Serpulids living deep: calcareous tubeworms beyond the abyss

Although the carbonate compensation depth (CCD) for calcite, generally located in the depth range 4000–5000 m, is often proposed as a physiological barrier to deep-ocean colonization, many organisms with calcareous exoskeletons are found in the deepest oceanic trenches. Serpulid polychaetes inhabiting unprotected calcareous tubes are unlikely deep-sea inhabitants, yet, they are found at all oceanic depths from intertidal to hadal. Here we review and revise the published and unpublished records of Serpulidae from below 5000 m depth. We also describe tube ultrastructure and mineralogical content of available deep-sea serpulid tubes to obtain insights into their biomineralisation. Species belonging to the genera Bathyditrupa, Bathyvermilia, Hyalopomatus, Pileolaria (spirorbin) and Protis were found at depths from 5020 to 9735 m. However, only specimens of Protis sp. were truly hadal (>6000 m) being found at 6200–9700 m. Hadal specimens of Protis have irregularly oriented prismatic tube microstructure similar to that found in more shallow-water representatives of the genus. Initial EDX analysis suggested a mostly calcitic composition (i.e., the most stable CaCO3 polymorph) on the basis of high Mg levels. Surprisingly, however, tubes of Bathyditrupa hovei and a species of Protis analysed using the more reliable method of laser Raman spectroscopy were found to be composed of aragonite. The compensation depth for this less stable CaCO3 polymorph in the oceans is usually 2000–3000 m. We found no obvious structural adaptations to life at extreme depths in the studied serpulid tubes and how serpulids are able to biomineralise and maintain their tubes below the CCD remains to be explained.