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What lives off the Northwest coasts of Spain and shrinks to one-fifth of its size when alarmed? Enter the accordion worm. This recently identified marine ribbon worm, colored brown to dark green, measures 110–250 mm long and 3–4 mm wide in its unbothered state and can expand and contract its body to form regular dark-colored ring-like folds, much like the bellows of its namesake musical instrument.

The new species that was first spotted during sport dives at Ría de Arosa, Spain, belongs to the phylum Nemertea, a group of predatory worms that use venom to immobilize their prey. It has been named Pararosa vigarae in a recent study by researchers from Spain, Guam, and the United States in Royal Society Open Science.

Taxonomy is the science of systematically classifying and naming living organisms, which not only makes it easier communicate biological information but also helps us understand the evolutionary histories of organisms and their relationships with other species and the environment.

Scientists have always found it difficult to classify ribbon worms because, traditionally, species were identified based on their external features. But it turns out these organisms have very few visible traits to rely on. Alternatively, researchers began exploring the use of internal anatomy as an identifier, but the process is slowly being abandoned due to its tedious and complex nature.

Consequently, ribbon worms remain a lesser-known species with researchers suspecting that the true diversity of these nemerteans is likely much higher than the currently described 1,350 species.

For this study, specimens of the newly discovered species were collected from the subtidal zone at a depth of about 30 meters. In total, six specimens were found living under rocks and shells. To overcome the challenges of previous identification and classification methods, the team adopted a new approach that combined both visible traits and DNA analysis.

The specimens were kept alive for imaging and morphology processes and then placed in RNAlater—a non-toxic, aqueous, reagent that stabilizes and protects RNA in unfrozen samples—and stored at −80°C for DNA extraction.

Molecular and genetic analysis using small sections of specific genes—16S rRNA, 18S rRNA, 28S rRNA, COI, and histone H3—extracted from the posterior end of each specimen, revealed a distinct lineage of the worm in the Lineidae family.

Imaging experiments revealed that the newly discovered Pararosa vigarae, or accordion worm, contracts its body to form regular rings that are annular epidermal constrictions that remain visible even when the animal is fully stretched. The number of rings varies with the size of the worm—for instance, the largest specimen collected had 60 rings when fully stretched.

With the identification of Pararosa vigarae, the study expands the catalog of described ribbon worms in the often-overlooked invertebrate phylum. The researchers also emphasize that combining morphological and molecular data is key to accelerating our understanding of nemertean diversity and evolution.

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