrates without the shaft, related to parasitic behaviour, Hyrrokkin might feed directly on adjacent external host tissue. H. sarcophaga is known along the North Atlantic continental margin from polar to subtropical latitudes and H. carnivora occurs on the continental margin of Mauritania, Congo and Guinea. In the Mediterranean we could document the parasitism of H. sarcophaga from Last Glacial A. excavata. Traces or detached foraminifer tests occur in Early Pleistocene cold-water coral deposits on Sicily and Rhodes. Recent H. sarcophaga has not been observed above 11°C and is scarce near 5°C water temperature. Hyrrokkin sp. was reported from the Canadian Pacific on fossil sponges and was observed on Acesta patagonica in the Beagle Channel (Chile).