Upside-down behaviour of certain ribbon worms (phylum Nemertea)

Ribbon worms in the genus Balionemertes from Vietnam, the Philippines, Australia, and Guam—as well as Cephalothrix suni from Vietnam—were examined. Our observations indicate that the worms crawl mostly with their ventral surface upwards (the ventral surface being where the mouth opens), a behaviour that has not been documented in previous literature. Like many other worm species with colour patterns, they have a darker-coloured and/or more intensely patterned behavioural dorsal surface (= anatomical ventral surface in Balionemertes and C. suni) than the other side. This type of behavioural dorsoventral body-axis inversion among vermiform benthos seems to be rare—not having hitherto been known at least in the phylum Nemertea—and may be related to their feeding strategy, which should be observed in future studies.