Solving a marine worm mystery: a detective story.

Identifying seaworms in the genus Terebellides was proving difficult due to a mystery surrounding the true identity of one particular species described over 180 years ago. Because this group of seaworms can be very abundant and plays a crucial role in seafloor ecosystem health, knowledge of their true diversity is important.

Way back in 1831, Michael Sars described a seaworm from a remote Norwegian locality which had a very distinctive type of gills, however the description was brief and the illustrations very basic. Over the next 180 odd years this species, Terebellides stroemii (Family Trichobranchidae), was reported as occurring all around the world, from cold arctic waters to the tropics, and from the shallow intertidal zone to deep waters. This was a very unlikely scenario for a seaworm!

This rampant misidentification was because this species of seaworm was described as having gills with four lobes consisting of numerous lamellae (thin plate-like structures) joined together and attached by a thick stalk to the body. All other species of the family were thought to have a different, branched or otherwise simpler gill arrangement. As a result, any species with this distinctive gill type was identified as Terebellides stroemii, and the “species” was reported from every corner of the seafloor. No other characters of the seaworms were even considered in identifying them!

<i>Terebellides akares</i>
Previously known as Terebellides stroemii - but now Terebellides akares. A new species described by Hutchings et al. 2015. Image: Alexander Semenov
© Alexander Semenov

In my work on seaworms, I became confused as to the real identity of the Australian species of Terebellides. One of the biggest problems was that it was really difficult to figure out exactly what Terebellides stroemii was: with so much variation in what people referred to as this species, it was hard to be sure other, related species were different. In order to solve this mystery, I needed to determine exactly what was the species described by Sars way back in 1831.

I set out to answer this question with the help of my colleague Julio by comparing the published illustrations of this species from around the world. Over the past 180 odd years it became obvious that several species had been confused. Trawling through the collections in the Oslo Museum and locating labels in Sars' handwriting and carefully reading Sars' original notes made while he was a Minister of religion in this remote village, we found preserved specimens of the seaworms he had originally used to describe the species. With these in hand, we fully redescribed it using modern day techniques.

Terebellides lilasae with abundant buccal tentacles
Terebellides lilasae with abundant buccal tentacles. Terebellides lilasae from France (Laveseque et al. 2019). Reproduced with permission from copyright holder. Paper citation: Lavesque, N., Hutchings, P., Dafffe G., Nygren, A., Londono-Mesa, M. 2019. A revision of the French Trichobranchidae (Polychaeta), with descriptions of nine new species. Zootaxa 4664(2): 151-190. Image: © Magnolia Press
© © Magnolia Press

Our research confirmed that Terebellides stroemii is actually restricted to this remote Norwegian location. We also discovered that there are many other useful characters that could be used to identify species within this genus. Now armed with a detailed and comprehensive description of Terebellides stroemii we can move forward to document the amazing diversity of this group using both morphological and molecular data.

We are increasingly finding that there are far more worms out there than we previously thought. Each of these species plays an important role in seafloor ecosystems and communities with higher diversities appear to be able to withstand change better than those with lower diversities. So being able to identify seaworms can be incredibly important for the management of seafloor ecosystems. 

Dr Pat Hutchings
Senior Principal Research Scientist

More information:
Parapar, J.& Hutchings,P.A. (2014). Redescription of Terebellides stroemii (Polychaeta, Trichobranchidae) and designation of a neotype. Journal of the Marine Biological Association of the UK.