Tomopterids are small segmented worms of the order Aciculata, measuring about one centimetre long. They are almost transparent and swim in the open ocean away from the shore. They can be found on the shore but not in great numbers.
They live in temperate waters, with adults swimming deeper than juveniles which can be found at the surface. Tomopterids are one of the few marine animals that show bioluminescence, displaying yellow when disturbed.
They swim using parapodia (muscular bristle-bearing appendages) as paddles to move through the sea. They have a trailing aciculum (needlelike spines) on each side behind the head.
Tomopterids cannot swim against a current and are holoplankton, staying in the water column their entire lives. They feed on fish larvae, chaetognaths, and tunicates.