Symbiosis of epi- and endocuticular bacteria with Helicoradomenia spp. (Mollusca, Aplacophora, Solenogastres) from deep-sea hydrothermal vents

Abstract

Two species of Solenogastres (Mollusca, Aplacophora) from deep-sea hydrothermal vents of the East Pacific Rise, Helicoradomenia cf. acredema and a second as yet undescribed species of the same genus, were discovered to harbor epibiotic and endocuticular prokaryotes along their entire bodies. Helicoradomenia sp. 1 additionally harbored intracellular rods in the epidermis. TEM (transmission electron microscopy) and SEM (scanning electron microscopy) observations revealed 4 different bacterial morphotypes: rod-shaped, helical, unsheathed and sheathed trichomes. One conspicuous rod was found to contain membranes typical for type II methanotrophs. Applying in situ hybridization with group-specific Bacteria and Archaea probes, the majority of the prokaryotes associated with Helicoradomenia sp. 1 were identified as -Proteobacteria, while others belonged to the -Proteobacteria or could not be associated with a definite bacterial clade. While solenogastres usually lack microbial epigrowth, the 2 Helicoradomenia species consistently are associated with similar epi-and endocuticular bacteria, indicating these are symbiotic associations.