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.