Treasure of the month: The "dragon worm" puzzle

Our treasure of the month is a single marine bristle worm: the holotype of Phyllodoce callirhynchus - the very reference specimen on the basis of which the species was originally scientifically described and defined. It was collected in August 1890 on the East Frisian Islands by Johann Wilhelm Michaelsen, the first curator of the annelid collection at the Museum der Natur Hamburg.

Bristle worms of the genus Phyllodoce are sometimes called "dragon worms" because of their shimmering colours and the leaf-like appendages along their bodies. They live on the seabed, moving sinuously between grains of sand or algae and are active predators that hunt smaller invertebrates. To human eyes, many species in this group look surprisingly similar - slender, segmented, with fine rows of bristles along the sides of the body.

No other specimen of this species has appeared since the original description - a truly unique specimen.

But anyone looking at the jar today will discover a surprise: later researchers came to the conclusion that this worm is probably not a separate species, but belongs to the already known Mediterranean species Phyllodoce lineata. This is why the specimen is usually labelled under this name today. How does an "East Frisian" worm acquire a Mediterranean identity? This is where a story begins that goes far beyond this single specimen.

To this day, the scientific categorisation of species follows a system developed by Carl von Linné in the 18th century. The first species to be described according to this system came mainly from Europe. As researchers increasingly travelled the world in the 19th century, they collected new organisms in all oceans - including countless worms.

In the 20th century, however, a practical assumption prevailed: If worms from different regions look similar, they probably belong to the same species. This seemed plausible because many marine bristle worms have free-swimming larvae that can spread with ocean currents. After all, the oceans are interconnected.

But worms see the world differently than we do - or rather, they hardly see it at all. Most of them can only distinguish between light and dark. Visual signals therefore presumably play a lesser role for them than chemical stimuli or touch, for example. Striking external differences that would immediately catch our eye are correspondingly rare - many bristle worms look astonishingly similar to human eyes. This is precisely what makes them so difficult to identify: what looks almost identical on the outside can turn out to be a completely different species genetically. For decades, similar animals from different regions of the world were therefore grouped together under the same, often originally European, species names.

Only modern genetic methods have shown that this picture was too simple. Marine worms are probably much more diverse than the approximately 12,000 currently recognised species would suggest. Seemingly familiar names often conceal entire groups of previously unrecognised species.

And so back to our treasure: today, the East Frisian worm is usually categorised as Phyllodoce lineata - but whether this classification is actually correct has not been conclusively clarified. Is it really the same species as the Mediterranean species? Or is this a separate species that has been overlooked for more than a century? Open questions like these show why historical collections are more important today than ever before.

In order to systematically solve these puzzles, researchers at the LIB are working together with experts in genomics and annelids from the Senckenberg Nature Museum Frankfurt and the University of Göttingen in the "EuroWorm" project. The aim is to analyse European worm species using modern genetic methods, describe them in more detail and make the data openly accessible. This will create a new basis for global biodiversity research - and perhaps one day provide the answer to a surprisingly difficult question: How many worm species are there actually?

Until then, our "dragon worm" will remain a silent witness in a jar - and a reminder that even old collection items can still tell completely new stories.