Epidermal ultrastructure and implications for sulfide tolerance in six species of deep‐sea polychaetes

. Ultrastructural characteristics of the epidermis of 5 deep-sea vestimentiferan tube worms and an orbiniid worm were similar to those described from other organisms inhabiting sulfidic environments. The integument was composed of a simple or pseudostratified layer of columnar or cuboidal cells, covered by a cuticle of varying thickness, and rested on an extracellular matrix. Unusual mitochondrial morphology was observed in the peripheral region of the epidermis, characterized by an electron-dense matrix and granules. The abnormal size, shape, and reduced array of cristae in these mitochondria may have resulted from environmental stress. Mitochondria in the inner epidermal layers, where they are more protected from sulfide exposure, displayed normal morphology. The seep vestimentiferans (Seepiophila jonesi and Lamellibrachia cf. luymesi) exhibited numerous electron-dense organelles, similar in morphology to cytolysosomes and previously described as sulfide oxidizing bodies, while in the 3 vent vestimentiferans (Riftia pachyptila, Tevnia jerichonana, and Oasisia alvinae) and the seep orbiniid Methanoaricia dendrobranchiata, fewer of these organelles were observed. Energy dispersive x-ray microanalysis of these electron-dense organelles indicated the presence of sulfur, iron, zinc, and copper, as well as lesser amounts of other elements. We propose that these epidermal cytolysosomes are morphologically similar to previously described sulfide oxidizing bodies that have been hypothesized to play a role in maintaining aerobically poised animal metabolism in sulfide-enriched habitats.