Reef forming Serpula vermicularis from Scotland and Ireland: Tube structure, composition and implications

Abstract

Serpula vermicularis, as with many other serpulid species, is normally found as separate individual tubes. A limited number of exceptions to this have been recorded where S. vermicularis forms isolated vertically oriented ‘reef’-like structures, of agglomerated intertwined tubes, up to ∼35 cm in height. Such reefs have mainly been noted from freshwater influenced sea lochs along the west coast of Ireland and Scotland. The ultrastructure of the tubes of reef forming S. vermicularis described herein, is more diverse than that previously described of tubes from solitary S. vermicularis from fully marine environments, and bears many similarities to that of Spirobranchus triqueter (as Pomatoceros triqueter). The tube comprises a mixture of ordered (O), unordered (U) and spherulitic-prismatic (SPHP) fabrics, with composition varying from Mg-calcite to aragonite. Ordered and unordered fabrics are used as replacements for previously used lamello-fibrillar and irregularly oriented prismatic fabrics, based on their more complex nano-structure, and in the case of ordered vs lamello-fibrillar fabrics, the latter should be restricted to usage amongst the Mollusca as originally described; as the two are likely to have had divergent origins/modes of formation. Both changes follow previous work on Spirobranchus and Spirorbis, and resurrect the descriptive terminology of ‘ordered chevron structure’ and ‘unordered chevron structure’. Examination of the detailed nano-structure of the lath-like components of ordered and unordered fabrics illustrates that both comprise previously unrecognised spherical calcitic structures, arranged into bundles of parallel to sub-parallel rods. The latter has important implications for mode of formation for both S. vermicularis and other serpulid tubes with similar microfabrics such as Spirobranchus, reopening the debate over a simple plastering mode of tube formation versus that of an organic sheet or organic matter mediated origin.