Tomopterids are pelagic polychaetes—segmented worms that swim in the water column, never touching the seafloor. They are relatively common in the deep mesopelagic, or twilight zone. They are often only a centimeter or two long, but can grow to be quite large—over 30 centimeters (one foot). Tomopterids have been observed spewing yellow bioluminescent mucus, presumably to deter predators. Yellow bioluminescence is rare in the deep and scientists don’t yet know why this worm has adapted this ability. Image: Rob Sherlock © MBARI

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