Fossil tube microstructure helps to address evolutionary unknowns of deep-sea tubeworms

Collaboration between an AMRI zoologist and a palaeontologist colleague sheds light on evolution of calcareous tubeworms in the abyss.

A morphological taxonomic revision was undertaken to understand affinities of very unusual tubeworms of the deep and an ultrastructural (examining structures at the microscopic level) study addressed the question whether the fossil tetragonal tubes of past worms belong to direct ancestors of the worms inhabiting such tubes today. Research on this topic is important for understanding the evolutionary history of tubeworms and ultimately assisting in the calibration of the molecular clock and revealing crucial information about evolutionary timelines.

Fossils are the records of life preserved in stone, but usually only the hard parts of animals such as bones and shells fossilize. As a result, soft organisms such as worms leave almost no footprints in the fossil record, while shelled invertebrates such as mollusks and corals are by far the most common fossil remains in the marine environment.

The notable exception among worms is the calcareous tubeworms of family Serpulidae. Because of their hard mineral tubes, these animals have the best fossil record of all worms. However, there is a problem – worms are recognized by the morphology of their soft bodies. As the serpulid tubes are the only structures that fossilize and thus are available for species recognition, the question arises whether the general tube shape is enough to link today’s species with their 230 to 65 million year old relatives. Luckily, recent studies showed that serpulid tubes often have a number of layers made of variably arranged calcium carbonate crystals. That is, looking at the tube wall very closely under a scanning electron microscope differentiates the tubes that might look the same at a first glance or, in some cases, justify the affinity of dissimilar tubes.

Serpulid tubes are typically cylindrical in shape and are attached to hard substrates mostly in shallow seas. This study focused on an unusual group that preadapted to inhabit soft muddy sediments of the deep sea by living in unattached polygonal (having four or more straight sides) tubes. Until now, we knew of only three such species from the deep sea: Spirodiscus grimaldii with quadrangular spirally coiled tubes, Bathyditrupa hovei with quadrangular tusk-shaped tubes, and Ditrupa groenlandica with octagonal tusk-shaped tubes. Similar unattached fossil tubes with tetragonal cross-sections, both coiled and tusk-shaped, were described from shallow-water Mesozoic deposits of Jurassic and Cretaceous age (172 – 66 million years old). But are animals living in such recognizable tubes closely related and should they be therefore placed in one genus? Do such Mesozoic tubes belong to the direct ancestors of the worms found in the deep sea today?

To answer these questions, we first revised the soft body morphology of deep-sea serpulids with tetragonal tubes. We then compared their tube ultrastructures using scanning electron microscopy (SEM) with those of morphologically similar fossils.

The taxonomic revision has revealed six species in 5 genera, including 3 new species and 1 new genus: Spirodiscus grimaldii, S. groenlandicus comb. nov., Bathyditrupa hovei, Bathyvermilia gregrousei sp. nov., Hyalopomatus dieteri sp. nov. and Zibrovermilia zibrowii gen. et sp. nov. Moreover, we discovered significant differences in fine tube structure between tubes of Recent deep-sea worms and their shallow-water Mesozoic counterparts.

Therefore, the answers to both question addressed in the study are negative. Similar tetragonal shapes of Recent tubes appear to be a result of parallel adaptations to soft-sediment habitats of the deep sea, not an indication that such animals are closely related. And based on ultrastructural data none of the Recent worms with tetragonal tubes appear to be direct descendants of Mesozoic species.

The results of the study highlight the importance of concerted efforts of paleontologists and zoologists for gaining further understanding of evolutionary history of marine organisms. Studying the fossil record of tubeworms can provide an invaluable tool for calibration of molecular clocks not only in serpulids, but by extrapolation also in other annelid groups that lack a fossil record.

Dr Elena Kupriyanova
Senior Research Scientist, AMRI

More information:

• Kupriyanova, E.K., Ippolitov, A.P. 2015. Deep-sea serpulids (Annelida: Polychaeta) in tetragonal tubes: on a tube convergence path from Mesozoic to Recent. Zootaxa: 4044 (2), 151-200